Transmission

Abstract

To provide a transmission that can secure output torque during low-speed rotation, and can restrain an increase in temperature of hydraulic oil.SOLUTION: A transmission 21 comprises an input shaft 22, an output shaft 23, a planetary gear mechanism 26, a hydraulic pump 33, a first main pipe line 35A, a second main pipe line 35B, a hydraulic motor 34, a first variable relief valve 36A, a second variable relief valve 36B, a first check valve 37A, a second check valve 37B, a hydraulic oil tank 38, a charge pump 39, a charge pipe line 40, and a charge relief valve 41. The charge pipe line 40, more specifically, a connection pipe line 52 connecting the first variable relief valve 36A and the second variable relief valve 36B is provided with an additional tank 51 different from the hydraulic oil tank 38.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a transmission device mounted on a vehicle such as a wheel loader. [Background technology]

[0002] Vehicles (work vehicles) that perform excavation work, such as wheel loaders, perform excavation work by operating a loading device such as a bucket while moving forward at a low speed with a large driving force. Here, the wheel loader uses the vehicle's driving force to strongly push the bucket against the excavation target and scoop it up. At this time, by taking a larger amount of the excavation target into the bucket, the excavation work can be carried out more efficiently. For this reason, wheel loaders require a large vehicle driving force when working at a low speed.

[0003] The transmission installed in a wheel loader may be, for example, a hydrostatic continuously variable transmission (hereinafter also referred to as "HST: Hydraulic Static Transmission") that combines a planetary gear mechanism. The HST includes, for example, a hydraulic pump, a hydraulic motor, and a pair of pipes that connect these together in a closed circuit. The planetary gear mechanism includes three elements (rotating elements): a first element, a second element, and a third element. For example, the first element connected to the input shaft of the HMT is driven by the engine (prime mover). The second element drives the hydraulic pump of the HST. The third element is connected to the output shaft of the HMT.

[0004] This type of HMT, i.e., an input-split HMT in which all of the power from the engine is first input to the first element of the planetary gear mechanism, has a problem of reduced output torque (driving force) during low-speed rotation (low-speed driving). Specifically, if the hydraulic pump of the HST cannot fully receive the torque output from the second element of the planetary gear mechanism during low-speed rotation, the torque output from the third element of the planetary gear mechanism, and ultimately the torque output from the output shaft of the HMT (output torque), will decrease. In response to this, Patent Document 1 describes a transmission for an input-split HMT in which the capacity (maximum displacement) of the hydraulic motor of the HST is made larger than the capacity (maximum discharge) of the hydraulic pump to ensure a large output torque (driving force). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2016-512871 (Patent No. 6449233) Summary of the Invention [Problem to be solved by the invention]

[0006] If the HMT's hydraulic pump can adequately receive the torque output from the second element of the planetary gear mechanism to the HST's hydraulic pump during low-speed rotation (low-speed driving), the torque output from the third element of the planetary gear mechanism, and therefore the HMT's output torque (driving force), can be ensured. For example, to increase the torque that the HST's hydraulic pump can withstand, it is possible to increase the capacity of the hydraulic pump beyond the capacity corresponding to the target gear ratio of the HMT, and to ensure output torque during low-speed rotation by relieving the necessary amount of hydraulic oil with a variable relief valve while maintaining pressure within the HST's closed circuit. However, in this case, there is a risk that the HST's hydraulic oil will reach a high temperature in a short period of time due to the hydraulic oil passing through the variable relief valve.

[0007] An object of the present invention is to provide a transmission that can ensure output torque during low-speed rotation and can suppress temperature rise of the hydraulic oil. [Means for solving the problem]

[0008] The present invention preferably relates to a hydraulic system including an input shaft connected to a power source, an output shaft connected to a load, an input element provided between the input shaft and the output shaft and driven by the input shaft, and a planetary mechanism having two output elements that rotate based on rotation of the input element, a hydraulic pump driven by one of the two output elements, a hydraulic motor connected to the hydraulic pump via a pair of main lines, that is, a first main line and a second main line, a hydraulic oil tank that stores hydraulic oil, a charge pump that supplies hydraulic oil from the hydraulic oil tank to the first main line and the second main line, a charge line that connects the charge pump to the first main line and the second main line, and a charge relief valve that maintains pressure in the charge line at or below a set pressure. a first relief valve that is provided between the first main line and the charge line and opens when the pressure in the first main line reaches a set pressure; a first check valve that allows hydraulic oil to flow from the charge line side to the first main line side and blocks hydraulic oil from flowing from the first main line side to the charge line side; a second relief valve that is provided between the second main line and the charge line and opens when the pressure of hydraulic oil in the second main line reaches a set pressure; and a second check valve that allows hydraulic oil to flow from the charge line side to the second main line side and blocks hydraulic oil from flowing from the second main line side to the charge line side; and an additional reservoir that stores hydraulic oil separate from the hydraulic oil tank is provided in the charge line. [Effects of the Invention]

[0009] According to the present invention, it is possible to ensure output torque during low-speed rotation and to suppress a rise in temperature of the hydraulic oil. [Brief explanation of the drawings]

[0010] [Figure 1]1 is a left side view showing a wheel loader equipped with a transmission according to an embodiment. FIG. [Figure 2] 2 is a partially cutaway side view showing the transmission in FIG. 1. FIG. [Figure 3] 1 is a configuration diagram showing a transmission according to an embodiment; [Figure 4] FIG. 4 is a cross-sectional view showing an additional storage section (additional tank) in FIG. [Figure 5] FIG. 2 is a configuration diagram showing a transmission according to a first modified example. [Figure 6] FIG. 10 is a diagram showing the configuration of a transmission according to a second modified example. [Figure 7] FIG. 3 is a characteristic diagram showing the relationship between the HST speed ratio (speed ratio) and the HMT speed ratio (speed ratio). DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, a transmission according to an embodiment and a modification thereof will be described in detail with reference to the accompanying drawings, taking as an example a case where the transmission is applied to a vehicle (wheel loader).

[0012] Figures 1 to 4 show an embodiment. In Figure 1, a wheel loader 1 is a representative example of a vehicle (work vehicle). The wheel loader 1 is configured as an articulated work vehicle in which a front body 3, on which left and right front wheels 2 are mounted, and a rear body 5, on which left and right rear wheels 4 are mounted, are connected so that they can bend in the left-right direction. In other words, the front body 3 and the rear body 5 form the body of the wheel loader 1. A center hinge 6 and a steering cylinder (not shown) are provided between the front body 3 and the rear body 5. The front body 3 and the rear body 5 bend in the left-right direction around the center hinge 6 as the steering cylinder extends and retracts. This allows the operator to freely steer the wheel loader 1.

[0013] A loading device 7, also called a front device or a working device, is mounted on the front body 3 of the wheel loader 1 so that it can be raised and lowered. The loading device 7 is equipped with a loader bucket 7A. Meanwhile, the rear body 5 of the wheel loader 1 is equipped with a cab 8, the interior of which serves as a driver's compartment, an engine 9, a hydraulic pump 10, a speed change device 21, which is a transmission (power transmission device), and the like. Inside the cab 8, there is provided an accelerator pedal 8A as an operating member for accelerating the vehicle, and a forward / reverse switch lever 8B (hereinafter referred to as FNR lever 8B) for switching between forward and reverse movement and gear positions of the vehicle. Also provided inside the cab 8, although not shown, is a driver's seat, a steering wheel, a brake pedal, a parking brake switch, and the like.

[0014] The engine 9 is the power source (prime mover) of the wheel loader 1. The power source can be configured by the engine 9 alone, which is an internal combustion engine, or, for example, by an engine and an electric motor, or by an electric motor alone. The hydraulic pump 10 is connected to the engine 9. The hydraulic pump 10 is a hydraulic source for operating the cargo handling device 7.

[0015] A front axle 12 extending in the left-right direction is provided below the front vehicle body 3. Left and right front wheels 2 are attached to both ends of the front axle 12. Meanwhile, a rear axle 13 extending in the left-right direction is provided below the rear vehicle body 5. Left and right rear wheels 4 are attached to both ends of the rear axle 13.

[0016] The front axle 12 is connected to the transmission 21 via a front propeller shaft 14. The rear axle 13 is connected to the transmission 21 via a rear propeller shaft 15. The transmission 21 changes the speed (increases and / or decreases the speed) of the driving force of the engine 9 and transmits it to the front propeller shaft 14 and the rear propeller shaft 15. That is, the power from the engine 9 is transmitted to the transmission 21 coupled to the engine 9.

[0017] The power from the engine 9 has its rotation speed and rotation direction adjusted by the transmission 21, and is then transmitted from front and rear output shafts 23A, 23B of the transmission 21 to the front axle 12 and rear axle 13 via the front propeller shaft 14 and rear propeller shaft 15. That is, as shown in FIG. 2 , the transmission 21 includes an input shaft 22 connected to the engine 9, a front output shaft 23A connected to the front propeller shaft 14, and a rear output shaft 23B connected to the rear propeller shaft 15. The transmission 21 changes speeds and switches between forward and reverse rotation between the input shaft 22 and the output shafts 23A, 23B by switching the power transmission path within the transmission 21.

[0018] Next, a transmission 21 according to an embodiment will be described. In order to avoid complicating the drawing, FIG. 3 simply shows the output shaft 23 of the transmission 21 as a common output shaft 23 (=output shafts 23A, 23B) that transmits power to both the front axle 12 and the rear axle 13. That is, FIG. 3 does not show a configuration that splits power between the front output shaft 23A and the rear output shaft 23B via, for example, a center differential mechanism. FIG. 3 also does not show the hydraulic pump 10 that operates the loading device 7, i.e., the hydraulic pump 10 connected to the engine 9.

[0019] The transmission 21, which is a power transmission device for a vehicle, is made up of a hydromechanical continuously variable transmission 24 (hereinafter referred to as HMT 24) that combines a planetary gear mechanism 26 and a hydrostatic continuously variable transmission 31 (hereinafter referred to as HST 31). That is, the transmission 21 is equipped with the HMT 24 and a controller 25. The HMT 24 is equipped with an input shaft 22 as an input member, an output shaft 23 as an output member, the planetary gear mechanism 26 as a planetary mechanism, and the HST 31. The input shaft 22 is connected to the drive shaft of the engine 9, which is a power source (prime mover) mounted on the vehicle (wheel loader 1). The input shaft 22 is also connected to the planetary gear mechanism 26.

[0020] The planetary gear mechanism 26 and the HST 31 are provided between the input shaft 22 and the output shaft 23. Power input to the transmission 21 (HMT 24), i.e., power input to the input shaft 22, is output from the output shaft 23 via the planetary gear mechanism 26 and the HST 31 or via the planetary gear mechanism 26. The planetary gear mechanism 26 and the HST 31 change the speed of the driving force of the input shaft 22 and transmit it to the output shaft 23. The output shaft 23 is connected to a load 27 that serves as a running device for the vehicle.

[0021] The load 27 corresponds to the front axle 12, rear axle 13, front wheels 2, rear wheels 4, etc. of the wheel loader 1. In other words, the output shaft 23 outputs rotation to the front wheels 2 and / or rear wheels 4 via the front axle 12 and / or rear axle 13. Although not shown, a multi-speed change mechanism serving as a stepped speed change mechanism (auxiliary speed change mechanism) may be provided between the output shaft 23 and the load 27. Also, a direct coupling mechanism that directly connects the input shaft 22 and the output shaft 23 may be provided between them. Also, a clutch that switches the power transmission state may be provided.

[0022] Next, the HMT 24 will be described with reference to FIG.

[0023] The HMT 24 includes an input shaft 22, an output shaft 23, a planetary gear mechanism 26 as a planetary mechanism, and an HST 31. The HMT 24 also includes a first planetary output shaft 28 as a first transmission member, a second planetary output shaft 29 as a second transmission member, and a hydraulic output shaft 30 as a third transmission member. The first planetary output shaft 28 connects the planetary gear mechanism 26 with a hydraulic pump 33 of the HST 31. One end of the first planetary output shaft 28 has a first sun gear 26B, and the other end is connected to a rotary shaft 33A of the hydraulic pump 33 of the HST 31.

[0024] The second planetary output shaft 29 connects the planetary gear mechanism 26 and the output shaft 23. A second sun gear 26C is provided at one end of the second planetary output shaft 29, and a planetary output gear 29A is provided at the other end. The second planetary output shaft 29 is cylindrical, and the first planetary output shaft 28 is rotatably inserted inside. The hydraulic output shaft 30 connects the hydraulic motor 34 of the HST 31 and the output shaft 23. A hydraulic output gear 30A is provided at one end of the hydraulic output shaft 30, and the other end is connected to a rotating shaft 34A of the hydraulic motor 34 of the HST 31.

[0025] The output shaft 23 is equipped with an output gear 23G, and the output gear 23G is in mesh with a planetary output gear 29A of a second planetary output shaft 29 and a hydraulic output gear 30A of a hydraulic output shaft 30. The second planetary output shaft 29 transmits the power of the planetary gear mechanism 26 to the output shaft 23, and the hydraulic output shaft 30 transmits the power of a hydraulic motor 34 of the HST 31 to the output shaft 23.

[0026] The planetary gear mechanism 26 is connected to the input shaft 22 on the engine 9 side. The planetary gear mechanism 26 is connected to the HST 31 (hydraulic pump 33) on the first output side via a first planetary output shaft 28. The planetary gear mechanism 26 is connected to the output shaft 23 (output gear 23G) on the second output side via a second planetary output shaft 29. The planetary gear mechanism 26 has three elements (members): an input element (first element) connected to the engine 9, which is a power source, a first output element (second element) connected to the hydraulic pump 33 of the HST 31, and a second output element (third element) connected to the output shaft 23 side. That is, the planetary gear mechanism 26 has the input element (first element) driven by the input shaft 22 and two output elements (second element, third element) that rotate based on rotation of the input element (first element).

[0027] In this embodiment, planetary gear mechanism 26 is configured with carrier 26A and two sun gears 26B, 26C (first sun gear 26B and second sun gear 26C). That is, planetary gear mechanism 26 includes carrier 26A corresponding to the input element (first element), first sun gear 26B corresponding to the first output element (second element), second sun gear 26C corresponding to the second output element (third element), planet gear 26D, and balance gear 26E. Note that power transmission between first sun gear 26B, second sun gear 26C, planet gear 26D, and balance gear 26E does not have to be via meshing of gears (cogwheels), and may instead be via friction between rollers (outer circumferential surfaces), for example.

[0028] Engine 9 is coupled to carrier 26A via input shaft 22. First sun gear 26B is connected to hydraulic pump 33 of HST 31 via first planetary output shaft 28. Second sun gear 26C is connected to output shaft 23 via second planetary output shaft 29. That is, the rotation of first sun gear 26B is output to hydraulic pump 33 via first planetary output shaft 28. The rotation of second sun gear 26C is output to output shaft 23 via planetary output gear 29A of second planetary output shaft 29 and output gear 23G of output shaft 23.

[0029] The first sun gear 26B meshes with the balance gear 26E. The balance gear 26E meshes with the planet gear 26D (gear portion 26D1). The second sun gear 26C meshes with the planet gear 26D (gear portion 26D2). The planet gear 26D and the balance gear 26E are supported by the carrier 26A. Therefore, the planet gear 26D and the balance gear 26E rotate while revolving around the central axis of rotation of the planetary gear mechanism 26. The planet gear 26D includes a gear portion 26D1 that meshes with the balance gear 26E and a gear portion 26D2 that meshes with the second sun gear 26C.

[0030] According to the embodiment, balance gear 26E is provided between first sun gear 26B and planet gear 26D. However, this is not limiting, and balance gear 26E may be provided, for example, between second sun gear 26C and planet gear 26D. Furthermore, balance gear 26E may be provided both "between first sun gear 26B and planet gear 26D" and "between second sun gear 26C and planet gear 26D," or it is not necessary to provide balance gear 26E at both locations.

[0031] The HST 31 includes a hydraulic pump 33, a first main line 35A, a second main line 35B, and a hydraulic motor 34. The hydraulic pump 33 is driven by a first sun gear 26B, which is one of two output elements (first output element) of the planetary gear mechanism 26. That is, a rotating shaft 33A of the hydraulic pump 33 is connected to the planetary gear mechanism 26, more specifically, to the first sun gear 26B, via a first planetary output shaft 28. The rotating shaft 33A of the hydraulic pump 33 corresponds to the input shaft of the HST 31. When the rotating shaft 33A is rotationally driven, the hydraulic pump 33 circulates pressure oil through the pair of main lines 35A, 35B. The hydraulic pump 33 is configured, for example, by a variable displacement, swash plate-type hydraulic rotary machine. The hydraulic pump 33 may be of either a bi-directional or a single-directional type.

[0032] The hydraulic pump 33, which is the first variator, is a hydraulic rotating machine (hydraulic pump motor) that functions as a hydraulic pump when power is input from the first planetary output shaft 28, and functions as a hydraulic motor when power is output to the first planetary output shaft 28. The hydraulic pump 33 has a regulator 33B (swash plate) for adjusting the pump capacity (motor capacity). The regulator 33B of the hydraulic pump 33 is variably controlled based on a command (command signal) from the controller 25.

[0033] A pair of main lines 35A, 35B connect a pair of supply / discharge ports of the hydraulic pump 33 and a pair of supply / discharge ports of the hydraulic motor 34. That is, the hydraulic pump 33 and the hydraulic motor 34 are connected via a pair of main lines 35A, 35B, i.e., a first main line 35A and a second main line 35B. The hydraulic motor 34 is rotated by pressure oil supplied from the hydraulic pump 33. That is, the hydraulic pump 33 and the hydraulic motor 34 transmit power by the hydraulic oil flowing between them via the pair of main lines 35A, 35B. The rotating shaft 34A of the hydraulic motor 34 is connected to the output shaft 23 (output gear 23G) via the hydraulic output shaft 30. The rotating shaft 34A of the hydraulic motor 34 corresponds to the output shaft of the HST 31. The hydraulic motor 34 is configured, for example, by a variable displacement, swash plate-type hydraulic rotary machine. The hydraulic motor 34 may be of either bi-directional or uni-directional rotation.

[0034] The hydraulic motor 34, which is the second variator, is a hydraulic rotating machine (hydraulic pump motor) that functions as a hydraulic motor when outputting power to the hydraulic output shaft 30 and functions as a hydraulic pump when power is input from the hydraulic output shaft 30. The hydraulic motor 34 has a regulator 34B (swash plate) for adjusting the motor capacity (pump capacity). The regulator 34B of the hydraulic motor 34 is variably controlled based on a command (command signal) from the controller 25.

[0035] The HST 31 also includes a first variable relief valve 36A as a first relief valve, a second variable relief valve 36B as a second relief valve, a first check valve 37A, and a second check valve 37B. The HST 31 also includes a hydraulic oil tank 38, a charge pump 39, a charge line 40, and a charge relief valve 41. The first variable relief valve 36A and the first check valve 37A are arranged in parallel. The second variable relief valve 36B and the second check valve 37B are also arranged in parallel.

[0036] The first variable relief valve 36A and the second variable relief valve 36B are variable proportional relief valves whose set pressures (relief set pressure, relief start pressure) can be changed. The first variable relief valve 36A is provided between the first main line 35A and the charge line 40 and second main line 35B. The first variable relief valve 36A maintains the pressure of the hydraulic oil in the first main line 35A at or below the set pressure. The first variable relief valve 36A closes when the pressure in the first main line 35A is below a predetermined pressure (set pressure) and opens when the pressure exceeds the predetermined pressure (set pressure). In other words, the first variable relief valve 36A opens when the pressure in the first main line 35A reaches the set pressure.

[0037] The second variable relief valve 36B is provided between the second main line 35B and the charge line 40 and first main line 35A. The second variable relief valve 36B maintains the pressure of the hydraulic oil in the second main line 35B at or below a set pressure. The second variable relief valve 36B closes when the pressure in the second main line 35B is at or below a predetermined pressure (set pressure), and opens when the pressure exceeds the predetermined pressure (set pressure). In other words, the second variable relief valve 36B opens when the pressure in the second main line 35B reaches the set pressure.

[0038] The adjustable relief valves 36A, 36B are configured as electrically operated relief valves (for example, electromagnetic relief valves) whose valve opening pressure (relief pressure) changes based on a command (command signal) from the controller 25. That is, the set pressures of the adjustable relief valves 36A, 36B are changed based on a command (command signal) from the controller 25.

[0039] First check valve 37A and second check valve 37B are check valves that allow pressure oil to flow in one direction and prevent pressure oil from flowing in the reverse direction. First check valve 37A allows hydraulic oil to flow from charge line 40 and second main line 35B toward first main line 35A, and prevents hydraulic oil from flowing from first main line 35A toward charge line 40 and second main line 35B. In other words, first check valve 37A allows pressure oil to flow from second main line 35B and charge line 40 toward first main line 35A, and prevents pressure oil from flowing in the reverse direction.

[0040] Second check valve 37B allows hydraulic oil to flow from charge line 40 and first main line 35A to second main line 35B, and prevents hydraulic oil from flowing from second main line 35B to charge line 40 and first main line 35A. In other words, second check valve 37B allows pressure oil to flow from first main line 35A and charge line 40 to second main line 35B, and prevents pressure oil from flowing in the opposite direction.

[0041] The hydraulic oil tank 38 stores hydraulic oil. That is, the hydraulic oil tank 38 stores hydraulic oil to be supplied to the HST 31, more specifically, to a closed circuit formed by the pair of main lines 35A, 35B, the hydraulic pump 33, and the hydraulic motor 34. The hydraulic oil may be any of the following: hydraulic oil dedicated to the HST 31, hydraulic oil dedicated to the HMT 24 (shared by the HST 31 and the planetary gear mechanism 26), hydraulic oil shared by the HMT 24 and an auxiliary transmission mechanism (not shown), and hydraulic oil shared by the HMT 24, the auxiliary transmission mechanism, and the cargo handling device 7. In other words, the hydraulic oil tank 38 may be a tank dedicated to the HST 31, or a tank shared by at least one of the planetary gear mechanism 26, the auxiliary transmission mechanism, and the cargo handling device 7.

[0042] Charge pump 39 supplies hydraulic oil from hydraulic oil tank 38 to first main line 35A and second main line 35B. Charge line 40 connects charge pump 39 to first main line 35A and second main line 35B. Charge relief valve 41 maintains the pressure in charge line 40 at or below a set pressure. As will be described later, in this embodiment, an additional tank 51 that stores hydraulic oil separately from hydraulic oil tank 38 is provided midway along charge line 40.

[0043] The controller 25 is configured by, for example, a computer (microcomputer) equipped with an arithmetic circuit (CPU), memory, etc. The controller 25 controls the rotation speeds of the hydraulic pump 33 and the hydraulic motor 34. The controller 25 also controls the variable relief valves 36A, 36B. That is, the controller 25 adjusts the pump displacement and motor displacement, and controls the set pressures of the variable relief valves 36A, 36B. The memory of the controller 25 stores processing programs used for these control processes, that is, processing programs used to control the displacement (pump displacement) of the hydraulic pump 33 and the displacement (motor displacement) of the hydraulic motor 34, and processing programs used to control the set pressures of the variable relief valves 36A, 36B.

[0044] However, the split-input HMT, in which all of the engine power is first input to the first element of the planetary gear mechanism as described above, has the following problem. Specifically, when the wheel loader is traveling at low speed, in other words, when the rotational speed of the HMT output shaft is low, the displacement of the HST hydraulic pump is reduced. This causes the HST to shift into a deceleration region where the rotational speed of the hydraulic motor is lower than the rotational speed of the hydraulic pump. Figure 7 shows the relationship between the gear ratio (speed ratio) of the HST, the gear ratio (speed ratio) of the HMT, the displacement (tilt) of the hydraulic pump, and the displacement (tilt) of the hydraulic motor. As shown in Figure 7, the "deceleration region" of the HST corresponds to a state in which the motor displacement (hydraulic motor displacement) is larger than the pump displacement (hydraulic pump displacement), and the "acceleration region" of the HST corresponds to a state in which the motor displacement is smaller than the pump displacement. Furthermore, a state in which the pump displacement and the motor displacement are the same corresponds to the "constant speed" of the HST. The gear ratio corresponds to the speed ratio (= output rotational speed / input rotational speed).

[0045] In the deceleration region of the HST, i.e., when the pump displacement is reduced, the pressure in the line connecting the hydraulic pump and hydraulic motor (i.e., the pressure of the hydraulic oil in the closed circuit) easily reaches the relief pressure even with a small torque. This prevents the hydraulic pump from fully receiving the torque input to the first element of the planetary gear mechanism (HMT input torque), which is divided by the planetary gear mechanism and output to the hydraulic pump from the second element. In other words, even if the torque output from the second element is small, the pressure of the hydraulic oil in the line connecting the hydraulic pump and hydraulic motor reaches the relief pressure, causing the HST to slip at low torque. Furthermore, if the second element of the planetary gear mechanism cannot fully receive the torque input to the first element, the torque output from the third element connected to the output shaft also becomes small. This results in a decrease in the torque output from the HMT, i.e., the output torque (driving force) during low-speed rotation (low-speed driving).

[0046] To address this issue, it is possible to ensure that the output torque during low-speed rotation is ensured by having the hydraulic pump fully receive the torque output from the second element. Specifically, the pump capacity is set to be greater than the capacity corresponding to the target speed ratio of the HMT, allowing the hydraulic pump to receive the required torque. Then, a variable relief valve installed in the line connecting the hydraulic pump and the hydraulic motor is used to relieve the required amount of hydraulic oil while maintaining the required pressure, thereby intentionally slipping the HST. Below, we will explain how this allows the hydraulic pump to fully receive the torque output from the second element.

[0047] The relationship between torque, displacement, and pressure of a hydraulic pump and hydraulic motor can be expressed by the following equation 1. In equation 1, T is torque, P is pressure in the closed circuit, and V is displacement.

[0048]

number

[0049] As shown in equation 1, the torque T that the hydraulic pump can withstand is determined by the product of the pressure P in the closed circuit and the pump capacity V. For this reason, if the pressure P in the closed circuit is constant, for example, the maximum set pressure (maximum allowable pressure) of the variable relief valve, the torque T that the hydraulic pump can withstand is determined by the pump capacity V. Therefore, in the range where the wheel loader speed (vehicle speed) is low (HST deceleration range), the pump capacity V that corresponds to the target speed ratio of the HMT is small, and the torque T that the hydraulic pump can withstand is also small.

[0050] Therefore, in order to increase the torque T that the hydraulic pump can withstand, the pump capacity V is set to a value greater than the capacity corresponding to the target speed ratio (vehicle speed) of the HMT, and the pressure P in the closed circuit is maintained at the maximum set pressure (maximum allowable pressure) while the required amount is relieved by the variable relief valve. This maximizes the product of pressure P and pump capacity V, thereby increasing the torque T that the hydraulic pump can withstand.

[0051] Note that any discrepancy with the vehicle speed, i.e., the amount by which the pump capacity V is increased beyond the capacity corresponding to the HMT's target gear ratio (vehicle speed), can be adjusted to the target gear ratio (vehicle speed) by relieving the necessary amount of hydraulic oil and slipping the HST (reducing the hydraulic motor's rotational speed). Furthermore, if the set pressure of the variable relief valve is Ps and the pressure loss (override pressure) of the variable relief valve due to the relief flow rate is Po, then the pressure P in the closed circuit is calculated as "P = Ps + Po." Therefore, by adjusting the set pressure of the variable relief valve using the relief flow rate, the pressure P in the closed circuit can be adjusted to the maximum set pressure (maximum allowable pressure).

[0052] In this way, when the output shaft of the HMT is rotating at low speeds, the HST hydraulic pump receives the necessary torque from the second element of the planetary gear mechanism, while the variable relief valve allows the HST to slip, ensuring a large torque output from the third element of the planetary gear mechanism and, ultimately, the torque output from the HMT. However, this state presents a problem. As the hydraulic oil relieved by the variable relief valve circulates within the HST's closed circuit, it converts the power equivalent to the slip into heat. At this time, the volume of hydraulic oil in the closed circuit is small, resulting in a low heat capacity.

[0053] For this reason, the power that causes the HST to slip (relieve the hydraulic oil) can cause the hydraulic oil to reach a high temperature in a short period of time, which can have an adverse effect on the hydraulic equipment. In other words, if the HST is intentionally relieved and slipped, the temperature of the hydraulic oil in the closed circuit will rise, and this state may only be maintained for a short period of time. Therefore, in the embodiment, an HMT that combines a planetary gear mechanism and an HST is provided with a mechanism (oil sump mechanism) for suppressing the rise in oil temperature so that the rise in oil temperature can be suppressed even when a large driving force is generated when the vehicle (wheel loader) is rotating at low speeds.

[0054] That is, to extend the time that a large output torque (e.g., maximum output torque) can be maintained when the HMT is rotating at a low speed (when the vehicle is traveling at a low speed), it is necessary to ensure the heat capacity and heat dissipation amount within the closed circuit of the HST. Therefore, in this embodiment, an oil reservoir mechanism that increases the amount of hydraulic oil is added to the circuit through which hydraulic oil circulates during relief operation (i.e., between the adjustable relief valve of one main line and the adjustable relief valve of the other main line). That is, in this embodiment, as shown in FIGS. 3 and 4, a tank separate from the hydraulic oil tank 38, i.e., an additional tank 51 serving as an oil reservoir mechanism, is provided between the first adjustable relief valve 36A of the first main line 35A and the second adjustable relief valve 36B of the second main line 35B. This increases the heat capacity within the circuit through which hydraulic oil circulates.

[0055] Furthermore, the oil sump mechanism (additional tank 51) is connected to the charge line and charge relief valve. This uses the excess flow rate from the charge to the HST to replace the high-temperature hydraulic oil in the oil sump mechanism and the closed circuit of the HST with low-temperature hydraulic oil charged from the hydraulic oil tank. In other words, when the wheel loader is operating under high load, low-load or no-load conditions may occur intermittently, and at these times the low-temperature hydraulic oil charged from the hydraulic oil tank flows into the oil sump mechanism, pushing out and replacing the high-temperature hydraulic oil stored in the oil sump mechanism.

[0056] In this embodiment, adding an oil reservoir mechanism (additional tank 51) to the HST closed circuit increases the heat capacity of the entire closed circuit. Furthermore, using the excess charge flow rate to replace the hydraulic oil in the HST closed circuit suppresses temperature increases in the hydraulic oil in the closed circuit. This significantly extends the operating time at low speeds and high torques. Furthermore, during low-load operation and no-load operation, where heat generation during high-load operation is small, most of the charge flow rate can be used to quickly replace the hydraulic oil in the oil reservoir mechanism and the HST closed circuit. Therefore, during intermittent work at low speeds and high torques, oil temperature increases can be stably suppressed, enabling continuous work. These points are explained in detail below.

[0057] As shown in Fig. 3, the transmission 21 is configured to include an input-split type HMT 24. That is, the transmission 21 is equipped with the HMT 24 and a controller 25. The HMT 24 is equipped with an input shaft 22 serving as an input member, an output shaft 23 serving as an output member, a planetary gear mechanism 26, and an HST 31. The HST 31 is equipped with a hydraulic pump 33 serving as a first variator, a hydraulic motor 34 serving as a second variator, a first main line 35A and a second main line 35B, a first variable relief valve 36A and a second variable relief valve 36B, an additional tank 51 serving as an oil reservoir mechanism (additional reservoir), a charge relief valve 41, a charge pump 39, an oil temperature gauge 57, and a hydraulic oil tank 38.

[0058] The input shaft 22 is connected to the input side of the transmission 21 (HMT 24), i.e., to the engine 9 which serves as the power source (prime mover) of the vehicle (wheel loader). The output shaft 23 is connected to the output side of the transmission 21 (HMT 24), i.e., to a load 27 (axles 12, 13, wheels 2, 4) which serves as the traveling device of the vehicle (wheel loader). The planetary gear mechanism 26 is provided between the input shaft 22 and the output shaft 23. The planetary gear mechanism 26 is configured around a carrier 26A which serves as a first element. That is, the planetary gear mechanism 26 is configured to include the carrier 26A which serves as the first element (input element), a first sun gear 26B which serves as a second element (first output element), and a second sun gear 26C which serves as a third element (second output element).

[0059] The hydraulic pump 33 is connected to the first sun gear 26B of the planetary gear mechanism 26. The hydraulic motor 34 is provided separately from the hydraulic pump 33. The first main line 35A and the second main line 35B are connected to the hydraulic pump 33 and the hydraulic motor 34, respectively. When the engine 9 is driving the load 27, the first main line 35A corresponds to a drive-side high-pressure flow path that is high-pressure, and the second main line 35B corresponds to a drive-side low-pressure flow path that is low-pressure. Conversely, when the engine 9 is "driven," that is, when the engine 9 is driven by the load 27 (during engine braking), the first main line 35A becomes low-pressure and the second main line 35B becomes high-pressure.

[0060] The first adjustable relief valve 36A is connected to the first main line 35A. The second adjustable relief valve 36B is connected to the second main line 35B. The additional tank 51 is provided between the first adjustable relief valve 36A and the second adjustable relief valve 36B. The charge relief valve 41 is connected to the additional tank 51. The charge pump 39 is connected to the additional tank 51. The additional tank 51 is maintained at or below the set pressure of the charge relief valve 41 by the charge pump 39 and the charge relief valve 41. In other words, the additional tank 51 corresponds to a pressurized hydraulic oil tank.

[0061] The oil temperature gauge 57 is connected to the additional tank 51. The oil temperature gauge 57 is an oil temperature sensor that detects the temperature of the hydraulic oil in the additional tank 51. The oil temperature gauge 57 is connected to the controller 25. The oil temperature gauge 57 outputs a signal (temperature signal) corresponding to the detected temperature of the hydraulic oil to the controller 25. The hydraulic oil tank 38 is connected to the charge pump 39 and the charge relief valve 41. The hydraulic oil tank 38 is a tank separate from the additional tank 51. The hydraulic oil tank 38 corresponds to all hydraulic oil tanks that store hydraulic oil for the HST 31.

[0062] The controller 25 is connected to the hydraulic pump 33 (regulator 33B), the hydraulic motor 34 (regulator 34B), the first variable relief valve 36A, the second variable relief valve 36B, the engine 9, the oil temperature gauge 57, etc. The controller 25 changes the rotation speed ratio between the hydraulic pump 33 and the hydraulic motor 34. That is, the controller 25 adjusts (controls) the rotation speed ratio between the hydraulic pump 33 and the hydraulic motor 34 by manipulating the swash plate tilt angle of the hydraulic pump 33 and the swash plate tilt angle of the hydraulic motor 34. The controller 25 also controls the relief pressure of the first variable relief valve 36A and the relief pressure of the second variable relief valve 36B. Furthermore, the controller 25 controls the rotation speed and torque of the engine 9.

[0063] The additional tank 51 is provided, for example, inside the case of the HMT 24. In this case, the additional tank 51 may be a tank integrated into the case of the HMT 24, or may be a tank separate from the case of the HMT 24. The location of the additional tank 51, i.e., the location of the additional tank 51 inside the case of the HMT 24, is not limited.

[0064] Planetary gear mechanism 26 is configured to include three gear elements: carrier 26A, first sun gear 26B, and second sun gear 26C. Carrier 26A, first sun gear 26B, and second sun gear 26C all have the same central rotation axis. A balance gear 26E, which serves as a counter gear, is provided within carrier 26A. Balance gear 26E causes the rotation directions of first sun gear 26B and second sun gear 26C to be the same as the relationship between the sun gear and ring gear of a typical planetary gear mechanism configured with three elements: a carrier, sun gear, and ring gear.

[0065] The first sun gear 26B is connected to a hydraulic pump 33 of the HST 31. The second sun gear 26C is connected to the output shaft 23 via a planetary output gear 29A of the second planetary output shaft 29 and an output gear 23G meshing with the planetary output gear 29A. The hydraulic motor 34 of the HST 31 is connected to the output shaft 23 via a hydraulic output gear 30A of the hydraulic output shaft 30 and an output gear 23G meshing with the hydraulic output gear 30A.

[0066] When power is input from engine 9 to carrier 26A of planetary gear mechanism 26, the torque and rotation speed are distributed to first sun gear 26B and second sun gear 26C. The torque and rotation speed distributed to first sun gear 26B drive hydraulic pump 33 of HST 31 as hydraulically transmitted power distribution. The torque and rotation speed distributed to second sun gear 26C drive output shaft 23 via planetary output gear 29A and output gear 23G as mechanically transmitted power distribution.

[0067] The hydraulic motor 34 of the HST 31 is driven by receiving a supply of hydraulic oil from the hydraulic pump 33. The hydraulic motor 34 drives the output shaft 23 via the hydraulic output gear 30A and the output gear 23G as a hydraulically transmitted power distribution. In this way, the mechanically transmitted power distribution and the hydraulically transmitted power distribution are combined at the output shaft 23, and the output shaft 23 is driven. In other words, the mechanically transmitted power distribution and the hydraulically transmitted power distribution are combined at the output shaft 23, and are output to the load 27 as the torque and rotation speed of the output shaft 23.

[0068] The following describes the distribution of torque in planetary gear mechanism 26. The input torque of planetary gear mechanism 26 is input to carrier 26A. The torque input to carrier 26A is distributed to first sun gear 26B and second sun gear 26C in a ratio determined by the number of teeth of first sun gear 26B, second sun gear 26C, and gear portion 26D1 and gear portion 26D2 of planet gear 26D.

[0069] The rotation speed and torque output from the split-input HMT 24 will now be described. The controller 25 changes the displacement of the hydraulic pump 33 and the hydraulic motor 34 by changing the tilt angle of the swash plates of each of the hydraulic pump 33 and the hydraulic motor 34. This controls the rotation speed ratio (gear ratio) between the hydraulic pump 33 and the hydraulic motor 34. The gear ratio of the HMT 24, i.e., the gear ratio of the output shaft 23 to the input shaft 22, is determined by changing the displacement of the hydraulic pump 33 and the hydraulic motor 34 and controlling the rotation speed ratio (gear ratio) between the hydraulic pump 33 and the hydraulic motor 34. This determines the rotation speed and torque output from the output shaft 23.

[0070] During normal traveling, the gear ratio is determined as described above, and the wheel loader 1 travels. Furthermore, when it is desired to ensure a large output torque when the wheel loader 1 is traveling at low speed, that is, when the HST 31 is in the deceleration range, unlike during normal traveling, the capacity (tilt) of the hydraulic pump 33 is ensured to be larger than the capacity corresponding to the target gear ratio of the HMT 24. This creates a state in which the hydraulic pump 33 can receive a large torque. Then, in order to set the gear ratio of the HMT 24 to the target gear ratio, an appropriate amount of hydraulic oil is relieved from the first variable relief valve 36A of the HST 31, causing the HST 31 to slip. This makes it possible to ensure a large output torque (driving force) during low-speed traveling.

[0071] The output torque of the HMT 24 is determined by the sum of the mechanically transmitted torque distribution and the hydraulically transmitted torque distribution. When the wheel loader 1 is traveling at low speed (the HST 31 is in the deceleration range), the hydraulically transmitted torque distribution is determined by the hydraulic pressure in the closed circuit of the HST 31 and the capacity (maximum capacity) of the hydraulic motor 34 that is at its maximum tilt during low-speed traveling. In this case, if the capacity of the hydraulic pump 33 is small during normal low-speed traveling, the hydraulic pressure will easily reach relief pressure or high pressure even if the torque input from the engine 9 and distributed by the planetary gear mechanism 26 is small. For this reason, the hydraulic motor 34 generates relief pressure torque, i.e., high torque, and the hydraulically transmitted torque distribution can easily ensure a large output torque (maximum output torque).

[0072] However, if left as is, the mechanical transmission torque distribution, which is the other output of the planetary gear mechanism 26, will become small. In other words, even if the torque distributed from the planetary gear mechanism 26 is small, the hydraulic pump 33 will easily reach relief pressure and will not be able to fully receive the distributed torque. This causes the planetary gear mechanism 26 to become stuck, and the mechanical transmission torque distribution will become small. For this reason, the torque that can be output with the capacity (tilt) of the hydraulic pump 33 during normal low-speed driving will be small, and the HMT 24 will not be able to generate large output torque, or maximum output torque.

[0073] Therefore, if it is desired to ensure a large output torque (maximum output torque) from the HMT 24 during low-speed travel in which the HST 31 is in a deceleration range, the capacity (tilting) of the hydraulic pump 33 is set to be larger than the capacity (tilting) during normal low-speed travel. This ensures that the hydraulic pump 33 is kept at a pressure close to the relief pressure with the distributed torque from the planetary gear mechanism 26, without reaching the relief pressure, and ensures that the hydraulic pump 33 can reliably receive torque, thereby preventing a stuck state from occurring in the planetary gear mechanism 26. As a result, the mechanical transmission torque distribution, which is the other output of the planetary gear mechanism 26, can be reliably transmitted. In other words, it is possible to transmit a large output torque (maximum output torque) from the mechanical transmission torque distribution, and it becomes possible to generate a large output torque (maximum output torque) from the HMT 24 during low-speed travel.

[0074] Note that ensuring a large capacity for the hydraulic pump 33 increases the output rotation speed of the HMT 24. Therefore, in order to match the output rotation speed of the HMT 24 to the output rotation speed that results in the target gear ratio, the first variable relief valve 36A is used to relieve an appropriate amount of hydraulic oil while maintaining the necessary pressure. This causes the HST 31 to slip, lowering the output rotation speed of the HMT 24. This makes it possible for the HMT 24 to generate a large output torque (maximum output torque) even when driving at low speeds.

[0075] However, when the HST 31 is slipped to transmit power from the HMT 24, the hydraulic oil discharged from the hydraulic pump 33 passes through the first main line 35A, and the hydraulic oil equivalent to the slippage is relieved from the first adjustable relief valve 36A. The hydraulic oil relieved from the first adjustable relief valve 36A (relief flow rate) returns to the hydraulic pump 33 via the second check valve 37B and the second main line 35B. In this circulation of the flow equivalent to the relief flow rate, when the hydraulic oil passes through the first adjustable relief valve 36A, the large hydraulic energy used for driving is converted into heat. As a result, the temperature of the hydraulic oil in the closed circuit of the HST 31 rises in a short period of time. This rise in oil temperature occurs when the amount of oil in the closed circuit is small, the heat capacity is very small, and the input hydraulic energy is very large.

[0076] To address this issue, in this embodiment, an additional tank 51 is provided between the first adjustable relief valve 36A and the second adjustable relief valve 36B. That is, the additional tank 51 serving as an additional reservoir for storing hydraulic oil is provided in the charge line 40, separate from the hydraulic oil tank 38. In this case, the additional tank 51 is provided midway through a connecting line 52 that connects the first adjustable relief valve 36A and the second adjustable relief valve 36B.

[0077] The charge line 40 includes a connecting line 52 that connects the first adjustable relief valve 36A and the second adjustable relief valve 36B. In this case, the connecting line 52 includes a first connecting line 52A that connects the first adjustable relief valve 36A and the additional tank 51, and a second connecting line 52B that connects the second adjustable relief valve 36B and the additional tank 51. The first adjustable relief valve 36A and the first main line 35A are connected by a first branch line 53A that branches off from the first main line 35A. The second adjustable relief valve 36B and the second main line 35B are connected by a second branch line 53B that branches off from the second main line 35B. With the above configuration, the heat capacity of the hydraulic oil in the closed circuit can be increased by the capacity of the additional tank 51, thereby suppressing a rise in temperature. Furthermore, the flow rates of both the high-temperature hydraulic oil flowing through the first connecting pipe 52A and the low-temperature hydraulic oil flowing through the charge pipe 40 decrease significantly when they enter the additional tank 51. As a result, the hydraulic oils of different temperatures mix effectively, and hydraulic oil that is sufficiently cooler than the hydraulic oil flowing through the first connecting pipe 52A flows out into the second branch pipe 52B.

[0078] A first bypass line 54A that bypasses the first adjustable relief valve 36A is provided between the first branch line 53A and the first connecting line 52A. A second bypass line 54B that bypasses the second adjustable relief valve 36B is provided between the second branch line 53B and the second connecting line 52B. A first check valve 37A is provided in the first bypass line 54A. A second check valve 37B is provided in the second bypass line 54B.

[0079] The capacity of the additional tank 51 is, for example, 10 times or more the volume of hydraulic oil discharged in one rotation of the hydraulic pump 33 (maximum pump volume). That is, the capacity of the additional tank 51 is preferably large from the viewpoint of ensuring heat capacity. For example, the capacity of the additional tank 51 is 10 times or more, preferably 50 times or more, and more preferably 100 times or more the maximum pump volume discharged in one rotation of the hydraulic pump 33. On the other hand, the capacity of the additional tank 51 is preferably small from the viewpoint of the mountability and strength (cost) of the additional tank 51. For example, the capacity of the additional tank 51 is 500 times or less, preferably 300 times or less, and more preferably 200 times or less the maximum pump volume discharged in one rotation of the hydraulic pump 33. The capacity of the additional tank 51 can be set in consideration of the required heat capacity, mountability, and strength (cost).

[0080] In the embodiment, the additional reservoir that stores the hydraulic oil is configured as the additional tank 51, that is, a container (tank) separate from the connecting pipe line 52 (first connecting pipe line 52A, second connecting pipe line 52B). However, the present invention is not limited to this, and the additional reservoir may be configured, for example, by a pipe line and / or a hose disposed between the first adjustable relief valve 36A and the second adjustable relief valve 36B. That is, the additional reservoir may be configured by providing a pipe line and / or a hose (a pipe line and / or a hose with a large inner diameter) between the first adjustable relief valve 36A and the second adjustable relief valve 36B that can secure a hydraulic oil capacity similar to that of the additional tank 51. In other words, the additional reservoir that stores the hydraulic oil may be configured by a pipe line and / or a hose that can serve as an oil reservoir for the hydraulic oil and perform the necessary heat exchange (suppression of temperature rise).

[0081] The charge pump 39 and the charge relief valve 41 are connected to the additional tank 51. That is, in addition to the connecting pipe 52, the charge pipe 40 is equipped with a supply pipe 55 that supplies hydraulic oil discharged from the charge pump 39 to the additional tank 51, and a return pipe 56 that returns hydraulic oil to the hydraulic oil tank 38. The upstream side of the supply pipe 55 is connected to the charge pump 39, and the downstream side of the supply pipe 55 is connected to the additional tank 51. The upstream side of the return pipe 56 is connected to the additional tank 51, and the downstream side of the return pipe 56 is connected to the hydraulic oil tank 38.

[0082] Hydraulic oil is supplied to additional tank 51 through charge pump 39. The pressure in additional tank 51 (and connecting pipe 52) is maintained at or below a set pressure by charge relief valve 41. Additional tank 51 is disposed upstream of charge relief valve 41, first check valve 37A, and second check valve 37B, and downstream of charge pump 39, first variable relief valve 36A, and second variable relief valve 36B.

[0083] In a high oil pressure state (relief pressure state) where the HMT 24 generates a large torque, the excess charge flow from the charge pump 39 pushes the high-temperature hydraulic oil in the additional tank 51 through the charge relief valve 41 and into the hydraulic oil tank 38. As a result, the high-temperature hydraulic oil in the additional tank 51 is replaced with low-temperature hydraulic oil. This prevents the oil temperature in the closed circuit from rising.

[0084] Furthermore, when heat generation is low between high-torque operations, i.e., when there is low oil pressure at low torque or no load, the hot hydraulic oil in the additional tank 51 is pushed into the hydraulic oil tank 38 by the remaining charge flow, which is mostly (almost all) surplus. As a result, the hydraulic oil in the closed circuit is replaced significantly in a short period of time. This significantly reduces the oil temperature in the closed circuit. Therefore, during short periods of low-load or no-load operation between high-load operations, the oil temperature in the closed circuit, including the additional tank 51, can be significantly reduced, enabling continuous high-load operation.

[0085] As described above, in this embodiment, the additional tank 51 is provided, to which the charge pump 39 and the charge relief valve 41 are connected. This allows the amount of oil in the closed circuit to be increased, thereby increasing the heat capacity. Furthermore, by retaining the flow of hydraulic oil in the additional tank 51, the amount of excess charge oil in each hydraulic state can be used to replace the high-temperature hydraulic oil in the closed circuit including the additional tank 51. Therefore, by replacing the hydraulic oil in the additional tank 51 with cooled hydraulic oil (hydraulic oil to be charged), a large margin of heat capacity can be secured for high-load operations.

[0086] The additional tank 51 also includes an oil temperature gauge 57. This allows the temperature of the hydraulic oil to be monitored by the oil temperature gauge 57. In this case, when the oil temperature rises, for example, when the oil temperature exceeds a preset threshold, the power transmission that causes the HST 31 to slip can be suppressed. Also, for example, the power input from the engine 9 can be suppressed. These controls, that is, controls to suppress "slip of the HST 31" and / or "power of the engine 9" until the oil temperature drops, can be performed by the controller 25.

[0087] As described above, according to the embodiment, the HST 31 includes the additional tank 51 as an additional storage section. The additional tank 51, which is a tank separate from the hydraulic oil tank 38, is provided in the charge line 40 (more specifically, the connecting line 52 that connects the first adjustable relief valve 36A and the second adjustable relief valve 36B). Therefore, the amount of hydraulic oil that can be stored in the additional tank 51 can be increased by the amount that can be stored in the closed circuit that is made up of the hydraulic pump 33, the hydraulic motor 34, the first main line 35A, the second main line 35B, and the charge line 40 (connecting line 52).

[0088] This increases the heat capacity of the hydraulic oil in the closed circuit, thereby suppressing a rise in temperature of the hydraulic oil. Therefore, even if the capacity of the hydraulic pump 33 during low-speed rotation is set to be greater than the capacity corresponding to the target gear ratio and the hydraulic oil in the main line 35A (35B) is relieved by the variable relief valve 36A (36B) while maintaining the pressure in the main line 35A (35B), it is possible to suppress a rise in temperature of the hydraulic oil and ensure output torque. Therefore, it is possible to ensure output torque during low-speed rotation and suppress a rise in temperature of the hydraulic oil.

[0089] According to the embodiment, additional tank 51 is disposed upstream of charge relief valve 41, first check valve 37A, and second check valve 37B, and downstream of charge pump 39, first adjustable relief valve 36A, and second adjustable relief valve 36B. Therefore, hydraulic oil relieved from first main line 35A through first adjustable relief valve 36A can be stored in additional tank 51. Also, hydraulic oil relieved from second main line 35B through second adjustable relief valve 36B can be stored in additional tank 51.

[0090] According to the embodiment, the capacity of the additional tank 51, which is the additional storage section, is 10 times or more the volume of the hydraulic oil discharged in one rotation of the hydraulic pump 33. Therefore, the heat capacity of the hydraulic oil in the closed circuit can be ensured by the hydraulic oil stored in the additional tank 51.

[0091] According to the embodiment, the first variable relief valve 36A and the second variable relief valve 36B are variable proportional relief valves with variable set pressures, which allows the variable relief valve 36A (36B) to ensure the pressure in the main line 35A (35B) while relieving the necessary amount of hydraulic oil from the main line 35A (35B).

[0092] In the embodiment, the HST 31 has been described as having an oil reservoir mechanism (additional tank 51) for suppressing an increase in oil temperature. However, the present invention is not limited to this. For example, the HST may have a heat dissipation mechanism (e.g., an oil cooler) for cooling the hydraulic oil stored in the oil reservoir mechanism in addition to the oil reservoir mechanism (e.g., a tank). That is, in a first modified example shown in FIG. 5, the HST 31 has an additional tank 51 and an oil cooler 61.

[0093] As shown in FIG. 5 , in the first modified example, an oil cooler 61 is added between the charge relief valve 41 and the hydraulic oil tank 38. In other words, the return pipe 56 is provided with the oil cooler 61 located downstream of the charge relief valve 41 to cool the hydraulic oil. According to this first modified example, the high-temperature hydraulic oil drained from the closed circuit to the hydraulic oil tank 38 can be effectively cooled by the oil cooler 61 before returning to the hydraulic oil tank 38. That is, according to the first modified example, the oil cooler 61 that cools the hydraulic oil is provided downstream of the charge relief valve 41, and therefore the hydraulic oil can be cooled by this oil cooler 61. This also makes it possible to suppress a rise in the temperature of the hydraulic oil.

[0094] In the first modified example, an example in which an oil cooler 61 is provided in the return pipe 56 has been described. However, this is not limiting, and for example, as in a second modified example shown in FIG. 6 , the oil cooler 61 and the oil pump 62 may be connected in series to the additional tank 51. That is, in the second modified example, the oil cooler 61 and the oil pump 62 are added to the additional tank 51. The hydraulic oil in the additional tank 51 discharged by the oil pump 62 flows into the oil cooler 61 and passes through the oil cooler 61. The hydraulic oil cooled by the oil cooler 61 then returns to the additional tank 51. This allows the hydraulic oil in the additional tank 51 to directly dissipate heat (cool).

[0095] That is, in the second modified example, a circulation line 63 that cools the hydraulic oil is connected to the additional tank 51 that serves as the additional storage section. In addition, an oil cooler 61 that cools the hydraulic oil and an oil pump 62 that serves as a cooling pump that supplies the hydraulic oil in the additional tank 51 to the oil cooler 61 are provided in series in the circulation line 63. Therefore, the hydraulic oil can be cooled by driving the oil pump 62 to guide the hydraulic oil in the additional tank 51 directly to the oil cooler 61.

[0096] In this case, the oil pump 62 can be configured to be activated when the temperature of the hydraulic oil reaches a predetermined value or higher. That is, when the temperature of the hydraulic oil is low, i.e., when the temperature of the hydraulic oil is below a first predetermined value, the oil pump 62 is not activated. Then, when the temperature of the hydraulic oil reaches or exceeds the first predetermined value, the oil pump 62 is activated to supply the hydraulic oil from the additional tank 51 to the oil cooler 61. This allows the hydraulic oil in the additional tank 51 to be cooled. Then, when the temperature of the hydraulic oil falls to or below a second predetermined value that is lower than the first predetermined value, the oil pump 62 is stopped. For example, if the maximum allowable temperature of the hydraulic oil is 80°C, the oil pump 62 can be started when the temperature of the hydraulic oil reaches 70°C and stopped when the temperature of the hydraulic oil drops to 50°C. According to this second modification, the oil pump 62 is activated when the temperature of the hydraulic oil reaches or exceeds a predetermined value, thereby allowing the hydraulic oil to be cooled when cooling is required.

[0097] In the embodiment, an example has been described in which the hydraulic output gear 30A of the hydraulic output shaft 30 connected to the rotating shaft 34A of the hydraulic motor 34 is configured to mesh with the output gear 23G of the output shaft 23. However, the present invention is not limited to this, and for example, the hydraulic output gear 30A of the hydraulic output shaft 30 connected to the rotating shaft 34A of the hydraulic motor 34 may be meshed with the second planetary output shaft 29 (planetary output gear 29A) that connects the planetary gear mechanism 26 (second sun gear 26C) and the output shaft 23. This also applies to the first and second modified examples.

[0098] In the embodiment, the planetary gear mechanism 26 has been described as an example of the planetary mechanism that constitutes the HMT 24. That is, in the embodiment, the planetary members of the planetary mechanism have been described as planetary gears. However, this is not a limitation, and the planetary members of the planetary mechanism may be composed of members other than gears, such as planetary rollers. Furthermore, the planetary gear mechanism is not limited to a planetary gear mechanism composed of three elements: a carrier, a first sun gear, and a second sun gear. For example, a planetary gear mechanism composed of three elements: a carrier, a sun gear, and a ring gear may be used.

[0099] Furthermore, it is possible to freely select which of the three elements of the planetary gear mechanism is connected to which member. That is, it is possible to freely select which of the three elements of the planetary gear mechanism is an input element driven by an input shaft, which is one output element (first output element) connected to a hydraulic pump, and which is another output element (the other output element, second output element) other than the one output element (first output element). For example, in the case of a planetary gear mechanism consisting of three elements, a carrier, a first sun gear, and a second sun gear, the carrier may be the input element and the two sun gears may be the two output elements, or a different combination may be used. Also, in the case of a planetary gear mechanism consisting of three elements, a carrier, a sun gear, and a ring gear, the carrier may be the input element and the sun gear and the ring gear may be the two output elements, or a different combination may be used. The same applies to the first and second modified examples.

[0100] In the embodiment, an example has been described in which the transmission 21 is mounted on a wheel loader 1. However, the present invention is not limited to this, and the transmission 21 may be mounted on work vehicles (construction machinery) other than wheel loaders, such as hydraulic excavators, hydraulic cranes, dump trucks, forklifts, tractors, agricultural vehicles, etc. Furthermore, the transmission is not limited to work vehicles, and can be widely applied as a transmission incorporated in various vehicles such as automobiles and railroad cars, or various industrial and general machinery. This also applies to the first and second modified examples.

[0101] Furthermore, the above-described embodiment and each modification are merely examples, and it goes without saying that partial substitution or combination of the configurations shown in different embodiments and each modification is possible. [Explanation of symbols]

[0102] 1 Wheel loader (work vehicle) 9 Engine (power source) 21 Transmission 22 Input shaft (input member) 23, 23A, 23B Output shaft (output member) 26 Planetary gear mechanism (planetary mechanism) 26A Carrier (Input or Output Element) 26B First sun gear (input element or output element) 26C Second sun gear (input element or output element) 27 Load 33 Hydraulic pump (first variator) 34 Hydraulic motor (second variator) 35A 1st main pipe (main pipe) 35B 2nd main pipe (main pipe) 36A First Adjustable Relief Valve (First Relief Valve) 36B Second variable relief valve (Second relief valve) 37A First check valve 37B Second check valve 38 Hydraulic oil tank 39 Charge Pump 40 Charging Pipe 41 Charge relief valve 51 Additional tank (additional storage section) 61 Oil cooler 62 Oil pump (cooling pump) 63 Circulation pipeline

Claims

1. an input shaft connected to a power source; An output shaft connected to a load; a planetary mechanism provided between the input shaft and the output shaft, the planetary mechanism having an input element driven by the input shaft and two output elements that rotate based on rotation of the input element; a hydraulic pump driven by one of the two output elements; a hydraulic motor connected to the hydraulic pump via a pair of main lines, that is, a first main line and a second main line; a hydraulic oil tank for storing hydraulic oil; a charge pump that supplies hydraulic oil from the hydraulic oil tank to the first main line and the second main line; a charge line connecting the charge pump to the first main line and the second main line; a charge relief valve that maintains the pressure in the charge line at or below a set pressure; a first relief valve provided between the first main line and the charge line, the first relief valve opening when the pressure in the first main line reaches a set pressure; a first check valve that allows hydraulic oil to flow from the charge line side to the first main line side and blocks hydraulic oil from flowing from the first main line side to the charge line side; a second relief valve provided between the second main line and the charge line, the second relief valve opening when the pressure in the second main line reaches a set pressure; a second check valve that allows hydraulic oil to flow from the charge line side to the second main line side and prevents hydraulic oil from flowing from the second main line side to the charge line side; In a transmission comprising: The charge line is provided with an additional reservoir for storing hydraulic oil separate from the hydraulic oil tank. A transmission characterized by:

2. the additional reservoir is disposed upstream of the charge relief valve, the first check valve, and the second check valve, and downstream of the charge pump, the first relief valve, and the second relief valve.

2. The transmission according to claim 1.

3. The additional reservoir is a tank.

2. The transmission according to claim 1.

4. The capacity of the additional storage section is 10 times or more the volume of hydraulic oil discharged in one rotation of the hydraulic pump.

2. The transmission according to claim 1.

5. the first relief valve and the second relief valve are variable proportional relief valves whose set pressures are changeable; 2. The transmission according to claim 1.

6. An oil cooler for cooling the hydraulic oil is provided downstream of the charge relief valve.

2. The transmission according to claim 1.

7. A circulation line for cooling the hydraulic oil is connected to the additional reservoir, An oil cooler that cools the hydraulic oil and a cooling pump that supplies the hydraulic oil in the additional reservoir to the oil cooler are provided in series in the circulation pipe.

2. The transmission according to claim 1.

8. The cooling pump is driven when the temperature of the hydraulic oil reaches or exceeds a predetermined value.

8. The transmission according to claim 7.

Images