Auxiliary blade rotation hydraulic system and control method thereof

Abstract

The present invention discloses an auxiliary blade rotation hydraulic system and a control method thereof, which belongs to the technical field of engineering machinery. It solves the defects of low efficiency, high fuel consumption and high operator requirements of traditional graders in the prior art. Its main structure includes a power source, a priority valve, a multi-way valve, a rotary mechanism, a steering system and a control system. The power source is respectively connected to the multi-way valve and the steering system through the priority valve, and the multi-way valve is connected to the rotary mechanism. An auxiliary valve is provided between the rotary mechanism and the priority valve, and the load feedback pressure port LS2 of the auxiliary valve is respectively connected to the load port LS1 of the priority valve and the steering system through a shuttle valve. The present invention is mainly used on graders.

Description

Technical field:

[0001] The present invention belongs to the technical field of engineering machinery, and in particular, relates to a hydraulic system for assisting blade rotation and a control method thereof. Background technology:

[0002] A motor grader is a typical traction-type working machine with a blade as the main tool and a variety of optional attachments. It is mainly used for leveling large areas of land, repairing roads, clearing scattered materials, scraping slopes, scraping ditches, repairing side channels and ditches, snow removal, farmland preparation, leveling mining roads, and stripping soil layers in open-pit mines. During leveling operations, flatness is a key indicator. In the initial working state, the blade position and blade angle are adjusted. Due to the complexity of the grader's operating conditions and the uncertainty of the resistance to leveling operations in front of the blade, when the material resistance in front of the blade becomes larger, the traction force of the current working gear cannot meet the demand, and manual gear shifting is required. At the same time, both hands control the left and right blade lifting handles to control the extension and retraction of the blade cylinder through the multi-way valve to achieve flatness adjustment. Sometimes it is also necessary to control the blade rotation handle to control the blade rotation, reduce the equivalent working width and reduce the operating resistance. Therefore, in order to ensure the flatness requirements during the operation, it is sometimes necessary to use both hands to control the left and right control handles, the blade rotation control handle, the gear change of the transmission, etc., which requires high skills from the operator, and the operator needs to pay attention to many factors at the same time. The experience level directly affects the flatness.

[0003] During the leveling operation, the operator needs to pay attention to adjusting the left and right blade lift cylinders at all times to ensure the flatness. However, sometimes the blade load is large, and only adjusting the left and right blade lift cylinders according to the flatness requirements cannot change the blade load. Sometimes it will even increase the blade load. At this time, it is necessary to control the blade rotation with compound priority, reduce the blade's equivalent operating width, and reduce the load. If the current gear does not meet the requirements, it is necessary to manually shift gears or adjust the blade inclination angle at the same time. This requires a high level of skill from the operator, and his skill level directly affects the flatness and work efficiency.

[0004] In order to improve the flatness and efficiency of operations and reduce dependence on the operator's skill level, a 2D or 3D leveling system is currently added. Although this system has greatly improved the operation accuracy, efficiency and reduced dependence on the operator's skill level, the cost is too high. It is only suitable for leveling construction in airports, large infrastructure, etc., but has poor economic benefits for ordinary leveling and other working conditions.

[0005] At the same time, most of the current graders are hydraulically driven, and the torque converter has a certain high-efficiency zone. In order to ensure operating efficiency and reduce fuel consumption, the change in blade load requires the operator to shift gears manually in the high-efficiency zone of the adjacent gear, which places high demands on the operator. In actual operations, most operators shift gears when the current gear cannot meet the traction characteristics requirements, causing the torque converter to operate in a low-efficiency zone, with low efficiency and high fuel consumption. Summary of the invention:

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an auxiliary blade rotation hydraulic system and a control method thereof. During the leveling operation, the operator only needs to focus on controlling the left and right blades and the lifting cylinder according to the flatness requirements, thereby reducing the operator's focus objects, reducing the complex control requirements, and lowering the operator's skill level requirements.

[0007] In order to achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0008] An auxiliary blade rotation hydraulic system includes a power source, a priority valve, a multi-way valve, a rotation mechanism, a steering system and a control system. The power source is respectively connected to the multi-way valve and the steering system through the priority valve, the multi-way valve is connected to the rotation mechanism, an auxiliary valve is provided between the rotation mechanism and the priority valve, and the load feedback pressure port LS2 of the auxiliary valve is respectively connected to the load port LS1 of the priority valve and the steering system through a shuttle valve.

[0009] Furthermore, a one-way valve is provided between the auxiliary valve and the CF port of the priority valve.

[0010] Furthermore, the control system includes an impeller speed sensor, a transmission gear information, a turbine speed sensor, an angle sensor, a controller, an electromagnetic valve A on an auxiliary valve, a transmission gear solenoid valve and an electromagnetic valve B on the auxiliary valve, and the controller is respectively connected to the impeller speed sensor, the transmission gear information, the turbine speed sensor, the angle sensor, the electromagnetic valve A, the transmission gear solenoid valve and the electromagnetic valve B.

[0011] Furthermore, the slewing mechanism is a slewing cylinder; the steering system includes a steering cylinder and a steering gear, and the steering gear is respectively connected to the steering cylinder, the CF port of the priority valve and the shuttle valve.

[0012] A control method includes the above auxiliary blade rotation hydraulic system, and the specific steps include:

[0013] 1) In the initial state of the operation, the operator adjusts the state of the blade, and the control system reads the current gear information S of the gearbox and the gear ratio i of the gearbox. g1 The speed ratio of the adjacent gear in the gearbox is i g2 ;

[0014] 2) Calculate the torque converter speed ratio i of a certain gear shift point when the torque converter is working in the high efficiency zone according to the stored torque converter characteristic curve c1 , torque converter speed ratio i of adjacent gear shift points c2 ;

[0015] 3) Read the turbine speed n t and torque converter pump impeller speed np , calculate the turbine speed n t and torque converter pump wheel speed n p The ratio i s ;

[0016] 4) Judgement i s with i c1 、i c2 The size relationship.

[0017] Furthermore, when i s ≥i c1 When , it means that the material load resistance in front of the blade is reduced, so that the torque converter works in the low-efficiency area, the solenoid valve A on the auxiliary valve is energized, and the hydraulic oil passes through the one-way valve and the auxiliary valve to the rotary cylinder to realize the rotation of the blade, so that the rotation angle β of the blade is reduced, the equivalent working width of the blade becomes larger, and the working resistance increases; when β≥γ2, the gear position of the transmission remains unchanged, the solenoid valve A on the auxiliary valve continues to be energized, and the rotation angle β of the blade is further reduced. When β≤γ1, the equivalent working width of the blade has increased to the maximum equivalent width, indicating that the current load resistance in front of the blade is still small, and the controller controls the gear position solenoid valve to realize upshifting, where γ2 and γ1 are the angle parameter values ​​set by the blade according to the working conditions and material characteristics.

[0018] Furthermore, when i s ≤i c2 When , it means that the material load resistance in front of the blade increases, causing the torque converter to work in the low-efficiency area. The solenoid valve B on the auxiliary valve is energized, allowing the hydraulic oil to pass through the one-way valve and the auxiliary valve to the rotary cylinder, thereby realizing the rotation of the blade, increasing the rotation angle β of the blade, reducing the equivalent working width of the blade, and reducing the operating resistance.

[0019] Furthermore, when β≤ε2, the gear position of the transmission remains unchanged, the solenoid valve B on the auxiliary valve is energized, and the rotation angle β of the blade is further increased, wherein ε2 is the maximum value of the angle set by the blade according to the working conditions and material properties.

[0020] Furthermore, when β≥ε1, the equivalent working width of the blade has been adjusted to the minimum allowable equivalent width, indicating that the current load resistance in front of the blade is still large. The controller controls the gearbox gear solenoid valve to downshift, and the gear information S=S-1, where ε1 is the angle parameter value set for the blade according to the working conditions and material characteristics.

[0021] Furthermore, after downshifting, if i s ≤i c2 , the controller continues to control the gearbox gear solenoid valve to achieve downshifting. When downshifting to first gear, if i s ≤i c2 , it means that the load in front of the blade is still large, and the torque converter enters the low efficiency zone. s≤i c3 , indicating that slippage is about to occur, the indicator light flashes, and the load cannot be lowered by adjusting the rotation angle of the shovel blade alone. The operator manually controls the lifting and lowering of the shovel blade to achieve load lowering; i c3 It is the ratio of the torque converter turbine speed to the torque converter pump wheel speed when the entire machine is slipping.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. During the leveling operation, the operator only needs to focus on controlling the left and right blades and the lifting cylinder according to the leveling requirements. According to the load size, the blade rotation is automatically controlled to adjust the blade rotation angle and the equivalent width. There is no need to pay attention to the blade rotation and the gearbox gear change (according to the load conditions, the load change is achieved by adjusting the blade rotation first, and then the load requirements are met by the gearbox up and down changes). This reduces the operator's focus, reduces the requirements for complex operations, and reduces the operator's skill level requirements, thereby improving the flatness of the operation, reducing the difficulty of operation, improving the operation efficiency, and reducing fuel consumption;

[0024] 2. When the blade rotation cannot meet the requirements, the gear shifting operation is automatically controlled. The automatic gear shifting is realized in the high-efficiency area according to the pump wheel speed information, turbine speed information and gear position information, ensuring that the torque converter works in the high-efficiency area, improving efficiency and reducing fuel consumption. Description of the drawings:

[0025] Figure 1 This is a schematic diagram of a hydraulic system according to Embodiment 1 of the present invention;

[0026] Figure 2 It is a structural schematic diagram of the rotation angle of the scraper blade in the present invention;

[0027] Figure 3 It is a flowchart of the control system in the present invention;

[0028] Figure 4 This is a flow chart of the control system of Example 2 of the present invention.

[0029] In the figure: 1. Pump wheel speed sensor; 2. Transmission gear information; 3. Turbine speed sensor; 4. Angle sensor; 5. Controller; 6. Solenoid valve A; 7. Transmission gear solenoid valve; 8. Solenoid valve B; 9. Steering cylinder; 10. Multi-way valve; 11. Pressure protection valve; 12. Rotary cylinder; 13. Auxiliary valve; 14. Check valve; 15. Shuttle valve; 16. Steering gear; 17. Priority valve; 18. Hydraulic pump; 19. Blade. Specific implementation method:

[0030] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0031] Embodiment 1:

[0032] like Figure 1 As shown, an auxiliary blade rotation hydraulic system includes a power source, a priority valve 17, a multi-way valve 10, a rotation mechanism, a steering system and a control system. The power source is connected to the multi-way valve 10 and the steering system respectively through the priority valve 17, the multi-way valve 10 is connected to the rotation mechanism, an auxiliary valve 13 is provided between the rotation mechanism and the priority valve 17, and the load feedback pressure port LS2 of the auxiliary valve 13 is connected to the load port LS1 of the priority valve 17 and the steering system respectively through a shuttle valve 15.

[0033] A one-way valve 14 is provided between the auxiliary valve 13 and the CF port of the priority valve 17 .

[0034] like Figure 3 As shown, the control system includes an impeller speed sensor 1, a transmission gear information 2, a turbine speed sensor 3, an angle sensor 4, a controller 5, a solenoid valve A6 on an auxiliary valve 13, a transmission gear solenoid valve 7 and a solenoid valve B8 on the auxiliary valve 13, and the controller 5 is respectively connected to the impeller speed sensor 1, the transmission gear information 2, the turbine speed sensor 3, the angle sensor 4, the solenoid valve A6, the transmission gear solenoid valve 7 and the solenoid valve B8.

[0035] like Figure 2 As shown, the angle sensor 4 is installed on the traction frame of the scraper 19 and is used to detect and feedback the rotation angle β of the scraper 19.

[0036] The power source is a hydraulic pump 18; the rotary mechanism is a rotary cylinder 12; the steering system includes a steering cylinder 9 and a steering gear 16, and the steering gear 16 is respectively connected to the steering cylinder 9, the priority port CF of the priority valve 17 and the shuttle valve 15; a pressure protection valve 11 is provided between the multi-way valve 10 and the rotary cylinder 12.

[0037] The hydraulic pump 18 supplies oil to the steering gear 16, the check valve 14 and the auxiliary valve 13 through the priority port CF of the priority valve 17. When the steering gear 16 and the auxiliary valve 13 are not working, the pressure oil of the hydraulic pump 18 is fed back to the load port LS1 of the priority valve 17 through the shuttle valve 15. Since there is no load pressure, the pressure oil of the hydraulic pump 18 is passed to the multi-way valve 10 through the EF port of the priority valve 17.

[0038] When the rotary valve in the multi-way valve 10 is controlled to work, the hydraulic oil flows to the rotary cylinder 12 through the pressure protection valve 11, thereby adjusting the rotary angle of the scraper 19.

[0039] When the solenoid valve A6 of the auxiliary valve 13 is energized, the auxiliary valve 13 works in the lower position, and the hydraulic oil passes through the priority valve 17, the one-way valve 14, the lower end of the auxiliary valve 13, and the pressure protection valve 11 to the rotary cylinder 12, thereby adjusting the rotary angle of the blade 19; at the same time, the load pressure of the rotation is fed back to the feedback load port LS1 of the priority valve 17 through the load feedback pressure port LS2 of the auxiliary valve 13 and the shuttle valve 15 to ensure flow priority; when the solenoid valve B8 is energized, the working principle is similar;

[0040] When the auxiliary valve 13 is in working state (when the lower solenoid valve A6 is energized or the upper solenoid valve B8 is energized), if the multi-way valve 10 is controlled to operate at the same time, the pressure oil of the multi-way valve 10 and the pressure oil of the auxiliary valve 13 work together to adjust the rotation angle of the scraper 19 through the rotary cylinder 12. At the same time, the one-way valve 14 prevents the pressure oil of the multi-way valve 10 from flowing back through the auxiliary valve 13 and affecting the priority valve 17.

[0041] Embodiment 2:

[0042] like Figure 4 As shown, a control method includes the above auxiliary blade rotation hydraulic system, and the specific steps include:

[0043] 1) In the initial state of the operation, the operator adjusts the state of the blade 19, and the control system reads the gear information S of the current gearbox and the gear ratio i of the gearbox. g1 The speed ratio of the adjacent gear in the gearbox is i g2 ;

[0044] 2) Calculate the torque converter speed ratio i of a certain gear shift point when the torque converter is working in the high efficiency zone according to the stored torque converter characteristic curve c1 , torque converter speed ratio i of adjacent gear shift points c2 ;

[0045] 3) Read the turbine speed n t and torque converter pump impeller speed n p , calculate the turbine speed n t and torque converter pump wheel speed n p The ratio i s ;

[0046] 4) Judgement i s with i c1 、i c2 The size relationship.

[0047] The step 2) calculates the torque converter speed ratio i of a certain gear shift point by the following steps: c1 , torque converter speed ratio i of adjacent gear shift points c2 :

[0048]

[0049] Among them, i c is the torque converter speed ratio; n t is the turbine speed, n p is the torque converter pump wheel speed, n e is the engine speed, i g is the gear ratio of the gearbox, n s is the gearbox output shaft speed ratio, η is the torque converter efficiency;

[0050] Because the torque converter pump wheel speed = engine speed;

[0051] From formula (I), we get:

[0052]

[0053] The intersection of the efficiency curves of each gear is the efficiency shift point. At this time, the output shaft speed of the gearbox and the efficiency of the torque converter are equal. From formula (Ⅱ), we can know that:

[0054]

[0055] Among them, η g1 is the torque converter efficiency at a certain gear (low gear), η g2 is the torque converter efficiency at the adjacent gear (high gear), i g1 is the gear ratio of the gearbox, i g2 is the speed ratio of adjacent gears in the gearbox;

[0056] From formula (III), we can deduce:

[0057]

[0058] Combining formula (IV) and formula (I), we can get:

[0059]

[0060]

[0061] Among them, i c1 is the torque converter speed ratio at a certain gear shift point, i c2 is the torque converter speed ratio of the adjacent gear shift points;

[0062] When the meshing curves are different:

[0063]

[0064] Combining formula (VI) and formula (I), we can get:

[0065]

[0066] Based on the efficiency shifting rule, shift delay is introduced to avoid shifting cycle, and the upshift and downshift shift delays are 5% each.

[0067] when i s ≥i c1 When , it means that the material load resistance in front of the blade 19 is reduced, so that the torque converter works in the low-efficiency area, the solenoid valve A6 on the auxiliary valve 13 is energized, and the hydraulic oil passes through the one-way valve 14, the auxiliary valve 13, and the pressure protection valve 11 to the rotary cylinder 12, so that the blade 19 rotates, the rotation angle β of the blade 19 is reduced, the equivalent working width of the blade 19 is increased, and the working resistance is increased; when β≥γ2, the gear position of the transmission remains unchanged, the solenoid valve A6 on the auxiliary valve 13 continues to be energized, and the rotation angle β of the blade 19 is further reduced. When β≤γ1, the equivalent working width of the blade 19 has increased to the maximum equivalent width, indicating that the current load resistance in front of the blade 19 is still small, and the controller 5 controls the gear position solenoid valve 7 to achieve upshifting, wherein γ2 and γ1 are angle parameter values ​​set for the blade 19 according to the working conditions and material characteristics.

[0068] During this process, the gear position of the transmission changes and the rotation of the blade 19 are automatically assisted according to the load conditions, and the operator only needs to focus on controlling the left and right blades 19 and the lifting cylinder control.

[0069] when i s ≤i c2 When , it means that the material load resistance in front of the scraper 19 increases, causing the torque converter to work in the low-efficiency area, and the solenoid valve B8 on the auxiliary valve 13 is energized, so that the hydraulic oil passes through the one-way valve 14, the auxiliary valve 13, and the pressure protection valve 11 to the rotary cylinder 12, so that the scraper 19 rotates, the rotation angle β of the scraper 19 increases, the equivalent working width of the scraper 19 becomes smaller, and the working resistance decreases.

[0070] When β≤ε2, the gear position of the transmission remains unchanged, the solenoid valve B8 on the auxiliary valve 13 is energized, and the rotation angle β of the scraper 19 is further increased, wherein ε2 is the maximum value of the angle set by the scraper 19 according to the working conditions and material properties.

[0071] When β≥ε1, the equivalent working width of the blade 19 has been adjusted to the minimum allowable equivalent width, indicating that the current load resistance in front of the blade 19 is still large. The controller 5 controls the transmission gear solenoid valve 7 to downshift, and the gear information S=S-1, wherein ε1 is the angle parameter value set for the blade 19 according to the working conditions and material characteristics.

[0072] After downshifting, if i s ≤i c2 , the controller 5 continues to control the gearbox gear solenoid valve 7 to achieve downshifting. When downshifting to the first gear, if i s ≤ic2 , it means that the load in front of the blade 19 is still large, and the torque converter enters the low efficiency zone. s ≤i c3 , indicating that slipping is about to occur, the indicator light flashes, and the load cannot be lowered by adjusting the rotation angle of the shovel blade 19 alone. The operator manually controls the lifting and lowering of the shovel blade 19 to achieve load lowering; wherein, i c3 It is the ratio of the turbine speed of the torque converter to the pump speed of the torque converter when the whole machine is slipping. The other parts are the same as those in Example 1.

[0073] i c3 It is the ratio of the turbine speed to the pump wheel speed of the torque converter (for example, 0.35) when the whole machine slips during the whole machine matching process (for example, when the traction of the whole machine is 72KN, the traction exceeds the traction determined by the ground adhesion coefficient of the whole machine, and the tire slips). This value is calculated based on the whole machine matching of the hydraulic transmission system. The calculated value will be different due to different performance requirements of the whole machine and different torque converters.

[0074] After using Example 1 and Example 2, in most working conditions of leveling operations, the operator only needs to control the left and right blades 19 and the lifting cylinder to ensure the leveling requirements according to the leveling control requirements. The blade 19 rotates first and the gearbox gear changes automatically according to the load conditions, reducing the operator's attention and control objects, thereby improving the leveling accuracy and improving the working efficiency.

Claims

1. A control method for an auxiliary blade rotary hydraulic system, comprising an auxiliary blade rotary hydraulic system, the auxiliary blade rotary hydraulic system comprising a power source, a priority valve (17), a multi-way valve (10), a rotary mechanism, a steering system and a control system, the power source being connected to the multi-way valve (10) and the steering system respectively through the priority valve (17), the multi-way valve (10) being connected to the rotary mechanism, an auxiliary valve (13) being arranged between the rotary mechanism and the priority valve (17), a load feedback pressure port LS2 of the auxiliary valve (13) being connected to a load port LS1 of the priority valve (17) and the steering system respectively through a shuttle valve (15); a check valve (14) being arranged between the auxiliary valve (13) and the CF port of the priority valve (17); The control system comprises a pump wheel speed sensor (1), a gearbox gear information (2), a turbine speed sensor (3), an angle sensor (4), a controller (5), a solenoid valve A (6) on an auxiliary valve (13), a gearbox gear solenoid valve (7) and a solenoid valve B (8) on the auxiliary valve (13); the controller (5) is respectively connected to the pump wheel speed sensor (1), the gearbox gear information (2), the turbine speed sensor (3), the angle sensor (4), the solenoid valve A (6), the gearbox gear solenoid valve (7) and the solenoid valve B (8); The slewing mechanism is a slewing cylinder (12); the steering system comprises a steering cylinder (9) and a steering gear (16); the steering gear (16) is respectively connected to the steering cylinder (9), the CF port of the priority valve (17) and the shuttle valve (15); Features: Specifically include: 1) In the initial state of the operation, the operator adjusts the state of the blade (19), and the control system reads the gear information S of the current gearbox and the gear ratio i of the gearbox. g1 The speed ratio of the adjacent gear in the gearbox is i g2 ; 2) Calculate the torque converter speed ratio i of a certain gear shift point when the torque converter is working in the high efficiency zone according to the stored torque converter characteristic curve c1 , torque converter speed ratio i of adjacent gear shift points c2 ; 3) Read the turbine speed n t and torque converter pump impeller speed n p , calculate the turbine speed n t and torque converter pump wheel speed n p The ratio i s ; 4) Judgement i s with i c1 、i c2 The size relationship of when i s ≥i c1 When γ2 is ≥ γ2, the gear position of the transmission box remains unchanged, the solenoid valve A (6) on the auxiliary valve (13) continues to be energized, and the rotation angle β of the blade (19) is further reduced. When β ≤ γ1, the equivalent working width of the blade (19) has increased to the maximum equivalent width, indicating that the load resistance in front of the blade (19) is still relatively small. The controller (5) controls the gear position solenoid valve (7) to achieve upshifting, wherein γ2 and γ1 are angle parameter values ​​set for the blade (19) according to the working conditions and material characteristics. when i s ≤i c2 When , it indicates that the material load resistance in front of the blade (19) increases, causing the torque converter to operate in a low-efficiency zone, and the solenoid valve B (8) on the auxiliary valve (13) is energized, causing the hydraulic oil to flow through the one-way valve (14) and the auxiliary valve (13) to the rotary cylinder (12), thereby realizing the rotation of the blade (19), increasing the rotation angle β of the blade (19), reducing the equivalent working width of the blade (19), and reducing the working resistance.

2. The control method of the auxiliary blade rotation hydraulic system according to claim 1, characterized in that: When β≤ε2, the gear position of the transmission remains unchanged, the solenoid valve B (8) on the auxiliary valve (13) is energized, and the rotation angle β of the scraper (19) is further increased, wherein ε2 is the maximum value of the angle set by the scraper (19) according to the working conditions and material characteristics.

3. The control method of the auxiliary blade rotation hydraulic system according to claim 1, characterized in that: When β≥ε1, the equivalent working width of the shovel blade (19) has been adjusted to the minimum allowable equivalent width, indicating that the load resistance in front of the current shovel blade (19) is still relatively large, and the controller (5) controls the gearbox gear solenoid valve (7) to achieve downshifting, and the gear information S=S-1, wherein ε1 is the angle parameter value set for the shovel blade (19) according to the working conditions and material characteristics.

4. The control method of the auxiliary blade rotation hydraulic system according to claim 3, characterized in that: After downshifting, if i s ≤i c2 , the controller (5) continues to control the gearbox gear solenoid valve (7) to achieve downshifting. When downshifting to the first gear, if i s ≤i c2 , it means that the load in front of the blade (19) is still large and the torque converter enters the low efficiency zone. s ≤i c3 When the load is lowered, it indicates that slipping is about to occur, the indicator light flashes, and the load cannot be lowered by adjusting the rotation angle of the shovel blade (19). The operator manually controls the lifting and lowering of the shovel blade (19) to lower the load. c3 It is the ratio of the torque converter turbine speed to the torque converter pump wheel speed when the entire machine is slipping.

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