Hydrostatic drive

Abstract

Hydrostatic drive with a variable displacement pump (12), with two hydraulic motors (10, 11) arranged in series and supplied with hydraulic fluid by the variable displacement pump (12) in a closed hydraulic circuit, and with a flow control valve (20) through which a hydraulic fluid flow rate flows from a line section (15) between the two hydraulic motors (10, 11) to a pressure sink, characterized in that the flow control valve (20) is integrated into the housing (30) of a first of the two hydraulic motors (10), wherein the hydraulic fluid flow rate limited by the flow control valve (20) flows into the housing (30) of the first hydraulic motor (10) and flows back to a tank (24) via its leakage port (L).

Description

The invention relates to a hydrostatic drive with a variable displacement pump, with two hydraulic motors arranged in series and supplied with hydraulic fluid by the variable displacement pump in a closed hydraulic circuit, and with a flow control valve through which a hydraulic fluid flow flows from a pipe section between the two hydraulic motors to a pressure sink. Such hydrostatic drive systems can generally be used in vehicles, but especially in agricultural vehicles such as tractors. From DE 10 2010 015 498 A1, a hydrostatic drive system is known in which several hydraulic motors in a closed hydraulic circuit are supplied with hydraulic fluid by a common variable displacement pump. Each wheel of the front and rear axles of a vehicle is assigned a hydraulic motor (wheel motor), two of which are connected in series, so that in an all-wheel drive system the left and right wheels, respectively, are driven by two hydraulic motors connected in series. Multi-stroke radial piston motors, such as those known from DE 38 28 131 A1, are frequently used as hydraulic motors mounted directly on the wheels. In the well-known hydrostatic drive system, an external valve block is used for the flow control valve. Its manufacture and installation on a vehicle component or hydraulic motor incur considerable costs. Further prior art is also known from US 2007 / 0 034 077 A1, which discloses an arrangement for controlling a hydraulically driven motor that is part of a hydraulic system in which pressurized hydraulic fluid forms a main flow through a main line to which the motor is connected. The motor drives a changing load, and one or more valves are designed to control, on the one hand, the flow of hydraulic fluid through the motor during operation and, on the other hand, the starting and stopping of the motor. One of the valves consists of a flow control valve connected in the main line downstream of the motor outlet, and the flow control valve is integrated into the motor housing. The invention aims to further develop the known hydrostatic drive system in such a way as to reduce the costs of its provision. This objective is achieved by a hydrostatic drive system equipped with the features of the preamble of claim 1, in which the flow control valve is additionally integrated into the housing of a first of the two hydraulic motors, and the hydraulic fluid flow rate limited by the flow control valve flows into the housing of the first hydraulic motor and returns to a tank via its leakage port. Thus, the hydraulic fluid flow rate flowing through the flow control valve contributes to the cooling of the first hydraulic motor. Advantageous embodiments of a hydrostatic drive system according to the invention can be found in the dependent claims. Preferably, the flow control valve draws the regulated hydraulic fluid flow directly from the first port of the first hydraulic motor, which is connected to the pipe section between the two hydraulic motors. This draw-off allows a specific slip to be set for the first hydraulic motor relative to the second hydraulic motor, provided the first hydraulic motor is the downstream hydraulic motor. It can be advantageous if the hydraulic fluid flow rate, limited by the flow control valve, flows to the branch of the hydraulic circuit located between a second port of the first hydraulic motor and the variable displacement pump. In particular, the hydraulic fluid flow rate, limited by the flow control valve, can flow directly to the second port of the first hydraulic motor. In this case, a simple bore between the two ports of the first hydraulic motor is possible. Advantageously, a check valve is arranged in series with the flow control valve, which blocks the section of pipe between the two hydraulic motors. This prevents hydraulic fluid from flowing through the flow control valve when the first hydraulic motor is the upstream of the two hydraulic motors. The drawings show two exemplary embodiments of a hydrostatic drive system according to the invention and another related to the invention as circuit diagrams, as well as a longitudinal section of a usable flow control valve. The invention will now be explained in more detail with reference to the drawings. Fig. 1 shows the circuit diagram of the first embodiment of a hydrostatic drive according to the invention with a drain of the pressure medium volume flowing through the flow control valve to a tank, Fig. 2 shows the circuit diagram of a second embodiment not belonging to the invention of a further hydrostatic drive with a drain of the pressure medium volume flowing through the flow control valve to a connection of a hydraulic motor, and Fig. 3 shows a flow control valve usable in the drive systems according to Fig. 1 and Fig. 2. As shown in Figures 1 and 2, a first hydraulic motor 10 and a second hydraulic motor 11 are arranged in series in a closed hydraulic circuit, which is greatly simplified compared to closed hydraulic circuits used in practice. For example, the replenishment of hydraulic fluid, which replaces the fluid lost through flushing and leakage, is not shown in detail. A variable displacement pump 12 is also located in the closed hydraulic circuit. A first working line 13 runs between the first hydraulic motor 10 and the variable displacement pump 12, and a second working line 14 runs between the second hydraulic motor 11 and the variable displacement pump.The variable displacement pump 12 is adjustable between a maximum positive stroke volume and a maximum negative stroke volume, so that, while maintaining the direction of rotation of the variable displacement pump, the working line 13 can be the high-pressure line and the working line 14 the low-pressure line, or vice versa, and the hydraulic motors can be driven in opposite directions of rotation. This allows a vehicle to move forwards and backwards. The two hydraulic motors 10 and 11 are each assigned to a wheel of a vehicle axle and drive the wheel directly. A section 15 of the hydraulic circuit runs between them. The first hydraulic motor 10 is associated with a flow control valve 20, which, as is customary, has a measuring orifice 21 and a pressure balance 22 connected in series with the measuring orifice. The control piston of the pressure balance is acted upon in the closing direction by the pressure upstream of the measuring orifice and in the opening direction by the pressure downstream of the measuring orifice and a spring 23. The pressure equivalent of the spring 23 thus sets a specific pressure drop across the measuring orifice 21. Together with the size of the flow cross-section of the measuring orifice, this results in the constant hydraulic fluid volume flow through the flow control valve. Both the flow control valve 20 according to Fig. 1 and the one according to Fig. 2 are connected with their input to the connection A of the first hydraulic motor, which is connected to the line section 15. Furthermore, in the two embodiments according to Fig. 1 and Fig. 2, the flow control valve 20 is integrated into the housing 30 of the first hydraulic motor 10, which is indicated in the figures by a dashed rectangle. In the embodiment shown in Fig. 1, the hydraulic fluid flowing from port A of the first hydraulic motor 10 via the flow control valve 20 enters the housing 30 and flows back to a tank 24 via its leakage port L. As it passes through the housing, the hydraulic fluid flowing through the flow control valve can absorb heat and dissipate it into the tank. It therefore contributes to cooling the first hydraulic motor 10. In the embodiment shown in Fig. 2, which is not part of the invention, the outlet of the flow control valve 20 is connected to the second port B of the hydraulic motor 10. The hydraulic fluid flow from port A of the first hydraulic motor 10 via the flow control valve 20 therefore does not flow through the housing 30, but rather via port B into the working line 13. However, this requires that the working line 13 is the low-pressure line. If the working line 13 is the high-pressure line, no hydraulic fluid should flow from port B via the flow control valve 20 to port A of the hydraulic motor. For this purpose, a check valve 25 is arranged in the channel between port B and the flow control valve, which blocks flow from port B to the flow control valve 20. The check valve could also be arranged between port A and the flow control valve. The flow control valve according to Fig. 3 is designed as a built-in valve that can be inserted into a corresponding bore in the housing 30 of the hydraulic motor 10. The valve has a cartridge-like housing 40 with a blind bore in which the can-like control piston or can-like pressure balance 22 is axially movable and controls a flow cross-section between its open end face and several radial housing bores 41. An axial bore, which forms the measuring orifice 21, is located centrally in the base of the pressure balance 22. A spring 23 loads the pressure balance 22, thereby increasing the flow cross-section at the radial bores 41. The inlet of the flow control valve is labelled A, the outlet B. When flow passes through valve 20 from A to B, the pressure in channel A exerts a force on the pressure balance, reducing the flow cross-section at the radial bores 41. The pressure balance then enters its regulating position, reducing the flow cross-section at the radial bores 41 until the forces exerted by the pressure in the spring chamber and by the spring 23 balance the force exerted by the pressure in channel A. At this point, the pressure in the spring chamber is exactly the pressure equivalent of the spring 23 lower than the pressure in channel A. The pressure balance 20 thus regulates a constant pressure drop across the measuring orifice 21 and consequently a constant hydraulic fluid flow rate. In a flow direction from B to A, the return flow occurs freely via the bore 21, which then only serves as a throttling point. To prevent this return flow, the check valve 25 is provided in the embodiment according to Fig. 2.

Claims

Hydrostatic drive with a variable displacement pump (12), with two hydraulic motors (10, 11) arranged in series and supplied with hydraulic fluid by the variable displacement pump (12) in a closed hydraulic circuit, and with a flow control valve (20) through which a hydraulic fluid flow rate flows from a line section (15) between the two hydraulic motors (10, 11) to a pressure sink, characterized in that the flow control valve (20) is integrated into the housing (30) of a first of the two hydraulic motors (10), wherein the hydraulic fluid flow rate limited by the flow control valve (20) flows into the housing (30) of the first hydraulic motor (10) and flows back to a tank (24) via its leakage port (L). Hydrostatic drive according to claim 1, characterized in that the flow control valve (20) takes the controlled pressure medium volume flow directly from the first connection (A) of the first hydraulic motor (10) connected to the line section (15) between the two hydraulic motors.

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