Abstract: A hydraulic vehicle drive, having a closed hydraulic circuit consists of a variable displacement pump and one or several hydraulic motors. The variable displacement pump is driven by a diesel engine. During the braking or pushing operation of the vehicle, the hydraulic motor becomes a pump and drives the variable displacement pump which, in turn, acts as a motor upon the diesel engine and acts to accelerate the latter. Thus, the intended braking effect prevents the danger that the diesel engine will reach non-permissible high revolutions per unit time, which is particularly dangerous for diesel engines with turbochargers. In order to prevent non-permissible high revolutions per unit time, a pressure regulator acting as a throttle is placed in the return line between the hydraulic motor and the variable displacement pump. The valve throttles the rate of flow when the pressure of the hydraulic fluid in the line exceeds a certain desired value during the braking operation.

Abstract: A hydrostatic loop dump valve for reducing the starting torque required for a hydrostatic unit includes a normally biased open dump valve connecting one or more of the system pressure lines to the reservoir. The dump valve(s) will automatically open and oil will drain out by gravity when the engine is not running. When the engine turns the hydrostatic unit at sufficient speeds to generate significant charge pressure, the dump valves will close and the unit will operate as normal. Thus, the input torque required to start the engine and the hydrostatic unit is minimized without damaging the hydrostatic unit.

Abstract: A method for determining system parameters, such as the operating speed, working pressure and pivot angle of an axial piston unit in which the operating speed, working pressure and pivot angle are determined by means of a frequency analysis of a detected solid-borne sound signal from an axial piston unit of a hydrostatic drive mechanism. A liquid-borne sound signal of airborne sound signal can be used, in which case a respective system parameter is determined by means of a frequency analysis of a detected liquid-borne sound signal or airborne sound signal from the axial piston unit.

Abstract: A piston for use in the cylinder block of a hydraulic power unit. The piston has an elongated cylindrical body and one end terminating in a spherical surface. The piston is composed of a body and insert which are made from different materials, preferably metallic materials, and joined together. The piston body takes the form of a solid cylinder or a hollow cylindrical tube. The spherical surface takes the form of an internal spherical socket or an external spherical ball. The insert takes an elongated or abbreviated form providing for both open cavity and closed cavity piston configurations. The piston includes a conduit which traverses the body and insert, providing a path for pressurized fluid. The body and the insert are preferably formed by a metal injecting molding process utilizing a molding material comprised at the time of molding of a mixture of metallic grains and a binder.

Abstract: The present invention relates to a filled piston assembly for a hydraulic pump or motor. The filled hydraulic piston assembly includes a blank piston body having first and second ends and a cavity or compartment in the piston body extending inwardly from one of the ends. The piston is filled with a lightweight solid insert element which is placed into the compartment of the piston. The material of the blank piston body is then cold formed around the insert element to encapsulate the element. The piston body is then further cold rolled to receive a spherical ball in one end. It is then cut to length, machine finished, and a longitudinal center bore is drilled therein.

Abstract: A specific type of hydraulic fan drive system is provided for engine drive machines, such as on road and off road machines, such as buses, construction equipment, agricultural equipment, and so on. The hydraulic fan drive systems turn a cooling fan (39) which provides airflow to cool the engine on such machines. This system uses a proportional pressure reducing valve (34) (or combination of valves providing a pressure reducing function) between the pump (12) and the fan motor (38). This valve (34) or (combination of valves) controls the pressure drop across the fan motor (38) based on a control signal from an electronic controller (50). The valve allows as little or as much flow to pass as is required to maintain this pressure drop at the load (the fan motor (38)).

Abstract: A closed cavity piston assembly includes a piston body, a separately formed piston cap sealingly joined to the body, and an elongated stem integrally formed with one of the piston body or the cap. In one embodiment, the stem is integrally formed with the piston body. In another embodiment, the stem is integrally formed with the piston cap and slidably journaled in and sealed to a hole in the bottom of the piston body. The cap of the piston assembly also has a surface for engaging a slipper. In either case, a fluid passageway extends through the stem, body, and cap of the piston assembly. The passageway is remote from the side wall of the piston body and fluidly isolated from the main interior cavity of the hollow piston body once the cap and body are sealingly joined together. The piston assembly can be pivotally attached to a slipper that has its own fluid passageway, which registers with the passageway in the cap, to provide fluid for slipper lubrication and balance.

Abstract: A piston for use in the cylinder block of a hydraulic power unit. The piston has an elongated cylindrical body and one end terminating in a spherical surface. The piston is composed of a body and insert which are made from different materials, preferably metallic materials, and joined together. The piston body takes the form of a solid cylinder or a hollow cylindrical tube. The spherical surface takes the form of an internal spherical socket or an external spherical ball. The insert takes an elongated or abbreviated form providing for both open cavity and closed cavity piston configurations. The piston includes a conduit which traverses the body and insert, providing a path for pressurized fluid.

Abstract: A tandem fixed displacement pump circuit with torque control includes first and second pumps that draw fluid from a reservoir to supply respective load circuits, pressure relief valves for each of the load circuits, and a pilot operated sequence valve that monitors the pressure for each of the circuits and reduces the pressure of one of the circuits as the pressure in the other circuit rises. The sequence valve modulates to control the pressure in the secondary priority circuit so that a predetermined input torque value is not exceeded.

Abstract: A description is given of an adjustable hydrostatic axial piston machine whose angle can be varied by means of a servo-piston (9) to which actuating pressure can be applied and which is connected to an electrohydraulic control valve. The control valve has a control piston (15) and a control sleeve (22) which can be moved axially relative to one another. The control piston (15) can be axially adjusted with a stepper motor (10), and the control sleeve (22) is coupled mechanically or hydraulically to an adjusting mechanism (9, 7) for the angle for the purpose of feeding back the angle.

Abstract: A drive train comprises an internal combustion engine, a clutchless gearbox, which can be shifted by means of an electronic control unit and by means of actuating devices, without a mechanical synchronizing device, and a hydrostatic gearbox with a hydraulic pump and a hydraulic motor, which are connected in a closed circuit. The rotational speed of the hydraulic motor being controllable in dependence on a control signal to the adjusting device of the hydraulic pump and/or to the adjusting device of the hydraulic motor. The control unit calculates the control signal by means of setpoint input via an accelerator pedal or as a function of the rotational drive speed of the internal combustion engine, with shifting of the transmission stages being possible by means of the actuating devices.

Abstract: A method of manufacturing a hollow piston including: fixing a core insert formed of a non-metallic material in a desired location within a mold cavity, injecting a mixture of non-metallic binder material and metal particles into the cavity so as to surround at least a portion of the core insert and form a piston, and eradicating the core insert from the piston.

Abstract: A loop flushing circuit for a hydrostatic transmission module includes a three position, three port pilot pressure operated loop flushing valve having a first inlet port fluidly connected to the first system pressure line, a second inlet port fluidly connected to the second system pressure line and an outlet port fluidly connecting the loop flushing valve to the case drain through a drain line having a drain orifice therein. A loop flushing and lubrication line fluidly connected to the drain line upstream of the drain orifice extends within the casing and has an open end directed toward the rotating elements of one of the pump and motor. The three position valve has an intermediate position operatively located between the opposite end positions. System pressure from at least one of the first and second system pressure lines is always routed to the loop flushing and lubrication line through the valve regardless of which of the three positions the valve is in.

Abstract: A variable displacement bent axis hydraulic unit includes a control mechanism having a control housing with a displacement feedback sleeve and spool connected by a spring. A computer controls a proportional solenoid fluid valve that is in operative fluid engagement with the end of the spool opposite its spring connection with the displacement feedback sleeve. The displacement feedback sleeve is urged by the spring into engagement with a cam member on the yoke of the bent axis unit.

Abstract: A special ball joint for servo piston actuation in a bent axis unit includes an elongated actuator rod having a substantially spherical external surface on a first end and a servo piston on a second end. Also included is a single-piece yoke adapted to carry a cylinder block and rotate about an axis of rotation. A socket having a substantially spherical surface is interposed between the yoke and the spherical surface on the rod such that the actuator rod rotates the yoke with forces residing in at least two planes when hydraulic pressure is applied to the servo piston. Two servo pistons and two such ball joints can be applied to the yoke to obtain rotation of the yoke which is up to greater than plus or minus 45° from a neutral position. The external surface on the first end of the actuator rod and the socket must be mating so as to provide freedom of movement. The socket can be incorporated into the yoke itself. The foregoing servo pistons are received in servo bores of a one-piece control housing.

Abstract: A positive displacement hydraulic unit includes a housing having a chamber formed therein with a circumferential surface bounded by a pair of spaced apart end walls. A rotary element for displacing fluid rotatably mounts in the chamber. A layer of abradable coating material is positioned between an end wall of the chamber and the adjacent side wall of the rotary element so as to provide a near-zero side clearance for the rotary element and thereby high volumetric efficiency for the hydraulic unit.

Abstract: A filled hydraulic piston assembly includes a piston body having first and second ends and a compartment in the piston body extending inwardly from the second end toward the first end. The piston is filled with an insert element which is inserted into the compartment of the piston and then held therein by pushing an annular lip on the second end of the body inwardly and against the adjacent end of the insert element. The insert element is comprised of a material that is less dense than the material of the body but has a higher bulk modulus than hydraulic oil. Oil channels can be formed in a variety of shapes on or around the insert so that oil may flow through the filled piston. The piston insert element can be formed from a variety of materials, such as plastic, magnesium, aluminum or other nonferrous metals. The body is generally comprised of steel.