Abstract: A fluid circuit includes a pressure fluid source configured to supply pressure fluid, multiple actuators connected to the pressure fluid source, a direction switching valve configured to switch a supply destination of the pressure fluid supplied from the pressure fluid source, and a discharge amount control mechanism configured to control the output pressure of the pressure fluid source such that a pressure difference ?P between the output pressure of the pressure fluid source and the maximum load pressure of the load pressures of the multiple actuators reaches a target value ?Pt. The fluid circuit further includes an accumulator configured to accumulate part of return fluid from the actuators.

Abstract: The invention relates to a method for operating a vibrating ram arrangement comprising a hydraulic assembly with a hydraulic pump driven by an internal combustion engine, a vibrator connected to the hydraulic assembly in a hydraulic circuit, a hydraulic clamping device connected to the vibrator for a material to be rammed, and a controller, with which a fluid flow through a hydraulic motor of the vibrator can be activated and deactivated and with which the hydraulic clamping device can be closed and opened. According to the invention, the controller requests at least one operating state value during a pause in the operation when the vibrator is deactivated and automatically stops the internal combustion engine if the operating state value corresponds to a specified value. Additionally, the controller automatically starts the internal combustion engine in order to resume the vibrating ram operation in the event of a user signal.

Abstract: Provided is an abnormality detecting device for a construction machine that can estimate an abnormality occurring to a component (engine, pump, etc.) of the construction machine based on the relationship among a plurality of pieces of sensor information and thereby prevent machine failure. A correlation coefficient calculation unit 102 calculates correlation coefficients between time-series sensor values acquired by a plurality of sensors 101. A correlation coefficient comparison unit 103 compares the correlation coefficients and calculates the degree of difference between each correlation coefficient and other correlation coefficients. An abnormality judgment unit 104 judges that an abnormality has occurred to a part related to a sensor when the degree of difference calculated in regard to the sensor exceeds a preset value.

Abstract: One embodiment of the system comprises a towable machine having at least one hydraulic motor arranged on the machine and operable to drive at least one wheel arranged on the machine. At least one hydraulic propelling pump is arranged to supply hydraulic fluid flow via a hydraulic circuit to the hydraulic motor to selectively drive the wheel. The system has a non-operative configuration wherein hydraulic fluid flow is blocked between the hydraulic motor and the hydraulic pump. A pressure sensor is associated with the hydraulic circuit to detect an improper pressure load when the circuit is in a non-operative configuration, and operates to trigger an alarm element.

Abstract: In one embodiment, a steam valve apparatus includes: a hydraulic cylinder including an internal space sectioned into first and second chambers by a piston operated by a hydraulic liquid; a first passage to supply the hydraulic liquid to the first chamber; a second passage connecting the first and second chambers; a third passage to drain the hydraulic liquid from the second chamber; an electromagnetic valve switched between first and second states; a first cartridge valve opening the first passage when the electromagnetic valve is in the first state and closing the first passage when the electromagnetic valve is in the second state; and a second cartridge valve closing the first passage when the electromagnetic valve is in the first state and opening the first passage when the electromagnetic valve is in the second state.

Abstract: The invention relates to an electropneumatic position controller having a controllable valve group (9, 10, 13) arranged in the position controller and having at least one valve (9, 10), by means of which in normal operation either compressed air supplied by means of a compressed air supply line (6) can be fed to an exhaust connection (8) or air fed by means of the exhaust connection (8) can be discharged to the outside, depending on the actuation of the valve group. In order for the valve group to be brought in a safe position, even in the event of a leak in one of the components thereof, said group is arranged in a gas-tight housing (14) within the position controller and means (15) are provided for monitoring the pressure in the housing (14) and for actuating the valve group (9, 10, 13) such that if the pressure in the housing (14) exceeds a predefined threshold value, said group is brought in the predefined safe position.

Abstract: A lock valve switching section switches a lock valve from a locked state to a released state when a lock member is switched from a locked position to a release position. An erroneous operation monitoring section maintains the lock valve in the released state when the pilot pressure is equal to or more than a predetermined pressure when elapsed time, which is from a point in time where the lock member is switched from the locked position to the release position, is equal to or more than the predetermined time. The erroneous operation monitoring section switches the lock valve to the locked state when the pilot pressure is equal to or more than the predetermined pressure when the elapsed time is less than the predetermined time.

Abstract: A hydrostatically driven vehicle has an engine operating a variable displacement propel pump, a displacement of which can vary based on an angle of a rotating swashplate, such that a fluid flow impelled by the pump transfers power to at least one propel motor rotating a wheel of the vehicle. An electronic controller of the vehicle senses an operating parameter of the system, for example, the angle of the rotating swashplate or the direction and speed of rotation of the propel motor with a sensor to yield an actual signal, and relays the actual signal to an electronic controller. The controller determines a desired angle for the rotating swashplate based on the control signal, and compares it to the actual signal from the sensor. Motion of the vehicle is stalled when the angle signal differs from the desired angle by a predetermined extent and for a predetermined period.

Abstract: A drive apparatus having an actuator assembly mounted on a control device is disclosed. The drive apparatus comprises an input, a variable drive system, a control device, and an actuator assembly. The drive apparatus is mounted on a vehicle that comprises an operating system. A prime mover drives the input, which subsequently drives one or more axle shafts. The variable drive system is capable of operation in forward and reverse directions. The control device determines the direction of operation and speed of the variable drive system. The actuator assembly is mounted to a component of the control device. The actuator assembly determines the state of operation of the output device with respect to the forward or reverse operation of the variable drive system.

Abstract: In a NOx purification system 1 provided with a NOx occlusion reduction catalyst 20 and a control unit 30 which executes a NOx regeneration control and a sulfur poisoning regeneration control, a binary ? sensor 26 and a reducing agent concentration sensor 25 for detecting a concentration of the reducing agent are disposed downstream of the NOx occlusion reduction catalyst 20. The level of deterioration of the NOx occlusion reduction catalyst 20 caused by sulfur poisoning is estimated on the basis of a time period Ta from a time point T3 at which a marked decrease of oxygen concentration is detected by the binary ? sensor 26 to the time point T2 at which the marked increase of the oxygen concentration is detected by the reducing agent concentration sensor 25 under the NOx regeneration control.

Abstract: A wheelchair lift (100) for a vehicle having a barrier (128) moveable between an access position and a blocking position. The wheelchair lift includes an actuator (152) coupled to the barrier, the actuator adapted to apply an actuation force upon the barrier to move the barrier from the access position to the blocking position. When the barrier is in an unloaded state, the actuation force causes the actuator to move in a first direction resulting in the barrier moving from the access position to the blocking position. When the barrier is in a loaded state, the actuation force causes the actuator to move in a second direction a predetermined amount without causing substantial movement of the barrier.

Abstract: A control device for a construction machine according to the present invention is configured so that in the construction machine provided with an automatic stop function which automatically stops an engine by an engine controller upon a predetermined automatic stop condition being met, and a hydraulic lock function which sets a hydraulic actuator locked in an inactive state by a hydraulic lock controller, there is provided a restart switch, where after an automatic stop of the engine, when the restart switch is operated, a restart command is transmitted to the engine controller via a route independently of that of an engine switch and independently of a hydraulic unlock condition, and the engine is restarted.

Abstract: A hydraulic circuit for a working machine includes hydraulic actuators which respectively drive a plurality of kinds of working devices and which are driven by a drive circuit having circuit states that correspond to the working devices, a directional control valve unit which, when one of the working devices is attached to the attachment, switches in response to a signal from a controller to select one of the circuit states, a detection sensor that detects the actual circuit state, and a display operated by the controller. The controller activates the display when the detected actual circuit state and the selected circuit state do not match.

Abstract: A hydraulic control arrangement is disclosed for the control of a consumer, comprising at least one mechanically operated continuously adjustable distribution valve with a subsequent LUDV pressure compensator down the line. In order to lock the consumer the control arrangement is provided with a spring holding the pressure compensator piston in a closed position. Furthermore, the LS line carrying the highest load pressure of all consumers is connected to a reservoir by means of a flow regulator, wherein the pump control may also be relieved by the flow regulator in the sense of a reduction of the pumped volume. According to the invention, the LUDV pressure compensator is pressure-compensated by means of a nozzle through which a connection between the LS line and a portion of the pressure medium flow path downstream of the pump and upstream of the outlet of the pressure compensator is generated. Said nozzle is preferably integrated in the pressure compensator piston.

Abstract: The present invention provides an exhaust gas treatment catalyst characterized by containing a compound oxide expressed by the following general formula (1), AMxOy ??(1) (wherein A represents an alkali metal or alkaline earth metal, M represents Cr, Mn, Fe, Co or Ni, O represents oxygen, and 0<x?4 and 0<y?8) and an exhaust gas treatment method using the said catalyst. According to the present invention, there can be provided an exhaust gas treatment compound oxide that can effectively remove particulates and NOx contained in exhaust gas with a high oxygen concentration that is discharged from a diesel engine etc., and also has excellent resistance to sulfur poisoning.

Abstract: An electro hydraulic actuator with built-in fail safes is provided. Multiple accumulators are integrated into the actuator to improve reliability and redundancy. One or more accumulators can fail and the remaining accumulators provide sufficient energy to move the actuator to its fail-safe condition.

Abstract: Dual neutral start switch configurations for preventing vehicle users from starting or activating a vehicle engine under certain circumstances and designed to be part of the control arm or return-to-neutral configuration are disclosed. These configurations may be used with pumps, transmissions or transaxles in a variety of different vehicle applications. Also disclosed is a combination neutral start switch and reverse mower blade cutoff switch.

Abstract: A neutral start switch for preventing vehicle users from starting or activating a vehicle engine under certain circumstances and for use in connection with a control arm mechanism, which may be attached to a casing of a hydraulic pump.

Abstract: A neutral start switch arrangement is provided for a work vehicle having a motor driving a hydrostatic pump, the hydrostatic pump driving a hydraulic motor which drives a wheel. The hydrostatic pump typically has an input control shaft, the rotation of which moves the hydrostatic pump from the neutral position to the forward or reverse drive position. A selectively rotatable control arm is fixed to the input control shaft. A first arm is mounted for rotation about the input control shaft axis. A second arm is mounted for rotation about the input control shaft axis. The control arm and the first and second arms are configured for the control arm to be rotated from a neutral drive position to rotate the first arm in a forward drive rotary direction or the second arm in a second rotary direction corresponding to the reverse drive. A first leg extends from the first arm and a second leg extends from the second arm, and a spring is stretched between the first and second legs.

Abstract: A forward/backward switching control apparatus for a hydraulic drive vehicle that includes the following structures. A variable displacement type hydraulic pump driven by an engine for discharging pressure oil in both directions. An electromagnetic pump inclination angle control mechanism making the discharge displacement of the variable displacement type hydraulic pump variable and for causing the same to discharge in both directions. An electric control outputting signals to run the hydraulic drive vehicle forward, backward and forward/backward. A control device outputting, when the electric control is switched forward/backward to output the forward/backward signal and when the time for switching the forward/backward signal is not longer than a preset time, a first modulate signal for delaying the return times from the forward position to the neutral position and from the backward position to the neutral position to the electromagnetic pump inclination angle control mechanism.

Abstract: A circuit (10) for operating a bi-directional air motor (12) of a valve closure system for enabling a tank valve to be discretionally operated to either open or closed condition either at the tank by attending personnel or from a remote location, with the capability of closing the tank valve in the event of emergency from a remote location in the absence of attending personnel. The circuit can be constructed using two commercially available valve assemblies ((24, 26).

Abstract: A method and apparatus for operating a load lifting device to move a load and hold the load steady is disclosed. In the conventional arts, a lifted or lowered suspended load had a tendency to move from its desired position, as hydraulic fluid in the lifting system cooled, e.g. a stick slip condition. The present invention monitors a fluid pressure in the lifting system and compensates the fluid pressure to accommodate any pressure drop due to fluid cooling. Thereby, the load can be effectively held steady at a desired position, and the stick slip condition can be avoided.

Abstract: A system for steering the wheels of a vehicle. The system includes a steering gear provided with a steering rack reciprocable in a power cylinder. A steering shaft has a pinion meshing with the steering rack. A hydraulic fluid system assists the reciprocation of the steering rack. The hydraulic fluid system includes a power piston reciprocable in the power cylinder. A control valve directs hydraulic fluid from a pump to the power cylinder at one side or the other of the piston, depending on the direction of rotation of the steering shaft. A lock valve is operable when closed to lock the rack against reciprocation to prevent steering of the wheels by preventing fluid flow in the hydraulic fluid system. The lock valve is controlled by a solenoid-operated electronic controller.

Abstract: A control circuit for a construction machine using a control lever for revolving a boom swing in common with a traveling motor and switching pilot pressure via a switching valve has a switch for supplying a switching signal to the switching valve. A pressure switch detects whether the pilot pressure is output from a remote control valve or not. A switching control circuit switches a position of the switching valve to a position for the boom swing cylinder when the switching signal is output from the switch and holds the position of the switch at the position for the boom swing cylinder when the pilot pressure is detected by the pressure switch, to thereby prevent wrong operation in switching.

Abstract: A system for steering the wheels of a vehicle. The system includes a steering gear provided with a steering rack reciprocable in a power cylinder. A steering shaft has a pinion meshing with the steering rack. A hydraulic fluid system assists the reciprocation of the steering rack. The hydraulic fluid system includes a power piston reciprocable in the power cylinder. A control valve directs hydraulic fluid from a pump to the power cylinder at one side or the other of the piston, depending on the direction of rotation of the steering shaft. A lock valve is operable when closed to lock the rack against reciprocation to prevent steering of the wheels by preventing fluid flow in the hydraulic fluid system. The lock valve is controlled by a solenoid-operated electronic controller.

Abstract: An actuator lock switching valve 50 is provided which communicates a drain line 52 and a pilot line 53 with each other when the valve 50 is in a position C, and which communicates pilot lines 51, 53 with each other when it is shifted to a position D. The pilot line 51 is connected to a delivery line 7 of a hydraulic pump 10, and the pilot line 53 is connected to pressure receiving sections 28a, 28b provided at ends of the pressure compensating valves 21a, 21b on the side acting in the closing direction. The actuator lock switching valve 50 has a pressure receiving section 55 connected to the output side of a pilot lock switching valve 43, and is switched over in interlock with shifting of the switching valve 43.

Abstract: A safety device for a moving system with a drive in which a pneumatically operated piston, by linear movement in a cylinder, moves a transported object past a stationary part in at least one direction has the aim of generating a switching signal at every point of a through-opening when an obstacle is located between the moving object and the edge of the through-opening, regardless of the movement direction of the object and the geometrical construction of the through-opening. The cylinder has, at its ends, devices for measuring the pressure of the air flowing out of the cylinder in the movement direction of the piston, wherein a drop in the otherwise substantially constant pressure below a threshold value serves as a switching signal at least for switching off the system. The device is useful for safety purposes during the transport of objects, particularly when the objects form shearing edges with neighboring objects.

Abstract: A device for preventing the unintentional restarting of motors or machines is proposed, it being possible to control the supply of energy to the motor or to the machine by means of a control valve with a mechanically adjustable control element and it being possible to connect the control valve non-detachably to the motor or to the machine, the energy for the motor being generated by means of compressed air, and in that a pneumatic piston, to which the primary air pressure prevailing upstream of the motor and upstream of the control valve can be applied, being arranged as a restarting inhibitor between the control valve and the motor, and in that, in order to apply the primary air pressure, a connecting line being arranged between the motor and the piston.

Abstract: The invention concerns a hydraulic line coupling arrangement of a consumer, with a pump line, a tank line and a safety valve in the pump line, the valve element of the safety valve being acted upon in the opening direction by an opening pressure from the pump line and in the closing direction by a closing pressure of a closing pressure connection. In such an arrangement it is desired to prevent overloading of the consumer. For this purpose, a pilot valve is provided for controlling the closing pressure, the position of the pilot valve being controlled by the pressure in the tank line.

Abstract: The invention relates to a hydraulic control circuit that allows operation to continue, when the working portion approaches the cab during work, and allows the working portion to stop when the working portion reaches the limit position, whereby the working efficiency is further improved. Electromagnetic proportional pressure reducing valves carry out the feeding and interruption of pilot pressurized oil into control valves on the basis of commands from the control unit, and electromagnetic selector valves are provided, wherein the electromagnetic proportional pressure reducing valves and electromagnetic selector valves are actuated when the working portion reaches the interference prevention area and limit position, whereby the hydraulic cylinders are stopped, and avoidance operation can be carried out.

Abstract: In a theft deterrent security device for trailers and like equipment having a dual chamber brake system that operates with compressed air an electro mechanical device, such as servomotor or a solenoid, is mounted within the interior of the dual chamber brake system housing in which an expanded high spring-rate spring biases and keeps a brake actuator in a locked “brakes on” position when there is no compressed air supplied to the system. The servo motor or solenoid is connected to members which in one position mechanically block movement of the brake actuator to prevent its normal operation even when there is compressed air available to the system and the high spring-rate spring is compressed. In a second position of the servo motor or solenoid the members do not block movement of the brake actuator and thereby allow normal operation of the brakes. The servo motor or solenoid is responsive to coded signals which control its position and which are normally available only to an authorized user.

Abstract: A rotary air tool includes an axially moving automatic speed control valve which responds to changes in rotary speed by varying the amount of air fed to its air motor. When the speed tends to increase under light load, less air is fed to the air motor to maintain a generally constant rotary speed. A safety device is triggered when the speed exceeds a predetermined maximum due, for example, to a malfunction in the speed control valve. The safety device includes a weight which is radially movable at the predetermined maximum speed to release a latch. The latch permits resilient actuation of the speed control valve toward its closed position. In one embodiment, a second latch maintains the safety device in the actuated condition so that the air tool remains substantially shut off until the reason for the overspeed condition is investigated. In a further embodiment, the speed control valve is not fully closed by actuation of the safety device so that at least a small amount of air is admitted to the air motor.

Abstract: A hydrostatic transaxle assembly is drivingly connected to a power source and the power source is activated through an electric starter circuit. The hydrostatic transaxle includes an axial type hydrostatic transmission module having a pump which includes a plurality of axially arranged cylinders each including a piston therein. A tiltable swash plate includes a lateral surface rotatable within the hydrostatic transmission casing. The swash plate is in engagement with the plurality of pistons wherein tilting of the swash plate effectuates fluid displacement of the pump. A neutral switch is fixed relative to the transmission casing and includes a registering portion engaged with the swash plate to detect a neutral position and correspondingly allow starting of an electric starter through the starter circuit. A neutral adjustment assembly includes a control rod rotatably supported by the casing and a two piece shift lever is attached thereto.

Abstract: A safety device for a moving system with a drive in which a pneumatically operated piston, by linear movement in a cylinder, moves a transported object past a stationary part in at least one direction has the aim of generating a switching signal at every point of a through-opening when an obstacle is located between the moving object and the edge of the through-opening, regardless of the movement direction of the object and the geometrical construction of the through-opening. The cylinder has, at its ends, devices for measuring the pressure of the air flowing out of the cylinder in the movement direction of the piston, wherein a drop in the otherwise substantially constant pressure below a threshold value serves as a switching signal at least for switching off the system. The device is useful for safety purposes during the transport of objects, particularly when the objects form shearing edges with neighboring objects.

Abstract: When a working implement comes close to a cab during operation, for efficiency, the operation should continue insofar as possible yet it is essential the working implement be prevented from striking the cab. To that end, a pilot pump oil line capable of supplying pilot pressurized oil to a solenoid proportional pressure reducing valve in a state where a control member is not manipulated, is provided in a stick contraction-side pilot oil line. When the working implement enters an interference prevention area, defined around the cab, the stick is automatically moved out so that the operation can continue yet the working element avoids the interference prevention area.

Abstract: A hydraulic control arrangement which has a directional control valve, a hydraulic pump and a two-way cartridge valve constructed as a seat valve. The directional control valve is connected to an inlet line and to a discharge line leading to a tank. Leading off from it is a load line leading to a hydraulic load. The hydraulic pump draws pressure medium from a tank and discharges it into the inlet line. The two-way cartridge valve is arranged in the inlet line and, in a closed position, isolates a second inlet line section leading off from the latter to the directional control valve from a first inlet line section running between the two-way valve and the hydraulic pump.

Abstract: A pump drive assembly is disclosed. The assembly includes a driver, a pump drive member driven by the driver, and a decoupler that decouples the drive member from the driver when a force acting upon the drive member exceeds a threshold force. The pump drive member can be a piston rod, and the decoupler can be a magnet attached to the piston rod and coupled to a magnetically attracted plate driven by the driver. When a force resisting movement of the piston rod exceeds the magnetic force coupling the magnet to the plate, the magnet decouples from the plate, and the pumping operation ceases.

Abstract: A regulator (20) comprises a servo piston 21 and a tilting control valve (22) which is made up of a spool (22a), a spring (22b), a control piston (22d) and a first pressure bearing chamber (22e). With these components, the regulator (20) controls a pump tilting such that a pump delivery rate is reduced as delivery pump pressure rises. The tilting control valve (22) also includes a second pressure bearing chamber (22f). When a gate-lock lever (31) is operated to switch over a lock valve (30), a flow control valve (6) is disabled from operating not to move even if a control lever (11a) is erroneously touched, and the machine is surely kept from coming into operation. At the same time, pilot primary pressure from a pilot pump (3) is introduced to the second pressure bearing chamber (22f) of the tilting control valve (22), causing the pump tilting to reduce down to a minimum tilting (q.sub.min).

Abstract: A hydraulic circuit enables a plurality of hydraulic actuators to be locked or unlocked at the same time no matter whether control valves controlling pressurized oil feed to the hydraulic actuators are pilot-controlled or not. To this end, the circuit is provided with oil lines leading from hydraulic pumps to an oil reservoir without passing through the control valves and with first and second selector valves 20 and 24 opening or closing the oil lines.

Abstract: A control device for controlling and monitoring two fluid valves, wherein each fluid valve comprises a drive means which is adapted for activation by an associated control signal, and a movable valve element which is biassed toward a first position in which fluid flow through the valve is prevented and which can be driven by said drive means to a second position in which fluid is able to flow through the valve, a first switch which is actuable by the displaceable element of the first valve, a second switch which is actuable by the displaceable element of the second valve, wherein each switch is biassed towards a first position and each switch can be driven to a second position when the displaceable element of the associated valve takes its second position.

Abstract: A device for controlling a hydraulic motor (10) comprises a directional valve (15) connected to a pressure fluid pump (11; 50) via a feed passage (17) and a pilot valve (19) activated safety shut-off valve (18) incorporated in the pressure fluid feed passage (17) between the pump (11; 50) and the directional valve (15) and shiftable from a normally open position to a closed position to, thereby, block the fluid communication between the pump (11; 50) and the directional valve (15) in emergency situations. A shunt valve (16) for short circuiting at least a part of the pump flow to a tank or, alternatively a displacement adjusting means (51) of a variable displacement pump (50) are activated by a load sensing pressure derived from a load sensing port (15e) on the directional valve (15), which load sensing pressure is discharged to tank (12) via the shut-off valve (18) as the latter is shifted to its closed position.

Abstract: A fault management apparatus and method are described for a system that includes at least one electro-mechanical or electro-hydraulic component, such as a system valve or main discharge pump in a process plant or water transmission line. An actuator is electrically coupled to the component for application of control signals to selectively actuate or de-actuate the component. A standby power supply, in the form of a uninterruptible power supply (UPS) stores sufficient electrical energy to energize the valve and/or the discharge pump to change the states thereof from one operational state to another. The UPS has sufficient electrical capacity to close a system valve to its fail-safe condition. The actuator can be actuated in a number of different ways, including sensing of failure of main power, manual activation or sensing of some emergency condition.

Abstract: A control system for the hydraulic operating circuit associated with a vehicle lifting device in which the circuit is provided with pressure fluid control valves and flow sensors which are operative on the vehicle lowering cycle to establish substantial evenness of the lowering mechanism and protection against malfunction of an operating component to arrest the travel of the vehicle lifting device in its lowering cycle.

Abstract: A hydrostatic transmission including a pump and motor unit wherein the pump unit is displaceable effectuated by an operative member integral to the pump unit. A neutral start switch communicating with a carrying machine is mounted in the pump unit to contact the operative member in a manner causing the neutral start switch to become operative only when the operative member is assuming a neutral position.

Abstract: A fluid control circuit includes a source of pressurized fluid (10) adapted for connection to a hydrostatic transmission (23) upon actuation of a vent control valve (16). The vent control valve (16) is connected to a control and preconditioning valve (17) which functions to ensure that a control lever (14) is in its "vent" position upon starting of an engine and provides for the continuous preconditioning of the control circuit subsequent to such start-up. The control circuit is relatively non-complex in structure and function and does not require positioning of the control lever (14) in a special reset position upon shifting thereof between its "run" and "vent" positions.

Abstract: A fluid control cirucit for selectively varying the displacement of a variable displacement pump (P), such as the type employed in a hydrostatic transmission, comprises a primary control circuit connecting a pressurized fluid source (10) with the pump (P) and adapted to be actuated to selectively vary the displacement of the pump (P) between minimum and maximum values. A separate override control circuit is connected to the primary control circuit to override it to selectively vary the displacement of the pump (P).