Hydrostatic transmission overspeed prevention circuit

In a material handling machine having a closed loop hydrostatic power transmission system driven by a prime mover to move a load using a hydraulic motor and a variable displacement hydraulic pump coupled together by two fluid conduits;

• a reservoir to collect leakage and to store fluid for use by the transmission system;
• and a servomechanism to regulate the quantity and direction of flow from the hydraulic pump by changing the angle of tilt on a swash plate within said hydraulic pump, a hydraulic circuit to prevent over-speeding the prime mover when decelerating the inertial load, comprising:
• (a) a fluid charging system connected to said reservoir for operating said servomechanism;
• and
• (b) pressure limiting valve means operationally associated with said fluid charging system, for de-energizing said servomechanism to limit the operation of said swash plate in response to the pressure in said main conduits and the direction of rotation of said motor, said pressure limiting valve means including:
• reference means for producing a pre-set maximum pressure value, said pre-set maximum pressure value being representative of the maximum allowable output torque or pressure loading from said pump;
• and
• first and second parallel valve pilots associated with said pressure limiting valve means for comparing said pre-set maximum pressure value against the pressure in each of said two conduits and for de-energizing said fluid charging system in the event that the pressure in either of said two conduits is greater than said pre-set maximum pressure value, or if the sum of the pressures in said two conduits is greater than said pre-set maximum pressure value, whereby
• said prime mover is precluded from being loaded by the operation of said servomechanism to decelerate the motion of said load in the event that the pressure in said conduits produced by the rotation of said hydraulic motor due to the moving inertia of said load with said hydraulic pump unloaded is greater than said pre-set maximum pressure value or in the event that the pressure in said conduits produced by said hydraulic pump and said hydraulic motor due to the rotational inertia of said load taken together is greater than said pre-set maximum pressure value.

In a material handling machine having:

• a closed loop hydrostatic power transmission system driven by a prime mover to rotate a load using a hydraulic motor and a variable displacement hydraulic pump fluidly coupled together by two fluid conduits;
• a charging system to make up fluid lost by leakage from said transmission;
• a reservoir to collect leakage and to supply fluid to said charging system;
• and a servomechanism, powered by fluid supplied from said charging system through a pipe, for controlling the quantity and direction of flow from said pump to said motor, a hydraulic circuit to prevent overspeeding the prime mover driving said pump, comprising:
• (a) a valve in said pipe, for supply of hydraulic fluid from said charging system to actuate said servomechanism;
• (b) operating means, operationally associated with said valve and powered by the fluid in said conduits, for opening and closing said valve to control the supply of fluid to said servomechanism in response to the direction of rotation of said motor and the pressure in said conduits, said operating means including:
• biasing means for urging said valve to the open position;
• first pilot actuator means, opposing said biasing means, for closing said valve, said first pilot actuator means being supplied with fluid from that one of said two fluid conduits which is at the higher pressure;
• and
• second pilot actuator means, opposing said biasing means and in parallel with said first pilot actuator means, for closing said valve, said second pilot actuator means being supplied with fluid from that conduit which, for a given direction of rotation of said motor, would be at the lower pressure if said pump were used to accelerate the load in said given direction, whereby
• the speed of the prime mover induced by the operation of said servomechanism is limited by the torque or pressure loading on said pump and the direction that said motor is rotating;
• (c) direction sensing means for sensing the direction of rotation of said motor and controlling said second pilot actuator means;
• and
• (d) control valve means responsive to said direction sensing means for transmitting pressure from a selected conduit to said second pilot actuator means.