Closed loop-open loop circuit for hydro-static transmissions

Claim 2

A hydrostatic transmission, comprising: a positive displacement hydraulic pump; a positive displacement hydraulic motor; a sump; a first conduit connecting the pump outlet to the motor input; a second conduit connecting the sump to the pump inlet; a third conduit connecting the motor output to the sump; a fourth conduit connecting the motor outlet to the pump input; the improvement which comprises valve means in the third and fourth conduits and operative to close the third conduit and open the fourth conduit only when the motor is operating in an overrunning manner.

Claim 3

The improvement of claim 1 wherein the valve means includes a first normally open valve in the third conduit and a spring-biased check valve in the fourth conduit, the opening pressure of the spring-biased check valve being greater than the pressure drop across the first valve when the first valve is open.

Claim 4

The improvement of claim 2 wherein the first valve is a pilot operated check valve, the operation of the first valve being controlled by the pressure in the first conduit.

Claim 5

The improvement of claim 1 further including a check valve in the second conduit operative to permit fluid flow from the sump to the inlet of the pump while preventing reverse flow.

Claim 6

A hydrostatic transmission including a positive displacement pump and a positive displacement hydraulic motor, comprising: means to drive the pump, means driven by the motor; a first conduit connecting the outlet of the pump to the inlet of the motor; a hydraulic fluid sump; a second conduit connecting fluid in the sump to the inlet of the pump; a third conduit connecting the outlet of the motor to the sump; a fourth conduit connecting the second and third conduits; and a normally open valve in the third conduit between the sump and the point of connection of the fourth conduit thereto; a normally closed valve in the fourth conduit between the second and third conduit connections thereto; the normally open valve closing and the normally closed valve opening directly dependent upon the fluid pressure in the first conduit.

Claim 7

A hydrostatic transmission including a positive displacement pump and a positive displacement hydraulic motor, comprising: means to drive the pump, means driven by the motor; a first conduit connecting the outlet of the pump to the inlet of the motor; a hydraulic fluid sump; a second conduit connecting fluid in the sump to the inlet of the pump; a third conduit connecting the outlet of the motor to the sump; a fourth conduit connecting the second and third conduits; and a normally open valve in the third conduit between the sump and the point of connection of the fourth conduit thereto; a normally closed valve in the fourth conduit between the second and third conduit connections thereto; the normally open valve closing and the normally closed valve opening directly dependent upon the fluid pressure in the first conduit.

Claim 8

The hydrostatic transmission according to claim 5 wherein the normally closed valve is a spring-biased check valve having an opening pressure greater than the pressure across the normally open valve when the latter is open.

Claim 9

The hydrostatic transmission according to claim 5 wherein the normally open valve is a pilot controlled check valve responsive to hydraulic pressure in the first conduit.

Claim 10

The hydrostatic transmission according to claim 7 wherein the normally open valve closes when the pressure in the first conduit falls to approximately 0 psi.

Claim 11

The hydrostatic transmission according to claim 5 which includes: a fifth conduit connected to the first conduit, and to the sump, and a normally closed spring loaded check valve in said fifth conduit permitting flow only from the sump to the first conduit dependent on the fluid pressure in the first conduit.