GB2114272A - Hydrostatic transmission - Google Patents

Abstract

An hydrostatic transmission for a vehicle having a closed loop hydraulic system hydraulically interconnecting a variable displacement hydraulic pump 1 and a hydraulic motor 2. The closed loop hydraulic system includes first and second circuits 3, 4 forming the closed loop. The first circuit 3 is connected with a first relief valve 7a having high and low setting pressure and the second circuit 4 is connected with a second relief valve 7b having high and low setting pressure. An hydraulic control pump 11 is provided for controlling the setting pressure of the first and second relief valves and a pilot-operated change over valve 9 is disposed between the control pump and the first and second relief valves. For each direction of travel the relief valve for the flow line (3 or 4) from the pump to the motor is set at a high pressure and that for the return line (4 or 3 respectively) is set at a low pressure to limit hydrostatic braking, prevent cavitation and prevent overspeeding of the prime mover during braking. <IMAGE>

Description

SPECIFICATION Hydrostatic transmission This invention relates to an hydrostatic transmission of the closed loop type wherein a variable displacement hydraulic pump is connected with the hydraulic motor in a closed loop and the hydraulic motor is driven by the pressurized fluid delivered by the hydraulic pump.

The hydraulic dynamic braking force obtained by the control circuit of the hydraulically driven vehicle will increase as the relief pressure within the closed loop increases.

In a conventional hydraulically driven vehicle, the relief pressure in the closed loop is set at a high value to increase the maximum force of traction when the vehicle is driven, and if hydraulic dynamic brakes are applied under such a condition, then the following difficulties or troubles will be encountered thus affecting the manoeuverability and reliability of the vehicle: (a) The peak pressure generated when the brake is applied will be high thus increasing the shock caused by deceleration; (b) Because the set relief pressure is high, there is a significant tendency to cavitation when the brake is applied; (c) The over-run of the engine when the brake is applied may be relatively high.

The present invention has arisen from our work seeking to avoid, mitigate or overcome the above problems.

In accordance with the present invention, there is provided an hydrostatic transmission of the closed loop type wherein a variable displacement hydraulic pump is connected with the hydraulic motor in a closed loop circuit, the hydraulic motor being arranged to be driven by pressurized fluid delivered by the hydraulic pump; the closed loop circuit comprising a first circuit and a second circuit; a first pump being provided for charging hydraulic fluid into said closed loop hydraulic system; first relief valve means being connected with said first circuit and having first means responsive to hydraulic fluid pressure to change over the setting pressure of said first relief valve means between low and high setting pressures; second relief valve means being connected with said second circuit and having a second means responsive to hydraulic fluid pressure to change over the setting pressure of said second relief valve means between low and high setting pressures; and pilot-operated change over valve means being disposed between said first and second means and a second pump for selectively connecting either one of said first and second means with said second pump while connecting the other of said first and second means to drain thereby changing over one of said first and second relief valve means to have high setting pressure and the other relief valve means to have low setting pressure.

The present invention will now be described below by way of example only with reference to the single figure of the accompanying drawing, in which there is schematically shown a simplified control circuit for an embodiment of hydrostatic transmission according to the present invention for an hydraulically driven vehicle. For clarity and ease of explanation of the present invention in a number of component parts which would normally be associated with such a circuit, such as shuttle valves interposed between the first and second conduits forming the closed loop, have been omitted from the drawing.

In the drawing, reference numeral 1 denotes a hydraulic pump, and 2 a hydraulic motor, both of which are connected through a first circuit 3 and a second circuit 4 in a closed loop.

As we shall explain, the circuit includes means for changing over the circuits 3 and 4 depending upon the direction of rotation of the hydraulic motor 2, the pressure in the first circuit 3 being kept higher than that in the second circuit 4 during forward running of the vehicle and the pressure in the second circuit 4 being kept higher than that in the first circuit 3 during backward running of'the vehicle.

Reference numeral 5 denotes a charge circuit, and 6 a charge pump. Reference numerals 7a and 7b indicate relief valves connected with the first circuit 3 and the second circuit 4, respectively. The setting pressures of the relief valves 7a and 7b can be changed over to either a high value or a low value by means of setting pressure change-over means 8a and 8b, respectively, which can be actuated by pilot pressure.

Such pilot pressure is provided via a signal valve 9 interposed between pilot pressure applying portions 1 0a and 1 Ob of the setting pressure change-over means 8a and 8b, and a control pump 11, the position of signal valve 9 being selected depending upon whether the motor is to be driven in forward or reverse senses. Signal valve 9 has a neutral position "a" where the pressurized fluid in the pilot pressure applying portions 1 0a and 1 Ob is drained, a forward running position "b" where the pressurized fluid is supplied into the pilot pressure applying portion 1 0a of the setting pressure change-over means 8a for forward running, and a reverse running position "c" where the pressurized fluid is supplied into the pilot pressure applying portion 1 0b for reverse running.The forward and reverse running signal valve 9 has at its respective ends pilot pressure applying portions 9a and 9b which are connected, respectively, with signal conduits 1 2 and 1 3 providing pressure signals indicating forward running and reverse running, respectively.

Although not specifically illustrated, the signal conduits 1 2 and 1 3 may be connected with a speed control valve of the vehicle.

When the speed control valve is shifted to the forward running position, hydraulic fluid is supplied into the signal conduit 1 2 through the speed control valve thus changing over the forward and reverse running signal valve 9 to its forward running position. Similarly, when the speed control valve is shifted to the reverse running position, hydraulic fluid is supplied into the signal conduit 1 3 thus changing over the forward and reverse running signal valve 9 to its reverse running position.

In the above-mentioned arrangement, when the vehicle is run forwardly by controlling the hydraulic pump 1 and the hydraulic motor 2 so that the pressure in the first circuit 3 is kept higher than that in the second circuit 4, hydraulic fluid is supplied to the signal conduit 1 2 for forward running to thereby change the forward and reverse running signal valve 9 over to the forward running position b.

Consequently, the pressurized fluid delivered by the control pump 11 is supplied into the pilot pressure applying portion 1 0a of the setting pressure change-over means 8a for forward running so as to change the relief valve 7a for forward running over to the high pressure side thereby changing the setting pressure in the first circuit 3 over to the high pressure side.

At the same time, the setting pressure of the relief valve 7b for reverse running remains at a lower value.

When the vehicle is run backwards, the afore-mentioned operation is reversed, i.e., the relief valve 7bfor reverse running is changed over to the high pressure side thereby setting the second circuit 4 at a pressure higher than that in the first circuit 3.

To summarise: when the vehicle is run forwards, the pressure in the first circuit 3 is set at a higher value, whilst the pressure in the second circuit 4 is set at a lower value, and during the reverse running of the vehicle, the reverse setting is made.

Thus, when a hydraulic dynamic brake is applied while the vehicle is driven forwards or backwards, the circuit on the driven side can be kept at a lower pressure even if the circuit on the driving side is kept at a higher pressure so that the peak pressure which occurs when the brake is applied may be reduced thereby reducing the shock caused by deceleration and the tendency of cavitation to occur, and further reducing the over-run of the engine which tends to occur when the brake is applied.

Claims (4)

1. An hydrostatic transmission of the closed loop type wherein a variable displacement hydraulic pump is connected with the hydraulic motor in a closed loop circuit, the hydraulic motor being arranged to be driven by pressurized fluid delivered by the hydraulic pump; the closed loop circuit comprising a first circuit and a second circuit; a first pump being provided for charging hydraulic fluid into said closed loop hydraulic system; first relief valve means being connected with said first circuit and having first means responsive to hydraulic fluid pressure to change over the setting pressure of said first relief valve means between low and high setting pressures; second relief valve means being connected with said second circuit and having second means responsive to hydraulic fluid pressure to change over the setting pressure of said second relief valve means between low and high setting pressures; and pilot-operated change over means being disposed between said first and second means and a second pump for selectively connecting either one of said first and second means with said second pump while connecting the other of said first and second means to drain thereby changing over one of said first and second relief valve means to have high setting pressure and the other relief valve means to have low setting pressure.

2. An hydrostatic transmission according to Claim 1, wherein said pilot-operated change over valve means has a neutral position where both of said first and second means are connected to drain, a first offset position where said first means is connected with said second pump while said second means is connected to drain and a second offset position where said second means is connected with said second pump while said first means is connected to drain.

3. An hydrostatic transmission according to Claim 2, wherein said pilot operated change over valve means is coupled to a first signal conduit for applying pilot fluid pressure thereto to thereby change over the same to said first offset position, and is coupled to a second signal conduit for applying pilot fluid pressure thereto to thereby change over the same to said second offset position.

4. An hydrostatic transmission of the closed loop type, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.