GB2103841A - Hydraulic pump - Google Patents

Abstract

A hydraulic pump device with variable feed volume has a hydraulic flow regulating valve 17 for regulating the flow medium to flow at a constant rate by measuring the pressure difference at an orifice 18. The control piston 21 of the flow regulating valve is supported by a measuring spring 26 via an intermediate member 25 surrounded by the flow medium. The intermediate member 25 has a high degree of compressability and a heat coefficient of expansion corresponding to at least three times that of steel. During variations in viscosity as a result of temperature changes in the flow medium as well as during pressure variations within the system, a flow volume is achieved which lies within very tight tolerances. The intermediate member may be a block of P.T.F.E. of a mercury filled pressure chamber. <IMAGE>

Description

SPECIFICATION Hydraulic pump device with variable feed volume The invention relates to a hydraulic pump device with variable feed volume and a hydraulic flow regulating valve for regulating the flow medium to flow at a constant rate by measuring the pressure difference at an orifice, whereby the control piston of the flow regulating valve is supported by a measuring spring.

Pump devices of this type operate either with a constant pump and a variable hydraulic motor or with a constant motor and a variable hydraulic pump. The present invention relates to both possibilities.

Devices such as are disclosed for example in DE-OS 29 04 474, for example are used for the hydro-static drive of auxiliary engines in diesel locomotives, where different auxiliary machines, such as compressors, ventilators, a.c. current generators etc. have to operate at constant revolutions independent of the instantaneous hydraulic pressure. In these known devices the unavoidable change in oil temperature and the resulting change in oil viscosity leads to a substantial variation in the revolutions of the regulated drives, since the change in viscosity results in a displacement of the pressure difference at the orifice.A change in the number of revolutions is also caused by the switching on or off of further hydraulic motors since the quantities of leakage oil in the flow regulating valve and in the hydraulic motors deviate as a result of the pressure variations that take place, but deviate also when there are changes in the viscosity. As the oil temperature rises, the revolutions to be kept constant are reduced, and are increased when the pressure level is raised, for example by additionally switching on a compressor.

It is the aim of the invention to adapt a device of the aforementioned type in such a way that both when deviations occur in the viscosity as well as when deviations occur within the pressure system, the volume flow remains substantially constant within very narrow tolerances.

The invention achieves this aim by providing a hydraulic pump device having variable feed volume and a hydraulic flow regulating valve for regulating the flow medium to flow at a constant rate by measuring the pressure difference at an orifice, a control piston of the flow regulating valve being supported by a measuring spring, there being provided on the side of the measuring piston supported by the measuring spring an intermediate member having a high degree of compressibility and a coefficient of heat expansion which is at least three times that of steel.

The intermediate member is preferably arranged between the measuring spring and the control piston.

By means of such a surprisingly simple development of a regulating valve according to the invention both the said disadvantageous influences are countered so that by means of a suitable combination of the intermediate member, the measuring spring and the orifice, a sufficiently constant volume stream, i.e. a minimum displacement of the number of revolutions is achieved both for very substantial changes in viscosity and large variations in pressure, even for heavy demands. When the temperature increases, the intermediate member expands and displaces the control piston in the direction towards a higher number of revolutions.On the other hand, the intermediate member, when subjected to the hydraulic pressure, because of its good compressibility, i.e. is compressed because of its low modulus of elasticity, so that the control piston is displaced in the direction of a smaller number of revolutions.

In accordance with a preferred embodiment of the invention, the intermediate member comprises a block of temperature and pressure resistant synthetic material. Preferably this is made from polytetrafluoroethylene (PTFE). In the temperature region applicable here, this material has a linear coefficient of expansion which, although different depending upon the temperature, is at least 150 to 160x10-8, in other words approximately 14 times the value of that of steel. The modulus of elasticity is between 430 and 750, i.e. only a fraction of the modulus of elasticity of steel or even aluminium.

For a specific reconstruction the balance between the length of the intermediate member, the measuring spring and the orifice may be relatively simple, since both the measuring spring as well as the intermediate member can be easily fitted and removed. For this purpose there are determined measuring points with two different lengths of intermediate members spaced apart by relatively wide gaps and the required length is determined by linear interpolation of these values.

According to an even more advantageous alternative development, the intermediate member can also be adapted to be a closed metal pressure chamber which is at least partially filled with mercury. Mercury also has a relatively high coefficient of heat expansion which amounts to approximately 1 6.5 times that of steel.

By way of example, a specific embodiment of the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a hydrostatic drive for auxiliary devices in a diesel locomotive, having a hydraulic pump device according to an embodiment of the invention; Figure 2 is a longitudinal cross-section of the regulating valve of the device shown in Figure 1 with an intermediate member acting as the compensator according to the invention; and Figure 3 is a diagrammatic representation of the oil temperature superimposed upon the number of revolutions of the hydrostatically driven auxiliary devices using a regulating valve without a compensator and one with a compensator.

In the schematic diagram according to Figure 1 a variable revolution diesel engine 4 of a diesel locomotive drives a hydraulic pump 5 with variable feed volume. The pump through a pressure line 6 feeds a hydraulic motor 7 with constant feed volume which in turn drives a piston compressor 10 which can be inactivated via a bypass line 8 and a control valve 9. In series with the compressor drive is fed a further fixed displacement motor 11 which drives the drive engine ventilator 12. A combined control and pressure limiting valve 14 is arranged in a bypass line 1 3 to the fixed displacement motor 11.

Reference numberal 1 5 denotes the overflow-oil lines which lead out of both motors 7 and 11 and feed back into the oil sump 1 6. The hydraulic pump 5 is controlled by a flow regulating valve 17 which is controlled in response to the pressure differences measured at an orifice 1 8.

Flow regulating valve 1 7 with orifice 1 8 are represented in an enlarged form in Figure 2. The oil conveyed by pump 5 enters the orifice 1 8 through the pressure line part 1 9. Before the orifice 1 8 a pipe 20 extends into an annular chamber 22 formed by the control piston 21 of the regulating valve and behind this orifice 1 8 a pipe 23 extends into the so-called spring chamber 24 of the regulating valve. In the spring chamber 24 the control piston 21 is supported by the intermediate member 25 acting as the compensator and made from polytetrafluoroethylene and the measuring spring 26 is supported by the valve housing 27.The pressure line 28 leads from the flow regulating valve to the adjusting mechanism of the pump and the metered oil and the overflow oil through line 29 and by means of pump 5 is fed to the sump 1 6. A line 30 leading off from the pressure line 20 also leads into the chamber 31 before the control piston 21.

As the oil temperature changes, the viscosity of the oil changes and therefore there is a pressure difference at the orifice 1 8. This and the loss through overflow oil leakage both at the control piston 21 and in the motors 7 and 11 without compensator 25 at increasing temperature would result in the control piston 21 being displaced towards the lefthand side (according to Figure 2) in the sense of a reducing number of revolutions.

However, the oil bathed compensator 25 counters this tendency by its linear expansion as a result of it being heated, and compensates it substantially.

In the same way the displacement tendency of the control piston during an increase in the pressure level, as a result of adding the compressor in the sense of an increase in the number of revolutions (to the right according to Figure 2), is similarly compensated by the increased pressure acting on the compensator 25, whereby it is linearly reduced. By a suitable combination of the PTFE compensator, the measuring spring and the orifice it is possible to keep the variation in the number of revolutions within very tight tolerances.

In the diagram according to Figure 3 the measuring values ascertained in tests are applied by a regulating valve both without compensator (index 1) as well as with a compensator (index 2) by way of the oil temperature v over the number of revolutions n. The curves shown respectively represent the data when using the ventilator V1 or V2 alone, and the curves in broken lines represent the data when using the ventilator + compressor (V+K), or (V+K)2.

When operating the ventilator alone, the pressure p=85 bar and when operating the ventilator + compressor p=260 bar.

The diagram clearly shows that the revolutions decrease rapidly at increasing oil temperature without the compensator, whereas in operation at an oil temperature in excess of 300C with the compensator the number of revolutions deviates only + or - 1% from the nominal number of revolutions n. At an increase of pressure from 85 to 260 bar as a result of including the compressor, without compensator at the same temperature the number of revolutions increase by 5% and more, whereas with compensator the variation in the number of revolutions is only insignificant, approximately 1%.

The invention is not restricted to the details of the foregoing embodiment.

Claims (7)

Claims

1. A hydraulic pump device having variable feed volume and a hydraulic flow regulating valve for regulating the flow medium to flow at a constant rate by measuring the pressure difference at an orifice, a control piston of the flow regulating valve being supported by a measuring spring, there being provided on the side of the measuring piston supported by the measuring spring an intermediate member having a high degree of compressibility and a coefficient of heat expansion which is at least three times that of steel.

2. A device according to Claim 1, in which the intermediate member is arranged between the measuring spring and the control piston.

3. A device according to Claim 1 or Claim 2, in which the intermediate member comprises a block of temperature and pressure resistant synthetic material.

4. A device according to Claim 3, in which the intermediate member is made from polytetrafluoroethylene.

5. A device according to Claim 1 or Claim 2, in which the intermediate member is adapted to be a closed metal pressure chamber which is at least partially filled with mercury.

6. A method for determining the length of an intermediate member of synthetic material required for a specific application in accordance with any one of Claims 1 to 4, in which the measuring points of the device are determined by two different lengths of intermediate members spaced apart by a relatively wide gap and the required length is determined by linear interpolation of these values.

7. A hydraulic pump constructed and arranged substantially as herein described with reference to the accompanying drawings.