GB2104961A - Motor vehicle tilting cab actuator - Google Patents

Abstract

The actuator has a first line 21 for pressurised fluid to the cylinder space 4 on the front side of the piston 2 to extend the ram. A normally closed valve 25 is opened by the fluid for extending the ram. A second line 22 is provided for pressurised fluid to the cylinder space on the rear side of the piston to retract the ram. A by-pass line 23, when the ram is retracted, interconnects the two cylinder spaces, and contains a normally open valve 27 which is closed by the pressure fluid supplied to extend the ram, which pressure is derived from a point in the line 21 upstream of the valve 25. The second line 22 acts on a piston 29 also opening the valve 25 to allow return flow in the line 21 when the piston retracts. Each line has a ball valve 24, 26 with restricted counterflow when closed. <IMAGE>

Description

SPECIFICATION Improvements in and relating to motor vehicles This invention relates to motor vehicles and more particularly to motor vehicles having a driving cab mounted on the vehicle frame at a position at least partly over the vehicle engine.

Such an arrangement is common in so-called heavy goods vehicles and, in order to service the vehicle engine, the driving cab is mounted so that it can be tilted with respect to the vehicle frame for access to the engine. It is normal practice to provide lifting jacks for effecting cab tilting, which jacks are usually in the form of hydraulic rams comprising a piston displaceable in a cylinder.

For travel comfort, it is desirabie that the driving cab be mounted on some form of shock absorbing suspension system. However, this presents a difficulty, particularly where hydraulic rams are used for cab tilting, since it will be appreciated that the cab must be free to move on its mountings and this requires freedom of movement of the piston in its retracted position.

Thus, a conventional hydraulic circuit and ram having a single piston could not conveniently be used, since it would present an incompressable column of hydraulic fluid preventing free movement of the piston, and hence of the cab on its mountings.

It has been proposed to provide an arangement for overcoming the disadvantages inherent in conventional hydraulic rams intended for use with tilted driving cabs, which arrangement comprises a by-pass for hydraulic fluid between the cylinder spaces on the front and rear sides of the ram piston when the piston is in its retracted position, fluid pressure operated control valve means being provided which, in use, are subjected to the pressure of the hydraulic fluid so as to close said by-pass whenever the ram is required to be extended. Thus, it was thought that, in use, the bypass between the two sides of the ram piston when in its retracted position would enable the piston to float and so permit free movement of the driving cab on its suspension.

In the aforesaid arrangement it was proposed that the control valve means be in the form a spool valve displaceable in a valve chamber and resiliently biased to maintain said by-pass open via ports in the valve chamber, the pressurised supply of hydraulic fluid to extend the ram causing the spool valve to be displaced against its resilient bias to close off communication between said ports and hence close the by-pass and to open simultaneously a port in the valve chamber leading to the cylinder space on the front side of the piston to enable the hydraulic fluid to extend the ram.

However, in practice it has been found that the above proposed arrangement does not operate exactly as thought. Thus, over a prolonged period of use, due to cab movement on its suspension system, it is thought that a pumping effect is created in the ram cylinder due to the piston being free to oscillate in its cylinder in response to vehicle movement, and because a check valve is necessarily provided in the pressure fluid supply circuit to prevent the pressure fluid from flowing back towards its source, there is a tendency to unbalance the pressure on the spool valve, and thereby overcome the effect of the biasing means, so that the spool valve moves from its position where it maintains said by-pass open to a position in which it closes off the by-pass so that the piston is no longer permitted free movement in its cylinder.

An object of the invention is to provide an arrangement which ensures that free floating movement of the piston in the retracted position of the ram (and hence of the cab on its suspension) is permitted over prolonged periods of use.

According to the invention there is provided a vehicle cab tilting arrangement including a hydraulic ram which is arranged to be extended to effect tilting of the driving cab of a motor vehicle relatively to the vehicle frame and which comprises a piston displaceable in a hydraulic cylinder, wherein (i) a first fluid flow line is provided for the pressurised flow of hydraulic fluid to the cylinder space on the front side of said piston to extend said ram, said first line containing first, normally closed valve means which is opened by the pressure of the supplied fluid for extending said ram and (ii) a by-pass line is provided which when said ram is in its retracted position interconnects the cylinder spaces on the front and rear sides of the piston, said by-pass'line containing second normally open valve means which is arranged to be closed by the pressure of hydraulic fluid supplied to extend said ram, which pressure is derived from a point in said first line upstream of said first valve means. Thus said second valve means is not subjected to any varying pressure in the portion of said first line between said first valve means and the cylinder space on the front side of the piston tending to close said second valve means, said varying pressure being due to the pumping effect caused by the free oscillation of the ram piston when it is floating in its retracted position.In fact it can be readily arranged that the pressure in said first line at the point from which the actuating pressure for the second valve means is derived, since it is upstream of the first valve means, is at atmospheric pressure when the ram piston is floating, so ensuring that the second valve means can not be unintentionally closed.

One cab tilting arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 shows an axial section through a hydraulic ram of the arrangement, and, Figure 2 is a circuit diagram of the arrangement.

The hydraulic ram comprises a hydraulic cylinder 1 in which a ram piston 2 is displaceably mounted. The piston 2 is fixed to one end of a piston rod 3 whose other end passes out of the cylinder 1 for connection to the vehicle driving cab. The hydraulic fluid is fed from a pressurised source into the cylinder space 4 on the front side of the piston 2 to extend the ram and so raise the cab into its tilted position on the vehicle frame.

Conversely for retracting the piston and so lowering the cab, the hydraulic fluid is fed into the annular space 5 on the rear side of the piston.

The pressurised source comprises an electric motor driven pump assembly 6 which draws the hydraulic fluid from a reservoir 7 (Figure 2) and pumps it to the ram. For convenience of drawing the circuit diagram the reservoir has been shown at various places in the diagram, but it is to be understood that only a single reservoir 7 is provided. The pump is operable so that it can pump fluid along line 8 into the cylinder space 4 on the front side of the piston or along the line 9 to the cylinder space 5. The line 8 has a junction 11 in it from which branch lines 12 and 13 lead to the reservoir 7 via a normally open solenoid operated valve 14 and a pressure relief valve 1 5 respectively.Similarly the line 9 has a junction 1 6 from which branch lines 1 7 and 1 8 lead to the reservoir 7 via normally open solenoid operated valve 1 9 and pressure relief valve 20 respectively.

When the pump is operated to supply hydraulic fluid along line 8 valve 14 is closed and when it is operated to supply hydraulic fluid along line 9 valve 1 9 is closed to prevent the hydraulic fluid being pumped directly into the reservoir 7.

The line 8 includes a port 21 in the head of the cylinder 1 from which port the line 8 continues as internal ducting in the cylinder head leading to the cylinder space 4. The line 9 includes a port 22 in the cylinder head from which the line 9 continues as ducting in the cylinder head and then as an external pipe 9a connecting into the cylinder space 5. A by-pass line 23 is provided to interconnect the cylinder spaces 4 and 5 when the piston 2 is in its retracted position. The by-pass line 23 comprises external piping 23a and internal ducting 23b which has a T-junction with the line 8 so that a portion 23c of the by-pass is constituted by a portion of the line 8.

Downstream of the port 21 the line 8 includes a normally closed one-way restrictor valve 24. This comprises a ball valve spring biased to a normally closed position and having a restricted flow path past it for hydraulic fluid as represented diagrammatically in Figure 2. The line 8 also includes a ball valve 25 spring biased to a normally closed position and whose purpose will be explained hereafter. Downstream of the port 22 the line 9 includes a normally closed one-way restrictor valve 26 similar to the valve 24. The bypass line 23 includes a valve 27 which is spring biased to a normally open position. The valve 27 is pilot operated by pressurised hydraulic fluid supplied through line 8 to extend the ram enabled by the pressure drop across valve 25.

As can be seen from Figure 1 and by the dotted line in Figure 2 the point in the line 8 from which the actuating pressure for the valve 27 is derived is between the port 21 and the valve 24, i.e. upstream of both the valves 24 and 25, the actuating pressure actirig on plunger 28 forming part of the valve to cause the valve 27 to close off the by-pass. As can be seen the valve 27 has a conical end for engaging its valve seat in the ducting 23b, so that it presents a relatively small surface area to hydraulic pressures tending to unseat it when closed. The valve 25 is opened directly by the pressure of hydraulic fluid supplied along line 8 to extend the ram and provides the pressure drop which ensures satisfactory operation of the valve 27.It is also opened by pilot operation by the pressure of hydraulic fluid supplied along line 9 to retract the ram as indicated by dotted line in Figure 2, this pressure acting on plunger 29 which has a pin-form extension 30 contacting the ball of the valve 25.

In operation of the arrangement, to extend the ram pressurised fluid is supplied along line 8 which closes valve 27 to close off the by-pass 23 and opens the valves 24 and 25. Hydraulic fluid displaced from the cylinder space 5 passes along the line 9 via the restrictor valve 26 and then via the normally open solenoid valve 1 9 to the reservoir 7. To retract the ram pressurised fluid is supplied along line 9 which opens valves 26 and 25. Hydraulic fluid displaced from the cylinder space 4 passes along line 8 via the valves 24 and 25 and then via the solenoid valve 14 to the reservoir 7. At this time the valve 27 is open since there is no actuating pressure on the plunger 28.

When the piston 2 is in its retracted position shown in Figure 1 and the pump assembly 6 is inoperative so that no pressurised hydraulic fluid is supplied along either line 8 or line 9 the valves 24, 25 and 26 are closed and the valve 27 is open to open the by-pass 23. Also the solenoid valves 14 and 1 9 are open. Thus the piston 2 can float, i.e.

oscillate freely about a mean position so that the vehicle cab is free to follow its suspension movements. During this oscillation, on the inward stroke of the piston 2 fluid is displaced from the cylinder space 4 to the cylinder space 5 via the bypass 23 and excess fluid flows back to the reservoir 7 through the line 9 via the restrictor valve 26, port 22 and solenoid valve 19. On the outward stroke of the piston 2 fluid is displaced from the cylinder space 5 to the cylinder space 4 and make-up fluid may be drawn from the reservoir 7 through the line 9 via the solenoid valve 1 9 the port 22, the valve 26 and the cylinder space 5.

It is to be noted that when the piston 2 is in its floating condition, there can be no actuating pressure applied to valve 27 since the pressure in the line 8 at port 21 is at reservoir pressure, i.e.

atmospheric pressure, the solenoid valves 14 and 1 9 both being open. Any pumping action in the line 8 between the cylinder space 4 and the valve 25 will tend to provide fluid pressures assisting to keep the valve 27 open.

The normally closed valve 25 is as described above open during both extension and retraction of the ram. It does however, in addition to its primary operational function described above conveniently provide a fail safe feature, in that if there should be a loss of pressure in either of the lines 8 or 9, the valve 25 will automatically close to lock the ram in position and so prevent the cab from crashing down on to the vehicle frame.

Claims (6)

1. A vehicle cab tilting arrangement including a hydraulic ram which is arranged to be extended to effect tilting of the driving cab of a motor vehicle relatively to the vehicle frame and which comprises a piston displaceable in a hydraulic cylinder, wherein (i) a first fluid flow line is provided for the pressurised flow of hydraulic fluid to the cylinder space on the front side of said piston to extend said ram, said first line containing first, normally closed valve means which is opened by the pressure of the supplied fluid for extending said ram, (ii) a second fluid flow line is provided for the pressurised flow of hydraulic fluid to the cylinder space on the rear side of the piston to retract said ram and (iii) a by-pass line is provided which when said ram is in its retracted position interconnects the cylinder spaces on the front and rear sides of the piston, said by-pass line containing second, normally open valve means which is arranged to be closed by the pressure of hydraulic fluid supplied to extend said ram, which pressure is derived from a point in said first line upstream of said first valve means.

2. A vehicle cab tilting arrangement according to Claim 1, wherein said pressurised flow of hydraulic fluid is supplied by a pump from a reservoir which is at atmospheric pressure and said point in said first line from which pressure is derived for closing said second valve means is located so that it is at atmospheric pressure when said pump is inoperative.

3. A vehicle cab tilting arrangement according to Claim 1 or 2, wherein said second valve means has a conical valve end for engaging its valve seat so that it presents a relatively small surface area to hydraulic pressures tending to unseat it when closed.

4. A vehicle cab tilting arrangement according to any preceding claim, wherein said first fluid flow line includes a normally closed one-way restricter valve connected between said point from which fluid pressure is derived for closing said second valve means and said first valve means and which is opened by the pressure of the supplied fuel for extending said ram.

5. A vehicle cab tilting arrangement according to any preceding claim, wherein said first valve means is arranged to be opened by pilot operation from the pressure of hydraulic fluid in said second fluid flow line supplied to retract said ram.

6. A vehicle cab tilting arrangement substantially as hereinbefore described with reference to the accompanying drawings.