GB2048383A

GB2048383A - Improvements in and relating to road vehicles

Info

Publication number: GB2048383A
Application number: GB8010345A
Authority: GB
Original assignee: Armstrong Patents Co Ltd
Current assignee: Armstrong Patents Co Ltd
Priority date: 1979-03-28
Filing date: 1980-03-27
Publication date: 1980-12-10
Also published as: GB2048383B

Abstract

For use in connection with road vehicles of the type having a driving cab mounted on the vehicle frame above the vehicle engine and in which the cab is capable of being tilted to provide access to the engine, the invention provides a cab tilt mechanism in which a hydraulic by-pass (20) is arranged across the ram piston (12) to avoid the transmission of vibration to the cab through the "solid" oil column otherwise existing in front of the piston. When the cab is to be tilted, the hydraulic pressure applied to the piston for that purpose also closes a valve spool (32) against a seat (30) to render the by-pass (20) ineffective. <IMAGE>

Description

SPECIFICATION Improvements in and relating to road vehicles This invention concerns road vehicles and relates more particularly to such vehicles of the type having a driving cab mounted on the vehicle frame above the region of the frame at which the vehicle engine is also situated.

Such an arrangement is common in modern commercial or so-called heavy goods vehicles and servicing requirements for the vehicle engine normally demand that the driving cab shall be capable of being tilted with respect to the vehicle frame in order to provide access to the engine. It is the usual practice to provide lifting jacks for cab tilting purposes and more often than not such lifting jacks take the form of hydraulic rams.

In the interests of greater travel comfort, it is an increasing practice in road vehicles of the type described above to mount the driving cab on some form of anti-vibration or shock absorbing suspension units which sometimes may be rubber mounted but may alternatively be in the nature of hydraulic shock absorbers or dampers. A suspension cab mounting of this nature, however, creates difficulties in connection with lifting jacks by which the cab is tilted, since it will be appreciated that the cab must be free to move on its mountings.

Generally speaking, this precludes the use of mechanical lifting jacks and in the case of hydraulic rams it demands that the ram shall not create any resistance to the desired freedom of movement. This requirement can mean that a standard hydraulic ram is unsatisfactory due to the presence of a "solid" column of oil or other hydraulic fluid in front of the ram piston.

It is an object of the invention to provide a simple and inexpensive means of overcoming this disadvantage of conventional hydraulic rams intended for use in connection with suspended driving cabs.

According to its broadest aspect, in a cab tilt mechanism employing a hydraulic ram for tilting the driving cab of a road vehicle relative to the vehicle body, the present invention provides a hydraulic ram fitted with a valve controlled hydraulic by-pass connected between the hydraulic pressure space in front of the ram piston and the hydraulic pressure space behind it and fluid pressure operated control valve means which is subjected to the pressure fluid supplied to the hydraulic pressure space in front of the ram piston to close said by-pass whenever said ram is required to be extended.

By virtue of the invention, the by-pass between the two sides of the ram piston enables the latter freely to follow the movement of the driving cab on its suspension since under such circumstances the control valve means maintains the by-pass in an open condition.

Whenever the cab is to be tilted to provide access to the engine, however, the application of pressure fluid to the control valve means causes the latter to close the by-pass so that the pressure fluid is directed only to one side of the ram piston, namely to the front or lifting side of the piston.

The control valve means may conveniently be arranged in an end closure for the hydraulic cylinder of the ram and preferably takes the form of a spool valve of which a resiliently biased valve spool is displaceable in a valve chamber situated transversely of the cylinder to communicate with the hydraulic by-pass and connected by suitably axially directed ports in the end closure to the cylinder space in front of the ram piston, the by-pass and the ports being selectively opened in dependance upon the valve spool displacement in the valve chamber.

For ease and economy in manufacture, the hydraulic by-pass may with advantage take the form of a tube situated externally of the hydraulic cylinder and serving to connect the valve chamber with a cylinder port situated behind the position of maximum extension of the ram piston in the cylinder.

The invention will be described further, by way of example, with reference to the accompanying drawing which is an axial section of a part of a hydraulic ram embodying the invention.

The ram shown in the drawing will be seen to include a hydraulic cylinder 10 in which there is displaceable a ram piston 1 2 carried at one end of a piston rod 14, the other end of which (not shown) passes out of the cylinder 10 for connection to a vehicle driving cab.

In the usual arrangement of such hydraulic rams, a hydraulic fluid which is normally an oil is passed under pressure into the cylinder space 1 6 in front of the piston in order to raise the cab into a tilted position on the vehicle frame. Conversely, when the cab is to be lowered, pressure fluid is supplied to the cylinder space 1 8 behind the piston and is allowed to pass from the cylinder space 1 6 back into a conventional hydraulic reservoir (not shown). A fluid flow restrictor 54 is incorporated in the flow path between the cylinder space 1 6 and the reservoir in order to control the rate of descent of the cab.

However, since an uninterrupted column of oil is always present between the fluid pressure source and the front of the piston 12, the ram in effect acts as a solid connection between the vehicle frame and the driving cab.

A solid connection of this nature is not acceptable where the cab is mounted on shock absorbing or damping suspension units and the invention therefore provides a hydraulic by-pass 20 which is shown as being a tube or pipe connecting a port 22 in the cylinder wall 10 behind the piston 1 2 which a co-operating port 24 provided in an end closure generally designated 26 and serving to close the cylinder end in front of the piston.

The port 24 will be seen to be a transverse bore in the end closure 26 and opens out into a counterbore 28, a frusto-conical valve seat 30 being provided at the junction of the port 24 and counterbore 28. The counterbore 28 in fact constitutes a valve chamber for a valve spool 32 and the valve chamber is connected to the cylinder space 1 6 by transversely spaced ports 34 and 36 provided in the end closure 26.

The end of the valve spool 32 adjacent the seat 30 is of frusto-conical shape intended to engage snugly with the seat 30 when so required but a spring 38 in the port 24 normally acts to bias the spool 32 away from the seat 30. The end of the valve spool remote from the tapered end is of slightly reduced diameter relative to the intermediate section of the spool and co-operates with that intermediate section to define a shoulder 40 which, in the illustrated position of the spool stands to the left of the port 34. Immediately adjacent the shoulder 40, the spool is formed with a circumferential recess 42 which communicates with a diametral bore 44 in the spool, the latter in turn intersecting an axial bore 46.The reduced diameter end of the spool 32 is formed hollow to receive a check valve assembly 48 of which a valve ball 50 is normally resiliently biased on to a co-operating valve seat in order to close the axial bore 46 and diametral bore 44.

As has already been noted, with the spool valve 32 in its illustrated position, the hydraulic by-pass 20 is effective to allow free movement of the ram piston 1 2 relative to the cylinder 10 so that the hydraulic ram does not interfere with suspension movement of the driving cab on its suspension mountings. If it is required to tilt the cab to provide access to the vehicle engine, the application of hydraulic pressure to the spool valve assembly via a connector 52 will displace the valve spool 32 to the right until it closes on to the seat 30, thus in turn closing the hydraulic by-pass 20.

Due to this movement of the spool 32, the shoulder 40 is displaced past the port 34 and hydraulic pressure fluid passes around the end clearance of the spool and through the port 34 into the cylinder space 16, thus resulting in a lifting movement of the ram piston.

Conversely, when the cab is to be lowered pressure fluid is supplied to the cylinder space 18, with the consequence that the valve spool 32 is displaced to the left as viewed in the drawing, until such time as the groove 42 in the valve spool registers with the port 34. A fluid flow path back to the hydraulic reservoir is thus established from the cylinder space 1 6 through the port 34 and diametral bore 44 and by way of the axial bore 46 into the interior of the valve spool 28. The resultant fluid pressure lifts the valve ball 50 of the check valve assembly and permits fluid to exhaust to the reservoir through a conventional fluid flow restrictor 54 which may be a simple throttling constriction somewhere in the fluid flow path or which may be a flow restricting valve.

Claims

1. A vehicle cab tilt mechanism including a hydraulic ram comprising a piston displaceable in a hydraulic cylinder for tilting the driving cab of a road vehicle, relative to the vehicle frame, in which the hydraulic ram is fitted with a valve controlled hydraulic by-pass connected between the hydraulic pressure space in front of the ram piston and the hydraulic pressure space behind it and with fluid pressure operated control valve means which is subjected to the pressure fluid supplied to the hydraulic pressure space in front of the ram piston to close said by-pass whenever said ram is required to be extended.

2. A vehicle cab tilt mechanism as claimed in claim 1, in which the control valve means is arranged in an end closure for the hydraulic cylinder of the ram.

3. A vehicle cab tilt mechanism as claimed in claim 2, in which the control valve means is a spool valve comprising a resiliently biased valve spool displaceable in a valve chamber situated transversely of the cylinder to communicate with the hydraulic by-pass and connected by axially directed ports in the end closure to the cylinder space in front of the ram piston, the by-pass and the ports being selectively opened in dependence upon the valve spool displacement in the valve chamber.

4. A vehicle cab tilt mechanism as claimed in claim 3, in which the hydraulic bypass is a tube situated externally of the ram cylinder and serving to connect the valve chamber with a cylinder port situated behind the position of maximum extension of the ram piston in the cylinder.

5. A vehicle cab tilt mechanism constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.