GB2055971A - Ram and Vehicle Cab Tilting Arrangement Incorporating Same - Google Patents

Abstract

A hydraulic ram 10 especially suitable for controlling a tiltable cab of a motor vehicle which passes through an over-centre intermediate position has a lost motion coupling slot 46 of which the effective length is controlled by a movable end member 40. In the rest position end member 40 is retracted permitting a limited amount of free movement between parts interconnected by the ram. In an operating stroke pressurised fluid supplied to chamber 14a of ram cylinder 12 passes through a valve- controlled passage in an internal partition 18 and preferentially activates a spring-loaded secondary piston 38 to displace end member 40 and take up the lost motion before operating smaller diameter driving piston 22 to extend the ram. Valve 72 closes to trap pressurised fluid when piston 22 moves so that lost motion slot 46 remains closed up and prevents lost motion, thereby maintaining rigid control and preventing any jerk in any change between compression and tensile loading of the ram, throughout the entire operating stroke. <IMAGE>

Description

SPECIFICATION Ram and Vehicle Cab Tilting Arrangement Incorporating Same This invention relates to ram devices for connecting between and controlling displacement of relatively movable parts, such ram devices being of the kind which are actuated by pressurised fluid and which are provided with lost motion slot means for permitting a limited amount of free movement between the parts connected, in use, by the ram device.

In some cases, in use, such ram devices are required to function under compression as a variable length strut over part of their range of movement and under tension as a variable length stay over another part of their range of movement, but at the point of changeover between these different functional modes the lost motion slot means may have an unwanted and undesirable effect of producing a sudden jerk or lurching movement in an otherwise smoothly controlled steady displacement movement of the interconnected relatively movable parts.

A specific example of this problem is to be found in arrangements incorporating such ram devices for controlling tilting of the cabs of heavy goods vehicles. These cabs are commonly mounted so that they can be tilted forwards on the vehicle chassis so as to expose the engine to facilitate servicing, and for controlling this operation the cab and chassis are interconnected by a ram device of the kind referred to, the lost motion slot means being required to permit a certain amount of free up and down movement of the cab in its normal in-use position on the chassis for accommodating springing motion, the cab generally being sprung independently on the chassis to provide acceptable driver comfort.

When the cab is being tilted, it is first raised angularly by the ram device extending and acting as a strut until it reaches a point of balance, after which the cab moves further forwards and downwards under its own weight to reach the final forwards tilted position with the extending ram device then acting as a stay controlling this movement. However, on passing through the point of balance, the lost motion slot means results in a sudden lurching motion of the cab which tends to produce unacceptable stresses in the cab and anchor pins of the mechanical linkage.In consequence, the extent of lost motion which can be permitted in the lost motion means is severely limited and, in practice at the present time, it is customary to restrict the length of the lost motion slot to a maximum of 25 mm, although from the point of view of providing better springing and driver comfort a longer slot would be desirable so as to accommodate a greater range of springing motion of the cab in the normal in-use position.

One object of the present invention is to provide a solution to the above problem and provide an improved form of ram device especially, although not exclusively, suitable for incorporating in a vehicle cab tilting arrangement.

According to the invention, a ram device of the kind referred to for connecting between and controlling displacement of relatively movable parts is characterised by the fact that it includes fluid actuated means operative throughout the stroke of the ram to control and shorten the length of the slot of the lost motion slot means thereby to prevent any free movement of lost motion and enable the ram device to exercise a rigid control throughout any change from compression loading to tensile loading during its stroke.

In preferred embodiments, the ram device comprises a cylinder having a first piston assembly for extending the ram and a second piston assembly which controls the position of a movable end member in a slot of a connecting member at one end of the cylinder providing the lost motion means, the second piston assembly having an actuating piston of larger crosssectional area than the actuating piston of the first piston assembly whereby, on supplying pressurised fluid in a rest condition of the device before an operating stroke, the second piston assembly is preferentially activated and operates to close up the slot by displacing the movable end member so as to take up the lost motion before the second piston assembly is activated to carry through the driving stroke, valve means being provided to trap pressurised fluid and lock the first piston assembly in position throughout substantially the entire driving stroke of the first piston assembly irrespective of whether the device is subjected to compressive or tensile forces.

Other preferred features and details of construction will be apparent from the following description of a particular embodiment.

By way of example, one form of hydraulic ram device in accordance with the invention and its use in a cab tilting arrangement of a heavy goods vehicle is illustrated in the accompanying drawings wherein: Figure lisa longitudinal section of the ram device shown in its inoperative retracted condition; Figure 2 is a fragmentary longitudinal sectional view on an enlarged scale showing the right-hand portion of the device of Figure 1 at the end of a preliminary first stage of operation; Figure 3 is an elevational view from the righthand end of Figure 1; and Figure 4 shows schematically the manner in which the ram device is incorporated and arranged in the cab tilting arrangement.

Referring first to Figures 1 and 2, the device 10 comprises a cylinder 12 divided internally into two chambers 14 and 1 6 by a partition 18 near to one end 20 such that the length of chamber 14 is considerably greater than that of chamber 1 6.

Chamber 14 is further sub-divided into two chamber portions 1 4a and 1 4b by a piston 22 therein. Piston 22 is connected to a piston rod 24 projecting through the opposite end 26 of the cylinder which is closed by a gland 28 provided with seals 30. The projecting end portion of the piston rod 24 carries a stop member 32 and terminates in a tongue 34 provided with an aperture 36 to receive an anchor pin for connecting, when in use, to one of two relatively movable parts to be controlled by the device.

The other smaller chamber 16 contains a piston 38 integrally connected to a piston rod 40 which extends through an end cap 42 closing the end 20 of the cylinder.

The end cap 42 is firmly secured to the cylinder and carries an axially projecting tongue 44 formed with a longitudinal slot 46 of which the open inner end is in axial alignment with the bore 48 in the end cap 42 which affords passage for the piston rod 40. As shown the piston rod 40 extends into the slot 46 and has an arcuate end face 50 forming an end wall closing the inner end of the slot 46 of which the effective length is determined by the axial position of the piston rod 40 and piston 38.

The slot 46 is also fitted with a bush 52, slidable along the slot, to receive a transverse anchor pin for connecting the device to the other of the relatively movable parts to be controlled thereby, the purpose of the slot 46 being to provide a lost motion arrangement.

Connectors 54 and 56 are provided for connecting to a pressurised hydraulic fluid supply circuit. Connector 54 communicates directly with the portion 1 4a of chamber 14 through a port 58 immediately adjacent the partition 1 8 and the piston 22. Connector 56 communicates with the portion 1 4b of chamber 14 through a feed tube 60 and port 62 adjacent the gland 28 at the other end of the chamber.

As shown, the partition 18, which is in the form of a separate disc-shaped member fitted against an internal shoulder 64 of the cylinder 12 and held in place by a circlip or retaining ring 66, provides for communication between portion 1 4a of chamber 14 and chamber 16 through a central passage way 68 having an enlarged counterbore portion 70 housing a ball valve 72 which is urged by a spring 74 towards an internal valve seating 76 provided by the inner end of the counterbore portion. When the piston 22 is in its rest position at the right-hand end of chamber 14 as seen in Figures 1 and 2, however, a valve control pin 80 on the piston extends, with clearance, into the passageway 68 and holds the ball valve 72 off its seating 76.

The piston 38 is spring loaded by a spring 82 which is seated in the end cap 42 and which urges this piston towards the partition 1 8 at the left-hand end of chamber 1 6. A recess 84 is provided in the face of piston 38 to clear an axially projecting central boss 86 of the partition which accommodates the ball valve assembly.

It should be particularly noted that the chamber 14 has a slightly smaller diameter than the chamber 16, the cylinder 12 being stepped internally adjacent port 58, so that the piston 38 has 2 somewhat larger cross-sectional area than the piston 22.

In the cab tilting arrangement of a heavy goods vehicle illustrated in Figure 4, the ram device 10 is connected at its ends between anchorages 90 and 92 on the cab 94 and chassis 96 respectively and is shown in its extended position as when in use.

The operation of the device will now be described in more detail with particular reference to the use of the device in the cab tilting arrangement of Figure 4.

Starting from the rest or retracted condition when the cab 94 is in its normal in-use position, both pistons 22 and 38 lie adjacent the partition 18 as seen in Figure 1 with the end of the piston rod 40 towards the inner end of the slot 46 at its furthermost position so that the slot 46 has a maximum effective length. In this embodiment, this maximum effective length can conveniently be 50 mm which allows for this extent of lost motion and of free movement of the cab on the chassis to accommodate up-and-down springing motion of the cab as the bus 52 fitted with the anchor pin connecting to the anchorage 92 on the chassis slides up and down the slot 46.

To tilt the cab, pressurised fluid is introduced into the cylinder 12 through port 58 and passes through the passageway 68, the ball valve 72 being held open by pin 80, into chamber 16 where it forces piston 38 to the right finally reaching the position shown in Figure 2. It will be noted that due to its larger cross-sectional area, the piston 38 is the first to react and during this preliminary first stage the piston 22 does not move.

As piston 38 moves to the right, so does the piston rod 40 thereby reducing the effective length of the slot 46 and engaging the sliding bush 52 which is pushed to the closed outer end of the slot in the final position when all free motion is taken up.

In a second stage, the fluid pressure continues to rise and eventually the piston 22 begin to move to the left, extending the projection of the piston rod 24 and so raising the cab. In so moving, the valve control pin 80 is carried with the piston 22 and the ball valve 72 closes to lock the piston 38 in position under the pressure of the fluid trapped in the chamber 1 6.

The ram device is thus then rigid under both compression and tension and can be fully extended to take the cab over centre into the forwardly tilted position shown in Figure 4, without any sudden jerk or lurching when passing the point of balance as occurs in conventional arrangements in which the lost motion slot means of the ram is not put out of action.

In returning the cab to its normal in-use driving position on the chassis, pressurised fluid is introduced into portion 1 4b of chamber 14 through the feed tube 60 and port 62 so that the piston 22 is forced back, retracting the piston rod 24. The rigid control is maintained, however, until the piston 22 again reaches its initial rest position when the valve control pin 80 again engages and lifts the ball valve 72 off its seating, so opening the passageway 68 and permitting the trapped fluid in chamber 1 6 to escape and exhaust through port 58. Piston 38 then returns to its original position under the force of its bias spring 82 and the slot 46 is restored to its original maximum effective length so that the bush 52 is again free to slide up and down providing a certain extent of free motion.

The fact that this arrangement provides complete rigid control in all raised positions of the cab and overcomes the problem of lurching in moving over the point of balance is especially advantageous in that it allows the use of a longer effective length of slot and hence a larger range of free motion in the normal lowered driving position of the cab, thereby enabling a more effective and better independent springing of the cab on the chassis to be used to improve driving comfort.

Although one specific form of ram device and its use in a particular field of application has been described in some detail, it will of course be understood that numerous modifications in the details described can be made within the scope of the invention.

Claims (7)

Claims

1. A pressurised fluid actuated ram device for inter-connecting and controlling displacement of relatively movable parts, said ram device incorporating lost motion coupling means having a lost motion slot to permit a limited amount of free relative movement between said interconnected parts when the ram is in an initial rest condition, characterised in that the ram device also includes fluid actuated means arranged to be automatically operative during the operating stroke of the ram so as to control and shorten the effective length of said lost motion slot thereby temporarily to take up the lost motion and prevent said free relative movement during said operating stroke such as to enable the ram device to exercise a rigid control over the displacement of the relatively movable parts throughout any change from compression loading to tensile loading at an intermediate position in said operating stroke.

2. A ram device as claimed in Claim 1, comprising a cylinder provided with a first piston assembly for extending the ram and with a second piston assembly that controls the position of a movable end member within the lost motion slot which is provided in a connecting member of the lost motion means disposed at one end of the cylinder, the second piston assembly having an actuating piston of a larger cross-sectional area than an actuating piston of the first piston assembly whereby, on supplying pressurised fluid in a rest condition of the device before an operating stroke, the second piston assembly is preferentially activated and operates to close up the lost motion slot by displacing the movable end member so as to take up the lost motion before the first piston assembly is activated to carry through the operating stroke of the ram, valve means being provided to trap pressurised fluid and lock the second piston assembly in position throughout substantially the entire operating stroke of the first piston assembly irrespective of whether the device is subjected to compressive or tensile forces.

3. A ram device as claimed in Claim 2, wherein the first and second piston assemblies are arranged in opposed axial relationship one either side of a transverse partition which divides the cylinder internally into two chambers, a first chamber containing the actuating piston of the first piston assembly and a second chamber containing the actuating piston of the second piston assembly, said partition being formed with a passageway therethrough which is controlled by said valve means and being disposed adjacent to a connector leading into said first chamber for connecting to a pressurised hydraulic fluid supply circuit.

4. A ram device as claimed in Claim 3, wherein the valve means is biased to a closed position in which it closes off communication through the partition passageway between said first and second chambers so as to trap the pressurised fluid and lock the second piston assembly during an operating stroke of the first piston assembly but is engaged and held open by a valve control member component of said first piston assembly when the actuating piston fo the latter is in its initial rest position towards the inner end of the first chamber which is defined by said partition.

5. A ram device constructed and arranged substantially as herein described and illustrated with reference to the accompanying drawings.

6. An arrangement of relatively movable parts interconnected by a ram device which controls displacement of said parts, the arrangement being such that over a first portion of its range of movement the ram device functions under compression as a variable length strut between the parts and over a second portion of its range of movement the ram device functions under tension as a variable length stay between the parts, characterised in that said ram device is constructed as claimed in any one of the preceding claims.

7. A motor vehicle having a cab pivotably mounted on a chassis for tilting movement about a transverse axis and incorporating a ram device connected between the cab and chassis for controlling the tilting movement and lifting and lowering of the cab, the arrangement being such that in moving to a forwardly tilted position the cab passes through an intermediate position of balance whereby the ram device is subjected to compression loading during a first portion of its operating stroke and to tensile loading during a second portion of its stroke, characterised in that said ram device is constructed as claimed in any one of claims 1 to 5.