Air suspensions for vehicles

Abstract: An air suspension unit for a vehicle has: a member (3), e.g. a leaf spring, to which an axle (4), or stub axle, is secured; a first air spring (11) mounted on the member (3) to act between that member and a chassis of the vehicle; a movable member (12), e.g. a mounting plate, between the first air spring (11) and the chassis permitting relative vertical movement between the first air spring and the chassis, and a second air spring (14) which acts between the movable member (12) and the chassis and can be inflated to raise the chassis beyond the maximum height attained by extension of the first air spring (11). The first and second air springs (11; 14) are preferably connected to the same pressure air source. A shock absorber (15) limits the extension of the first air spring (11). Inadvertent extension of the second air spring (14) may be prevented by a latch between the movable member (12) and the chassis.

Description: This invention relates to air suspensions for vehicles wherein air springs are used and in which a long potential travel for the suspension is required to facilitate lifting of the vehicle chassis or frame. An example of such a vehicle is a freight carrying road trailer adapted for rail travel. One such vehicle is referred to in our British patent application no. 9107098.7. A number of air suspensions of different designs and configurations are known. These have the common factors that they have a member or members to which a vehicle axle (or stub axle) is secured and the member is mounted to the vehicle and movable relative thereto to define the locus of the axle/stub axle relative to the vehicle, there being at least one air spring interposed between the member and the vehicle to provide a resilient connection. One arrangement of a suspension system is known from US 2 627 403 which discloses a suspension system comprising an upper air spring superimposed upon a lower air spring. A saddle is provided intermediate each air spring and is held in position by a guide bar pivotally attached to the vehicle frame. The saddle retains the two air springs, which are individually inflated, in proper working relationship whilst permitting their working movement. A further double air spring suspension system is known from US 4 783 096. This discloses a system corresponding to the preamble of claim 1, having a very high (height) carrying and/or lift capability in which a pair of stacked, expandable air bags are connected to the frame of a vehicle such that there is a lower beam for supporting the stack and an intermediate beam with plate member for stabilising the connection between the stacked airbags. It is an aim of the present invention to provide an improved air suspension system. According to the present invention there is provided an air suspension unit for a vehicle comprising a member adapted to be movably mounted to a vehicle and to have a vehicle axle or stub axle secured to it, a first air spring mounted on the member and adapted to act between a chassis of the vehicle and the member, a movable member adapted to be interposed between the first air spring and the chassis, a second air spring adapted to act between the movable member and the chassis, the first and second air springs being pneumatically connected and characterised in that the characteristics of the second air spring are such that it does not extend to lift the chassis over the normal working pressure range of the first air spring. The first and second air springs may be selected so that, when they are pneumatically connected, the second air spring does not extend over the normal working pressure range of the first air spring due to the size of their respective effective areas over their working strokes. Means may be provided to restrict the extension of the first air spring. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:- Figure 1 is a side elevation showing an air suspension unit, two such units being required to make up a complete air suspension for one axle; Figure 2 is a graph showing the relationship between effective area and extension for a first air spring of the air suspension unit; and Figure 3 is a similar graph for a second air spring of the unit. Referring to Figure 1, a hanger bracket 1 is secured to and depends from a chassis member 2 of a trailer (not shown). A member in the form of a two leaf spring 3 has its front end pivotally mounted to the lower end of bracket 1. An axle 4, shown in section, has a spring seat 5 secured to it as by welding. The spring seat 5 and a top plate 6 are clamped to the spring 3 by U bolts 7 and nuts 8. A piston 9 of a first air spring 11 is secured as by bolts to the rear end of the lower leaf of the spring 3. The upper end of the first air spring 11 is secured to a mounting plate 12 which engages the trailer chassis, abutting the trailer chassis at its front and rear ends, as shown in Figure 1. A second air spring 14 is mounted between an upper surface of the mounting plate 12 and the chassis 2. A shock absorber 15 is pivotally mounted, as at 13, 13A, between the mounting plate 12 and the spring seat 5. The suspension unit which has been described is duplicated at the other side of the trailer. If the trailer wheels are independently suspended then there will be one suspension unit (which is likely to be of different design but utilising a first air spring and a member to which a stub axle is secured which controls the locus of the stub axle in use) on each side of the trailer. This arrangement will be repeated for each axle/pair of stub axles but the number of second air springs will depend on the lifting capacity required. In operation the first and second air springs 11, 14 may be independently supplied with pressure air whereby the first air spring 11 is pressurised for normal operation and the second air spring 14 is independently supplied with pressure air when it is desired to raise the chassis 2 further than extension of the first air spring 11 permits. However, it is preferred that the air springs 11, 14 are both connected to the same source of pressure air. Referring now to Figure 2, this is a graph showing a typical relationship between the effective area and the bag height for an air spring in a suspension such as has been described with reference to Figure 1. R denotes the normal ride height position and S1 the full bump position, i.e. closure of the air spring to its minimum operational bag height. SN denotes the normal maximum extension of the air spring, at which it will be seen the effective area has reduced and that increasing pressure is required to obtain further extension. During such further extension the effective area continues to decrease, further increasing the required pressure for additional extension. This is seen, for example, in the region of AS, S2 on the graph where S2 denotes the maximum rebound position, defined by maximum extension of the shock absorber 15 or by a check strap. The effective area over most of the operational range is substantially constant, indicated by A1. Referring now to Figure 3, the horizontal scale has been reversed so that increasing extension is indicated towards the origin. The condition A2, S2 corresponds to the condition A2, S2 on Figure 2 and it will be seen that increasing pressure extends the second air spring 14 in a second stage of lift causing separation of the mounting plate 12 from the chassis 2 whereby the chassis 2 is raised. A3, S3 indicates the maximum extension of the second air spring 14, which is dictated by the available air pressure in the system or, if desired, by a mechanical restraint before the air spring 14 is fully extended. On release of air pressure the air springs 11, 14 will close due to the weight of the chassis and any load it carries. The air feed connection between the air springs 11, 14 is preferably of low cross-sectional area in order to reduce the possibility of high pressure spikes generated during rough road running causing premature extension of the second air spring 14. If desired, a mechanical latch could be provided between the mounting plate 12 and the chassis 2 to prevent inadvertent extension of the second air spring 14. This latch might be spring engaged and disengaged manually or, for example, by operation of a pneumatic, electric or hydraulic motor. The second air spring 14 is shown as a convoluted type bag. Preferably this air spring is one which is tolerant of angular articulation so as to avoid the internal chafing which might occur with a conventional rolling lobe type bag if used at a very long stroke and a consequent high angle of articulation. The tow air springs 11, 14 have been shown mounted one vertically above the other. However, this is not necessary and they may have lateral/longitudinal displacement from one another, subject to the requirements of effective area and pressure for operation which will change with change of moment arms about the pivot. It will be appreciated that any restraint on extension of the first air spring 11 must permit extension of the second air spring 14 to achieve the second stage lift. In the embodiment which has been described the shock absorber 15, on full extension, acts as a restraint to limit the extension of the first air spring 11.

Claims: An air suspension unit for a vehicle comprising a member (3) adapted to be movably mounted to a vehicle and to have a vehicle axle or stub axle secured to it, a first air spring (11) mounted on the member (3) and adapted to act between a chassis of the vehicle and the member (3), a movable member (12) adapted to be interposed between the first air spring (11) and the chassis, a second air spring (14) adapted to act between the movable member and the chassis, the first and second air springs (11;14) being pneumatically connected and characterised in that the characteristics of the second air spring (14) are such that it does not extend to lift the chassis over the normal working pressure range of the first air spring (11). An air suspension unit as claimed in claim 1 characterised in that restriction means is provided in the pneumatic connection between the first and second air springs (11; 14), the restriction means being adapted to attenuate transient peak pressures travelling from the first air spring (11) to the second air spring (14) during road travel of a vehicle to which the unit is fitted for use. An air suspension unit as claimed in Claim 1 or Claim 2 characterised in that a shock absorber (15) is pivotally mounted to act between the movable member (12) and the member (3), or an axle or stub axle carried thereby in use, the shock absorber (15), when fully extended, limiting the extension of the first air spring (11). An air suspension unit as claimed in any preceding claim characterised in that a releasable mechanical lock is provided adapted to act between the movable member (12) and the chassis the lock being engagable during road travel of a vehicle to which the unit is fitted for use, and being releasable to enable the chassis to be lifted. A vehicle characterised in that it has an axle (4) secured to the vehicle chassis (2) by two air suspension units each as claimed in any preceding claim. A vehicle having two aligned stub axles characterised in that each stub axle is secured to the vehicle chassis (2) by an air suspension unit as claimed in any one of claims 1 to 4.

Inventors: DIXON ALAN GEOFFREY, DIXON, ALAN GEOFFREY
Applicants: RUBERY OWEN ROCKWELL LTD, ROR ROCKWELL LIMITED
Application year: 1991
Main classification: B60G11
IPC codes: B60G17, B60G11