GB2032054A

GB2032054A - Resilient mounting

Info

Publication number: GB2032054A
Application number: GB7934066A
Authority: GB
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1978-10-11
Filing date: 1979-10-02
Publication date: 1980-04-30
Also published as: CA1130261A; DE2941274A1; DE2941274C2; SE7908317L; AU5166079A; FR2438768A1; FR2438768B1; SE443621B; GB2032054B

Abstract

A resilient mounting comprising a tubular bush (3) of elastomeric material acting between and bonded to confronting surfaces (10, 12) of rigid inner and outer members (1, 2). The bush (3) is formed with a cut-out (13) on that side which would otherwise be subjected to tensile forces in use and the relative spacing between the members (1, 2) is subsequently reduced by either expanding the inner member 2 or contracting the outer member 1 to close at least part of the cut-out (13) and selectively pre-load the elastomeric material in the unloaded condition of the mounting. The reduction in spacing can be controlled to give the required pre-loading and may be sufficient to ensure that the elastomeric material is always under compression in use thereby avoiding the occurrence of tensile forces with consequent improved fatigue life. Preferably the mounting incorporates resilient limit stops (15, 16) to control relative movement between the members (1, 2) both in the axial direction of the bush (3) and vertically under a working load. <IMAGE>

Description

SPECIFICATION Resilient mountings This invention concerns improvements in or relating to resilient mountings suitable for supporting a working load which load is subject to dynamic variation. Such mountings may be used for mounting vehicle cabs, engine mountings etc.

Resilient mountings are known which, in their simplest form comprise a tubular bush of resilient material acting between and bonded to the confronting surfaces of rigid inner and outer members. A disadvantage of this type of mounting is that when supporting a working load a portion of the elastomeric material is subjected to tensile forces and an opposed portion is'subjected to compression forces such that in use dynamic variation in the working load, for example when a vehicle incorporating the mounting passes over a surface irregularity, causes relative movement to occur between the rigid inner and outer members resulting in a variation in the tensile and compression forces experienced by the portions of elastomeric material.Depending on the direction of relative movement the tensile and compression forces may be increased or decreased, for example under rebound those portions which were subjected to tensile forces may be subjected to compression forces and vice versa. The existence of tensile forces in the elastomeric material of the mounting is particularly undesirable as the fatigue life of the mounting is considerably reduced and this problem is exaggerated by the change in forces, i.e. from tensile to compression and vice versa which the portions of the elastomeric material may experience.

It has already been proposed to form mountings of the above type in which the portion of the elastomeric material which is initially subjected to tensile forces when the working load is applied is cut away either partially or completely.

Although this construction reduces the tensile forces experienced in the cut-away portion of the elastomeric material and therefore increases the fatigue life of the mounting it does not completely eliminate the occurrence of tensile forces in the elastomeric material of the mounting in use. In addition under conditions in which relative movement between the rigid members is such as to first close the cut-out, e.g. during rebound, the amount of movement which occurs before either the members contact one another or the remaining. portion of the elastomeric material in the region of the cut-out is compressed is greater than the radial width of the cut-out in the unloaded condition. As a result the ride characteristics of the mounting under rebound are poor.

It is an object of the present invention to provide a resilient mounting which reduces or eliminates at least some of the above-described problems and disadvantages of the known mountings.

According to the present invention a resilient mounting comprises a tubular bush of elastomeric material acting between and bonded to the confronting surfaces of rigid inner and outer members wherein a portion of the elastomeric material which would otherwise be subjected to tensile forces under a working load is formed with a cut-out and the relative spacing between the inner and outer members is subsequently reduced to close at least part of the cut-out and selectively pre-load the elastomeric material in the unloaded condition.

The cut-out may be formed within the elastomeric material e.g. by the use of a removable core. Preferably the cut-out is provided at an interface between the elastomeric material and the adjacent surface of the inner and/or outer member, more preferably at the interface with the inner member, either by the use of removable core or by selective masking of the surface of the member to prevent bonding of the elastomeric material.

Conveniently the confronting surfaces of the inner and outer members are cylindrical and where a cut-out is provided at an interface with one of the members the cut-out preferably extends over approximately one third of the circumference of the member.

The elastomeric material at the lateral edges of the cut-out may be further cut away so as to reduce the possibility of the elastomeric material tearing in those zones.

The degree of precompression may be varied depending on the required rebound characteristics.

The required precompression may be obtained by mechanically deforming one of the rigid members to reduce the relative spacing between the members. For example the outer member may be contracted but more preferably the inner member is expanded.

The mounting may include limit stops to further modify the ride characteristics. The limit stops may comprise resilient blocks made of elastomeric material provided on the members to limit relative movement between the members. In the case of a resilient mounting for use as a vehicle cab mounting the blocks may be arranged to fit inside a structural channel of the cab to simplify the mounting assembly.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings wherein: Figure 1 is an end elevation of a mounting as moulded; Figure 2 is a transverse cross-section of Figure 1; Figure 3 shows the mounting of Figure 1 in the finished manufactured state; Figure 4 is a transverse cross-section of Figure 3; Figure 5 shows the mounting of Figure 3 under steady load; Figure 6 is a plan of the mounting of Figure 5; Figure 7 is a cross section of Figure 5; Figures 8 and 9 show two characteristics of load/deflection for versions of the mounting, and Figure 10 is a sectional side elevation of part of another mounting in accordance with the present invention.

The mounting shown in Figures 1 to 7 of the accompanying drawings comprises outer and inner rigid members 1 and 2 respectively spaced relative to one another by means of an eccentric cylindrical rubber bush 3 acting between and bonded to the members. The mounting is suitable for use as a cab mounting in which the outer member 1 is adapted to be secured to the vehicle frame (not shown) and the inner member 2 is adapted to be secured to the cab (not shown) and is subjected to the working load.

The outer member 1 comprises a flat base 4, a pair of parallel vertical sides 5, angled shoulders 6 and a flat top 7. The base 4 projects laterally beyond the sides 5 and is formed with mounting holes 8 to receive bolts (not shown) for securing the member 1 to a vehicle. The member 1 is formed with a through bore 9 defining a cylindrical surface 1 0.

The inner member 2 comprises a steel tube 11 having a cylindrical outer surface 12. The tube 11 is located within the bore 9 and has its longitudinal axis parallel to but offset from the axis of the bore 9.

The bush 3 is formed by moulding rubber in the space between the confronting surfaces 10 and 12 of the members 1 and 2 respectively, the surfaces 10 and 12 each being prepared by conventional means to effect bonding of the rubber to the surfaces.

As shown in Figure 1 the radial thickness of the rubber below the inner member 2 which is subjected to compression forces by a load applied to the inner member is substantially greater than that of the rubber above the inner member 2 while the rubber above the inner member 2 is formed with a cut-out comprising an aperture 13 at the interface with the outer surface 12 of the inner member to eliminate tensile forces therein due to the applied load. The aperture 1 3 is of generally dumb-bell shape in transverse cross-section having edge portions 1 3a of circular cross-section interconnected by a narrow centre portion 1 3b.

The aperture extends axially over the length of the inner member 2 and circumferentially over approximately one-third of the inner member 2.

The aperture 13 is formed by means of a pair of removable cores (not shown) to which the rubber does not bond. The cores are inserted, one from either end of the bore 9, between the members 1 and 2 before moulding. The cores are constructed so as to leave a narrow circumferentially extending bridge piece 14 (Figure 2) between the adjacent ends thereof. The bridge piece is ruptured when the rubber is flexed following removal of the cores.

A pair of rubber end stops 1 5 and a rubber bump stop 1 6 provided on the outer surface of the outer member 1 are moulded simultaneously with the bush 3 and are integral therewith. The end stops 1 5 limit relative movement between the members in an axial direction while the bump stop 1 6 limits relative movement due to increased ioading of the mounting.

After moulding the inner member 2 is expanded until the centre portion 1 3b of the aperture is closed (Figure 3). This expansion precompresses the rubber in the regions marked A in Figure 3 i.e.

at either side of the bush 3 adjacent to the end portions 1 3a of the aperture. The mounting is then as shown in Figures 3 and 4.

Typical static cab deflection is 5 mm with a further 5 mm of movement allowed before the bump stop 1 6 is contacted. Inner member expansion is determined largely by the radial thickness of the centre portion 1 3b of the aperture as moulded. In the case where the centre portion has a radial thickness of 3 mm an increase of the order of 10% in the diameter of the inner member 2 is sufficient to close the centre portion 1 3b so that the rubber above the inner member contacts the outer surface 12 of the inner member in the unloaded condition.

The mounting in its normal working condition carries the static load of a vehicle cab and the loaded condition is shown in Figures 5, 6 and 7.

The outer member 1 is bolted to the vehicle frame (not shown) by means of a bolt (not shown) passing through the mounting holes 8 in the base 4. The inner member 2 is secured to the vehicle cab (not shown) by means of a bolt (not shown) which extends axially through the bore of the inner member 2 and is attached at each end to a respective one of a pair of parallel lugs 1 7 which extend downwardly from a mounting flange 18 bolted to the vehicle cab. The static weight of the cab downwardly deflects the inner member 2 as shown in Figures 5 and 7 thereby increasing the compression of the rubber below the inner member 2, opening the centre portion 1 3b of the aperture and lessening the precompression in the rubber zones A.

Dynamic variation in the load applied is allowed by the bush in the usual manner. Excessive movement of the inner member 2 in the downward direction (i.e. increasing load) is resisted by the upper flange 1 8 contacting the rubber stop 1 6 which causes a steep rise in spring rate. Excessive rebound movement of the inner member (i.e. upwards as shown or decreasing load) results in the centre portion 1 3b of the aperture 1 3 closing and the inner member 2 contacting the rubber above the inner member which acts as a rebound stop to give a steep rise in rebound spring rate. The resultant load deflection characteristics of the mounting is shown in Figure 8 in which deflection x is the static deflection. The mounting is thus an effective low rate mounting in the normal working range YZ yet has positive cushioning of excessive deflection of the cab in either direction. Furthermore relative movement between the members in the axial direction of the inner member 2 i.e. directions P and Q in Figure 7 is restricted by the end stops 1 5 contacting the lugs 1 7.

It will be appreciated that by expanding the inner member 2 so as to close the centre portion 1 3b of the aperture in the unloaded condition increases the range of the ioads which can be applied to the mounting without subjecting the rubber above the inner member to tensile forces as compared with the known mountings in which cut-outs provided in the rubber are open in the unloaded condition. Furthermore the control of the mounting under rebound conditions is improved since the amount of movement of the inner member necessary to close the centre portion of the aperture is reduced thus giving a considerable improvement in the ride characteristics.

It will be apparent from the foregoing that if the diameter of the inner member is increased even further, for example of the order of 20%, then in addition to closing the centre portion 1 3b of the aperture the portion of the rubber above the inner member will be precompressed giving a mounting having the spring characteristic shown in Figure 9.

This characteristic has the same nominal stiffness in the working zone YZ as the same mounting in which the diameter of the inner member is increased by 10% but the rebound stop now acts at Y, giving even greater control under rebound.

Furthermore it will be understood that the degree of expansion of the inner member may be selected so that in addition to the above described improved characteristics of the mounting the rubber below the inner member which is subjected to compression forces under the applied load and the rubber in side zones A is sufficiently precompressed that it remains under compression during rebound. The rubber below the inner member is therefore never subjected to tensile forces with consequent increase in the fatigue life of the mounting as compared with known mountings in which the rubber below the inner member is not precompressed prior to application of a load and may therefore be subjected to tensile forces under rebound.

The invention is not restricted to the abovedescribed embodiment which may be modified in a number of ways, for example the aperture 13 may be formed at the interface with the outer member or at a position between the inner and outer members such that thickness of rubber is bonded to the confronting surfaces of both members. The outer member may be adapted to carry the working load with the inner member being fixed to the vehicle frame. In this arrangement the cut-out would be provided in the rubber below the inner member and the rubber above the inner member would be subjected to compression forces under the working load.

The outer member may be subjected to a contraction operation in addition or alternative to the expansion of the inner member to produce the required reduction in the relative spacing between the members. This method is particularly suitable if the inner rigid member has a non-uniform crosssection, for example an inner member having the shape shown in Figure 10 in which the profile of the outer surface of the inner member 102 is selected to give a particular characteristic under dynamic conditions, A rubber bush 103 is bonded to the inner member 102 and an outer member (not shown). An aperture 113 is formed at the interface with the inner member and the outer member is subsequently contracted to close the centre portion 1 3b of the aperture.

The cut-out in the rubber may be formed by selective masking of the surface of the inner and/or outer members to prevent bonding of the rubber to the members over a selected region. The cut-out is formed by the contraction of the rubber on cooling following moulding which results in the rubber moving away from the surface of the member in the masked region.

The mounting may be connected to the cab and frame by any appropriate method for example the base 4 and mounting flange 1 8 could be secured by welding although the use of bolts is preferred as providing for easy replacement of the mounting when necessary.

In addition to the end stops 1 5 and rebound stop 16 rubber may be moulded onto other surfaces of the outer member to provide other characteristics as required.

Finally the mounting of the present invention has applications other than as a cab mounting for example as an engine mounting, and may be applied where a resilient mounting with fully controllable yet resilient rebound control is required.

Claims

1. A resilient mounting comprises a tubular bush of elastomeric material acting between and bonded to the confronting surfaces of rigid inner and outer members wherein a portion of the elastomeric material which would otherwise be subjected to tensile forces under a working load is formed with a cut-out and the relative spacing between the inner and outer members is subsequently reduced to close at least part of the cut-out and selectively pre-load the elastomeric material in the unloaded condition.

2. A mounting according to claim 1 wherein the cut out is within the elastomeric material.

3. A mounting according to claim 1 wherein the cut-out is at an interface between the elastomeric material and the adjacent surface of one of the members.

4. A mounting according to claim 3 wherein the cut-out is at the interface with the inner member.

5. A mounting according to any one of the preceding claims wherein the cut-out extends in both the axial and circumferential direction of the bush.

6. A mounting according to claim 5 wherein the cut-out is at an interface with one of the members and extends circumferentially over one-third of the surface of the member.

7. A mounting according to any one of the preceding claims wherein the cut-out is further cut away at the edge portions.

8. A mounting according to claim 7 wherein the centre portion of the cut-out is of uniform radial thickness.

9. A mounting according to claim 8 wherein the centre portion of the cut-out is closed in the unloaded condition by the reduction in the relative spacing between the members.

10. A mounting according to any one of the preceding claims wherein at least one of the members is cylindrical and has been expanded or contracted to reduce the relative spacing between the members.

1 A mounting according to claim 10 wherein the outer member has a cylindrical through bore and the inner member comprises a cylindrical tube located in the bore and having its axis parallel to the axis of the bore.

12. A mounting according to any one of the preceding claims wherein the bush is eccentric having a greater radial thickness on that side which is subjected to compression forces by the working load.

13. A mounting according to any one of the preceding claims including resilient end stops to limit relative movement between the members in the axial direction of the bush.

14. A mounting according to any one of the preceding claims including a resilient bump stop to limit relative movement between the members under the working load.

1 5. A mounting according to any one of the preceding claims wherein the reduction in spacing between the members is sufficient to just close the cut-out in the unloaded condition.

1 6. A mounting according to any one of claims 1 to 14 wherein the reduction in spacing between the members is sufficient to close the cut-out and pre load the portion of the elastomeric material in which the cut-out is formed in the unloaded condition.

1 7. A resilient mounting substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.

1 8. A resilient mounting substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings and as modified by Figure 10 of the accompanying drawings.