Elastic bushing

DESCRIPTION

The invention relates to an elastic bushing, particularly for use in the field of autovehicles.

More in detail, the invention pertains to an elastic bushing for the connection of a rotating and/or vibrating axis or pin to a frame or support. A common example is the connection of the eye of a leaf spring to the frame of a commercial vehicle, van, truck, etc...

A known and widespread embodiment provides two coaxial metal sleeves (also called "tubes") which are joined by an elastic rubber ring. The assembly is generally forced: the outer sleeve or tube is forced into a suitable mounting seat, and the pin (or axis) is in turn forced into the inner sleeve of the bushing.

This kind of bushing may suffer a fatigue failure of the elastic ring, due to variable and/or alternate stress of compression and torsion, induced by relative rotation between the pin and the frame.

Some embodiments have been proposed to solve this problem, including a bushing with no outer rigid sleeve, made essentially by an inner self-lubricating sleeve and a rubber ring free to rotate and fitted with an internal reinforcing sleeve (see EP 1368572 B1 of the same applicant).

However, when the bushing has to bear severe loads (e.g. in medium or large-sized commercial vehicles), a bigger and more

robust solution is generally adopted, e.g. the above-mentioned all- metal construction with the elastic ring firmly fixed to both the inner and outer sleeve.

The problem of fatigue failure in this case is faced with a drawing of the outer sleeve, in order to prestress the elastic ring. It has been found that fatigue life is noticeably extended by said prestress of the elastic ring, substantially because it avoids the "stretching" of the elastic fibres and induces an axial orientation of the same fibres.

The drawing of the bushing, to be obtained through a conical drawbench, is however an expensive process.

Another disadvantage is the assembly by interference (forcing) between the metal surfaces of the bushing and the seat; it is known that such assembly requires strict tolerances, which can only be met through expensive tooling. Further to this, the metal parts are also responsible for increased weight and cost of the bushing.

The invention is aimed to eliminate these disadvantages. A first aim, in particular, is to realize a bushing for medium/high loads, obtaining the prestress of the elastic ring without the aforesaid expensive drawing; another aim is to reduce the required machining precision both for the bushing and the seat; a further aim is to reduce the overall weight and cost of the bushing.

The aims are reached with an elastic bushing comprising at least an inner metal sleeve, an outer sleeve coaxial to said inner sleeve and an elastic ring substantially having a tubular form, provided between

said inner sleeve and said outer sleeve, characterized in that the outer sleeve is made of a plastic material and is adapted to buckle during the assembly of said bushing into a suitable seat, for allowing to force the bushing into said seat, and to establish a prestress of compression of said elastic ring.

According to a preferred aspect, the side surface of the outer plastic sleeve has a convex portion, with a substantially "barrel-like" profile. The overall diameter of the bushing is then increased by the curvature of said convex portion, allowing to force the bushing into the respective seat.

The mounting seat is commonly made of metal, so that the assembly results in the surface of the plastic sleeve deformed and substantially flattened; following this deformation, the inner surface of the plastic sleeve is curved towards the axis of the bushing, thus compressing the elastic ring.

In a first embodiment, the elastic ring is integral (e.g. vulcanized) with the inner sleeve and substantially fixed also to the outer sleeve; the bushing is able to bear radial stress as well as torsional stress.

In a second embodiment, the inner sleeve is free to rotate relative to the elastic ring, and the bushing comprises a self-lubricant layer between the two aforesaid elements. By this degree of freedom, the sleeve (connected to the pin) is torsionally disconnected from the elastic ring, which is subjected to substantially radial stress only.

More in detail, this second embodiment provides two self- lubricating collars, preferably made of Movelon® (a registered

trademark of the same applicant), said collars being inserted between the inner sleeve and the elastic ring.

According to a further aspect of the invention, said self-lubricating collars have a head with a conical portion, suitable to give an axial support for the elastic ring: through this support, a greater axial load can be supported by the bushing.

The elastic ring is normally made of rubber; the outer plastic sleeve may be realized, for example, with nylon.

It can be understood that the invention makes use of the deformation of the outer plastic sleeve, when assembly takes place, to obtain a prestress of the elastic ring without the expensive drawing which is indispensable in prior-art metal bushings.

It is also to note that the bushing is mounted by forcing a plastic surface on a metal one, namely the side surface of the outer sleeve and the inner surface of the mounting seat (for example, the eye of a leaf spring). This kind of assembly does not require the same surface and dimension tolerances of a forcing of two metal parts, thus reducing the machining cost.

The applicant has also found that a plastic outer sleeve, and particularly made of nylon, with a suitable mounting interference, gives a well sufficient slip resistance, and can fully replace a metal sleeve.

The plastic outer sleeve allows to reduce the cost and weight of the bushing; advantage from this reduced weight is not negligible, mainly in a commercial vehicle provided with many bushings of this kind. The invention is now described in greater detail, with the help of the

enclosed drawings of some non limitative examples, wherein:

Fig. 1 is a cutaway view of a bushing according to the invention;

Fig. 2 shows the mounting position of the bushing of Fig. 1 ;

Fig. 3 is a cutaway view of a bushing according to the invention, in an embodiment alternative to that of Fig. 1 ;

Fig. 4 shows the mounting position of the bushing of Fig. 3.

Referring now to the figures, an elastic bushing is shown comprising a metal inner sleeve 1 , an elastic ring 2 having a substantially tubular form, and an outer sleeve 3 made of a plastic material and coaxial to the inner sleeve 1.

The elastic ring 2 is provided between the coaxial sleeves 1 and 3, and is made of rubber or other material having equivalent elastic features; the plastic sleeve 3 is preferably made of nylon.

There are a couple of drawn shaped washers 4, made according to known art, at the end portions of the inner sleeve 1.

The outer sleeve 3, according to the invention, is able to buckle during the assembly of the bushing into a suitable seat, to allow to force the bushing in the seat and to prestress the elastic ring 2.

Referring in particular to Fig. 1 , the outer sleeve 3 has a convex side surface, with a "barrel-like" profile; more in detail, the side surface has a convex portion 5 between two substantially cylindrical portions 6 and 7. Preferably, the convex portion 5 is extended to the major part of the axial dimension of the bushing (Fig. 1).

Internal side surface 8 of the outer sleeve 3, contacting the elastic ring 2, is preferably cylindrical and plane.

The outer sleeve 3 has a nominal diameter d _n , taken at cylindrical portions 6 and 7 (Fig. 1), that generally corresponds to the nominal diameter of the mounting seat; the convex portion 5, as an effect of the curvature, has a greater diameter up to a maximum of d _max . The increased diameter allows to force the bushing into the mounting seat. The difference between maximum diameter d _max and nominal value d _n is determined so that the assembly is easy but, at the same time, resistant to slip. In a common case, the two values differ by a few millimetres; as an example, a bushing for the wheel suspension of a commercial vehicle having d _n = 65 mm, and sleeve 3 made of nylon, can be realized with maximum diameter d _ma χ= 70 mm.

Fig. 2 shows the bushing 1 mounted in a metal eye 100 of a leaf spring. The figure shows the plastic sleeve 3 in the deformed state after the assembly: the outer side surface of the sleeve 3 is flattened against the inner surface of the eye 100, while the inner side surface 8 is deformed and bent towards the axis of the bushing. In practical terms, the curvature of the convex portion 5 is passed, through the deformation, to the inner side surface 8.

Due to buckling of the side surface 8, elastic ring 2 is subjected to a permanent prestress of compression, thus improving its performance especially in terms of fatigue life and response to torsional stress.

In embodiments of Figs. 1 and 2, the elastic ring 2 is vulcanized to the inner sleeve 1 , and the outer sleeve 3 is substantially integral with the elastic ring 2. The bushing is then able to bear a torsional stress, through the deformation of the elastic ring 2.

Figs. 3 and 4 refer to another embodiment, wherein there is a self- lubricating layer between elastic ring 2 and inner sleeve 1 , and said inner sleeve 1 is free to rotate relative to the elastic ring.

Said self-lubricating layer is preferably realized by providing two collars 9 and 10 made of Movelon®, between the inner sleeve 1 and the elastic ring 2.

The bushing of Figs. 3 and 4 can substantially absorb only radial stress but not torsional stress. As a consequence, the stress of the elastic ring 2 is noticeably less severe compared to embodiment of Figs. 1, 2; the compression of the elastic material, even with radial load only, is however still advantageous in terms of an increased life of the bushing.

Collars 9, 10 may be equally replaced by a single, cylindrical piece, providing a self-lubricating seat for the inner sleeve 1. Another useful feature of collars 9 and 10 is represented by head portions 11 , 12 (Fig. 3) having a conical shape, forming an axial support for the elastic ring 2. This solution gives the bushing a better reaction to axial stress.

The assembly is made by forcing the bushing into the seat, e.g. the eye 100 of Figs. 2 and 4. Due to interference between the bushing and the seat, the bushing is held into the seat and elastic ring 2 is prestressed by the above mentioned buckling of the outer sleeve 3.

The bushing achieves the above aims and purposes, and is particularly suitable for leaf springs, vehicle suspensions and noticeably for use in commercial vehicles and vans.