GB2338522A

GB2338522A - Gearbox shift fork arms with concave engagement ribs

Info

Publication number: GB2338522A
Application number: GB9913309A
Authority: GB
Inventors: Dominique Benoit; Eric Prique; Stephane Kippelen
Original assignee: MGI Coutier SA
Current assignee: Akwel SA
Priority date: 1998-06-16
Filing date: 1999-06-09
Publication date: 1999-12-22
Anticipated expiration: 2019-06-09
Also published as: DE19927344A1; GB9913309D0; FR2779795A1; FR2779795B1; ITMI991302A0; GB2338522B; ITMI991302A1

Abstract

A gearbox shift fork comprises a body 1 with two arms 3, 4 forming a cylindrical shell with an internal surface provided with a rib 5 having two sides 51, 52 which cooperate with a sliding component. The sides 51, 52 comprise several portions 510a, 510b, 511a, 511b, 512 which come progressively into contact with the sliding component in accordance with an operating force. The two sides of the rib 5 have a substantially concave shape relative to a reference plane (PR, fig 2), the shape providing a normal contact surface 510a, 510b, a progressive contact surface 511a, 511b, which may be a straight or curved gradient, and an abutment surface 512. Preferably the rib 5 is symmetrical about the reference plane (PR, fig 2), however it may be asymmetrical (see fig 3B). The shift fork may be made of plastics, aluminium or alloy and manufactured by moulding, injection moulding or gravity casting.

Description

1 1 1 1 GEARBOX FORK 2338522 The present invention relates to a gearbox

fork comprising a body receiving the shaft and carrying two arms in a U-shape for driving the sliding component.

It is already known to produce gearbox forks from plastics material. Those forks known from document JP 59218522 comprise an insert in the arms and, at the end of the arms, there are grippers or shoes for entering the groove of the sliding component and pulling or pushing that component to change the speed ratio.

However, those known solutions, using a fork partly of plastics material, are not entirely satisfactory owing to the insert reinforcing the arms. Manufacture is complicated owing to the insert, which occupies an important and delicate position in the composite article. In addition, it would be especially difficult and tricky to hold an insert in the fork during the injection of the plastics material to obtain a complete and regular coating with the plastics.

The object of the present invention is to overcome those disadvantages and proposes to simplify the manufacture of a fork while lightening its weight and at the same time enabling it to withstand exceptional forces, caused by some drivers, applied to the knob and consequently applied to the fork, in order to dimension the article as a function of normal forces and to set up the present invention for exceptional forces in order to obtain a cheaper and lighter article, and improving lubrication by avoiding "breaking" the film of oil between the sliding component and the shoes.

2 To that end, the present invention relates to a fork of the type defined above, characterised in that the two arms and the body are in the form of a cylindrical shell having a cross-section in the shape of an arc of a circle extending substantially on a semicircle, the cylindrical shell being parallel to the axis of displacement of the fork, - the internal surface of the shell is provided with a rib which has a rectangular cross-section and the two sides of which constitute, at least partially, and in accordance with the force to be transmitted, the surface of contact with the sliding component, the two sides of the rib have a concave shape relative to a reference plane perpendicular to the axis of translation of the fork, the cross- section of the rib, taken in the direction of the axis, decreasing from each end towards the middle, while forming on each side of the middle:

a normalcontact surface, parallel to the reference plane, for transmitting a normal force to the sliding component, a progressive gradient approaching the reference plane to form a progressive contact surface, coming progressively into contact with the ever increasing surface of the sliding component, in accordance with the force to be transmitted, an abutment surface substantially parallel to a reference plane and arranged in the middle of the rib, joining the progressive gradient of each arm.

The fork according to the invention produced from plastics material, or from aluminium, or from an alloy by injection moulding or gravity casting, permits the transmission of substantial gearchanging forces without requiring exceptional oversizing of the known fork of plastics material.

3 The fork according to the invention is very simple to manufacture because it is not provided with an insert extending in the two arms of the fork and having the disadvantages discussed above.

On the contrary, the invention permits compensation for any tolerances caused by the irregular shrinkage of the plastics material, because the contact surface between the fork and the sliding component increases progressively if necessary. In the case of aluminium or an alloy, that problem does not arise but the cross-section of the article may be even further reduced.

The fork according to the invention may be either a fork mobile on a fixed guide shaft or a fork provided with its own shaft, the whole sliding in fixed bearings.

Advantageously, the two sides of the rib have a symmetrical shape. It is also possible to favour one of the sides relative to the other in accordance with the difference in the forces to be transmitted in one translation direction or the other for passage from the first or the second gear, for example.

The rib is advantageously terminated at each end by a chamfer facilitating the engagement of the rib in the groove of the sliding component.

According to another feature, the progressive surface is a straight gradient or a curved gradient.

i 1 4 In the case of a fork produced from aluminium or from a light alloy, as the modulus of elasticity is far larger than that of the plastics materials used within the scope of the invention, it is possible, for the same result, to reduce the inertia of the article, that is to say, to make it lighter and thus less expensive and to compensate for its resilient deformation by means of the progressive surface.

The present invention will be described hereinafter in more detail with reference to an embodiment represented diagrammatically in the appended drawings in which:

- Figure 1 is a perspective view corresponding to Figure 2, - Figure 2 is a bottom view of a fork, in a direction parallel to the axis of translation of the fork, - Figure 3A is a developed view of the rib of the fork according to the invention, Figure 3B is a diagrammatic view of the rib deformed by the application of an exceptional force, Figure 3C shows the cross-section of the groove of the sliding component.

According to Figures 1 and 2, the invention relates to a fork comprising a body 1 which is either mounted on or carried by a shaft 2, that is to say, sliding on that fixed shaft, or which, as in the example shown, is movable in translation in a unitary manner with the shaft 2.

The body 1 carries two arms 3, 4 forming a U-shaped component. The body 1 also comprises a sleeve or another means receiving the shaft 2 or rendering the sleeve unitary with the shaft 2. Since that means does not form part of the present invention, it has not been described in detail.

The two arms 3, 4 and the corresponding portion of the body 1 form a cylindrical shell having a cross-section in the shape of an arc of a circle extending substantially on a semicircle, the generating line of the shell being parallel to the axis of displacement XX of the fork.

The internal surface of the shell, which is smooth, is provided with a rib 5 which has a rectangular cross-section and the two sides 51, 52 of which, arranged one on each side of a reference plane PR, constitute the support surfaces cooperating with the sliding component.

The reference plane PR is a plane perpendicular to the axis XX corresponding to the axis of translation of the fork and thus perpendicular to the plane of Figure 2. The plane PR may constitute the plane of symmetry of the rib 5, which is the most general case. The reference plane enables the concave shape of the sides 51, 52 of the rib 5 to be described, the base 53 of the rib being of no particular importance to the present invention. The base 53 is the surface of the rib 4 that is generated by a generating line parallel to the axis Xx.

According to Figures 1 and 2, and especially Figure 1 which shows the various portions of the side 51 of the rib 5 by separating lines, the rib 5 is in general substantially symmetrical relative to a plane of symmetry PS passing through the shaft 2 perpendicular to the plane of Figure 2; it has, on each side, a contact surface 510a, 510b corresponding to "normal" support, that is to say, a normal operating pressure exerted on the fork. The two surfaces 510a, 510b, which are arranged one on each side of the plane of symmetry PS, are parallel to the reference plane PR.

1 6 Following the surfaces 510a, Slob and approaching the plane of symmetry PS, there is a progressive gradient 511a, 511b which approaches the reference plane PR in accordance with a preferably linear plot, that is to say, a straight gradient. That gradient may also be curved in accordance with the desired progress for the transmitted force. The progressive gradient 511a, 511b forms a progressive contact surface between the rib 5 and the sliding component, which surface increases progressively as a function of the force transmitted. Finally, beyond the two progressive gradients Slia, 511b, an abutment surface 512 extends on each side of the plane of symmetry PS. The abutment surface 512 constitutes the end surface for a major force regarded as the maximum. At that moment, all of the surfaces 510a, Slob, 511a, 511b and 512 are arranged, by deformation, in a plane parallel to the reference plane PR, that is to say, perpendicular to the axis of translation XX of the fork. That plane corresponds to one of the faces of the groove of the sliding component.

In general, the rib 5 is preferably symmetrical, that is to say, its other side 52 also has two normal-contact surfaces 520a, 520b followed in each case by a gradient 521a, 521b and by a central portion 522 parallel to the reference plane and constituting the limit abutment surface. However, the progressive surfaces 512, 522 may also be different.

As shown especially in Figure 1, the rib 5 is terminated at its two ends by a chamfer 54, 55 for facilitating the positioning of the fork in the groove of the sliding component.

7 Figure 3A shows the development of the rib 5 with the various surfaces described above and bearing the same refer ences.

Figure 3C shows the crosssection of the sliding component 6 with its two contact surfaces 61, 62 against which the one or the other face 51, 52 of the rib 5 is supported for gear changing manoeuvres.

Figure 3B, which is placed directly below Figure 3A, shows the deformation of the rib 5 coming against the contact surface 61 under the effect of an operating force F regarded as maximum.

8

Claims

1. Gearbox fork comprising a body receiving the shaft and carrying two arms in a U-shape for driving the sliding component, characterised in that - the two arms and the body are in the form of a cylindrical shell having a cross-section in the shape of an are of a circle extending substantially on a semicircle, the cylindrical shell being parallel to the axis (XX) of displacement of the fork, the internal surface of the shell is provided with a rib which has a rectangular cross-section and the two sides of which constitute, at least partially, and in accordance with the force to be transmitted, the surface of contact with the sliding component, - the two sides of the rib (5) have a concave shape relative to a reference plane (PR) perpendicular to the axis of translation (XX) of the fork, the cross-section of the rib, taken in the direction of the axis (XX), decreasing from each end towards the middle, while forming on each side of the middle: a normal-contact surface, parallel to the reference plane (PR), for transmitting a normal force to the sliding component, a progressive gradient approaching the reference plane (PR) to form a progressive contact surface, coming progressively into contact with the ever increasing surface of the sliding component, in accordance with the force transmitted, an abutment surface substantially parallel to a reference plane (PR) and arranged in the middle of the rib, joining the progressive gradient of each arm.

9

2. Gearbox fork according to claim 1, characterised in that the progressive surface is a straight gradient.

3. Gearbox fork according to claim 1, characterised in that the progressive surface is a curved gradient.

4. Gearbox fork according to claim 1, characterised in that the progressive surface is different from the progressive surface.

5. Gearbox fork according to claim 1, characterised in that the body and the arms are moulded from plastics material.

6. Gearbox fork according to claim 1, characterised in that the body and the arms are produced from aluminium or from an alloy, by injection moulding or gravity casting.

1

7. A gearbox fork substantially as hereinbefore described with reference to or as shown in Figs. 1 to 3C.

0:'SPECSUGT588 A

7. A gearbox fork substantially as hereinbefore described with reference to or as shown in Figures 1 to 3C.

1\ 1 1 0 0 to Amendments to the claims have been filed as follows CLAIMS 1. Gearbox shift fork comprising a body adapted to receive a shaft and having two arms in a U-shape for driving a sliding component of a gearbox, characterised in that the two arms and the body are in the form of a cylindrical shell having a cross-section in the shape of an arc of a circle extending substantially on a semicircle, the cylindrical shell being parallel to an axis (XX) of displacement of the fork, an internal surface of the shell is provided with a rib which has a rectangular crosssection, two sides of which rib constitute a contact surface for contacting the sliding component of the gearbox, the two sides of the rib have a concave shape relative to a reference plane (PR) perpendicular to the axis of displacement (XX) of the fork, the cross-section of the rib, taken in the direction of the axis (XX), decreasing from each end towards the middle, such that the sides of the rib form on each side of the middle: a normal-contact surface, parallel to the reference plane (PR), for transmitting a normal force to the sliding component, a progressive gradient approaching the reference plane (PR) to form a progressive contact surface, said progressive contact surface adapted to come progressively into contact with a surface of the sliding component according to the force transmitted thereto by the shift fork, and an abutment surface substantially parallel to the reference plane (PR) and arranged in the middle of the rib, joining the progressive gradient of each arm.

0:\SPECS\JG\P588 t 0 a 0 0 a a.

1 2. Gearbox fork according to claim 1, characterised in that the progressive surface is a straight gradient.

4. Gearbox fork according to claim 1, characterised in that the progressive surfaces on 1 each side of the rib are different.