Technical Field

The present invention relates to a method and a device for preventing chattering and drawbacks associated therewith, which can arise between intermeshing gear wheels, primarily with regard to the type of chattering which arises when the device is intended to operate within a given temperature range and whereby a lubricant is used whose viscosity in the upper temperature range is less than the viscosity in the lower temperature range.

Background art and problems

In vehicles, for example, it is a general aim to reduce noise levels. This aim is not based solely on purely hygienic aspects (increased comfort) but also on the fact that noise often leads to vibrations which, for" constructional reasons (such as fatigue) , are not desirable. An example of such are manual gearboxes which are generally used nowadays in vehicles.

A gearbox comprises a plurality of gear wheels which, preferably with help of bearings, are rotatably arranged on shafts. When such a gear wheel is unloaded, chattering usually arises because the gear teeth of the two intermeshing gear wheels impact each other. This is a drawback which is undesirable.

The drawback is due to the fact that the lubricant which is used in the gearbox cannot have a viscosity which is too high when the gearbox is at a low temperature. The higher the viscosity, the thicker the liquid becomes and thus it

becomes more difficult to execute movements of components which are in the liquid/lubricant. Accordingly, the viscosity of the lubricant at the lowest imaginable operating temperature (for example -40°C) must be such that it is still possible to engage a gear (move a gear wheel) without great difficulty. The viscosity of the lubricant is, however, temperature dependent and increased temperature gives lower viscosity (= more flowable) . Accordingly it becomes easier to execute movements of components within a lubricant-filled space since the temperature rises. As a consequence, a gear wheel, for example, in a gearbox at the operating temperature runs very freely, which leads to the above mentioned undesirable drawback, namely gear teeth chatter.

It is already known from JP-A-59-205067 to prevent teeth chatter by arranging a spiral spring between a gear wheel and its shaft. When the shaft rotates in a certain direction, the spring is influenced so that the gear wheel runs more slowly, thereby reducing or eliminating the chattering. There are several disadvantages with this known arrangement. One is that it gives rise to increased resistance, even when this is not desired, i.e. even when it is cold and the oil is viscous. Furthermore, it is relatively lengthy to affix and relatively complicated to integrate within a bearing housing.

Solution and advantages

The object of the present invention is to provide a method and a device which eliminates the above mentioned problem.

Said object is achieved by means of a device and a method according to the present invention, whereby a friction ring is arranged between said gear wheel and said shaft, and the method is characterized in that a friction ring is arranged between said gear wheel and said shaft, at least parts of

which friction ring consist of radially extending members which, at least together with the base part on which it may be arranged, at least in the radial direction l _h at a temperature increase from T ₀ to T ₂ expands quicker than the radial distance l _h in the gap where the friction ring is arranged, dl _f /dT>dl _h /dT, so that the friction ring at the upper temperature applies a larger radial force between the shaft and the gear wheel than at the lower temperature. In this way, it is assured that an increase of the frictional resistance only arises when it is desired, i.e. when chatter would otherwise occur. Furthermore, it is extremely cheap and simple to affix.

A further object of the invention is to provide a friction ring which, depending on conditions, can be combined with a bearing in an optimal manner.

Brief description of the drawings

The invention will be described in the following with help of an example and with reference to the attached drawings in which

Fig. 1 shows in perspective a needle bearing which, in a preferred case, is used together with the subject of the invention.

Fig. 2 shows a friction ring in accordance with the invention in perspective, and

Fig. 3 shows the friction ring and the needle bearing fitted between a shaft and a gear wheel in a gearbox.

A conventional needle bearing 3 is shown in Fig. 1, which consists of a cage 30 in which a plurality of rollers 31 are arranged. The rollers 31 are provided in two

circu ferentially extended rows. The needle bearing is made with a plurality of rectangularly shaped openings 32. The significance of these openings for the invention will be explained later.

A preferred embodiment of a friction ring according to the invention is shown in Fig. 2. The friction ring 1, in this preferred case, consists of a single unit which is suitably made from a thermoplastic, preferably polyamide. The unit consists of a homogenous, ring shaped base part 12 from which radial protrusions 11 project. The radial protrusions 11 are intended to be arranged in the notches 32 in the needle bearing 3. Furthermore, it can be seen that the friction ring 1 is divided with help of a split 13. This split makes it possible to apply the friction ring around a shaft by prizing it apart, thanks to the use of a flexible, preferably elastic, material in the friction ring 1. The needle bearing 3 is also made in a corresponding way in order to simplify the fitting thereof. In this case, however, a plastic other than polyamide is used, namely preferably a cured plastic.

Fig. 3 shows a part of a gearbox in cross-section. Here, a section of one of the shafts 4 can been seen, upon which a gear wheel 2 is arranged, with its tooth faces 21. Between the gear wheel 2 and the shaft 4 is arranged a unit consisting of a needle bearing 3 and the friction ring 1. The friction ring is hereby arranged in such a way that between each and every opening 32 in the needle bearing 3, a radially projecting protrusion 11 extends from the friction ring 1. It is desirable that the thickness of the uniform ring 12 of the friction ring 1, i.e. the distance between its inner peripheral surface 15 and the peripheral surface 14 arranged between the protrusions, substantially corresponds to the distance between the inwardly directed peripheral surface of the friction ring 3 and the distance

with which the rollers 31 project from the latter mentioned surface. In this manner, a unit with a substantially constant inner diameter is achieved. The total length of the protrusions 11, 12 (l _f ) substantially corresponds to the distance (l _h ) between the gear wheel 2 and the shaft 4 in the gap 5 in which the friction ring 1 is inserted. It should be noted that these latter mentioned lengths only exactly correspond with each other at a certain given temperature (T,) , since the coefficients of expansion of the friction ring 1 on the one hand and the shaft 4 and gear wheel 2 respectively on the other hand are considerably different. The coefficient of expansion for the material of the friction ring 1 significantly exceeds the value of the coefficient of expansion for material in the surrounding parts 2,4.

The invention operates in the following manner. At low temperatures (approx. T ₀ ) the radial maximum height of the friction ring 1 (l _f ) is less than the distance (l _h ) in the gap in which the friction ring is inserted. This means that the gear wheel 2 during movement is principally only affected by the viscous forces from the lubricant in which it is arranged. When the temperature in the gearbox after a period of use rises, the viscosity of the lubricant decreases in accordance with that which was described earlier. At the same time, the friction ring 1 is affected by the increased temperature and, thanks to its higher coefficient of expansion, it expands more quickly than that which is the case for the gap (l _h ) in which it is arranged. At a certain given temperature T, (for example 40°C) the friction ring will have expanded so much that its radial maximum length (l _f ) coincides with the radial distance in the gap (l _h ) . If the temperature rises further (T>Tj) the friction ring begins to apply a radial force in the gap between the gear wheel 2 and the shaft 4. The higher the temperature, the higher the force since the expansion of

the friction ring in the gap is higher than the surrounding parts. Thanks to this radial force which is generated by the friction ring, a certain braking moment is applied to the gear wheel 2, which braking moment prevents the gear wheel from running too freely and thereby eliminates the origin of the undesired chattering.

The radial maximum distance (l _f ) of the friction ring may be selected with regard to its coefficient of expansion in relation to the surrounding material and be optimized so that the two radial distances (l _f , l _h ) are the same at a temperature _λ which is determined with regard to the normal operating temperature (approx. T ₂ ) for the gearbox so that an optimal braking effect is achieved at the normal operating temperature (approx. T ₂ ) . The choice of a thermoplastic is not only advantageous with regard to manufacturing and the coefficient of expansion, but also with regard to its good wear properties.

The skilled man will realize that the invention is not restricted to the above-described example, but can be varied within the scope of the following claims. Accordingly it is not necessary to form the friction ring 1 in a single unit, but it is imaginable that in certain circumstances it can be more suitable, for example, to combine a metal part with a thermoplastic to form the friction ring, such as an inner metal ring 12 to which the outer thermoplastic layer 11 is moulded. Furthermore, the shown combination of friction ring and needle bearing is not restrictive for the invention. It is possible to combine a totally uniform friction ring 1 (without projection 11) with two side-by-side needle bearing parts. The split 13 is of course also solely a preferred embodiment. The shown example is preferred from the supposition that the needle bearing 3 is a preferably already existing component. In certain cases it is however

possible to provide the needle bearing 3 and the friction ring integrated already during manufacture, whereby the cage 30 and the friction ring l is suitably formed as an integrated unit. Although the invention has been described in connection with gearboxes for vehicles, the skilled man will realize that also further fields of application, gearboxes of the type for large fixed machines, etc., are possible.