DESCRIPTION

The present invention relates to a driving arrangement particularly for gearing applications.

The invention can be used in any gearing application but has particular use in arrangements in which a gear wheel is driven into rotation from a stationary state by a toothed rack.

Known systems use pointed laterally extending teeth on a rack to engage radially extending pointed teeth on a gear wheel so as to set the gear wheel in motion. However, problems are encountered in satisfactorily phasing the leading teeth into engagement: jamming may occur and if the initial rack acceleration is too rapid then the leading tooth shears and correct engagement is severely impaired.

According to the present invention, there is provided an arrangement for rotatably driving a toothed pinion wheel, the arrangement comprising a rack arranged to be driven tangentially to the pinion wheel and having a plurality of individual laterally extending driving teeth for cooperating with the pinion wheel teeth to rotatably drive the pinion wheel, the rack comprising a leading engagement tooth which is smaller than the following driving teeth on the rack. For example, the leading engagement tooth may be truncated compared to subsequent driving teeth.

This helps to phase engagement between the rack and the pinion wheel .

Preferably the leading rack tooth extends laterally less distance than the subsequent teeth and less than the depth of the spacing between the teeth on the pinion wheel, preferably less than half the depth.

According to one embodiment of the present invention the

leading rack tooth has a substantially planar outer edge profile, and optionally all rack teeth may have this trapezium shape in profile. In another embodiment the teeth profile, particularly of the leading tooth has rounded edges in profile.

According to yet a further embodiment the rack teeth have an involute, for example a bulbous profile in cross-section. The circular thickness of the teeth can be readily varied by altering the number of teeth and the tooth height .

Known systems use pointed teeth to prevent jamming of the drive but these are weak and since the initial load is, in the worst case, taken on the tip of the tooth, they often shear or break. The preferred embodiments of the present invention using flat or rounded teeth tips gives this new system more strength.

A variety of profiles may likewise be employed for the teeth of the pinion wheel.

The second tooth on the rack may also be truncated compared to subsequent teeth in case breakage or deformation of the first tooth occurs .

Two racks may be provided at opposite sides of the pinion wheel to move tangentially to the wheel, in opposite directions, to provide more initial driving force for the pinion wheel and accelerate it from stationary more rapidly. The two racks may be mobilised simultaneously or their engagement phased, for example by electronic means.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, in which:

Figure 1 shows one embodiment of the driving arrangement of the invention; and

Figure 2 shows another embodiment of the invention.

In Figure 1 a pinion wheel 1 is illustrated which has 20 teeth 2 radially extending at spaced intervals around its periphery. Two racks 3 are shown at opposed sides of the pinion wheel 1 arranged for movement in respectively opposed directions illustrated by arrows A and B tangential to the pinion wheel 1. The racks 3 have laterally extending teeth 4 dimensioned and spaced to cooperate with and engage the pinion wheel teeth 2 to rotate the pinion wheel .

As is clearly shown in Figure 1 the leading tooth 5 on each rack 3 is truncated compared to the following teeth. The height of the truncated tooth 5 is such that it can only contact one tooth of the pinion and will thus correctly phase the engagement of the subsequent rack teeth. As shown the leading tooth extends around half the height of the subsequent teeth. The exact length and shape of the first tooth is dependent on the size and number of teeth on the pinion and is optimised to give sufficient pinion tooth strength and optimum engagement phasing. By correctly phasing engagement of the teeth a positive and speedy engagement is ensured even when the rack is accelerated very fast. Thus, the pinion wheel can be set in motion, from a stationary state very rapidly.

In Figure 2 a pinion wheel 1 is illustrated with only twelve teeth 2. Like parts are identified with reference numbers corresponding to those used in Figure 1. In this embodiment the rack teeth 4 are of a more robust design since a smaller number of larger teeth are provided on the pinion, also allowing for a higher truncated tooth.

In both Figures, the outer dotted circle X represents the pitch diameter being the nominal contact circle between the rack and pinion. This is defined by the size or module of the teeth and the number of the teeth. The inner dotted circle Y is the

base circle from which the involute defining the tooth profile is constructed. This base circle is defined by the pitch diameter X cosine pressure angle. Below the base circle diameter, rolling contact between the rack and the pinion ceases. Typically the pitch diameter is given by nm + 2k and the base diameter by (nm + 2k) cos θ where m = tooth size or module

= 2 x both circular thickness at the pitch diameter n = number of teeth k = correction coefficient a = addendum coefficient d = dedendum coefficient θ = pressure angle Also the root diameter = (nm + 2k) cos θ where the tooth diameter is the diameter of a circle drawn tangentially to the troughs of the pinion teeth.

In one embodiment these variables would typically take the values : m = 0.8mm n = 12 k = 0.2 a = 1 d = 1.4 θ = 20 Of course, many combinations of variables as well as of tooth shape and size are envisaged within the scope of the present invention.

This invention is particularly useful for ensuring rapid, positive acceleration from stationary of a piston wheel. One application would be for a pretensioning device as used in safety

restraints in the automotive industry when it is desired to improve the restraining power of a safety belt by reeling in excess safety belt webbing in a crash situation. Response times in such an application must be extremely rapid and for example movement of the rack or racks would be initiated in the event of a crash situation being detected, and the pinion wheel would be connected to the shaft of a safety belt winding mechanism such as a retractor spool. In this way the invention provides for a compact, rapid response pretensioning device to be constructed.