Cam Gears Limited

Trw Cam Gears Limited

Philip A Downing

Frederick J Adams

A rack and pinion gear and a method of assembling such a gear. The gear has a rack bar 1 the rack 4 of which is biased into engagement with the pinion 6 by a spring loaded support 7 which slidably (or rolling) engages a bearing surface 8 of the rack bar. The bearing surface has a central region 9 parallel to the rack axis and tapering end regions 10 and 11 which reduce the thickness of the rack bar from the central region 9. The length C of the region 9 corresponds to 1 to 1.5 revolutions of the pinion 6 while the operative length A of the rack corresponds to at least three revolutions of the pinion. The pinion rotates about an eccentric axis 14 which varies the rack bar tightness. To determine the position of maximum tightness in which the spring loading of the support 7 is to be adjusted, the pinion is rotated from a central position on the rack 4 clockwise and counter-clockwise to displace the rack bar in each instance through a distance corresponding to 0.5 revolutions of the pinion from the said central position. During this displacement the reaction on the support 7 is observed to determine the position in which the maximum tightness of the rack bar is encountered where the spring loading of the support means is finally set. The invention is applicable to supports 7 which engage the rack bar in one or more positions around the support surface.

Rack and pinion gear and a method of assembling such a gear

A variable ratio rack and pinion gear having a rack bar (1) the rack (8) of which engages with the pinion (10) so that rotation of the pinion displaces the rack member (1) longitudinally relative to the housing (6). A yoke (25) supports the member (1) with its teeth in engagement with the pinion and straddles the member (1) to restrain displacement of that member relative to the housing (6) in a direction parallel to the axis of rotation for the pinion (10). The rack member is restrained from rotation relative to the housing, and (1) has a longitudinally extending non-rectilinear grooved track of one or more curved portions (14) which is engaged by a peg (17) rotatably carried in a mounting block (21). The block (21) retains the yoke (25) in the housing with the peg (17) located within and extending through the yoke (25). The peg (17) engages with the track (14) so that during longitudinal displacement of the rack member (1) the reaction between the groove (14) and peg (17) imparts limited controlled rotation to the member (1) to vary the ratio of the gear. Consequently, by appropriately shaping the track the gear assembly can be provided with a predetermined variable gear ratio. For example, the track may be a helix of variable pitch to provide the variable gear ratio. The peg can rotate in its mounting block (21) to alleviate friction between the peg and rack member (1) during displacement of the latter and can be spring biased to urge and support the rack member for engagement with the pinion. In a modification, the grooved track (14) is engaged by the peripheral edge of a rotatably mounted roller or wheel located within the yoke so that engagement between the roller and rack member (1) imparts rotation to the rack member to vary the gear ratio during displacement of the rack member. The teeth of the rack member can extend part way only around the periphery of that member while the non-toothed peripheral portion provides a slideway within which the groove (14) is machined and with which the yoke (25) slidably engages.

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Bristol, United Kingdom

Philip A Downing