GB2383622A

GB2383622A - Rack and pinion drive having two tapered pinions

Info

Publication number: GB2383622A
Application number: GB0125592A
Authority: GB
Inventors: Philip Robert Edwar Warrington
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-10-25
Filing date: 2001-10-25
Publication date: 2003-07-02
Anticipated expiration: 2021-10-25
Also published as: GB2383622B; GB0125592D0

Abstract

A rack and pinion drive has two tapered pinions C mounted with the large end of the taper on opposite sides of the rack L. The large end of the tapers being preloaded by springs D to bias the pinions C into contact with the rack L, thus eliminating any backlash.

Description

Variable Pitch Drive This mechanism has been developed with stair lift chair manufacturing in mind, but has a far wider application, i. e. to production line technology.

The Problem : With a straight cut pinion running in a matched straight track, there is always a line of pressure contact between adjacent flanks of their teeth. If however, a linearly cut rack is subsequently curved to fit a location then the pitch of the rack teeth will alter.

In the most difficult case of a rack being curved laterally, the rack will bend at it's weakest points, i. e. across the roots of the teeth, so that the curve is approximated by line segments. Therefore the cut part of the teeth containing the dedendum will be wedge shaped, giving a larger pitch on the outside of the curve, but the teeth thickness remain parallel. Hence there are now two tooth patterns running alternately, parallel cut and wedge cut. If a straightcut pinion is used in this situation, it will have to'slide'on the top of the next rack tooth on the outer side, while point contacting'on the inner side and even riding up the rack causing upwards force, and jamming of the mechanism in close fitting set-ups.

Hence in this case there is a need for a pinion which has teeth cut parallel alternate wedge, but depending on whether curvature is to the left or right the wedge part must be cut to that hand. This suggests that two'handed'pinions are used, assuming that it was possible to cut such a pinion and the curve which

determines the angle of the wedge tooth were constant. The straight cut pinion used in this scenario is subject to poor engagement from both sides: whereby the pinion pivots about the tooth on the inboard side of the curve to allow that tooth to slide over the rack tooth on the outer side resulting in rapid wear, and from the outset noisy operation.

In the case where the rack is curved vertically, and assuming the rack is kept purely in the vertical plane the flanks of the teeth remain parallel, but the cut part of the teeth will : (a) Close up when curved upwards. i. e. the crests of the teeth will move together.

(b) Open up when curved downwards, i. e. crests of the teeth will move apart.

In the case of (a) the pinion will tend towards running on the crests of it's teeth forcing the pinion upwards, causing again an up force.

In the case of (b) the pinion will roll over one tooth then slide/skip over the next.

Hence there is a need for a pinion with parallel cut teeth but alternating 'standard form'A thin standard form'or'standard form/'thick standard form'.

The problem is further compounded by the fact that both lateral and vertical conditions usually occur simultaneously and are not mutually exclusive.

One further disadvantage to overcome is variation in the theoretical height of the rack to the load bearing rails.

The Solution:

THfJc-To F * < To address the problem of lateral curvature of the rack, a tapered pinion C L has been used to fit the need for a wedge tooth form. But since curvature can be to the left or right then two taper pinions have been used parallel, reversed and

reloaded against one another sandwiching the rack between them. In operation the appropriate pinion fills the wedge form before an equilibrium in the forces is achieved, giving maximum engagement of at least one pinion.

For vertical curvature a pinion with two forms of alternating parallel cut teeth is required. To achieve this, the two taper pinions of the device then close together to coarsen the tooth pitch, and similarly move apart to reduce pitch. Then operating in these conditions the pinions move in and out, relative to one another, between consecutive rack teeth.

To this end the following mechanism has been devised.

I A The unit consists of a cradle which is in some way bolted to the load carrier. This "3" carries two freely rotating spindles with parallel axes. Each spindle has mounted on "C""M" it one tapered pinion which is keyed (splined) to it's own shaft but allowed to slide.

SEE Ft 6 2- The pinions are mounted so that when assembled the larger diameter of one is

adjacent to the smaller diameter of the other. Mounted on each spindle between "iC the large end of the pinion and cradle body is a spring and thrust race. Each of these springs are used to preload the pinion against opposite top sides of the rack: the thrust race ensures that the spring does not rotate.

These pinion carrying shafts extend outboard of the cradle sufficiently to allow the

-clfixing one wormwheel to each. Above the axes of these are are mounted two worms on a common driven shaft, one worm to each wormwheel. Thus both spindles are simultaneously driven. The driving wormshaft is for rigidity supported "" by two line bearings (ball) as close to the worm as possible. The shafts are held

captive in the cradle between'pinned'collars and wormwheels.

Advantages : I) Since there are two driven pinions sharing the load tooth wear is halved.

2) As the pinions are tapered the line of contact increases with wear, thus giving the availability of using a wider rack if desired.

3) The preload eliminates backlash and prevents"rocking"from back face of tooth to front face of adjacent tooth when changing from gradient to level or vice versa.

4) Any irregularities between the height of the rack and load carrying track can be absorbed by the pinions a) Closing together for a falling discrepancy or b) Opening up for a rising datum.

5) Using pre-loaded tapered teeth also overcomes any inaccuracy in the manufacturing of pitch of any of the following : a) rack b) pinions c) worm and wormwheels 6) Variable pitch also overcomes meshing problems between adjoining track/rack sections, reducing demands in accurate coupling.

7) This system also allows for the rack to be made wider. giving longer life.

8) The mechanism automatically adjusts to overall change in pitch from required curvature.

9) Relaxes necessity for very accurate setting of pinion to rack.

Design Points : FIGI I) The distanceX (see diagram) should be kept as close together as possible to minimise the turning moment of the cradle, ensuring that there is still clearance between the two pinions when operating over a curve drop in the vertical plane when the rack is simultaneously below it's designed datum from the carrier track. (i. e. it's extreme operating scenario).

2) The taper of the pinions is minimal, but is primarily a function of the minimum radius of the rack which determines the'wedge angle'of cut part of the teeth. Within machining tolerances the included angle ( ) of the taper of the pinion is calculated by:

where, t = pitch of rack teeth and r = radius of curvature of rack where t, r are in similar dimensions.

E. g.

For a 10mm. pitch rack curved at 150 mm. radius

gives "" } ='

3) The preload (f) of the pinion spring is determined by : f = m x Tan o < /2. where m = load and = included angle of pinion but since the load is distributed over two pinions then the preload is halved.

E. g. For the previous example of I Omm. pitch and 150mm. radius rack # = 3. 82 Then for a typical 150kg. load this gives a preload (f) thus:

This can then be multiplied by an appropriate safety factor.

-c='

Claims

Claims 1. A variable pitch gear for rack and pinion systems that consists of a cradle supporting two parallel driven splined shafts upon which tapered pinions are allowed to slide but mounted in reverse to one another with a preload spring behind each of the pinions larger diameters.
2. A mechanism as claimed in Claim 1 where the rack is sandwiched between the two oppositely reloaded pinions which because of their taper adjust pitch to suit a linearly cut rack subsequently curved in any plane, also eliminating backlash.