AU671751B2 - Stepless detent apparatus - Google Patents

Description

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AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

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Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Class Int. Class 1 a t Applicant(s): S M.T.M. PTY. LTD.

Department of Defence Anzac Park West CANBERRA A.C.T. 2600 Address for Service is: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA o. Complete Specification for the invention entitled: a" "STEPLESS DETENT APPARATUS" Our Ref: IRN 334434 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1 y 1 2r STEPLESS DETENT APPARATUS The present invention relates to detent apparatus such as ratchet and pawl mechanisms. Such mechanisms are used in park braking systems of motor vehicles and other arrangements in which elements are to be releasably secured against a counteracting force.

A conventional design of a parking brake detent mechanism is shown in Fig. 1. This consists of a steel base 10, a lever handle 11 and a small pawl 12 with activating rod 13 which interferes with or engages teeth 14 formed in an associated ratchet sector 15. The teeth "14 on ratchet sector 15 need to be machined precisely to ensure that the parking brake cannot release under influence of vibrations or other forces unless a release button 16 is actuated. On the other hand forces necessary to actuate the release button 16 should be relatively low to facilitate easy release of the parking brake by means I of a finger actuated button. Therefore the angle under 20 which pawl 12 and teeth 14 interact is very important and the permissible tolerance is small. The teeth 14 on the ratchet sector 15 and the tip of the pawl 12 therefore have to be machined, fine blanked or made by sintering.

These are all relatively expensive procedures which i 25 significantly add to the cost of manuacting a .conventional parking brake detent mechanism, A further disadvantage of prior art ratchet and pawl mechanisms is that the teeth of the associated ratchet sector are separated by preset distances. This factor places an upper limit on the number of detent positions along the ratchet sector which can be engaged by the pawl. As a consequence the securing or holding force which a ratchet and pawl mechanism can apply to a given braking system or other arrangement cannot be continuously i or steplessly adjusted. This can be a disadvantage in ii some instances eg. where successive positions of teeth if along the ratchet wheel give rise to securing or holding 7 forces which are too low and too high respectively.

An object of the present invention is to at least j, MJP LH1 -3alleviate the disadvantages of the prior art. A further object of the present invention is to provide a detent apparatus which may be steplessly or continuously adjustable.

The present invention contemplates a stepless detent apparatus in which the role of the toothed ratchet sector is replaced by a base element having a relatively smooth first surface including a cylindrical, semi-cylindrical or other preferably curved surface. The action of the pawl 1 may be replaced by a detent element such as a sliding, rolling or similar element having a relatively smooth second surface for engaging the first or base surface.

;o The detent element may be arranged such that it engages the base surface with a net force which is oriented at an angle relative to the normal at the point of contact of the detent element with the base surface (ie. normal to the tangent at the point of contact). It may be shown that if the angle p at which the net force is applied is less than or equal to the inverse tangent of the 20 coefficient of friction between the first and second surfaces, self jamming, gripping or locking together of the detent element and the base surface will take place.

The above relationship may be expressed alternatively in the form 25 tan p j.

where p represents the angle of the force and represents the coefficient of friction between the materials of the first and second surfaces.

The detent element may be arranged such that a small degree of movement thereof under control of the release button is sufficient to move the detent element away from the base surface and hence to release the detent element.

This may result in a correspondingly small travel of the release button.

The actuating force on the release button may be reduced by incorporating a lever arrangement providing a I mechanical advantage between the release button and the detent element. 39 Since the forces between the detent element and base I 4 surface may be relatively large, eg. of the order of kilo Newtons in a typical parking brake situation, a pressure bracket may be provided to more reliably exert such a force. In one form the pressure bracket may be provided in a pivoting arrangement on the same axis on which a hand operated park brake lever pivots.

A further advantage of the present arrangement is that it may avoid "ratchet" noise that is associated with a park brake when it is being engaged.

According to the present invention there is provided a detent apparatus including: I'**So a base means having a reatively smooth first surface; a detent means having a relative4r smooth second 15 surface for engaging said first surface at least at a point of contact; and means for urging said second surface against said first surface with a net force such that the angle of said j o force relative to the normal at said point of contact is 20 not greater than the inverse tangent of the coefficient of friction between said first and second surfaces.

Preferred embodiments of the present invention will I now be described with reference to the accompanying drawings wherein: ij 25 Figure 1 shows a prior art parking brake detent mechanism as described above; Figure 2 shows one form of detent apparatus according to the present invention; Figure 3 shows a further form of detent apparatus according to the present invention; Figure 4A shows a still further form of detent apparatus according to the present invention; i Figure 4B shows a sectional view on IVB-IVB in Figure 4A; and i Figure 4C shows a still further form of detent 0 apparatus according to the present invention.

S n)l Referring to Figure 2 the detent apparatus includes a semi-cylindrical base surface 20 and sliding element 39 21. The centre of curvature of base suface 20 is along

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lever axis 22. Pressure bracket 23 and hand lever 24 are arranged to pivot together about lever axis 22. Sliding element 21 is arranged such that it is captured between base surface 20 and pressure bracket 23.

Lever 24 is manufactured in a conventional way from formed sheet metal. Alternatively it may be moulded from engineering plastics material. Lever 24 may be moulded in two halves split longitudinally. One lever half may have moulded into it provisions for component parts such as the release button 25, return spring 26, connector rod 27, pressure bracket 23 etc. The various parts may be easily assembled into the one moulded lever half and then covered by the second moulded lever half. This may improve efficiency of assembly. One piece lever configurations S 15 are also possible.

.o Brake engaging cable 28 is attached t3 lever 24 such that a tensile force is created in cable 28 upon rotating lever 24 (pressure bracket 23 and sliding element 21 rotate therewith) clockwise with respect to pivot axis 20 22. The geometry of the arrangement is such that the tensile force in cable 28 gives rise to a reaction force 00between sliding element 21 and base surface :::According to the present invention the arrangement is such that the direction of the net reaction force S 25 between sliding element 21 and base surface 20 is oriented at an angle p with respect to the normal at the point of contact with base surface 20, which is less than the inverse tangent of the coefficient of friction between sliding element 21 and base surface Where sliding element 21 and pressure bracket 23 are formed from hardened steel the angle p preferably is less than 5 degrees bearing in mind that a lubricant such as grease may be applied to the surfaces of slider 21 and/ori base surface 20 intentionally or accidentally. i Self jamming or locking together or sliding element 21 and base surface 20 will be maintained until the mechanism is released. The mechanism may be released by means of an L-shaped lever 29 pivoting on axis 30. Lever 4 39 29 is actuated via rod 27 and release button 25. Lever 29

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6 engages sliding element 21 via coupling 32. Coupling 32 is arranged to move sliding element 21 linearly away from base surface 20 to release sliding element 21.

To provide a mechanical advantage, the distance from axis 30 to the point of action of rod 27 on lever 29 is greater than the distance from axis 30 to the point of action of lever 2q upon coupling 32 and sliding element 21.

The embodiment shown in Fig. 3 operates on a similar principle to that of Fig. 2 and like parts are labelled consistently. In the embodiment of Fig. 3 sliding element 21 of Fig. 2 is replaced with a pawl element 33 arranged 9 to pivot about axis 34. Rod 27 is pivotably connected to pawl 33 via pivot axis 35. Pawl 33 is released via a release button (not shown) acting upon rod 27. Hand lever 15 24 and pawl pivot axis 34 are arranged to pivot together about lever axis 22. Pawl 33 includes a cammed surface 36 for engaging base surface 20. The radius of curvature of cammed surface 36 to the right of a nominal point of contact between base surface 20 and cammed surface 36 is 20 less than or at most is equal to the shortest distance between axis 34 and base surface 20. The radius of curvature of surface 36 to the left of the nominal contact point is greater than the said distance between axis 24 and base surface 25 Brake engaging cable 28 is attached to hand lever 24 such that a tensile force is created in cable 28 upon rotating lever 24 clockwise with respect to lever pivot axis 22. The arrangement is such that the tensile force in cable 28 gives rise to a reaction force which tends to move axis 34 on which pawl 33 pivots, counter clockwise with respect to lever axis 22. Movement of pivot axis 34 counter clockwise and friction between cammed surface 36 and base surface 20 causes pawl 33 to rotate counter clockwise with respect to pivot axis 34. i, The distance between pivot axes 22, 34 is fixed and t a pressure bracket (not shown) may be provided to maintain i constant the distance between axes 22, 34 under w application of forces which arise in a typical parking i brake situation.

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7 00 0 6.: 4 The increased radius of curvature of surface 36 to the left of the nominal contact point causes self jamming or locking together of pawl 33 and base surface 20. The geometry of the arrangement is such that the direction of the net -ce between camming surface 36 of pawl 33 and base surface 20 is oriented at an angle with respect to the normal at the point of contact with base surface which is less than the inverse tangent of the coefficient of friction between camming surface 36 and base surface Self jamming or locking together of pawl 33 and base surface 20 will be maintained until the mechanism is released. The mechanism may be released by movement of rod 27 to the right causing pawl 33 to rotate clockwise around axis 34 and removal of the net force between camming surface 36 of pawl 33 and base surface The embodiments shown in Figs. 4A and 4C operate on a similar principle to that of Figs. 2 and 3 and like parts are labelled consistently. In the embodiment of Fig. 4A sliding element 21 of Fig. 2 is replaced with a cylindrical rolling element 37. Rolling element 37 is biased via spring 38 and spring cap 39 towards a wedge shaped space 40 not unlike that found in a free wheeling clutch. Hand lever 24, roller 37, spring 38, cap 39 and wedge shaped space 40 are arranged to pivot together about lever axis 22. The geometry of the arrangement, including the angle of attack of wedge shaped space 40, is such that the direction of the net force between rolling element 37 and base surface 20 is oriented at an angle with respect to the normal at the point of contact with base surface 20, which is less than the inverse tangent of the coefficient of friction between rolling element 37 and base surface With this arrangement self jamming or locking together of rolling element 37 and base surface 20 will be maintained until the mechanism is released. The mechanism may be released by movement of rod 27 to the right causing pawl 41 to pivot clockwise around axis 42. Pivoting of pawl 41 clockwise causes it to dislodge rolling element 37 39 away from the shallow end of wedge shaped space 40 and

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4 i i;: -8removal of the net force between rolling element 37 and base surface To minimize slippage between rolling element 37 and base surface 20, a friction promoting surface such as a knurled or fluted band 43 as shown in Figure 4B may be formed around rolliig element 37. Band 43 may be formed at least partly around the circumference of rolling element 37. Band 43 may be formed at one end of rolling element 37 as shown in Fig. 4B or it may be formed at both ends or intermediate the ends of rolling element 37. A corresponding friction promoting surface may be formed as 'o a path along base surface 20 (not shown). The friction promoting surface or path is formed along base surface such that it is out of the plane of cap 39, wedge shaped *99 15 space 40, and pawl 41. :I The embodiment of Fig. 4C is similar to that of Fig i 4A and like parts are labelled consistently. In the embodiment of Fig. 4C rolling element 37 and cap 39 of o Fig. 4A are replaced with a mushroom head plunger 44. The *9c* S 20 manner of operation of the embodiment of Fig. 4C is almost identical to that of the embodiment of Fig. 4A except that self jamming or locking together occurs between mushroom head plunger 44 and base surface 20. As before the geometry of the arrangement is such that the direction of *9* S 25 the net force between mushroom head plunger 44 and base surface 20 is oriented at an angle with respect to the normal at the point of contact with base surface 20, which is less than the inverse tangent of the coefficient of friction between mushroom head plunger 44 and base surface 20. Release of the mechanism via rod 27 and pawl 41 is S, identical to that described with reference to Fig. 4A.

It is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit ofi the invention.

For example, in the embodiments shown in Figs. 2-4C the first surface associated with the base means may be 39 relocated to one or both sides of the ratchet sector, i.e.

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i~ 4 -9there may be a pair of first surfaces lying at right angles to base surface 20. In one form the first surface may include a semi-annular region formed on one or each side of the ratchet sector. The second surface associated with the detent means may also be relocated to engage the relocated first surface.

Where the first surfaces are formed on both sides of the ratchet sector in the manner of a "disc brake", the detent element may be provided with a pair of second surfaces, each for engaging a respective first surface of the ratchet sector. Alternatively, a pair of detent J elements may be provided each having a respective second surface for engaging the respective first surface of the ratchet sector.

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Claims (14)

1. A detent apparatus including: a base means having a smooth first surface; a detent means having a smooth second surface for engaging said first surface at least at a point of contact; and means for urging said second surface against said first surface with a net force such that the angle of said force relative to the normal at said point of contact is not greater than the inverse tangent of the coefficient of friction between said first and second surfaces.

2. A detent apparatus according to claim 1 wherein said first surface is adapted to provide a continuum of contact points for said second surface.

3. A detent apparatus according to claim 1 or 2 wherein said first surface includes an arcuate portion.

4. A detent apparatus according to claim 3 wherein said first surface includes a semi-cylindrical portion.

A detent apparatus according to claim 1 or 2 wherein said first surface includes at least one semi-annular portion.

6. A detent apparatus according to any one of the preceding claims wherein 20 said detent means includes at least one sliding element and said means for urging includes means for restraining the or each sliding element.

7. A detent apparatus according to any one of claims 1 to 5 wherein said detent means includes at ieast one pivotable pawl element and said means for urging includes means for pivoting the or each pawl element.

8. A detent apparatus according to any one of claims 1 to 5 wherein said detent means includes at least one rolling element and said means for urging includes at least one wedge shaped space.

9. A detent apparatus according to claim 8 wherein the or each rolling element is substantially cylindrical.

10. A detent apparatus according to claim 8 wherein the or each rolling element is substantially spherical. 4 0*. 4. *i *4 V 4 *44. >I 6 'i CCi 44 it i J ii it ft I ii I *I- A 1A i ii i: /a A< -11

11. A detent apparatus according to any one of claims 1 to 5 wherein said detent means includes a combination of elements selected from a sliding element, a pivotable pawl element and a rolling element.

12. A detent apparatus according to any one of the preceding claims wherein at least one of said first surface and said second surface includes means for minimizing slippage therebetween.

13. A detent apparatus according to any one of the preceding claims including means for releasing said urging means.

14. A detent apparatus according to Fig. 2 or Fig. 3 or Figs. 4A and 4B or Fig. 4C of the accompanying drawings. A park braking system a motor vehicle incorporating a detent apparatus according to any one of the preceding claims. r i DATED: 9 July 1996. PHILLIPS ORMONDE FITZPATRICK Attorneys for: M.T.M. Pty. Ltd. et 0 0r 0t 00 0 000 0 0 *o 00 00 0 *00 00 l~e 4000 1; i0000 >p 7 1 ~i-i- i-i I 1 i i ABSTRACT A stepless detent apparatus suitable for use in park braking systems of motor vehicles is disclosed. The detent apparatus includes a base element having a relatively smooth first surface a detent element (21, 33, 37, 44) having a relatively smooth second surface (36) for engaging said first surface (23) at least at a point of contact; and means (23, 28, 34, 40) for urging said second surface (36) against said first surface with a net force such that the angle of said force relative to the normal at said point of contact is not greater than the inverse tangent of the coefficient of friction between saij first (20) and second (36) surfaces. 00 09 8a* 0 990 9000o 09 p0 i MJP 7 1' I- i I: