GB2347986A

GB2347986A - Variable pitch leadscrew

Info

Publication number: GB2347986A
Application number: GB9906008A
Authority: GB
Inventors: Pierrot Nool Germaine Catry
Original assignee: Youngflex AG
Current assignee: L&P Swiss Holding GmbH
Priority date: 1999-03-16
Filing date: 1999-03-16
Publication date: 2000-09-20
Anticipated expiration: 2019-03-16
Also published as: GB9906008D0; GB2347986B

Abstract

A variable pitch leadscrew comprises a helical groove (15,16 fig 2) the pitch of which varies along the axial length of the groove. A nut 3 engaging the leadscrew comprises one or more arcuate teeth 18 each to engage a corresponding groove (15,16 fig 2), and has trailing and leading edges that taper away from an intermediate point of the tooth 18 (especially see the tooth profile shown in fig 10).

Description

VARIABLE PITCH LEADSCREW This invention concerns improvements in and relating to a variable pitch leadscrew of the kind intended for conversion of rotary movement into an axial translatory movement the rate of which is not constant in relation to the angular movement of the leadscrew.

Although variable pitch leadscrews are known in various contexts it is an object of the invention to provide a novel arrangement that is capable of implementation in a relatively compact arrangement, more especially, but not exclusively, where the components of an adjusting device are to be formed of synthetic plastics material.

Particularly in the context of the adjustment of motor vehicle seats, for example to provide lumbar adjustment in the backrest of the seat, various arrangements are known that require a force to be provided on a supporting element or elements of the seat in order to cause flexing or distortion thereof with a view to varying the lumbar support provided. A particularly effective means of applying such a force has proved to be the use of a Bowden cable or Bowden cables tensioned between respective anchorage points of an element to be flexed. Furthermore, a convenient means of applying force to the Bowden cable has proved to be a leadscrew and nut device wherein a nut moveable relatively to the leadscrew is coupled to one or more Bowden cables. Leadscrew devices hitherto proposed for this purpose have been of constant pitch, whereas, owing to the geometrical arrangement of the lumbar or other adjusting means, the force exerted on the Bowden cable or cables has proved to be non-linear as the leadscrew and nut undergo relative translatory movement.

An effective means of varying the mechanical advantage of such a leadscrew device would therefore be a significant advance in the art.

In accordance with the invention there is provided a leadscrew and nut device, wherein the leadscrew has a thread form comprising a helical groove of which the axial crosssection is substantially constant whilst the helix pitch of the groove varies along the length of the leadscrew, whereas the nut engaging the leadscrew has a male threaded portion extending over a limited arc of rotation with respect to the axis of the leadscrew, the cross-section of said male portion at an intermediate point on said arc corresponding substantially to the axial cross-section of said helical groove, whereas sections of said male portion extending respectively away from said intermediate point have cross-sections that reduce in a direction away from said intermediate point, in such a manner that a first pair of flanks of said male portion that extend away from said intermediate point in opposite directions and are arranged to engage opposite surfaces of said groove are angled at a pitch substantially corresponding to the maximum pitch of said helical groove and that a second pair of flanks of said male portion that respectively lie opposite said first pair to engage opposite surfaces of said groove are angled at a pitch substantially corresponding to the minimum pitch of said leadscrew.

An arrangement in accordance with the invention has the advantage that forces to be transmitted between the helical groove of the leadscrew and the nut device are distributed over different angularly spaced portions of the nut as the pitch of the leadscrew changes and thus the force capable of being transmitted between the screw and nut and the corresponding wear resistance of the leadscrew device is increased in comparison with a simple worm and peg device.

Preferably, the leadscrew has a twin start thread with identical helical grooves relatively displaced by 180 about the axis of the leadscrew, and the nut device correspondingly has two diametrically opposed male portions for engagement with the respective grooves.

Further preferred features and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which: Figure 1 is an exploded perspective view of an actuating mechanism incorporating a leadscrew device in accordance with the invention taken from one axial direction; Figure 2 is a similar view taken from the other axial direction; Figure 3 is a plan view of the leadscrew of the mechanism shown in Figs. 1 and 2; Figure 4 is a section on the line IV-IV of Fig. 3; Figure 5 is an enlarged view of a fragment of Fig. 4; Figure 6 is an end view taken in the direction of the arrow X, of Fig. 3 ; Figure 7 is a section on the line VII-VII of Fig. 3; Figure 8 is an end view of a nut of the mechanism shown in Figs. 1 and 2, taken from the end that is visible in Fig. 1; Figure 9 is a section on the line IX-IX of Fig. 8; Figure 10 is an enlarged view of a fragment of Fig. 9 ; Figure 11 is a section on the line XI-XI of Fig. 8; Figure 12 is an enlarged view of a fragment of Fig.

11; Figure 13 is a section on the line XIII-XIII of Fig.

8, and Figure 14 is a section on the line XIV-XIV of Fig. 8.

Referring to Figs. 1 and 2 of the drawings the actuating mechanism illustrated comprises an outer housing 1, an end stop 2, a nut 3, a cover 4 and a leadscrew 5.

The housing 1 has on its outer end a pair of cylindrical bosses 6 that are adapted, in known manner, to receive outer sheaths of Bowden cables, not shown. The nut 3 is a sliding fit within the housing 1 and has cylindrical recesses 7 that are likewise adapted to receive nipples at the ends of inner cores of Bowden cables received in the bosses 6. Slots 8 and 9 in the housing 1 and nut 3 and apertures 10 in the housing 1 permit the Bowden cables to be attached to and released from the actuating mechanism in known manner.

The end stop 2 has a stub shaft 11 that engages in a bore 12 of the housing 1 and dogs 13 that engage in recesses 14 of the leadscrew 5. Thus the end stop 2 rotates with the leadscrew 5 and serves to transmit axial force between the leadscrew 5 and the outer housing 1.

The leadscrew 5 has twin-start helically screw-threaded grooves 15,16 each of which commences at one axial side wall of a corresponding one of the recesses 14.

The nut 3 has two diametrically opposed arcuate teeth 17 (see Figs.), 18 that engage the respective grooves 15,16.

The cover 4 comprises a cylindrical bore 19, with two diametrically opposite ramp-shaped arcuate teeth 20 that make snap-engagement with an annular groove 21 of the leadscrew 5. The cover 4 also has a pair of catches 22 that make snap-engagement with recesses 23 of the housing 1 to retain the leadscrew 5 and the assembled nut 3 and end stop 2 therein.

The shaft of the leadscrew 5 has a squared end 24 for engagement by an appropriate drive mechanism, not shown, that can be assembled to the housing 1 by fastening screws engaging in bores 25 of the housing 1.

All of the components illustrated in Figs. 1 and 2 are advantageously formed by injection moulding from a suitable synthetic plastics material such as a polyamide which has appropriate characteristics both of resilience and wear resistance.

The form of the grooves 15,16 in the leadscrew 5 is illustrated in more detail in Figs. 3-7.

As illustrated, each groove commences at one of the axial walls of a recess 14 and extends helically to the right hand end, as viewed in Figs. 3 and 4, of a cylindrical portion 30 of the leadscrew, each groove extending through an arc of rotation of 4 x 360 + 70 . Each of the teeth 17,18 of the nut 3 subtends an arc of 70 and thus the leadscrew 5 and nut~3 are permitted four complete turns of relative rotary movement when the mechanism is in the assembled condition. Furthermore the arrangement is such that the pitch of each groove 15,16 decreases linearly with the angular rotation of the groove from the left hand to the right hand end of the portion 30. In one example the pitch of the thread may be 7mm at the left hand end and 3mm at the right hand end of the screw, which has a left hand thread.

The form of the nut 3 will now be described in further detail with reference to Figs. 8 to 14 of the drawings. As can be seen from Fig. 9, the nut 3 comprises a cylindrical bore 40 of which the internal diameter substantially corresponds to the external diameter of the cylindrical portion 30 of the leadscrew such that the latter can be received as a sliding fit within the former. The disposition of the arcuate teeth 17 and 18 can be seen by reference to Figs. 8 and 9, it being understood that although Fig. 9 shows a view of the arcuate tooth 18 taken in the radial direction of the nut, the radial view of the tooth 17 when viewed in the reverse direction would be identical to that of the tooth 18 as shown in Fig. 9.

The configuration of the tooth 18 is shown in greater detail in Fig. 10 from which it will be seen that the cross-section of the tooth 18 is greatest where it intersects a plane 41 extending normally to the plane of the drawing. At this point, the cross-section of the tooth 18 has the form shown in Figs. 11 and 12.

Referring again to Fig. 10 it will be seen that the arcuate tooth 18 tapers away from the section of maximum crosssection, as it extends around the circumference of the nut 3 in respective directions away from the plane 41. Thus, the arcuate portion of the tooth extending to the left hand side of the plane 41 as shown in Fig. 10 is defined by a first surface 18A that has the form of one flank of a helical screw thread extending at a pitch of 7mm relatively to the axis of the cylindrical bore 40, and a second surface 18B that extends in the form of one flank of a screw thread having a pitch of 3mm relatively to the axis of the cylindrical bore 40. In like manner, a surface 18C extending on the opposite side of the plane 41 has a helical pitch of 3mm, whereas an opposite surface 18D has a helical pitch of 7mm. It will thus be seen that the surfaces 18A and 18D form complementary surfaces of a screw thread extending at a pitch of 7mm, whereas the surfaces 18B and 18C form complementary surfaces of a screw thread extending at a pitch of 3mm.

It will be noted that the arrangement of the complementary surfaces 18A and 18D on the one hand, and 18b and 18c on the other is such that notional extensions of the surfaces beyond the plane 41 would extend above the actual surfaces of the screw thread that are located on the opposite sides of the plane 41. Thus the respective pairs of surfaces of different pitch do not interfere with one another. This allows the arcuate tooth 18 to extend in any portion of the helical groove 16 without binding.

Figures 13 and 14 show the positions of the respective surfaces 18A, 18B and 18D, 18D, at the respective ends of the arcuate tooth 18.

As a result of the thread forms of the helical grooves 15, 16 and the arcuate teeth 17,18, it will be seen that axial forces transmitted between the grooves 15,16 and the teeth 17,18 are angularly distributed rather than being located at a single point, the regions of contact between, for example, the tooth 18 and the groove 16 progressively changing from the flanks 18A and 18D in the region of the groove of maximum pitch, to the flanks 18B and 18C in the region of minimum pitch.

It will be appreciated that although the respective flanks 18A and 18D only make full interfacial contact with the groove 16 in the region of maximum pitch, whereas the flanks 18B and 18C likewise only make full interfacial contact in the region of minimum pitch, the fact that the material of the leadscrew and nut is resilient allows for the spreading of the load over greater areas as the tooth 18 passes along portions of the groove having a pitch the value of which lies between the maximum and minimum values.

It will be appreciated that various alterations and modifications may be made to the illustrated embodiment of the invention without departing from the scope of the appended Claims. Thus, although the teeth 17 and 18 are defined only by four helical flanks pairs of which have the maximum and minimum pitch of the helical groove, a greater number of individual flanks may be provided in order to provide a transition between maximum and minimum pitch.

The cross-section of the helical grooves 15 and 16 may also be varied, with corresponding variation of the configuration of the arcuate teeth 17 and 18.

Additionally, although the embodiment of the leadscrew described has a twin-start thread, single start or multiple start threads may also be contemplated depending upon the particular context in which the device is to be used.

Claims

Claims : 1. A leadscrew and nut device wherein the leadscrew has a thread form comprising a helical groove of which the axial cross-section is substantially constant whilst the helix pitch of the groove varies along the length of the leadscrew, whereas the nut engaging the leadscrew has a male threaded portion extending over a limited arc of rotation with respect to the axis of the leadscrew, the cross-section of said male portion at an intermediate point on said arc corresponding substantially to the axial cross-section of said helical groove, whereas sections of said male portion extending respectively away from said intermediate point have cross-sections that reduce in a direction away from said intermediate point, in such a manner that a first pair of flanks of said male point that extend away from said intermediate point in opposite directions and are arranged to engage opposite surfaces of said groove are angled at a pitch substantially corresponding to the maximum pitch of said helical groove and that a second pair of flanks of said male portion that respectively lie opposite said first pair to engage opposite surfaces of said groove are angled at a pitch substantially corresponding to the minimum pitch of said leadscrew.
2. A leadscrew and nut device as claimed in Claim 1 wherein the leadscrew has a twin-start thread with identical helical grooves relatively displaced by 180 about the axis of the leadscrew, and the nut device correspondingly at two diametrically opposed male portions for engagement with the respective grooves.
3. A device according to Claim 1 or 2 wherein said nut is longitudinally slidably mounted within an extemal housing within which it is held against relative rotation, said leadscrew being mounted for rotation within said housing and being held captive therein by means restraining the leadscrew against axial movement relatively to the housing.
4. A device according to Claim 3, wherein said nut and said housing are adapted to provide anchorages respectively for the inner cable and outer sheath of at least one Bowden cable device, whereby axial movement of said nut upon said leadscrew is effective to apply tension to a cable attached thereto.
5. A device according to Claim 3 or 4, wherein the or each helical groove of said leadscrew terminates at one end thereof within an axial recess permitting initial engagement of said male threaded portion of the nut with said leadscrew, and said axial recess is engaged by a dog of a stub shaft engaging within said outer housing to transmit axial force between the leadscrew and outer housing.
6. A device according to any one of Claims 3 to 5, wherein said leadscrew is retained within said housing by means of a cover arranged for snap engagement with said housing at an end thereof axially opposite to the end engaged by said stub shaft.
7. A device according to Claim 6, wherein said cover comprises a cylindrical bore engaging around said leadscrew and incorporates internally projecting ramp-shaped teeth for snap engagement with a groove of said leadscrew in order to retain the latter within said housing.
8. A device according to any one of Claims 1 to 7, of which the components are formed by injection moulding from synthetics plastics material.
9. A leadscrew and nut device substantially as described herein with reference to the accompanying drawings.