GB2033048A

GB2033048A - A flexible worm for a worm gear

Info

Publication number: GB2033048A
Application number: GB7936278A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-10-21
Filing date: 1979-10-19
Publication date: 1980-05-14
Also published as: SE7908637L; DE2845903B2; GB2033048B; DE2845903C3; DE2845903A1

Abstract

A flexible worm for a worm drive comprises a helix which is produced by coiling a strand (11) provided with lateral toothed flanges whose teeth mesh with one another to prevent slipping between adjoining convolutions. To obviate a thrust, tending to urge the convolutions apart, being developed, the tooth flank angles alpha and beta differ and are shown such that the respective tooth flanks (12a and 12b) are at an angle delta no more than about 10 DEG to the surface lines parallel to the axis of the worm. <IMAGE>

Description

SPECIFICATION A flexible worm for a worm gear The invention relates to a flexible worm for a worm drive, comprising a helix which is produced by coiling a strandlike structure, the strandlike structure having at its edges teeth which taper towards their tips and which, in the coiled state of the strandlike structure, mesh with one another to prevent slipping between adjacent convolutions.

A worm of this kind is described in German Patent Specification No. 23 46 550. In such a worm, counter-rotation of the convolutions is impossible either in the sense of a tightening or in the sense of an opening-out of the worm since the form-locking engagment of the teeth in the tooth gaps of the adjacent convolutions prevents mutual displacement between the convolutions. The worm therefore always maintains its diameter and therefore fits with the provided clearance into the teeth of an associated worm wheel so that the damaging effects of jamming are avoided. As a result, a very much longer service life of the worm may be achieved. Flexible worms compared to rigid worms have the advantage that several worm convolutions can be brought fully into engagement with a worm wheel simultaneously.

In the above-mentioned Patent Specification, symmetrical teeth are illustrated which taper toward the tooth tips, namely trapezoidal teeth and teeth having curved flanks. This shape of tooth is advantageous for the coiling of the worm from the strandlike structure since the tooth gaps widen outwards in a funnel-like manner so that the teeth can be easily introduced into the tooth gaps. When the worm has to transmit a torque, the tooth flanks are pressed against one another. Because of their slope, force components are produced which tend to urge the worm convolutions apart. In order to prevent the worm convolutions from being forced apart, these convolutions may have to be axially braced which has, however, an adverse effect upon the flexibility of the worm.

An object of the invention is to construct the teeth of the strandlike structure in such a manner that the forces tending to force the convolutions of the worm apart are reduced or completely eliminated.

In accordance with the invention, a flexible worm for a worm drive comprises a helix which is produced by coiling a strandlike structure, the strandlike structure having at its edges teeth which taper towards their tips and which, in the coiled state of the strandlike structure, mesh with one another to prevent slipping between adjacent convolutions, the two tooth flanks of each tooth being of different steepness and being directed in such a manner that the less steep flanks because of the angle of lead of the worm form with axially directed surface lines of the worm a smaller angle than with planes which extend perpendicular to the longitudinal direction of the strandlike structure.

The coiling of a worm constructed in this manner presents no problem since the teeth also taper outwards so that the tooth gaps widen outwards and teeth can easily be introduced into tooth gaps. The position of the tooth flanks on the finished worm is more favourable than in the case of symmetrical teeth since both tooth flanks only form a small angle with surface lines of the worm. It is also conceivable for a tooth flank to lie parallel to axially directed surface lines. The tendency towards separation of the convolution is therefore slight or non-existent, namely if the angle formed between the tooth flanks and surface lines is equal to or less than the angle of friction, which depends upon the worm material selected and upon the state of lubrication. Slight forces tending to urge the worm convolutions apart can often be allowed for since such forces may be easily absorbed.

Both tooth flanks of each tooth are preferably straight. However, this is not absolutely necessary, i.e. the flanks could alternatively be curved. It is also possible to provide teeth having straight flanks on one edge of the strandlike structure and teeth having curved flanks on the other edge.

The teeth according to the invention may be provided with different cross-sections of the strandlike structure. However, a cross-sectional shape having lateral toothed flanges is particularly advantageous.

Within the framework of the invention various tooth shapes are possible. In the case of flanks perpendicular to the longitudinal direction of the strand like structure on one side of each tooth, this flank forms with surface lines of the worm an angle which is equal to the angle of lead of the helix. This shape will generally be used with worms in which the angle of lead is approximately equal to or less than the angle of friction. If one flank of each tooth is parallel to surface lines, in one direction of rotation of the worm, namely the direction of rotation in which these flanks parallel to surface lines are loaded, no axial forces can arise, not even when there is no friction, which practically never occurs. One flank of each tooth may alternatively be formed by an undercut.By doing this it is also possible, for example, to achieve that the flanks extend parallel to surface lines of the worm. It is also possible (not only with undercut flanks) for the angle formed with the surface lines to be, as it were, negative, i.e. to be so adjusted that under the influence of the torque a contraction of the worm convolutions occurs. The tooth shape may be trapezoidal, no trapezoidal angle being a right angle.

If torques of differing magnitude arise in the two directions of rotation of the worm, the worm is advantageously constructed in such a manner that the position of the flanks is more favourable in the case of the greater torque than in the case of the lesser torque, i.e. in the case of the greater torque the angle which the flanks form with surface lines of the worm is as small as possible, zero, or even negative.

The invention is further described, by way of example, with reference to the drawings, in which: Figure 1 is a side view of a worm gear having a flexible worm, Figure 2 is a section through the gear along the line ll-ll of Fig. 1, Figure 3 is a view of a strandlike structure from which the worm of the worm gear is produced, Figure 4 is a section along line IV-IV of Fig. 3, Figure 5 is a detail developed view of a worm according to the invention, Figure 6 is a larger-scale fragmentary view from Fig. 5 in the region of the frame VI outlined by dash-dot lines in Fig. 5, Figure 7 is a similar view corresponding to Fig. 6 with a different tooth shape, and Figure 8 is a view corresponding to Figs. 6 and 7 with another tooth shape.

The worm drive shown in Figs. 1 and 2 has a housing 1 made up of three superimposed plates 2, 3 and 4 which are attached to one another, for example, by means of screws (not shown). A worm wheel 5 is journalled between the outer plates 2 and 4. The worm wheel is supported on a shaft 6 having a square portion 6awhich engages in a square hole 5a of the worm wheel 5.

The worm wheel 5 is enclosed over part of its periphery by a flexible worm 7. The latter is guided in groovelike recesses 2a and 4a of the plates 2 and 3 and on a surface 3a of the plate 3 and its convolutions engage in tooth gaps 8 of the worm wheel 5.

To secure the worm axially, a collar 9 is provided which is firmly connected to the worm and engages in a window 10 in the housing 1.

The worm 7 is coiled from a strandlike structure 11, such as is illustrated in Figs. 3 and 4. This strandlike structure 11 has the cross-section which may be seen in Fig. 4.

The portion 11 a of the cross-section serves to form the worm spirals while the striplike flanges 11 b and 11 care provided with teeth (yet to be described in detail) which, when the initially straight strandlike structure is wound to form the worm, are brought into mesh with one another. In the case of a single-start worm, therefore, the teeth located on the edge flange 11 b engage in tooth gaps located on the flange 11 C. The finished worm then has the appearance of Fig. 5. The shape of the teeth is now described with reference to Fig. 6.

The teeth 1 2 are trapezoidal and have flanks 12aand 12b. The flanks are of differing slope, flank 1 2 a being steeper than flank 12b. The flank 12a forms with the base 13 of the adjacent tooth gap an angle a of 90 . The angle ss which the other tooth flank 126 formes with the tooth gap base 14 is greater than 90 and may be, for example, 110'.

The dash-dot line 1 5 represents a surface line of the worm, i.e. a line extending parallel to the worm axis. The angle of lead (or pitch angle) is accordingly y. It is assumed that in the case y has the magnitude of 10'. In the above and in the following part of the description an angular division is always spoken of in which a right angle is subdivided into ninety degrees of angle.Because of the pitch y of the worm, the less steep flanks 12bare tilted in such a manner that the angle 8 which these flanks form with surface lines 1 5 have the value - 90 - y, i.e. in the present example 8=10'. The steep flanks 12a, which in fact extend perpendicular to the longitudinal direction of the strand like structure 11, have in this case also a sloping position of 10 relative to surface lines 1 5.

The angle which the less steep flank forms with the plane 22, which extends perpendicular to the longitudinal direction of the strandlike structure 11, has the value 8 - 90'. This angle is greater than the angle 8, since 8=13-90' - y.

When a torque is exerted upon the worm in such a manner that the less steep flanks 1 2b are stressed, i.e. for example in the case of a clockwise rotation of the worm, a specific axial component arises which tends to urge the convolutions apart. In the opposite direction of rotation the steep flanks 12aare stressed and similarly a component arises which tends to urge the convolutions apart.

However, the angle 8 is approximately equal to the angle of friction, so that the convolutions are not spread apart. A fundamental factor of the invention is that the less steep flanks are so adjusted that the angle of lead of the helix aligns this flank, so to speak, towards the surface lines 1 5 of the worm.

In the embodiment of Fig. 7, the teeth are designated 16. The teeth have flanks 1 6a and 1 6 b. The flank 1 6b forms with the base surface 1 7 of the adjacent tooth gap, which extends parallel to the longitudinal direction of the strandlike structure, an angle a which is smaller than 90'. The angle is 90' - y so that in the finished helix the flanks 1 6a extend parallel to surface lines 1 5.

The flank 1 6 b forms with the adjacent tooth gap surface 18 the same angle ss as in the embodiment of Fig. 6. As a result, in the wound worm there is the same angle 8 between the flanks 1 6 a and the surface lines as in the embodiment of Fig. 6.

If the worm is stressed in such a manner that the flanks 1 6a are loaded, no axial com ponent can arise since the flanks extend parallel to surface lines. In the opposite direction of rotation, although conditions are not quite so favourable, there is still no spreading apart of the convolutions so long as the angle 8 is equal to or less than the angle of friction. A worm as in Fig. 7 is particularly advantageous when a substantially greater torque arises in one direction of rotation than in the opposite direction of rotation. The worm will then be constructed in such a manner that the flanks 1 6a are stressed with the greater torque.

In the embodiment of Fig. 8, the teeth are designated 19. These teeth too have a steeper flank 19 a and a less steep flank 19b. The angle which the flank 1 9 a forms with the adjacent tooth gap base 20 is greater than 90 and is designated a". The angle which the less steep tooth flank 1 9 b forms with the adjacent tooth gap 21 is likewise greater than 90 and is designated 1311. ss" is greater than owe".

Here too, the angle of lead y is 10 . The angles ss" and a" are so selected that the angle y at which the flanks 1 9 b are tilted relative to surface lines 1 5 of the worm is approximately equal to or less than the angle of friction. The corresponding angle e for the flanks 1 9a is slightly divergent from 8 but is similarly in the region of the angle of friction or less than it.

All the drawings show a state in which the worm is straightened out. When the worm is curved, the teeth are mostly apread apart to a greater or lesser extent, those teeth at the outside of the curve being spread the farthest apart while only those at the inside of the curve are in full engagement with one another.

Claims

1. A flexible worm for a worm drive comprising a helix which is produced by coiling a strandlike structure, the strandlike structure having at its edges teeth which taper towards their tips and which, in the coiled state of the strandlike structure, mesh with one another to prevent slipping between adjacent convolutions, the two tooth flanks of each tooth being of different steepness, and being directed in such a manner that the less steep flanks because of the angle of lead of the worm form with axially directed surface lines of the worm a smaller angle than with plates which extend perpendicular to the longitudinal direction of the strandlike structure.

2. A flexible worm as claimed in claim 1, in which the two tooth flanks form with the surface lines of the worm angles which are smaller or at least only slightly greater than the angle of friction between the tooth flanks.

3. A flexible worm as claimed in claim 1 or 2, in which both flanks of each tooth are straight.

4. A flexible worm as claimed in claim 1, 2 or 3, in which the strandlike structure has at its edges striplike flanges on which the teeth are located.

5. A flexible worm as claimed in any of claims 1 to 4, in which one flank of each tooth extends perpendicular to the longitudinal direction of the strandlike structure.

6. A flexible worm as claimed in any of claims 1 to 4, in which one flank of each tooth extends parallel to the surface lines of the worm.

7. A flexible worm as claimed in any of claims 1 to 4 or in claim 6, in which one flank of each tooth is formed by an undercut (Fig. 7).

8. A flexible worm as claimed in any of claims 1 to 4, in which both flanks of each tooth form obtuse angles with the base surfaces of the adjoining tooth gaps.

9. A flexible worm constructed substantially as herein described with reference to and as illustrated in the drawings.

10. A worm drive having a flexible worm as claimed in any preceding claim.