GB2295878A - One-way gear trains - Google Patents

Abstract

A gear train comprising two gears 10, 12 having intermeshing teeth T1, T2 is characterized in that said teeth are configured such that rotation or movement of said gears is permitted in one direction 24, 22 but prevented in the reverse direction 20, 26. As shown the teeth have 8 hook configuration which interengage to prevent reverse rotation. In an alternative embodiment one of the two gears may be linear rather than rotary. <IMAGE>

Description

GEAR TRAINS This invention relates to gear trains, that is to say to assemblies comprising gears having intermeshing teeth.

It is often desired, in gear trains, to have the facility that a gear can be moved (usually rotated) in one only of the two possible directions. Conventionally, this is achieved by the use of a ratchet arrangement comprising a toothed wheel and at least one pawl resiliently biased against the teeth of the wheel, the teeth being formed differentially such that they engage or lock with the pawl when the wheel is rotated in one direction, but not in the other.

A ratchet arrangement has the following disadvantages.

(a)In the direction in which rotation is possible, friction arises from the pressure of the pawl or pawls against the teeth.

(b)The ratchet arrangement comprises at least two moving parts (the toothed wheel and the at least one pawl); as well as a spring or equivalent to resiliently bias the pawl or pawls against the teeth.

(c)In the direction in which rotation is possible, the (or cach) pawl is displaced by the teeth, as a result of which the arrangement is inherently noisy.

According to the invention there is provided a gear train comprising two gears having intermeshing teeth, characterised in that those faces of the teeth of both gears that are urged together when one of the gears is driven in a first direction are configured so as to interengage when said one gear is driven in said one direction so as to stop relative movement of the gears.

If implemented in a form in which both the two gears are gear wheels and used in substitution for a conventional ratchet arrangement to provide a similar effect, namely the prevention of rotation in one sense, a gear train embodying the invention need, like the ratchet arrangement, have only at least two moving parts, namely the two gears: the number of moving parts of a system incorporating a gear train embodying the invention, instead of a conventional ratchet arrangement, is thus not increased. Further, with such a gear train embodying the invention there is the advantageous effect that a spring or the equivalent is not needed. Moreover, with such a gear train embodying the invention, the absence of a pawl leads to the advantageous effect that, in the sense in which rotation is possible, no friction arises from the pressure of a pawl or pawl against teeth.Further, the absence of a pawl also leads to the advantageous effect that, in the sense in which rotation is possible, no noise is generated by displacement of a pawl by teeth. In the sense in which relative rotation is possible, the gears can be designed to have as low a coefficient of friction as conventional gears and to be as silent as conventional gears.

Further, in cases where a gear train is to be provided with a ratchet effect, i.e.

it is desired that rotation in one sense be prevented, a gear train embodying the invention and implemented such that both the two gears are gear wheels enables the combination of a conventional gear train and a conventional ratchet arrangement to be replaced by a gear train only, in that a gear train embodying the invention has, in effect, a "built-in" ratchet feature. That is, in this case, the further advantageous effect arises that the number of components can be reduced by elimination of a separate ratchet arrangement.

Gear trains embodying the invention may be used in a variety of applications.

Possible applications include low cnd audio or video tape drive mechanism and camcorders.

The invention will now be further described, by way of illustrative and nonlimiting example, with reference to the accompanying drawings, in which: Fig. 1 shows a gear train embodying the invention; Figs. 2 to 4 are views on an enlarged scale of a region of the gear train of Fig.

1 at which teeth of respective gear wheels of the train intermesh, the three figures showing three respective positions of the gear train when one of the gear wheels is driven in a direction that causes faces of the teeth of the gear wheels to interengage to stop relative rotation of the gear wheels; Fig. 5 is an enlarged view of a part of Fig. 4 where a free edge of a projection of a tooth of one gear wheel contacts the underside of a projection of a tooth of the other gear wheel; Fig. 6 is a view corresponding to Fig. 5 and showing a modification in which the angles of the undersides of the teeth projections are changed with respect to Fig.

5; Figs. 7 and 8 are part views, corresponding to Figs. 5 and 6, of further modifications that use respective different hook configurations; Fig. 9 shows a gear train embodying the invention that is similar to that of Fig.

1, but has more than two gear wheels; and Fig. 10 is a view corresponding to Fig. 5 and showing a modification in which the interengagement of faces of the teeth of the gear wheels that stops relative rotation of the gear wheels is achieved without those faces being provided with hook configurations.

Fig. 1 shows a gear train that comprises two gear wheels 10, 12 having respective teeth T1, T2 which intermesh with each other in conventional manner.

In the illustrated arrangement, the number of the teeth T1 is equal to the number of the teeth Ti and the gear wheels 10, 12 are of equal diameter, whereby the gear wheels sere merely to transmit rotary motion. However, in a manner well known peruse, the numbers of the teeth T1, Ti may be unequal, as may also the diameters of the gear wheels 10, 12, to enable a mechanical advantage/velocity ratio (speed change) to be achieved.

Each of the teeth T1, Ti has a pair of opposed faces 14, 16. The tooth faces 14, shown in Fig. 1 as being rectilinear and extending substantially radially of their respective gear wheels from the viewpoint of Fig. 1, may in fact be so configured, though they may be instead be configured in any other suitable manner known to those skilled in the art. However, the tooth faces 16 are manufactured so as to be configured differently. Specifically, by virtue of each of the teeth T1, T2 having at its tip a projection 18 extending tangentially away from the face 16, the face 16 is formed to have a hook configuration.

If the gear wheel 10 is driven in the direction of an arrow 20 (or the gear wheel 12 is driven in the direction of an arrow 22) the faces 14 of the teeth of the driven wheel are urged against the faces 14 of the teeth of the other wheel and the rotary motion of the driven wheel is transmitted to the other wheel in conventional manner.

If the gear wheel 10 is driven in the direction of an arrow 24 (or the gear wheel 12 is driven in the direction of an arrow 26) the faces 16 of the teeth of the driven wheel are urged against the faces 16 of the teeth of the other wheel. In this event, possibly after a slight amount of rotation, the hook configuration of the face 16 of one of the teeth on one of the gear wheels will interengage and latch with the hook configuration of the face 16 of one of the teeth on the other of the gear wheels to stop rotation. Thus, the present gear train will operate in one direction only in response to driving of either one of the gear wheels 10, 12.

The way in which the latching of the hook configurations of the faces 16 of the teeth T1, T2 of the gear wheels 10, 12 acts to stop rotation in one direction will now be described in more detail with reference to Figs. 2 to 4, which show three respective positions of the gear train when one of the gear wheels is driven in that one direction, namely the direction that causes the teeth of the gear wheels to interlock.

Fig. 2 shows the gear train at rest, prior to a torque being applied to the gear wheel 12 in the direction of the arrow 26. One of the teeth Ti of the gear wheel 12, which tooth is referenced Tia in Fig. 2, is disposed between two of the teeth T1 of the gear wheel 10, which teeth are referenced Tla and Tlb in Fig. 2. (Fig. 2 shows, for the sake of clarity of explanation, a large clearance between the teeth 12a and T1, though this is not essential.) When a torque is applied to the gear wheel 12 in the direction of the arrow 26, the wheel 12 starts to rotate in the direction of the arrow 26. Due to the clearance between the tooth T2a and the teeth Tla and Tlb, the gear wheel 10 initially is not driven.After a small extent of rotation of the gear wheel 12 in the direction of the arrow 26, the nose 28 of the projection 18 of the tooth Tia contacts the face 16 (more specifically a part 30 of that face defined by the undercut portion of the tooth beneath the projection 18) of the tooth Tla: see Fig. 3. The gear wheel 10 is thereupon driven by the gear wheel 12 and, as rotation continues, the nose 28 of the projection 18 of the tooth T2a slides up the part 30 of the face 16 of the tooth Tla. Very soon, the underside 32 (specifically, in the illustrated arrangement, the free edge 36 thereof) of the projection 18 of the tooth Tia latches against the underside 34 of the projection 18 of the tooth Tla: see Fig. 4. The torque still being applied to the gear wheel 12 causes a force to be applied along a line defined by the free edge 36 of the underside 32 of the projection 18 of the tooth Tia to the underside 34 of the projection 18 of the tooth Tla. Since any further movement would involve extension of one of the teeth Tia and Tla, and since the teeth are substantially inextensible in that the gear wheels are made of a substantially rigid material, further movement is prevented.

That is, the gear train locks up: further movement of the gear wheel 12 in the direction of the arrow 26 (and further movement of the gear wheel 10 in the direction of the arrow 20 (Fig. 1)) is stopped.

Fig. 5 is an enlarged view of part of Fig. 4, specifically the part showing where the free edge 36 of the underside 32 of the projection 18 of the tooth Tia contacts the underside 34 of the projection 18 of the tooth Tla. It can be seen from Fig. 5 that, in the illustrated arrangement, sure latching is provided in that further rotation cannot be achieved without deformation of the projection 18 of the tooth Tla. This is due to the fact that the angle of the underside 34 of the projection 18 of the tooth Tla is such that the locus of movement of the free edge 36 in the event of further movement would necessitate penetration of the free edge into the projection 18 of the tooth na.

That is, "form locking" engagement of the teeth Tia, Tla is achieved.

Even more sure latching could be provided if the angle of the underside 34 of the projection 18 of the tooth Tla were increased in the direction of an arrow 38 in Fig. 5. (Correspondingly, since the teeth of the two gear wheels 10,12 are desirably symmetrical, the angle of the underside 32 of the projection 18 of the tooth Tia would preferably be increased to the same extent.) It will be appreciated from a study of Fig. 5 that, if the angles of the undersides 32, 34 of the projections 18 of the teeth Tia and Tla were increased by half the angle between those two undersides in Fig. 5, the two undersides would, in the latching position, be flush with one another, instead of being in line contact at the free edge 36, which would provide very secure latching.

Such an arrangement is shown in Fig. 6.

Conversely, if the angle of the underside 34 of the projection 18 of the tooth Tla were changed in the direction opposite to the arrow 38 in Fig. 5, there would come a point at which the force applied by the free edge 36 to the underside 34 of the projection of the tooth Tla would be normal to the underside 34. In that event, the "form locking" latching effect provided by the fact that further rotation cannot be achieved without deformation of the projection 18 of the tooth Tla would no longer be provided: the locus of further movement of the free edge 36 would be substantially along the underside 34 of the projection 18 of the tooth Tla. In that limit case, the free edge 36 could slide along the underside 34 of the projection 18 of the tooth Tla till it slid off the underside 34, whereupon further rotation would be possible.In fact, in this case, latching would still be provided, but in this case it would be provided by "friction locking", as opposed to form locking, in that sliding of the free edge 36 along the underside 34 of the projection 18 of the tooth Tla would be opposed by friction.

However, if the angle of the underside 34 of the projection 18 of the tooth Tia were further changed in the direction opposite to the arrow 38 in Fig. 5, there would eventually come a point at which the friction locking would not operate and no latching against further rotation would be provided.

Implementation of the hook configurations of the faces 16 of the teeth T1, T2 of the gear wheels 10, 12 is not restricted to that form described above, in which each configuration is defined in manufacture by a simple projection 18 extending like a step from the remainder of the face 16. Various other forms of hook configuration may be employed, two of which are shown in Figs. 7 and 8, which are part views corresponding to Figs. 5 and 6. Fig. 7 shows an arrangement in which the projections 18 are curved rather than angular. Fig. 7 shows an arrangement in which the projections 18 have stepped rather than angular undersides 32, 34.

Fig. 9 shows a gear train similar to that described above, but having more than two gear wheels. Specifically, Fig. 9 shows a freewheel mechanism comprising four gear wheels, namely two intermeshed gear wheels 10,12 as described above and two further gear wheels 40 and 42. The further gear wheel 40 has teeth like those of the gear wheels 10 and 12 and, as shown, has its teeth intermeshed with those of the gear wheel 12. The gear wheel 4 has internal teeth of conventional form which, as shown, are intermeshed with the teeth of the gear wheels 10 and 40. The gear wheel 12 is driven. The gear wheels 10 and 40 are idlers and are mounted so that their axes can rotate around the common axis of the gear wheels 12 and 42.That is, the gear wheels 10, 40 act as planetary or orbital gears whose axes orbit the axis of rotation of the gear wheel 12.

When the gear wheel 12 is driven to rotate in a clockwise direction as viewed in Fig. 9, both the gear wheels 10 and 40 will lock with the gear wheel 12. The three gear wheels 10, 12 and 40 will thus rotate together as an unit, and act like a single gear wheel to transmit torque applied to the gear wheel 12 to the gear wheel 42 and therefore rotate the gear wheel 42. However, when the gear wheel 12 is driven to rotate in an anti-clockwise direction as viewed in Fig. 9, the gear wheels 10 and 12 will be rotated in clockwise and anti-clockwise directions, respectively, and will cause no transmission of torque to the gear wheel 42.

As so far described, the gear train of Fig. 9 acts like a conventional bicycle freewheel mechanism. However, it is to be noted that, if the gear wheel 42 is driven to rotate in a clockwise direction, then the consequent driving of the gear wheels 10 and 12 will be such as to lock them with the gear wheel 12 and torque will be transmitted from the gear wheel 42 to the gear wheel 12. This is exactly the opposite of a conventional bicycle freewheel mechanism having a simple ratchet arrangement.

In the gear trains described hereinabove, the form locking or friction locking interengagement of the faces 16 of the teeth T1, T2 realised when one of the gear wheels is rotated in one direction is achieved by virtue of the faces 16 being provided with hook configurations. However, it is to be appreciated that such interengagcmcnt can be achieved without the faces 14 being provided with hook configurations. An example of such an arrangement will now be described with reference to Fig. 10, the showing of which corresponds to Fig. 5 in that it shows the position in which the tooth T2a of the gear wheel 12 has been driven so that its face 16 engages with the face 16 of the tooth Tla of the gear wheel 10.

In the arrangement of Fig. 10, the faces 16 are each (at least where they interengage) manufactured so as to be planar and inclined at an angle 0 to a tangent of the respective gear wheel. In this position, the faces 16 are in line contact along the free edge 36 of the tooth T2a. The situation is thus similar to that described above with reference to Fig. 5.The torque still being applied to the gear wheel 12 causes a force to be applied by the free edge 36 of the tooth T2a to the face 16 of the tooth Tla. Since any further movement would involve extension of one of the teeth Tia and Tla, and since the teeth are substantially inextensible in that the gear wheels are made of a substantially rigid material, further movement is prevented. That is, the gear train locks up: further movement of the gear wheel 12 in the direction of the arrow 26 (and further movement of the gear wheel 10 in the direction of the arrow 20 (Fig. 1)) is stopped. Sure latching is provided in that further rotation cannot be achieved without deformation of the tooth Tla.This is due to the fact that the angle o of at least that part of the face 16 of the tooth Tla contacted by the free edge 36 of the tooth Tia is such that the locus of movement of the free edge in the event of further movement would necessitate penetration of the free edge into the tooth Tla.

That is, "form locking" engagement of the teeth ra, Tla is achieved.

Even more sure latching could be provided if the angle e of at least that part of the face 16 of the tooth Tla contacted by the free edge of the tooth T2a were decreased.

Conversely, if the angle 8 of at least that part of the face 16 of the tooth Tla contacted by the free edge of the tooth tea were increased, there would come a point at which the force applied by the free edge 36 to the face 16 of the tooth Tla would be normal to that face. In that event, the "form locking" latching effect provided by the fact that further rotation cannot be achieved without deformation of the tooth Tla would no longer be provided: the locus of further movement of the free edge 36 would be substantially along the face 16 of the tooth Tla. In that limit case, the free edge 36 could slide along the face 16 of the tooth Tla till it slid off the face 16, whereupon further rotation would be possible. In fact, in this case, latching would still be provided, but in this case it would be provided by "friction locking", as opposed to form locking, in that sliding of the free edge 36 along the face 16 of the tooth Tla would be opposed by friction. However, if the angle 0 of at least that part of the face 16 of the tooth Tla contacted by the free edge of the tooth Tia were further increased, there would eventually come a point at which the friction locking would not operate and no latching against further rotation would be provided.

The invention can of course be embodied in other ways than those described above by way of example. For instance, at least one of the gears may be linear rather than rotary. That is, the invention could be embodied as a rack and penion arrangement having one gear wheel and one linear gear.

Claims (10)

1. A gear train comprising two gears having intermeshing teeth, characterised in that those faces of the teeth of both gears that are urged together when one of the gears is driven in a first direction are configured so as to interengage when said one gear is driven in said one direction so as to stop relative movement of the gears.

2. A gear train according to claim 1, wherein said faces of the teeth of both gears are configured such that said engagement is of a form locking nature in that relative movement of the gears is stopped by virtue of the fact that further movement of said one gear in said one direction relative to the other of the gears would necessitate deformation of the teeth.

3. A gear train according to claim 1, wherein said faces of the teeth of both gears are configured such that said engagement is of a friction locking nature in that relative movement of the gears is stopped by virtue of the fact that further movement of said one gear in said one direction relative to the other of the gears is resisted by friction where the teeth are interengaged.

4. A gear train according to claim 1, claim 2 or claim 3, wherein said faces of the teeth of both gears are provided with hook configurations that latch together when said one gear is driven in said one direction so as thereby to stop relative movement of the gears.

5. A gear train according to claim 4, wherein said hook configuration of each tooth is defined by a projection extending from said face of the tooth, said latching being provided by interengagement of an underside of a projection of a tooth of one of the gears with an underside of a projection of a tooth of the other gear.

6. A gear train according to any one of the preceding claims, comprising a third gear having teeth intermeshed with the teeth of said one gear, wherein faces of the teeth of the third gear against which said faces of the teeth of said one gear are urged when said one gear is driven in said one direction are configured so as to interengage with said faces of the teeth of said one gear when said one gear is driven in said one direction so as to stop relative movement of said one gear and said third gear.

7. A gear train according to claim 6, wherein said two gears and said third gear are all gear wheels, and including a fourth gear wheel having internal teeth intermeshed with the teeth of the other of said two gear wheels and with the teeth of said third gear wheel, whereby rotation of said one gear wheel in said one direction will result in transmission of torque to said fourth gear wheel by virtue of the other three gear wheels, which are unable to rotate relative to one another and thus rotate as a unit.

8. A gear train according to any one of claims 1 to 6, wherein both of said two gears are gear wheels.

9. A gear train according to any one of claims 1 to 6, wherein one of said two gears is a gear wheel and the other of said two gears is a linear gear.

10. A gear train substantially as herein described with reference to Fig. 1, Figs. 1 to 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9 or Fig. 10 of the accompanying drawings.