Technical Field of the Invention

The present invention relates to ratchets and in particular to a pawl for a ratchet and to a multi-pawl ratchet. Background to the Invention

A ratchet is a mechanical device which is operable to enable continuous motion in a first direction whilst preventing motion in a second direction. In particular, a ratchet comprises a ratchet wheel or ratchet rack provided with a series of asymmetric teeth and a pivotable pawl biased to engage the asymmetric teeth. When the wheel or rack moves in the unrestricted direction, the sloped forward face of the teeth urges the pawl to disengage by pivoting away from the teeth against the bias. When the gearwheel or rack starts to move in the restricted direction, the reverse slope on the rear face of the teeth in combination with the bias urges the pawl to engage with rear face of the teeth and thus lock the ratchet against further movement in the restricted direction.

In order for the ratchet to be effective in restricting movement in the restricted direction, the pawl and the rear face of the tooth must engage successfully and remain engaged. Where there is a failure to engage or where the bias, typically provided by a torsion spring or the like, fails the ratchet will not restrict movement as desired. Accordingly, it is known for ratchets be provided with additional safety mechanisms. One simple safety mechanism is to provide more than one pawl, the pawls being offset with respect to the teeth such that one pawl is always in position to engage thereby. Nevertheless in such arrangements, each pawl effectively works in isolation and thus an increase in reliability is provided by duplication of pawls rather than by improvement in the pawl design.

In an example disclosed in GB2501515 a ratchet is provided with a pair of pawls. Each pawl comprises a rigid member pivoted about its centre and with opposing end portions. A first, engaging end portion is adapted to engage with the ratchet teeth conventionally. The other end portion is a balancing end and is adapted to be pushed outward by the forward face of the gear teeth during motion in the unrestricted direction. The action of pushing the balancing end away from the teeth results in the engaging end being urged towards the gear teeth. Whilst this can help urge the engaging end into engagement with the teeth without additional bias, there is still only a single point of engagement between each pawl and the teeth and thus reliability is ensured by the provision of multiple pawls,

It is therefore an object of the present invention to provide a pawl and a ratchet incorporating such a pawl that at least partially overcomes or alleviates the above problems.

Summary of the Invention

According to a first aspect of the present invention there is provided a dual pawl suitable for use with a toothed ratchet wheel or rack, the dual pawl comprising: a first pawl arm having a base end and an engaging end for engaging with a tooth, the first pawl arm being rotatable about the base end; a second pawl arm having a base end and an engaging end for engaging with a tooth, the second pawl arm being rotatable about the base end; and wherein the first and second pawl arms are rotatable about a common axis and wherein cooperating features of the pawl arms interact with each other such that as one pawl arm disengages from the teeth, the other pawl arm is urged into engagement with the teeth.

The present invention thus provides a single pawl incorporating two engaging ends. This enables such a dual pawl to provide two points of engagement with a ratchet rack or wheel limiting the prospect of failure in use.

The engaging end of each pawl arm may be thicker than the remainder of the pawl arm. A thicker engaging end helps to increase the surface area for engagement with the teeth. In a preferred embodiment, the engaging end is wider than the teeth. This enables full engagement between the engaging end and the teeth even where the there is some misalignment between the teeth and the dual pawl.

The base end of each pawl arm may be rotatably mounted to a common pawl stand. The base end may comprise a bore adapted to be rotatably mounted to the pawl stand. The base end may be thinner than the remainder of the pawl arm. This allows each pawl arm to be mounted to a common pawl stand without being displaced from the teeth.

The cooperating features may comprise mating faces operable to interact with opposing mating faces on the other pawl arm. The mating faces may be provided adjacent to the bore within the base end. In a preferred embodiment one mating face may be provided to each side of the bore on each pawl arm. In such embodiments, the opposing mating faces are preferably arranged such that only one pair of mating faces can engage at any one time. In such embodiments, rotation of one pawl arm in a first direction about the pawl stand will cause the interaction of one pair of mating faces and thus the rotation of the other pawl arm in the same direction about the pawl stand. This enables disengagement of one pawl arm to urge the other pawl arm into engagement with the teeth.

The pawl arms may be provided with bias means. The bias means may comprise a spring. In a preferred embodiment, the bias means may comprise a torsion spring. The torsion spring may be mounted around the pawl stand. Each arm of the torsion spring may be adapted to act on a different pawl arm. The bias means thus act to move the engaging ends of each the pawl arms towards the teeth. This provides an additional mechanical urge for the pawl arms to engage with the teeth.

According to a second aspect of the present invention there is provided a ratchet comprising: a wheel or rack provided with a plurality of asymmetrical teeth and rotatable about a central axis; and at least one dual pawl according to the first aspect of the present invention.

The ratchet of the second aspect of the present invention may comprise any or all features of the dual pawl of the first aspect of the present invention. In a preferred embodiment, the rack or wheel may be provided with two dual pawls. In the case of a wheel, the two dual pawls are preferably provided on opposing sides of the wheel.

According to a third aspect of the present invention there is provided a hoist comprising a ratchet according to the second aspect of the present invention. The hoist of the third aspect of the present invention may comprise any or all features of the first two aspects of the present invention.

Detailed Description of the Invention In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 shows a ratchet according to the present invention incorporating two dual pawls according to the present invention;

Figure 2 is a side view of the ratchet of figure 1 in a with the engaging ends of a first pawl arm of each dual pawl engaging with the ratchet teeth;

Figure 3 is a side view of the ratchet of figure 1 in a with the engaging ends of a second pawl arm of each dual pawl engaging with the ratchet teeth; Figure 4 provides a series of views of the first and second pawl arms showing the respective mating faces;

Figure 5 provides a series of views illustrating the interaction between the mating faces and how this results in the engaging ends of the respective pawl arms alternatively engaging and disengaging with the ratchet teeth; and

Figure 6 shows (a) the ratchet mounted on a grooved pinion shaft, (b) a view of the disc hub of the ratchet, and (c) a view of the grooved pinion shaft.

Turning now to figure 1, a ratchet 1 comprises a ratchet wheel 2 having a plurality of asymmetric teeth 3 around its outer rim. Each tooth 3 has a shallow sloped forward face 4 and a steeply sloped reverse face 5. The ratchet wheel 2 locates onto a disc hub 6. The inner rim of the disc hub 6 is provided with a plurality of symmetric teeth 7 for engaging with a matching grooves 17 of a grooved pinion shaft 18 (shown in figure 6).

The ratchet 1 is operable to allow rotation of the shaft in a desired direction (clockwise in the example shown) and so as to restrict rotation of the shaft in the opposing direction (anti-clockwise). This is achieved by interaction between the asymmetric teeth 3 and a pair of dual pawls 8 according to the present invention. The skilled man will appreciate that whilst the present example utilises a pair of dual pawls according to the present invention, it is possible for a dual pawl according to the present invention to be implemented without a second dual pawl. Each dual pawl 8 is mounted on a pawl stand 9 and comprises a first pawl arm

10 and a second pawl arm 20. The first pawl arm 10 has a base end 11 rotatably mounted to the pawl stand 9, an engaging end 12 and, a body portion 13 linking the base end 11 to the engaging end 12. The second pawl arm 20 has a base end 21 rotatably mounted to the pawl stand 9, an engaging end 22, and a body portion 23 linking the base end 21 to the engaging end 22.

Additionally, a torsion spring 30 is mounted on the pawl stand 9. The respective arms 31, 32 of the torsion spring 30 are positioned so as to urge the respective pawl arms 10, 20 together.

Each dual pawl 8 is arranged such that the respective engaging ends 12, 22 are offset with respect to the teeth 3 of the ratchet wheel 2. As such, whilst the engaging ends 12 of the first pawl arms 10 are engaged with the rear face 5 of teeth 3, the engaging ends 22 of the second pawl arms 20 are disengaged from teeth 3. This configuration is illustrated in figure 2. Similarly, whilst the engaging ends 22 of the second pawl arms 20 are engaged with the rear face 5 of teeth 3, the engaging ends 12 of the first pawl arms 10 are disengaged from teeth 3. This configuration is illustrated in figure 3.

In use, the pawl arms 10, 20 are arranged such that as the engaging end 12 of the first pawl arm 10 disengages from the teeth 3, the engaging end 22 of the second pawl arm 20 is urged into engagement with the teeth 3 and such that as the engaging end 22 of the second pawl arm 20 disengages from the teeth 3, the engaging end 12 of the first pawl arm 10 is urged into engagement with the teeth 3. This operation and the cooperating features that allow this operation to be achieved are described further below with relation to figures 4 and 5.

Turing now to figure 4, an exploded view of the first and second pawl arms 10, 20 of a dual pawl 8 is shown in figure 4a. Each base end 11, 21 comprises a bore 14, 24 adapted to be mounted to and rotate around the pawl stand 9. Adjacent to the bore 14, the base end 1 1 comprises mating faces 15, 16. Adjacent to the bore 24, the base end 21 comprises mating faces 25, 26. The mating faces 15, 16, 25, 26 are highlighted in figure 4b.

Figure 4 also illustrates how the base ends 11, 22 are thinner than the body portions 13, 23 of the respective pawl arms 10, 20. This allows the pawl arms 10, 20 to fit around the pawl stand 9 such that as the pawl arms 10, 20 rotate with respect to one another the respective pairs of mating faces 15, 25 and 16, 26 can interact with each other. Figure 4 further illustrates that the engaging end 12, 22 of each pawl arm 10, 20 is thicker than the body portion 13, 23 of each pawl arm 10, 20. This helps compensate for any misalignment between the teeth 3 and the engaging ends 12, 22.

Turning now to figure 5a, an expanded view of the dual pawl 8 is illustrated with the pawl arms 10, 20 in transition. As the wheel 2 rotates clockwise interaction with the front face 4 of a tooth 3 urges the engaging end 22 of pawl arm 20 to pivot away from the wheel 2 as illustrated by arrow 101. This motion causes mating face 26 on the second pawl arm 20 to separate from mating face 16 on the first pawl arm 10 as is illustrated by arrows 102. In addition this motion cases mating face 25 of the second pawl arm 20 to come into contact with mating face 15 of the first pawl arm 10 as is illustrated by arrows 103. This contact interaction urges the first pawl arm 10 to move the engaging end 12 into engagement with the teeth 3. This movement is aided by the action of the torsion spring 30 (not shown in figure 5).

Ultimately, as is illustrated by figure 5b, as clockwise rotation of the wheel 2 continues, the engaging end 12 of first pawl arm 10 moves 104 into full engagement between a pair of the teeth 3. Alongside this motion, mating face 15 on the first pawl arm 10 separates from mating face 25 on the second pawl arm 20. Similarly, mating face 15 of the first pawl arm 10 comes into contact with mating face 25 of the second pawl arm 20. This interaction helps push the second pawl arm 20 to full disengagement from the teeth 3 against the bias of the torsion spring 30 (not shown in figure 5).

Turning to figure 5c, as clockwise rotation of the wheel 2 continues interaction with the front face 4 of a tooth 3 urges the engaging end 12 of pawl arm 10 to pivot away from the wheel 2 as illustrated by arrow 105. This motion causes mating face 16 on the first pawl arm 10 to separate from mating face 26 on the second pawl arm 20 as is illustrated by arrows 106. In addition this motion cases mating face 15 of the first pawl arm to come into contact with 10 mating face 25 of the second pawl arm 20 as is illustrated by arrows 107. This contact interaction urges the second pawl arm 20 to move the engaging end 22 into engagement with the teeth 3. This movement is aided by the action of the torsion spring 30 (not shown in figure 5).

Ultimately, as is illustrated by figure 5d, as clockwise rotation of the wheel 2 continues, the engaging end 22 of second pawl arm 20 moves 108 into full engagement between a pair of the teeth 3. Alongside this motion, mating face 25 on the second pawl arm 20 separates from mating face 15 on the first pawl arm 10. Similarly, mating face 25 of the second pawl arm 20 comes into contact with mating face 15 of the first pawl arm 10. This interaction helps push the first pawl arm 10 to full disengagement from the teeth 3 against the bias of the torsion spring 30 (not shown in figure 5).

Subsequent continued clockwise motion of the wheel 2 urges the engaging end 22 of pawl arm 20 to pivot away from the wheel 2 as described in relation to figure 5a at the start of the sequence.

In the event that an anticlockwise rotation is applied to the wheel 2 either the engaging end 12 of the first pawl arm 10 or the engaging end 22 of the second pawl arm 20 will engage with the rear face of a tooth 3 to restrict anticlockwise rotation. The engaging end 12, 22 that engages with the tooth 3 is determined by the position within the above sequence. In the position where the first pawl arm 10 is fully engaged (figure 5b) the first engaging end 12 will engage. In the position where the second pawl arm 20 is fully engaged (figure 5d) the second engaging end 22 will engage. In the intermediate transition position of figure 5a, the first engaging end 12 will be urged to remain in contact with the front face 4 of the present tooth 3 until anticlockwise rotation of the wheel 2 brings it into engagement with the rear face 5 of the adjacent tooth 3. In the intermediate transition position of figure 5c, the first engaging end 12 will be urged to remain in contact with the front face 4 of the present tooth 3 until anticlockwise rotation of the wheel 2 brings it into engagement with the rear face 5 of the adjacent tooth 3. The above embodiment is described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.