This invention relates to couplings such as, non-exclusively, splined couplings coupling two shafts, and to a guide for such couplings.

Splined couplings, where ridges on one shaft engage corresponding grooves in a bore in another shaft, are well known. However, they suffer from the problem that, when inserting one shaft into the other, the shafts must be correctly aligned in three degrees of freedom: the axes of the two shafts must be aligned linearly and angularly, and the shafts must be rotated so that the ridges on the shaft engage the grooves and not the intervening ridges in the bore. There is therefore a possibility for mis-engagement of the shafts. This is not so much of a problem if a user can see the shafts whilst they are being engaged, but if the shafts are hidden within, for example a piece of machinery, it can be difficult to engage the shafts correctly.

According to a first aspect of the invention, a coupling is provided, comprising a first shaft and a second shaft, the first shaft having an axis and a profile on a profiled end of the first shaft and the second shaft having an axis and a profiled bore being sized so as to be able to receive the profiled end of the first shaft and having a complementary profile complementary to the profile of the first shaft, such that rotational motion may be transferred between the first and second shafts, in which the coupling is provided with a guide mounted on the second shaft, the guide comprising at least one resilient elongate finger, each finger being positioned so as to guide the profile into the complementary profile as the profiled end is inserted into the profiled bore.

Thus, a guide is provided, which is separate to the engaging profiles of the first and second shafts that will accommodate for axial or linear misalignment of the shafts when they are brought together. Given the guide relies upon the use of at least one resilient finger, it can be made fairly simply and cheaply. Furthermore, the user does not need to see the position of the shafts relative to one other, once they are loosely in the correct orientation.

Each finger may be free to resiliently deform at least one of radially relative and tangentially relative to the axis of the second shaft. This allows each finger to exert an elastic spring force in the opposite direction to which it moves; thus, each finger can exert a force on the profile both tangentially (and hence circumferentially, if the shafts are not quite rotationally aligned) and radially (to accommodate for linear misalignment of the shafts). The profile comprises at least one ridge or groove and the complementary profile comprises at least one groove or ridge, each ridge or groove of the profile corresponding to a groove or ridge of the complementary profile. Typically, there will be grooves on the profile corresponding to ridges on the complementary profile, although the opposite situation is possible. As such, the guide can be used with a common splined coupling, in which the grooves and ridges are present around the entire circumference of both the profile and the complementary profile.

In many cases, the profiles of the first and/or the second shaft may be considered as having both ridges and grooves, but here we are only interested how the profiles of the shafts fit together, and therefore it is enough to focus on either the grooves or the ridges of one and the same profile.

Typically, each finger will be arranged to align with a groove or ridge of the profile as the profiled end is inserted into the profiled bore. Here, by, align, we include the cases where the finger is brought into a position parallel with the groove or ridge, as well as the case where the finger is positioned along the same line as the groove or ridge; either or both are possible. A force due to resilient movement of the fingers on insertion of the profiled end into the guide biases each groove or ridge of the profile rotationally about the axis of the first shaft into alignment with a corresponding ridge or groove in the second shaft. This can occur whether each finger is elongate along a direction parallel to the axis of the second shaft, or is skew to the axis of the second shaft.

Where, as in the preferred embodiment, each finger aligns with a groove of the profile, each finger may have a convex inner surface facing the axis of the second shaft. This inner surface may engage the groove as the first shaft is inserted into the second shaft, guiding the profile into engagement with the complementary profile. The convex shape may act to guide each finger into a corresponding groove. Alternatively, where the profile comprises ridges rather than grooves, each finger may align with a corresponding ridge by sliding along the ridge. This will still act to guide the ridge into the correct orientation.

When the fingers are arranged to engage with grooves on the profile, at least one groove and one finger may be required. When, on the other hand, the fingers are arranged to engage with ridges on the profile, at least two fingers and one ridge may be required.

Typically, each ridge and a corresponding groove have equal lengths, such that each ridge occupies an entire groove. However, in some cases each ridge may a length shorter than the length of a corresponding groove. In this case, each ridge may only take up a portion of the corresponding groove. This may be a substantial portion (greater than half), or may only be a small portion, say less than 25%, 10% or 5%. As such, the ridge may be formed as a tooth that engages the corresponding groove. Alternatively, the each ridge may have a length longer than the length of a corresponding groove.

The guide may be mounted over the second shaft by means of an angularly indexed mounting, such that the guide is fixed relative to the second shaft in a particular angular relationship. This allows the guide to be correctly rotationally aligned relative to the complementary profile, so that it can correctly guide the first shaft into the second shaft. The mounting may comprise a ridge on one of the internal surface of the guide and the external surface of the second shaft, and a complementary groove on the other of the internal surface of the guide and the external surface of the second shaft, the ridge and the groove extending parallel to the axis of the second shaft. The parallel extent of the ridge and groove makes the guide resistant to angular movement around the shafts, which could otherwise potentially lead to the guide being improperly aligned and so not bringing the shafts together in the correct angular orientation.

The coupling can be used in any situation where two shafts are to be coupled to transmit rotational movement. For example, it could be used in power tools where it is desired to transmit power from a motor or engine to a tool, possibly an interchangeable tool. Example tools include handheld power tools such as trimmers, brushcutters and the like. Other examples include couplings between motors and pumps or fans.

According to a second aspect of the invention, there is provided a guide for a coupling, comprising at least one resilient elongate finger arranged around an axis and arranged to guide a profiled end of a first shaft of the coupling into a complementarily profiled bore of a second shaft of the coupling.

This aspect therefore provides a guide that can potentially be used as the guide in the first aspect of the invention.

Each finger may be free to resiliently deform at least one of radially relative and tangentially relative to the axis. This allows each finger to exert an elastic spring force in the opposite direction to which it moves; thus, each finger can exert a force on the profile both tangentially (and hence circumferentially, if the shafts are not quite rotationally aligned) and radially (to accommodate for linear misalignment of the shafts).

Each finger may be elongate along a direction parallel to the axis, or along a direction skew to the axis. Where there are a plurality of fingers, all of the fingers may be skew to the axis at the same angle. Such a skew arrangement may allow the fingers to exert a greater torque as the fingers are rotated into a parallel arrangement by engaging the profile. Each finger may have a convex inner surface facing the axis. This inner surface may engage a groove of the first shaft as the first shaft is inserted into the second shaft, guiding the profile into engagement with the complementary profile. The convex shape may act to guide each finger into a corresponding groove. In an alternative embodiment, the fingers may have a flat inner surface facing the axis.

The guide may be provided with an angularly indexed mounting, such that the guide can be fixed relative to the second shaft in a particular angular relationship. This allows the guide to be correctly rotationally aligned relative to the complementary profile, so that it can correctly guide the first shaft into the second shaft. The mounting may comprise a groove on the internal surface of the guide and a ridge on the external surface of the second shaft. The groove will typically extend parallel to the axis of the second shaft. The parallel extent of the ridge and groove makes the guide resistant to angular movement around the shafts, which could otherwise potentially lead to the guide being improperly aligned and so not bringing the shafts together in the correct angular orientation.

A free end of each finger may be bent inwards or outwards in relation to the axis. Again, this can act to guide each finger into engagement with the profile.

There now follows, by way of example only, description of embodiments of the invention, described with reference to the accompanying drawings, in which:

Figure 1 shows a perspective exploded view of a coupling in accordance with an embodiment of the invention;

Figure 2 shows a perspective view of the coupling of Figure 1 , as the shafts are engaged; Figure 3 shows a perspective view of the guide of the coupling of Figure 1 ;

Figure 4 shows the guide of Figure 3 engaging a shaft of the coupling of Figure 1 ; Figure 5 shows an end elevation of the second shaft with a guide according to an alternative embodiment attached, viewed down the bore;

Figure 6 shows a perspective view of the guide of Figure 5 ; Figure 7 shows a perspective view of a guide according to a second alternative embodiment that could be used with the coupling of Figure 1 ; and

Figure 8 shows the guide of Figure 7 engaging a shaft of the coupling of Figure 1. Figures 1 to 4 of the accompanying drawings show a coupling in accordance with a first embodiment of the invention. In this embodiment, a coupling 20 comprises a first shaft 1 having an axis 4a and a second shaft 2 having an axis 4b to be coupled together.

The first shaft 1 carries a profiled section 5a of the form of splines forming alternating ridges and grooves along the length of the first shaft, the splines surrounding the circumference of the end of the first shaft 1. The second shaft 2 has a profiled bore 6 into which the first shaft is receivable. The bore has a complementary profile 5b, which is complementary to the profile 5a of the first shaft. Thus, a standard splined coupling has been defined so far, in which the splines of the profile 5a will engage with corresponding grooves in the complementary profile 5b. As such, when the splined portion of the first shaft 1 is introduced into the profiled bore 6, the profile 5a and the complementary profile 5b will engage, so as to be able to transmit rotational motion of the two shafts 1 , 2 about their axes 4a, 4b.

However, to introduce the first shaft 1 into the second shaft 2, the shafts 1 , 2 must be aligned in two regards. Firstly, their axes 4a, 4b must be aligned both linearly and angularly. Secondly, the rotational position of the shafts must be such that the ridges formed by the splined profile 5a engage with the corresponding grooves in the complementary profile 5b; otherwise, a ridge on the first shaft 1 could collide with a ridge in the complementary profile 5b of the profiled bore, thus making it difficult to insert the first shaft 1 into the second shaft 2.

In order to promote the correct alignment of the two shafts, a guide 3 is provided, mounted on the second shaft to protrude along the axis away from the front face 13 of the bore 6. This guide is of the form of a collar 12 supporting a plurality (here, six) of elongate fingers 7. Each finger extends away from the collar parallel to the axis 4b of the second shaft 2. The internal surface 10 of each finger 7 facing the axis is convex, and the ends of each finger 7 are turned out away from the axis.

Each finger is also elastically resilient, so that if displaced from its neutral position (shown in Figure 3 of the accompanying drawings) either radially or tangentially with respect to axis 4b, it will tend to return to that position and will exert an elastic force against anything preventing it doing so. The fingers 7 are aligned with grooves in the complementary profile 5b.

This means that, when the first shaft 1 is to be inserted into the second shaft 2, the splined portion 14 of the first shaft 1 will first encounter the fingers 7 of the guide 3. The fingers 7 will initially be forced radially outwards away from the axis 4b, but will tend to engage the grooves of the profile 5a, because the grooves are closer to the axis 4b. The convex face 10 of each finger will ease each finger 7 into the ridges, as it will allow each finger to easily slip tangentially.

Thus, with each finger 7 in a groove of the profile 5a, the guide 3 will be acting to correct for the various misalignments described above. Misalignment of the axes 4a, 4b whether linear, angular or rotational will cause an imbalance in the force applied by the fingers. The imbalanced forces will then tend to bias the axes 4a, 4b so that they overlap. Because the fingers are aligned with the ridges in the complementary profile 5b but engage the grooves in the profile 5a and have a neutral position along the axis 4b, the fingers will tend to bias the grooves in the profile 5a rotationally so that the grooves are aligned with the ridges in the complementary profile 5b. This is the correct alignment to allow insertion of the first shaft 1 into the second shaft 2. Experimental tests have shown that use of this guide can reduce the incidence of improper or difficult mating of the shafts from approximately 1 in 3 to approximately 1 in 20.

The guide is mounted on the second shaft by means of an angularly indexed coupling. This comprises a ridge 8 parallel to the axis 4b on the internal surface of the collar 12, which mates with a corresponding groove 1 1 on the end of the second shaft 2. This ensures that the guide 3 is correctly aligned rotationally relative to the second shaft 2. Furthermore, a further corresponding ridge 14 and groove 15 on the same members but running circumferentially around the second shaft 2 acts to secure the guide axially relative to the axis 4b.

An alternative embodiment of the guide 33 is shown in Figures 5 and 6 of the accompanying drawings. Equivalent features to those of the first embodiment are shown with corresponding reference numerals, raised by 30. Rather than the form discussed above, where the fingers 7 extend from the body of the guide along the axis 4b, so that the ends of the fingers 7 distal from the guide body point away from the guide body, the fingers 37 in this embodiment change direction through approximately a half turn, so that the distal ends of the fingers 37 point back towards the guide 33 body. The fingers 37 turn in towards the shaft 1 so that the radially (relative to axis 4b) inner surface 40 of a turned-in portion 41 of the fingers contacts the grooves of the profile 5a. This then provides the same guiding function as with the embodiment of the guide described above with reference to Figures 3 and 4 of the accompanying drawings.

The fingers 37 of this embodiment are still resilient against radial movement relative to the shaft axis 4b, but because of the approximate u-shape of each finger, the desired resilience can be generated in a short distance. Thus, this design can potentially be made smaller (at least axially) than the embodiment of the guide shown in Figures 3 and 4 of the accompanying drawings. Furthermore, it allows for a more robust guide that is less liable to breakage.

A further alternative embodiment of the guide 53 is shown in Figures 7 and 8 of the accompanying drawings. Equivalent features to those of the first embodiment of Figures 3 and 4 of the accompanying drawings are shown with corresponding reference numerals, raised by 50. The guide 53 still comprises elongate fingers 57, but instead of being aligned with the axis 4b in the neutral unloaded position, they are all skew (by the same angle) about the axis 4b. Particularly where the inner surface 60 is flat, the skew fingers 57 will be helpful in creating a torque about the shaft 1 in case of rotational misalignment of the shafts 1 , 2.