GB2379967A

GB2379967A - Shaft coupling for plastics pinion

Info

Publication number: GB2379967A
Application number: GB0122989A
Authority: GB
Inventors: Lucux Law
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 2001-09-25
Filing date: 2001-09-25
Publication date: 2003-03-26
Also published as: GB0122989D0

Abstract

A miniature permanent magnet direct current motor (10 Fig. 1) has an output shaft 22 arranged to drive a helically toothed plastics pinion (24) pressed onto the shaft. To increase pinion pull off force, the shaft has V shaped projections 28 on its surface, apex towards the free end. As the pinion is pressed onto the shaft the projections bite into and cut a narrow groove in the inside of a hole in the pinion. On removing the pinion material will be gathered into the belly of the V thereby resisting removal. The protrusions can be axially and/or circumferentially spaced on the shaft and are formed by forging, rolling or knurling.

Description

Electric Motor This invention relates to electric motors and in particular, to miniature electric motors having a plastic pinion.

Small electric motors are used to drive a multitude of different devices. Generally, a pinion is pressed onto the output shaft of the motor and used to drive the appliance through one or more gears meshed with the pinion. There is a tendency to use smaller motors. As the motors become smaller, so does the diameter of the output shaft and the pinion fitted thereon. The pinions were often made of brass but to reduce costs and noise, plastic pinions are becoming increasingly popular. However, the grip between the plastic pinion and the motor shaft is not as strong as with the brass pinion due to the nature of the material and the tendency for the plastic pinion to split if the interference between the shaft and the pinion is too great. This problem is exacerbated as the pinion and shaft diameter get smaller, due in part to the percentage increase in the size of the manufacturing tolerances required.

To overcome this problem, it is known to add axial knurling to the shaft. The knurling bites into the plastic as the pinion is pressed onto the shaft and prevents the pinion from rotating on the shaft. However, as the pinion is pressed onto the shaft axially, and the knurls extend axially, the knurls do not increase the force required to remove the pinion axially. This is a particular problem for pinions with helically cut teeth.

Adding adhesive to the shaft and pinion may improve the grip between the pinion and the shaft but it is messy to apply and increases the cost of the motor.

Therefore, there exists a need for a simple procedure to increase the grip between the pinion and the shaft in both the axial and circumferential directions so that the pinion will not slip or move along the shaft.

Accordingly, the present invention provides an electric motor as defined in Claim 1.

Preferred features of the invention are set out in the accompanying dependent claims 2 to 9.

One preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a side view of an electric motor having a pinion fitted in accordance with the present invention; Figure 2 is an enlarged view showing in detail a part of an output shaft of the motor of Figure 1 ; Figure 3 is an end view of the shaft of Figure 2; and Figure 4 is a view similar to Figure 2 of an alternative shaft.

Detailed Description of the Preferred Embodiment Figure 1 depicts a miniature PMDC motor 10 to illustrate the invention. The motor 10 has a housing 12 of the deep drawn cup shape type with an open end which is closed by a plastic end cap 14. The end cap supports motor terminals 16, brush gear (not shown) and a bearing 18. The housing 12 also supports a bearing 20 as well as the permanent magnet stator (not shown). The housing accommodates a wound rotor (not shown) which is mounted on a shaft 22 which is journalled in the bearings. The motor described thus far is conventional and the invention is not limited to any particular type of or construction of the motor.

Figure 1 also shows a pinion 24 fitted to an end of the motor shaft 22 extending from the housing. The pinion 24 is of plastics material and has a central through hole into which the shaft is 22 pressed. The pinion 24 also has a plurality of helical gear teeth 26 formed on its radially outer surface.

Figure 2 shows part of the exposed portion of the motor shaft 22 of Figure 1 without the pinion. The shaft has a V-shaped protrusion 28 on its surface in the area of the shaft 22 which is to be in contact with the pinion 24. The V is aligned axially and points in the direction away from the housing 12. Thus it points towards the direction from where the pinion 24 will be pressed onto the shaft 22.

As the pinion 24 is pressed onto the shaft 22, it engages the protrusion 28. As with an arrowhead, the protrusion 28 offers little resistance to the pinion 24, and cuts only a small groove in the inner surface of the hole in the pinion 24, the material of the pinion being partly resiliently deformed or pushed aside by the protrusion 28.

When the pinion 24 is pulled in the opposite direction as when the pinion 24 is to be removed from the shaft 22, the V-shaped protrusion 28 presents a wide obstacle and the deforming material is gathered into the belly of the V, requiring a large or wide groove to be gouged into the surface of the hole in the pinion, thereby significantly increasing the force required to remove the pinion 24 compared to the force required to fit the pinion 24.

The number of V-shaped protrusions 28 may be varied according to circumstances such as force required, diameter of the shaft 22, axial length of pinion 24, etc. There may be a single protrusion, two axially spaced circumferentially aligned protrusions, two, three or more circumferentially spaced axially aligned protrusions and combinations of the above.

Figure 3 depicts an end view of an output shaft 22 similar to the shaft of Figure 2 except that there are three V-shaped projections 28 equally spaced circumferentially about the shaft 22.

Figure 4 depicts a view very similar to Figure 2 where the output shaft 22 has two Vshaped projections 28 axially spaced along the shaft 22.

Varying the angle of the V will vary the push on and pull off forces as well. The wider the V, the greater the force required. If the V is too narrow, then the desired increase in pull off force may not be achieved.

The example shows a 200 angle between the arms of the V but satisfactory results are expected for angles in the range of 100 through 450 or even 90 .

The formation of the protrusion 28 could be formed by forging or shaping but the preferred method is to use a knurling or rolling technique. This is particularly effective for multiple circumferentially spaced protrusions.

As an example, on a 2.0 mm diameter shaft, the V-shape protrusion may be 2.0 mm long with a height of 0.06 mm and located at 2.5 mm from the end of the shaft, with a 200 angle between the arms of the V.

The embodiment described above is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims

Claims 1. An electric motor having an output shaft ; a plastics material pinion fitted to an end of the output shaft; wherein the output shaft has at least one V-shaped protrusion formed thereon and the protrusion is engaged with the pinion.
2. A motor according to claim 1, wherein the protrusion is aligned with the axis and points to the nearest end of the shaft.
3. A motor according to claim 1 or 2, wherein the protrusion has an included angle A where 5 < = A < = 90 .
4. A motor according to claim 3, wherein 10 < = A < = 450.
5. A motor according to claim 3, wherein the angle A is about 200.
6. A motor according to any one of the preceding claims, wherein there are two or more protrusions circumferentially spaced about the shaft.
7. A motor according to any one of the preceding claims, wherein there are at least two protrusions axially spaced along the shaft.
8. A motor according to any one of the preceding claims, wherein the axial length of the protrusion is equal to the diameter of the shaft.
9. A motor according to any one of the preceding claims, wherein the motor is a miniature permanent magnet direct current motor.
10. An electric motor with a plastic pinion substantially as hereinbefore described with reference to the accompanying drawings.