GB2373304A

GB2373304A - Tapered involute gear profile

Info

Publication number: GB2373304A
Application number: GB0104114A
Authority: GB
Inventors: Bryan Nigel Victor Parsons
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-02-20
Filing date: 2001-02-20
Publication date: 2002-09-18
Also published as: GB0104114D0

Abstract

A tapered involute gear profile for a parallel shaft transmission wherein the cross-sectional profile of the gear reduces across the axial distance of the gear, wherein the tooth thickness (T and t) measured at a constant radius, such as the pitch circle radius, reduces in linear proportion to the axial distance so that the flanks of the tooth in cross-sectional profile remain involute curves on a common base circle for all cross-sections. Tapered involute gears having a helical tooth profile are also discloses (figs 5-8).

Description

Tapered Gear Profile

This invention applies to gear profiles suitable for parallel shaft transmissions and particularly to gear profiles that are manufactured from a die process, such as injection moulding or forging.

Applying a simple draft angle to gear profile designed for parallel shaft transmission, such as spur gear, to facilitate manufacture by, for example, forging, would result in gear inaccuracy and non-conjugate motion between mating gears. This patent defines a profile that fulfils the requirements of the gears whilst providing a shape that can be easily forged or moulded.

According to the present invention a tapered involute gear profile for a parallel shaft transmission is characterised in that the cross-sectional profile of the whole gear reduces in accordance with the axial distance of the said cross-sectional profile from the maximum cross-sectional profile of the gear, wherein the tooth thickness measured at a constant radius, such as the pitch circle radius, reduces in linear proportion to the axial distance from the said maximum cross-sectional profile, so that the flanks of the tooth in cross-sectional profile remain involute curves on a common base circle for all cross-sections.

According to a preferred embodiment of the invention, the radial distance of the centre of the gear to the tips of the gears in cross-sectional profile reduces in linear proportion to the axial distance of the said cross-sectional profile to the maximum cross-sectional profile.

According to a further preferred embodiment of the invention, the radial distance of the centre of the gear to the root of the gear teeth reduces in linear proportion to the axial distance of the said cross-sectional profile to the maximum cross-sectional profile.

According to a further aspect of the invention the gradient of the variation of the tooth thickness with the axial distance from the maximum cross-section is substantially one module reduction for every five modules axial distance.

According to a further aspect of the invention the gradient of the variation of the tooth addendum with the axial distance from the maximum cross-section is substantially one module reduction for every ten modules axial distance.

According to a further aspect of the invention the gradient of the variation of the tooth dedendum with the axial distance from the maximum cross-section is substantially one module reduction for every ten modules axial distance.

According to a further aspect of the invention the mid plane of the tooth in section might be translated helically as the axial distance from the maximum cross-sectional profile increases.

According to a further aspect of the invention the helical translation of the mid plane of the tooth in section is such that the flank of one side of the tooth remains in axial alignment.

Various embodiments of the invention will now be described, by example only, with reference to the accompanying drawings, where: Figure I represents a plan transparent view of a gear designed in accordance with the invention with no helical translation of the section.

Figure 2 represents an isometric view of two gears designed in accordance with the invention, in mesh, with no helical translation of the section.

Figure 3 represents an isometric view of a gear meshing with a rack designed in accordance with the invention, with no helical translation of the section.

Figure 4 represents an isometric view of an internal gear designed in accordance with the invention with no helical translation of the section.

Figure 5 represents a gear, designed in accordance with the invention, with a helical translation where one flank of the tooth remains in axial alignment.

Figure 6 represents an isometric view of two gears designed in accordance with the invention, in mesh with a helical translation where the helical translation ensures one flank is axially aligned.

Figure 7 represents a gear, designed in accordance with the invention, with a helical translation of the profile equal to half of the angular pitch.

Figure 8 represents an isometric view of two gears designed in accordance with the invention, in mesh with a helical translation of the profile equal to half of the angular pitch.

Figure I represents the geometry of the gear profile of a gear with twenty teeth where outline 21 represents the gear tooth shape at one end of a gear whilst outline 22 represents the gear tooth shape at the opposite end of the gear to outline 21. Circle 31 is the addendum at the large end of the gear whilst circle 32 is the addendum at the small end of the gear. The circle 33 is the dedendum at the large end of the gear whilst circle 34 is the dedendum at the small end of the gear. The involute flanks of the gear teeth are developed from the base circle 36 for the whole gear profile. The pitch circle 35 depends on the base circle 36 and the nominal pressure angle of the gear A. The tooth thickness varies from T at the large end of the gear to t at the small end of the gear. The change in tooth tip height is denoted by H and the change in tooth depth is denoted by d. The line of action 37 of the gear is tangential to the base circle 36 and passes through the pitch point 40. the angle subtended from the centre of the gear between the pitch point and the tangent point of the line of action 37 to the base circle 36 is equal to the nominal pressure angle A. The thickness of the tooth form varies linearly with axial position between T and t. For a standard gear the tooth thickness t the pitch circle diameter would be equal to the pitch circle circumference divided by twice the number of teeth. The thickness T for the gear shown is equal to the standard gear tooth thickness plus one module, whilst the thickness t is equal to the standard thickness minus one module.

Where a module is equal to the pitch circle diameter divided by the number of teeth on the gear. The thickness of the gear is ten modules. The ratio of the change in thickness for the

tooth with axial distance is for this gear equal to 1 : 5. This gives an apparent draft angle of 60 for the tooth flank. The ratio of the change in the addendum for the gear with axial distance between the large and small ends of the gear is 1: 10. this gives an apparent draft angle for the tooth tips of 60. The ratio of the change in the dedendum for the gear with axial distance between the large and small ends of the gear is 1: 10. this gives an apparent draft angle for the tooth roots of 60. the gear has thus an overall draft angle of about 60 making the manufacture by means of forging, die casting or moulding easier than a conventional straight cut gear. The axial mid plane of the gear (halfway between large and small ends of the gear) corresponds to the standard proportions for a gear tooth in that the addendum is equal to one module whilst the dedendum is one and one quarter modules whilst the tooth thickness is half the circular pitch.

Figure 2 shows the gear AA of figure I in isometric elevation engaged with a similar gear BB being identical to gear AA except inverted to mesh the large end of gear AA with the small end of gear BB.

Figure 3 shows the gear CC of figure 1 in isometric elevation in mesh with a rack DD formed in accordance with the same condition for gradient for tooth thickness, addendum height and dedendum depth. The profile of the rack DD being inverted so that the large end of the gear CC meshes with the thin end of the tooth form of the rack DD.

Figure 4 shows an internal gear constructed to the same constraint on the tooth thickness, addendum height and dedendum depth. In this case the large end of the internal gear is shown at the top whilst the external surface of the gear is given a draft angle of 6 so that the largest external diameter is also at the top to enable the gear to be easily ejected from the forming die.

Figure 5 shows an external gear with substantially the same constraint on the tooth thickness, addendum height and dedendum depth but with a helical twist with axial position such that the flank 38 is axially aligned whilst the back face is helical.

Figure 6 shows an isometric elevation of the gear EE of figure 5 meshing with a mirror image gear FF of the gear EE. The large end of the gear EE meshes with the small end of the gear FF.

Figure 7 shows an external gear with substantially the same constraint on the tooth thickness, addendum height and dedendum depth but with a helical twist with axial position such that the helical twist is half of the pitch. In this case the angle between the mid-plane of tooth 42 and the mid-plane of tooth 43 is half of the angular pitch of the gear.

Figure 8 shows an isometric elevation of the gear GG of figure 5 meshing with a mirror image gear HH of the gear GG. The large end of the gear GG meshes with the small end of the gear HH. This arrangement is only really applicable to gears of equal tooth numbers, but might be an advantage in certain applications such as hydraulic gear pumps.

The general form of the gears allows the degree of backlash between a pair of gears to be adjusted by altering the relative axial position of the gears. This should have applications where zero backlash is a requirement for a transmission system.

Various modifications might be made without departing from the invention. For example the change in profile has generally been described as linear with axial position, however other degrees of variation might be used provided the inverse for the meshing gear can also be applied. A sinusoidal variation could be applied to the tooth thickness, addendum and dedendum parameters with axial position.

Claims

1. According to the present invention a tapered involute gear profile for a parallel shaft transmission is characterised in that the cross-sectional profile of the whole gear reduces in accordance with the axial distance of the said cross-sectional profile from the maximum cross-sectional profile of the gear, wherein the tooth thickness measured at a constant radius, such as the pitch circle radius, reduces in linear proportion to the axial distance from the said maximum cross-sectional profile, so that the flanks of the tooth in cross-sectional profile remain involute curves on a common base circle for all cross sections..

2. A gear according to claim I where the radial distance of the centre of the gear to the tips of the gears in cross-sectional profile reduces in a linear proportion to the axial distance of the said cross-sectional profile to the maximum cross-sectional profile.

3. A gear according to claim 1 where the radial distance of the centre of the gear to the root of the gear teeth reduces in a linear proportion to the axial distance of the said cross sectional profile to the maximum cross-sectional profile.

4. A gear according to claim 2 in which the gradient of the variation of the tooth thickness with the axial distance from the maximum cross-section is substantially one module reduction for every five modules axial distance.

5. A gear according to claims 2 in which the gradient of the variation of the tooth addendum with the axial distance from the maximum cross-section is substantially one module reduction for every ten modules axial distance.

6. A gear according to claim 3 wherein the gradient of the variation of the tooth dedendum with the axial distance from the maximum cross-section is substantially one module reduction for every ten modules axial distance.

7. A gear according to claim 6 in which the mid plane of the tooth in section might be translated helically as the axial distance from the maximum cross-sectional profile increases.

8. A gear according to claim 6 the helical translation of the mid plane of the tooth in section is such that the flank of one side of the tooth remains in axial alignment.

9. A gear designed according to any of the above claims in which the gradient of the tooth thickness with axial position is adjusted to provide optimum angle for manufacture.

10. A gear transmission according to any of the above claims in which the backlash is adjusted by axially adjusting the relative position of two meshing gears.

11. A gear pair in accordance with any of the above claims used to form a hydraulic gear pump