From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Page detailing design considerations for gear tooth patterns, bearing preloading, and manufacturing precision in axle units.
| Identifier | ExFiles\Box 114\1\ scan0081 | |
| Date | 11th January 1937 guessed | |
| In Figure 13 is shown contact pattern of the well designed axle unit. The variation in tooth pattern from no load to full load is caused by the relative displacement on gear and pinion due to deflections and mounting, and to some extent to the deformation of the gear teeth themselves. One criterion for good design is that under full load conditions the contact pattern must not show high pressures at the heel of the tooth or shall not show a heavy wipe at the top or bottom of the tooth. With the proper tooth pattern under maximum load conditions, the displacements must not be great enough to yield a too short pattern under no load conditions. If the tooth pattern is too short, the gears will be noisy under so-called floating loads or at very light loads in drive or coast. If, in case the displacements are too great and the gear and pinion are lapped for sufficiently long tooth contact pattern under light load conditions in order to avoid noise, then when maximum loads are applied, the tooth patterns will show concentrated loadings which, of course, means that the capacity of the unit is reduced because of the high unit pressures existing. In addition to gear and pinion displacements caused by the deformation of parts, it is obviously a matter of great importance to eliminate lost motion in the bearings and for this reason preloading of bearings is essential. Gears and pinions are lapped to a high precision during their manufacture and are lapped to the backlash to which the gear and pinion are intended to run in the axle. The gear and pinion, for instance, which has a proper tooth contact at backlash of .003 in. will show a marked different pattern at .006 in. backlash and if the gear and pinion were adjusted to give a proper bearing on one side of the tooth with the .006 in. backlash then the contact on the opposite side of the teeth would be out of position sufficiently to cause noise at light loadings and concentrated tooth pressures under maximum load. Preloading of bearings has the advantage of reducing the deformation displacements after the looseness is taken up, since the yield varies in rate, being greatest at low loads. It is obvious that initial seating in or wear in the bearings must be reduced to such low magnitudes that the displacement of the gear and pinion will not be affected. The general practice for many years has been to preload the differential bearings by screwing up the adjusting nuts so as to actually cause a slight spread at the bearings caps. This elastic yield is utilized in maintaining an appreciable axial loading on the bearings. For pinion mountings, bearing preloading has not been general, but it is now universal practice to have some preload, and in some types of design it is common to have from a 1000 to 1500 pounds axial preload. Much effort has been expended in order to limit the yield and looseness in bearings. Great improvement has been made to eliminate the seating in of new bearings which has been accomplished by high precision in manufacture and by surface finishes of extraordinary smoothness. In ball bearings it is common practice to have highly lapped finishes on the balls which are made to extraordinary precision for size and roundness and to form the grooves accurately and polish them to a mirror-like finish. In taper roller bearings for pinion mountings lapped rolls and mirror finish race surfaces are demanded. Years of effort has been expended to the design and manufacture of bearings which will deform the least possible amount under load. This has, to a large extent, been accomplished by refinements in design and specifications for great precision. As I have mentioned, after heat treatment the gears are lapped to a smooth finish and to a high precision of tooth to tooth spacing. Before the gears are assembled in the axle units, they are tested on machines in accurate running relationship. In a well built axle unit this relationship is maintained in the assembly, which is possible only when the parts on which the gears are mounted are accurately made and the carrier is accurately machined. The limits of accuracy - 6 - | ||