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 discussing the principles, mounting, and standardization of worm gearing.
| Identifier | ExFiles\Box 136\5\ scan0322 | |
| Date | 14th October 1937 guessed | |
| 8 Worm Gearing—contd. in trolley buses operated by various undertakings shows that a life of 250,000 miles may be expected and in some instances a mileage of 400,000 has been obtained. As the transmission loads are more severe in trolley buses than in most other classes of heavy automobile service, these figures may be safely regarded as representing the normal capabilities of worm gears. To secure these high mileages it is, of course, essential to design the gears with reasonable working stresses, to mount them rigidly in the axle and to lubricate them correctly. Mounting of worm gears. When it is considered that automobile worm gears are produced within a limit of accuracy of 0·001in. on tooth-form and accumulated pitch error, it will be understood that extremely rigid mounting is necessary to enable high-quality gears to show to the best advantage. The gear manufacturer recognises that no mounting can be quite unyielding and is able to modify the worm wheel tooth form in order largely to neutralise the distortions which occur under some particular load. Such modification can be strictly correct only for one load and therefore stiffness in the mounting is necessary in order to minimise change of distortion with change of load. The relative displacement of worm and worm wheel that has the most serious effect on contact conditions occurs in a direction parallel to the worm wheel axis. To minimise this, the worm wheel bearings must be given adequate axial support which, in a rear axle drive assembly, may be secured by arranging the bearing caps as parts of a substantial member fitting closely into the circular opening of the banjo. The main body of the gear unit fits into the other opening of the banjo, whose rigidity thus assists in holding the gears closely to their designed relative positions. In addition to the advantage of substantial mounting in making it easier to secure quiet running under all loads, the close approach to uniformity of contact conditions avoids such stress concentration as might otherwise occur and permits of higher nominal surface stresses in the gears. Fig. 10 illustrates a worm and differential gear assembly which is successfully used in high grade private cars where avoidance of noise is necessary. The favourable load distribution secured by its rigidity also commends the design for heavy vehicles in which lightening by reduction of dimensions is valuable. Standardisation of worm gears. Until comparatively recently there had been no attempt to establish any standard system of dimensions for worm gears and the only approach to uniformity was in respect of axial section of the worm. Very often this was given the same shape as a standard involute rack of 14½ deg., 15 deg. or 20 deg. pressure angle, but this system is not suitable for worms of high lead angle. The British Standard Specification, No. 721 (1937), wherein is laid down a logical system of design and dimensions, based on the involute helicoid worm, represents the fruit of long experience with worm gearing in many fields. All the general information which it contains is applicable to automobile worm gears, and design of rear axle drives in accordance with the standard principles now established may be undertaken with complete confidence. Printed by The Cornwall Press Ltd., Paris Garden, S.E.1. | ||