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 manufacturing of accurate and quiet gears, focusing on hobbing techniques and hob tolerances.
| Identifier | ExFiles\Box 136\5\ scan0336 | |
| Date | 15th January 1940 guessed | |
| - 2 - Figure #1 shows a method of calculating the arc of contact that any two mating gears have; also, the length of contact that the involute curve has with its mating gear. In order to produce gears as above, one of the many necessary requirements is to have accurate hobs. In following the hobbing process, it is of paramount importance that the lead of the hob be made accurate; by accurate lead we not only mean that the thread of the hob be advanced a given amount in one convolution, but that the lead must not deviate more than .0005" from the true helical path of the thread in making a convolution. "Quiet gears" being the watchword of the transmission builder of today, and since noisy gears are always found to be inaccurate; "How to manufacture accurate gears", is the problem that confronts the gear manufacturer of today, especially the transmission builder. The fact is that a gear having teeth, the contours of which comply with the geometrical laws underlying its construction, will make a better gear than one not having teeth so formed. Considering the many factors that enter into the making of accurate gears, if the fundamental facts as mentioned are kept in mind by the manufacturer, it will help a great deal to eliminate gear trouble, provided, of course, that the system of gearing used is theoretically correct. The constantly increasing demands as regards accuracy in gearing has given rise to the utmost accuracy in hob manufacture. The machine-relieved hobs formerly exclusively used will still suffice perfectly well for many purposes, but not in cases where silent running, most exact uniformity of rotation or close fitting is stipulated. For such purposes, relief-ground hobs should be used. They differ from the machine-relieved in that the flanks and tops of the teeth, often also the root, are ground after hardening. This has the effect of removing all inaccuracies occurring in hardening; the lead is improved and uniform tooth-form ensured for all teeth. Figure #2 shows the hob tolerances that are used by the hob manufacturers of today. This gives both Class "A" and Class "B" tolerances. Roughing as well as finishing hobs may be ground on the teeth, although it is usual to limit form-grinding of hobs to a special quality of work where real accuracy is required. It is not possible to lay down rules where ground hobs must be used or where machine-relieved will suffice. The deciding factor is the degree of accuracy needed by the producer of his gears or other components hobbed. Thus, for example, many spline-shaft hobs are form-ground, although most hobs for special profiles are only machine-relieved, - this despite the fact that the short, wide spline-hob teeth distort very little in hardening by comparison with other tooth-forms. Hob teeth that are to be ground are relieved at the back to clear the runout of the grinding wheel, as shown in Figure #3. By this type of relief the hob tooth is left just as strong as formerly without widening the flutes. There has been developed a number of test fixtures to control the manufacturing process of hobs based on the fundamental laws of gearing. These | ||