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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).
Technical paper on the lubrication requirements and design of hypoid gears, contrasting them with spiral bevel gears.

Identifier  ExFiles\Box 114\1\  scan0077
Date  11th January 1937 guessed
  
demanded the best and most expert service and who were inclined to regard service instruction with the fervor of those who appreciate the value of honest advice for their own best interests. For this reason Packard cars, very nearly to the extent of 100%, were serviced for rear axle lubrication in Packard service stations. This picture has been largely true on the Packard 120 until recent months with the introduction of hypoid axles by volume producers particularly in the lower price brackets.

It is well known that hypoid gears require special lubrication and, of course, the widespread adoption of hypoid gears creates a demand for the special type of lubricant required and for a widespread distribution of this lubricant. The promise of large volume business in hypoid lubricants has naturally awakened a definite interest among lubricant manufacturers who wish to participate in this business. This has brought about the introduction of new types of hypoid lubricants and consequently has focused much attention on the problems of the distribution of these lubricants and the servicing of hypoid axles in the field. Unfortunately, not all those who are engaged in the production and servicing of lubricants are equipped with the facilities or the technical personnel required to produce a satisfactory product and maintain the necessary uniformity and, much as I regret to say, some are so zealous to do business and are so indifferent to the best interests of their customers as to push the sale of lubricants which are entirely unsatisfactory, even ruinously injurious when used in hypoid axles. While to a small extent there may be some cause to suspect that inferior lubricants are being sold with deliberate intent, there are more cases where this is done through ignorance of the requirements of these lubricants and because of a woefully inadequate knowledge of the merits and behavior of the lubricant by its producer. In some few cases the producer has failed by believing that his product possessed exceptionally high merit and accounted for trouble by blaming the units in which it was used. Therefore, much of what is to be presented here is intended to clear up some of these misconceptions and to point out the great need for caution on the part of the makers and users of hypoid lubricants, in order to obtain long and satisfactory life from hypoid axles.

COMPARISON OF GEAR TYPES

Although hypoid gears have been used in regular production for more than ten years, I am frequently asked what a hypoid gear is and how it differs from the older spiral bevel gear. This can best be explained by referring to Figure 1 and 2 which show respectively a spiral bevel construction and a hypoid design. In Figure 1 the axis of the ring gear is at (0) and the axis of the pinion (a-a) is disposed perpendicularly to the axis of the gear through (0). In Figure 2 the gear axis is perpendicular to the pinion axis (a-a) but it is observed that the axes do not intersect but are displaced by the distance (A) which is called the hypoid offset. Pinion mountings may be of two types: The straddle type shown in Figure 2 or the overhung type shown in Figure 3.

Both bevel and hypoid gearing are based fundamentally on pitch surfaces which rotate about their axes with a uniform velocity ratio. In the case of the spiral bevel gears these pitch surfaces are cones, as shown in Figure 4, which obviously may rotate about their axes with pure rolling motion on the contacting surface. For gears operating about oblique but non-intersecting axes at a constant velocity ratio the pitch surfaces must also be surfaces of revolution but the enveloping elements form a hyperbolic curve and such pitch surfaces are known as hyperboloids of revolution, which for brevity are called hypoids, shown in Figure 5. Only a portion of the hypoid surface is required in the formation of the gears so that in appearance the pitch surfaces approximate sections of cones.

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