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).
Comprehensive survey on worm gearing for modern automobile drives.
| Identifier | ExFiles\Box 136\5\ scan0318 | |
| Date | 1st September 1937 guessed | |
| 1 WORM GEARING. A Comprehensive Survey of Modern Automobile Drives. By W. A.{Mr Adams} Tuplin, M.Sc. ALTHOUGH worm gears have long been used for the rear-axle drives of heavy automobiles, knowledge of their character and capabilities is not nearly so general as is the case with the spur and helical gears used in change-speed gear boxes. Nowadays the demand for quietness and low propeller-shaft line is again drawing attention to the worm drive for private cars. The characteristic which first led to the adoption of the worm drive by automobile designers was no doubt the facility with which it can give a high velocity ratio. In a bevel drive, the diameter of the wheel is equal to that of the pinion multiplied by the velocity ratio, and therefore a high ratio demands either a small pinion or a large wheel. Consideration of load capacity tends to exclude the former, with the result that a high-ratio bevel drive becomes unwieldy. In a worm drive, on the other hand, the diameter of worm and worm wheel are not immediately controlled by the velocity ratio, and a high ratio does not enforce either a small worm or a large wheel. Less restriction is placed on choice of ratio than is the case with any other type of gear. Another important point in favour of the worm drive is that a low propeller-shaft position is secured by placing the worm beneath the wheel, this effecting a height reduction of several inches as compared with the alternative bevel drive. By reason of the nature of the tooth contacts and of the materials from which it is usually made, the worm drive is the quietest running of any type of gear. This advantage is of increasing importance as engine noise and vehicle noise continue to diminish with improvement in design. In the trolley bus the worm gear provides a convincing demonstration of its value, for in that type of vehicle no transmission noise can fail to be detected, and only an axle drive of great strength and endurance can survive the severe loading conditions associated with rapid and frequent acceleration and retardation. Essential character. Worm gearing forms a means of connecting to non-parallel, non-intersecting shafts. Usually the shafts are perpendicular, but worm gearing is not confined to this condition, and it may be used for other angles down to about 60 deg.—a feature which may be useful for un-orthodox transmission arrangements. In principle the worm drive may be regarded as a screw engaging with a nut whose axis does not intersect that of the screw. Worm and worm wheel are both gears with helical teeth. An axial section of a worm wheel shows a rim profile which is curved to correspond with the circularity of the section of the worm on the same plane. An axial section of the worm may, or may not, be similarly curved. In the first case the worm is described as "globoidal" or "hollow-faced"; in the alternative it is called a "parallel" worm, any transverse section differing from any other only in angular position of the thread sections and not in shape or size. In the following, attention is confined to the parallel type of worm gear. Tooth and thread form. Considering the section of the drive on a plane containing the axis of the worm and lying perpendicular to that of the wheel, it is seen to consist of a rack and pinion. Usually the worm is of the parallel type, and in this case an axial section of the worm shows two similar diametrically opposed profiles, each composed of a number of identical teeth disposed at equal intervals along a straight line—in other words, two racks. Rotation of the worm causes the rack profile to move axially and the worm wheel rotates in such a way that its mid-section keeps pace with the rack. The same remark applies to the tooth action on any parallel plane contained within the width of the worm wheel. Now, the shape of the worm thread may be selected so that its axial section (i.e., the profile of the imaginary rack on the central plane) has any arbitrary shape. The section of the worm wheel teeth on the same plane must be of such a shape as to give constant angular velocity to the wheel when the worm has constant angular velocity. The determination of the shape of the central section of the worm wheel tooth, once that of the worm thread has been fixed, is a relatively simple problem. There is no difficulty in shaping the worm thread so that its axial section is such as to give satisfactory worm wheel tooth-form on the central plane. On planes parallel to the central plane different conditions arise, because the sections of the worm threads on such planes are different from that on the central plane. Fig. 1 illustrates this in respect of a worm whose threads are of "involute helicoid" form. It is seen that, in departing from the central plane, the sections of the thread profile change thus: (a) They depart more and more from their original symmetry. (b) The mean slopes of the two sides of each thread change in opposite directions, one tending to become more nearly parallel to the axis of the worm. The corresponding effects on the sections of the wormwheel teeth are: (a) They depart more and more from their original symmetry. (b) They tend to become subject to "interference" on the side which mates with the more nearly radial side of the worm thread. Such a section is unsatisfactory because it has a relatively short path of contact with the worm, and because it is mechanically weak. The greater the lead angle of the worm, the more pronounced do these effects become and the more serious, therefore, is the result of any error in the lateral position of the worm in relation to the worm wheel, any section of the wheel then meshing with a worm section with which it was not intended to make contact. On account of (b), it is necessary to take special precautions in the selection of thread-form of a worm of high lead angle. Line of contact. The points A₁A₂A₃A₄A₅ in Fig. 2 are points of simultaneous contact between worm and wheel on planes 1, 2, 3, 4 and 5. They lie on a line formed by the assemblage of similar points on the intervening planes. This line, which is in general a three-dimensional curve, is a line of contact between the worm and a particular tooth a of the worm wheel for a certain angular position. Now if the worm and worm wheel be rotated in unison, the line of contact A₁A₂A₃A₄A₅ moves bodily in such a way as still to lie on both the surface of the worm thread and the surface of the wheel tooth. To do this it has, in general, to [Image Caption 1] Sections on planes 1,2,3,4,5,&6 [Image Caption 2] Fig. 1. Sections of worm threads and wheel teeth on several planes parallel to central plane of wheel. [Footer] The technical information contained in this article is derived from the experience of David Brown & Sons (Hudd.) Ltd., Huddersfield, in the design and manufacture of worm gearing for all classes of automobiles. | ||