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 brochure for 'Cone-Ring' flexible couplings, detailing their functions, design principles, and advantages.
| Identifier | ExFiles\Box 159\6\ scan0057 | |
| Date | 13th March 1940 guessed | |
| ‘CONE-RING’ FLEXIBLE COUPLINGS Page 2 Functions of Flexible Couplings. Flexible couplings may be required to serve any or all of the following purposes: (a) Accommodation of errors in alignment of shafts (in respect of either eccentricity or angularity) which with a rigid coupling produce excessive bearing loads and bending moments in the shafts. (b) Accommodation of relative axial motion of two shafts in order to isolate each from the effect of end thrust from the other. (c) Torsional flexibility, in order to provide a cushioning effect against shock loads and cyclic torque fluctuations and thus to reduce the destructive effect on associated mechanism. (d) Damping of torsional vibration by dissipation of energy in the flexible element. Fig. 1. Comparison of Sleeve and Cone-ring Types of Rubber Bushes. The term “flexible” as applied to couplings therefore has a very wide range of meaning and may be misleading unless suitably qualified. Principles of Flexible Coupling Design. Couplings which are flexible in so far as they permit relative shaft movement axially or in respect of alignment can be designed on purely mechanical lines, but all involve joints and moving parts which are subject to wear and require lubrication. Couplings of this kind, unless very complicated, do not give compensation for alignment errors and axial freedom simultaneously, and give no torsional elasticity or damping. By the use of resilient material, joints and sliding surfaces may be avoided, relative motion taking place inside the resilient material itself. Two types of resilient material are applicable to this purpose, spring steel and rubber. When steel springs are used, they must be anchored securely and then give flexibility only in a limited degree, except in designs which although possible are not commercially practicable. Thus if strip steel springs are used to give torsional flexibility, axial motion involves rubbing between highly stressed surfaces, which in spite of lubrication will result both in wear and in the transmission of considerable axial loads when relative axial movement occurs. When steel is used to give torsional flexibility the amount of energy which it can absorb is definitely limited by the allowable stress and is equal to about 3-5 ft. lb. per lb. or 1·5 ft. lb. per cubic inch at 60,000 lb. per square inch bending stress. The weight of spring steel in most couplings of this type now available is comparatively small, and the flexibility of some couplings of this type is actually less than that of the length of shaft on which they are mounted. Further, unless the steel is stressed beyond the elastic limit (which is not feasible) it cannot give any appreciable hysteresis loss and cannot therefore serve to damp out vibrations. As a medium for absorbing energy, rubber, properly used, is a much more satisfactory material from the point of view of both specific energy absorption and hysteresis loss. The “David Brown” Flexible Coupling is based on the scientific use of the properties of rubber and combines all the functions of flexible couplings. It permits a degree of alignment error either in eccentricity or angularity which easily covers all errors normally met with in practice, and also gives freedom of axial movement without surface skidding. It has been developed from the conventional pin-and-bush type of coupling, but whilst resembling it in simplicity, ease of assembly and dismantling, differs from it fundamentally in the way in which the resilient material behaves. Rubber, when compressed, suffers practically no change in volume. Consequently, if it is so disposed as to be confined and incapable of free change of shape, its full resilient properties are not realised. This is very largely the case in the conventional pin-and-bush coupling since the longitudinal expansion of a long hollow cylindrical bush fitting over a pin and inside a hole is seriously restricted. Moreover, axial movement of the pin and bush induces, particularly if the length is large in proportion to the thickness, a severe condition of internal shear which causes a heavy axial load and possible skidding and wear of the parts, in spite of the high resistance of rubber to abrasion. If leather bushes are used, of course, the conditions become very much worse. Page 3 The modification introduced into the “David Brown” Flexible Coupling is simple, yet of profound significance; it consists only in the substitution, for a parallel bush, of a series of rings of conical section. Fig. 1 shows with some exaggeration, the comparison between the old and new forms, and the deformation produced by transverse load. At (a) is shown the conventional bush, in which most of the “flow” takes place circumferentially of the bush and is constrained by the small, unbroken section. At (b) is shown the change of shape of the cone-ring bushes. Here the material is free to deform in all directions and this, for a given pin pressure, gives over four times the torsional flexibility in a coupling of given leading dimensions. A further feature is the freedom of axial motion. This can take place with very little internal distortion producing stress, and therefore axial resistance, since the cone-rings are capable of a “rolling” motion, as shown in Fig. 2. Fig 2. Showing “rolling” of Cone-ring Bushes under axial motion. DAVID BROWN & SONS (HUDD) LTD HUDDERSFIELD Page 4 The torsional flexibility is greatest at light loads, and diminishes as the load increases. Fig. 3 shows a typical load/deflection curve, and the curve for increasing load DAC lies below that for decreasing load CBD. The area between these curves represents the hysteresis loss which is available for damping—obviously there is no loss so long as the load is constant; the actual loss from zero to maximum load and back is in itself small, but sufficient to restrain the building up of vibration. The practical advantages of the “David Brown” Flexible Coupling, are :— (1) It is easily assembled and dismantled and when the pins are removed one shaft and its attached parts can be withdrawn upwards or sideways without disturbing the other. (2) No lubrication or attention is required. (3) It is simple and has no working parts to wear except the bushes, which have a long life. (4) Replace bushes are cheap and easily and quickly fitted without dismantling or moving either coupled shaft. (5) Wear of the bushes only occurs with serious errors of alignment or severe torque fluctuation; under such conditions the cost of replacement is far less than the saving due to the lightening of the load and vibration on other parts. (6) It has been successfully used to reduce noise arising from severe torque fluctuation. MEDIUM DUTY TYPE. It frequently happens that the size of coupling is determined by shaft diameters of standard machinery and the stresses in the shafts are extremely low. In such a case, the size of standard coupling necessary for the shaft might have many times the torque capacity actually required. For this reason we have evolved the medium duty type (dimensions, ratings, etc., of which are given on page 5) in which the overall dimensions are identical with the heavy duty type, but only half the number of pins is provided, thus maintaining the required degree of flexibility and avoiding unnecessary cost. All “Cone-Ring” couplings are adequately shrouded and thus comply fully with the Factories Act of 1937 in that the heads of the pins do not project beyond the flanges. Fig. 3. Typical Load/Deflection Curve. | ||