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).
Comments on a memo regarding crankshaft torsional vibrations in B.60 and Flat 12 engines.
| Identifier | ExFiles\Box 132\2\ scan0206 | |
| Date | 16th December 1940 | |
| COPY. Wd{Mr Wood/Mr Whitehead}/GSB.11/RS.{Sir Henry Royce's Secretary}16.12.40. Comments on Memo on Crankshaft Torsional Vibrations in B.60 and Flat 12 engines. (Rm{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden}1/MT.9.12.40). ------------------------------------ A.{Mr Adams} On Page 2 the expression given for the proportional value of the amplitude (∝s) at the free end of the crankshaft is incorrect. The square root sign should be omitted, and the same remark applies to the radical over TN the tangential effort. This sign also should be removed, since displacement is proportional to force in all linear systems. The expression "equilibrium amplitude", would then read:- "Equilibrium amplitude" = (TN Σ∝) / (ω²ΣJ∝²) where ∝ is relative amplitude at any crank for unit amplitude at reference crank (say free end). J is M.{Mr Moon / Mr Moore} of I. of any crank or other body, including flywheel. TN = torque of order concerned at any one crank. ω = circular frequency of vibration. The summation in the numerator is vectorial and includes cranks only. The summation in the denominator includes cranks and any other bodies having kinetic energy, e.g. flywheel. To obtain the actual amplitude at the free end, the "equilibrium amplitude", which is simply equivalent to the static deflection under the exciting torques, must be multiplied by a magnifying factor which depends on the damping (oil, hysteresis, etc.) No mention is made in the memo of how this factor has been determined. A preferable method of treatment is to equate the exciting work per cycle to that lost in damping, also per cycle, as is done by Shannon ("Engineering" 20 and 27.12.35) and Carter (Report E.3586). It should be noted that the summations occurring in this method as used, rather differently, by both these investigators, apply only to the cranks. Shannon's method has been found to give good results agreeing well with practice, and in several instances, agreeing with Carter's method applied as a check. B. On Page 3 an expression is given for the optimum torque setting of a constant friction damper which does not contain any term involving the M.{Mr Moon / Mr Moore} of I. of the damper flywheel. This is obviously incorrect. The best damper (up to date), is the tuned vibration absorber with optimum damping. | ||