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).
Analysis of vibration waveforms, order forces, and component failures in relation to engine torque and inertia effects.
| Identifier | ExFiles\Box 142\1\ scan0278 | |
| Date | 20th October 1938 guessed | |
| (4) The various types of waveform are sketched in curve V.M.2, which shows the straight forward addition and modulation effects in curves (B) and (C) respectively. Although it is known that sixth order forces are present in the engine torque characteristics, calculation has shown that the third order is also serious and consequently we find it difficult to find an explanation for the presence of the sixth order only. A similar difficulty exists on account of the modulation effect, although it is recognised that such a waveform could arise from the beating effect of fifth and sixth orders. Again, a strict analysis of the combined gas and inertia forces makes it difficult to account for the presence of the fifth order. In an attempt to throw further light on this aspect some readings have been taken from the distributor screens on the Exe Star Unit, which are similarly disposed to those on the Vulture and where the operating forces are more easily analysed. Here it was found that the waveform corresponded to second order vibrations modulated at the ½ order. We were not able to trace the effect of the irregular firing - intervals but it seems that the modulation effect must correspond with the recurrence of the engine cycle. The type of waveform is illustrated in Curve (D). 5. GENERAL CONCLUSIONS.- The normal effects of vibration are well understood but it may be pointed out that where inertia effects are involved the stresses will be proportional to the square of the frequency and that apart from any considerations that the increase in stress will tend to reduce the number of reversals before failure, the time required for a failure by fatigue will be proportional to 1/frequency³. Consequently the presence of appreciable vibrations of an order as high as the sixth is to be considered dangerous and for a particular instance of a part stressed by inertia we may consider the distributor moulding, the base of which is required to support the mass of the coil. A number of failures at the thin section around the coil housing have occurred and are directly attributable to this effect of high frequency vibrations. In regard to the abnormal record of failures occurring on Vulture II - 4, the deduction from the experiments already mentioned is that no special features are to be noted on the bed and accordingly some consideration must be given to the possibility that abnormal conditions come into play on the hangar only. In this connection it is significant that all hangar failures occurred during the period at 2850 r.p.m. it being well known from the measurements of blade vibration stress that this speed is extremely close to the 1½ order critical which is the most serious in the speed range of the engine. In general the torsional conditions are certain to undergo considerable modification on brake running, where it is unlikely that this critical would appear at a similar speed. | ||