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).
Increasing the load on a 20/25 circlip and analysing piston failures.
| Identifier | ExFiles\Box 86\2\ scan0024 | |
| Date | 28th March 1933 | |
| X236. To BY.{R.W. Bailey - Chief Engineer} from E.{Mr Elliott - Chief Engineer} c. Hs.{Lord Ernest Hives - Chair} Hd.{Mr Hayward/Mr Huddy} E.2/HP.28.3.33. re Chassis GDP-49. ALLDRITT. With reference to your memo BY.4/G.27.3.33, we think it would be quite safe to increase the load on the 20/25 circlip right away. As the detail now stands there is a diametral load of 3 1/2 lbs.when in position in the piston and the diametral set is .016. The groove in the piston, however, has a tolerance of plus .010", and the ring a minus tolerance of .005", so that if the limits come all one way, which of course is very unlikely, the rings would have practically no set on at all. In the case of the Bensport pistons which failed in respect of this circlip fixing, we think the rings started to enlarge the groove by reason of having insufficient load and set to withstand their own inertia forces at the extra high speed at which this engine ran. The inertia force of half the circlip at 4500 r.p.m. is 1.3 lbs. Some of the pistons show the groove elongated up and down, and at the same time widened on both sides due to the rubbing movement of the ring. When this stage is reached the ring is perfectly free to jump up and down and then we think it fails itself under the effect of the fatigue stress due to being freely bounced up and down. We have asked the Experimental department to test these rings with the load increased from 3 1/2 to 8 lbs. This increases the set from .016 to .035, and thus makes the ring much safer from the point of view of any variations which might be experienced within the limits on the drawing. The stress corresponding to 8 lbs is not more than 140,000 lbs per square inch. Actually we have made some tests opening out these rings with a spring balance, and they do not take a permanent set until 16 lbs. diametral load has been exceeded. Moreover the temper of the wire is such that the ring can be safely distorted to get it into its groove, and it would then automatically register the stress of which it is capable | ||