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).
Investigation into the failure of the dumb bell link on the Phantom III front brakes.
| Identifier | ExFiles\Box 92\4\ scan0379 | |
| Date | 4th November 1937 | |
| By.{R.W. Bailey - Chief Engineer} from Rm{William Robotham - Chief Engineer}/AFM.{Anthony F. Martindale} c. Da.{Bernard Day - Chassis Design} c. RHC.{R. H. Coverley - Production Engineer} c. Hl. 306 DUMB BELL LINK, PH.III. FRONT BRAKES. The failure of the above has been attributed to a succession of light impacts against the side of the bell crank lever, when inadequate clearance has been allowed, due to the bad brake geometry on bump and rebound, wheels straight ahead. The failure on the road that was very surprising was the mild steel link (made in France) that broke after only a few hundred miles. Before going to the expense of changing all bell crank levers we thought it as well to see that the light impacts could break these links in the time available. We find that they can. The wheel patter frequency of Ph.III. front wheels is 550-680 per minute, and they patter almost continuously. This means that averaging 40 m.p.h. there would be 90,000 impacts per hundred miles. On our rig we could break the links with 180,000 impacts (200 miles). The test rig (picture attached) consisted of a connecting rod, having a set up low rating spring in its centre, driven by a crank rotating at 961 r.p.m. The connecting rod caused the test specimen to hit the side of a standard bell crank lever. The load of 15 lbs. corresponds to the pull off spring load on the link, plus the force necessary to accelerate the brake actuating mechanism. Preliminary tests indicated that if the dumb bell did not rotate, it would withstand 80 hours running, but that if it had a natural tendency to rotate, it would break after 5 or 6 hours. To confirm these observations we had two S/M.{Mr Moon / Mr Moore} specimens turned and heat treated together, and each specimen had a Brinell figure of 194. One was placed in the rig and allowed to rotate: it failed after 3 hours. The other we did not allow to rotate, and it withstood 50 hours running during which time it hammered a large flat on its side. (see picture). It follows, then, that to get maximum safety we should prevent the lower ball from turning. We are, however, taking no further action as if the foul does not occur the dumb bell does not break. We have satisfied ourselves that the action already taken will prevent further failures. Rm{William Robotham - Chief Engineer}/AFM{Anthony F. Martindale} | ||