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).
Summary of calculations and conclusions regarding the Dennis Crankshaft 34442 and its centre bearing.
| Identifier | ExFiles\Box 116\1\ scan0061 | |
| Date | 7th September 1938 | |
| 1020a Rm.{William Robotham - Chief Engineer} c.c. Rm{William Robotham - Chief Engineer}/Gry.{Shadwell Grylls} BY/B.2/G.7.9.38. Scott DENNIS CRANKSHAFT 34442 AND ITS CENTRE BEARING. (see their letter of 27.8.38.) SUMMARY. Even if shaft was free at ends and supported only at centre, the effect of inertia forces alone, although giving deflections at ends of shaft of .095", would only give deflection at each end of centre journal relative to centre of engine of .000525". When the centres of the three bearings are maintained in line, the end bearings exerting re-actions on the shaft, this centre journal deflection will only be .000087". Distortion effects due to inertia forces are therefore quite negligible. This is because of the symmetrical loading. Conditions are different, however, with the explosion force loading. Assuming a cylinder pressure of 600 lbs/sq.ins. when Crank 2 is at top of firing stroke, this will cause an angular deflection of the centre journal in the plane of the shaft, which, if the three bearings are assumed to keep their centres in line, will give a deflection at each end of centre journal relative to centre of .003". An initial diametral clearance of .006" will be fully taken up by this deflection, which is due to the three bearing construction (instead of five) with relatively weak webs. The maximum load on the centre bearing occurs when running light at 3000 r.p.m. when it amounts to 8190 lbs. which gives a pressure of 1450 lbs/sq.in. based on projected area,including radiused ends, or of 1620 lbs/sq.in. assuming radii not to bear any load. The maximum centre bearing load at low speed due to explosion force alone (600 lbs/sq.in) = 6700 lbs. Conclusions. As far as crankshaft is concerned, trouble is due to angular bending deflection under explosion loads,owing to lack of intermediate bearings. It is also possible that the centre bearing boss is too rigidly webbed to the crankcase. The oil grooving should be critically examined. BY/B. | ||