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 camshaft drive failure, calculating tooth load and stress on a fabroil gear and chain drive.
| Identifier | ExFiles\Box 136\1\ scan0314 | |
| Date | 12th April 1940 | |
| 1155 ¢ 1046 Timing gears General Rm{William Robotham - Chief Engineer}/JRS 1/Ck{Mr Clark} 12-4-40 B60 Camshaft Drive Because of the failure of the fabroil cam gear EB 3956, I decided to calculate the safe working tooth load for an engine speed of 4000 r.p.m. Professor Earle Buckingham of the M.I.T. gives a modified Lewis formula for helical gears of this material as: W=px.Fa.y.St Z. where- W=safe transmitted tooth load, lbs. St=bending endurance limit for material in lbs/in². px=normal circular pitch, inches. Fa=active face width, inches. y=tooth form factor(as for spur gears) V=pitch time velocity ft/min. The bending endurance limit for the material is given as 6000 lbs/in² Z is a speed factor and the most accurate results are given by.{R.W. Bailey - Chief Engineer} Z= (150 / (200+V)) + .25 The calculated figure for W on this basis comes to 60.3 lbs. corresponding to a driving torque of 16.87 lbs ft. At present we have no definite data for the camshaft driving torque but this would seem to be on the low side. In addition to the camshaft torque we have a torque due to the torsional vibration of the crankshaft. Taking a maximum vibration amplitude of +.8º and a natural frequency of 240 vibrations per second. Circular frequency=W=1510 radians/second. Moment of inertia of camshaft=.00179slug ft². The inertia torque due to vibration comes to 57.2 lbs ft which is hopelessly in excess of the safe torque. In the case of the chain the smallest cross sectional area is .082 in² so that when transmitting a torque of 100 lbs ft. to the camshaft the direct tensile stress is 4400 lbs/in². From this it would appear that the fabroil camwheel is impracticable whereas the chain drive should be satisfactory. The reason for the failure of the previous chain drive is due to the "no bend back" quality of the chain. The job of producing this quality falls solely on the two outside links of the chain and consequently produces a large bending stress in these two links. Also these 2 links are a closer fit on the pins and therefore take up the tensile load before the inside ones. A suitable fully flexible chain should rectify the fault. | ||