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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 12-cylinder engine crankshafts, vee angles, and torsional vibrations for a 6250cc engine.

Identifier  ExFiles\Box 179\2\  img216
Date  9th February 1932
  
To R.{Sir Henry Royce} From Ha/Gry.{Shadwell Grylls}
c. to Rg.{Mr Rowledge} Mor.
c. to E.{Mr Elliott - Chief Engineer} Rg{Mr Rowledge}
c. to PY.

ORIGINAL

Ha/Gry.{Shadwell Grylls}1/WJ.9.2.32.
X634

12 CYL. ENGINE CRANKSHAFTS & VEE ANGLE.

Numerical Results for 6250 cc; 3½" x 4½"

There is a choice for this type of engine of a crankshaft with 7, 4, or 3 main bearings. The latter two provide a shorter engine, stiffer crankshafts of less inertia, and require less balance weights. It is thought that in the 3 bearing shaft the bearings have no tendency to go out of line due to applied torques. We are trying to get confirmation of this. The various arrangements of the balance weights are shown on the accompanying sketch.

The effect of cylinder angle on the severity of torsional vibrations is shown in the table. The important vibrations are the 12th., 9th., 6th., 4½, and 3rd. order. The 90° engine has no periods that would not easily damp out until the 3 per rev. which would probably be above the speed range of the engine. Such a large angle is not easy to get under the bonnet.

It will be seen that a negligible advantage is obtained over the 60° engine by the use of 65°. With a 60° engine a damper is required for the 6th. order vibration (2000 - 2500 RPM.) but the higher period (4000 - 5000 RPM.) does not exist, and there would be no fear of crankshaft failure due to over-revving. With a 45° angle the 4½ and 6th. order vibration would require a damper, there seems no advantage in using this angle. A 30° engine would require a smaller damper for the 4½ order vibration, but again it would be necessary not to reach the 3 per rev. vibration. The 6th. order which is bad in a 60° engine is absent.

The natural frequency of the Phantom ll crankshaft is about 12,000 minute, the 3 per rev. vibration being at 4000 per minute. Using this size of crankshaft for a 12 cylinder engine would mean that the period would be raised, and would be in the neighbourhood of 4500 RPM. Additional stiffness would be gained by using less than 7 bearings so that the 30° engine could safely be used, if the crankshaft was not required to run above about 4600 RPM.

Ha/H.Grylls.
  
  


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