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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).
Technical discussion on engine vibrations, mountings, and methods for improving smoothness in 6-cylinder engines.

Identifier  ExFiles\Box 137\4\  scan0207
Date  16th August 1931 guessed
  
(2)

torsional flexibility of mounting with damping. The virtues of flexibility were fairly well known in the earliest days and we added the damping.

NOW our P. 2. mounting seems very inefficient for this purpose, and the diamond mounting the most efficient we know. Rubber is self damping and results should be better than the earliest attempt at this mounting with steel springs, and one thinks that we ought to get the best results without any other damper such as our own torque reaction dampers used on several of our earlier models.

After the torque reaction vibration appears to die away - i.e. at about 23 MPH., say 900 revs., or 2700 periods we must realise that the impulses are still there and are reacting upon the frame as much as ever, but we are now faster than the natural period of the engine in the frame, but if we have anything whose period is higher, say double, we shall set this vibrating when we come to its speed, but with less vigour. Our car here shews some vibration (very slight and no-one would complain) - Dodd says between 35 and 50 MPH.

Now as the speed increases the varying kinetic energy in the piston has a wonderful smoothing out effect on these gas pressures impulses, and at some speed according to the throttle opening the engine becomes remarkably smooth, but at these higher speeds we find that completely closing the throttle we get a definite vibration which we call the over-run, and is caused by the varying kinetic energy in the piston passing in and out of the flywheel and like the power impulses getting at the chassis via. the cyl. walls. (at 3 per rev.)

If what I am saying represents the facts we should get smooth running and an absence of all these low speed vibrations by removing the pistons and con. rods. Fitting lighter pistons might reduce the over-run but we feel it would only be to a small extent, and naturally they would not benefit those due to gas pressure, in fact, they might appear slightly worse.

So that, being natural to every 6 cyl. engine they can best be reduced by more efficient torsionally flexible mounting.

The foregoing vibrations cannot be improved by the following:

(1) Balance weights.
(2) Lighter and stiffer flywheel.
(3) Stiffer crankshaft (especially pins.)
(4) Stiffer crankchamber.
(5) Heavier crankchamber.
  
  


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