Backdrop to these articles

This is the first of three articles discussing some unpublished practical aspects of the bench type post war six cylinder crankshaft damper and camshafts. It was to have been the second in the sequence but requests from owners of these six cylinders to print some practical details at an early stage have prompted a re-think. Following articles will relate in particular the history of these crankshaft dampers during the period 1953 to 1958, nearly all of which has never been seen in print. To round the situation off an article will cover all the details of the different camshafts including valve timing and some oil tests.

Basic Background of the Damper and Spring Drive

Read any publication on technical aspects of Rolls- Royce or Bentley cars and you are almost bound to be overwhelmed by details of the slipper drive or crankshaft damper see Fig 1. About 100 years have now passed since Henry Royce realised the need for some sort of crankshaft damper. Considering that the final fitting of any device should naturally encompass all the latest modifications it is almost unbelievable that authors seem to be oblivious to the company data between 1953 and 1959. No detailed mention is made anywhere of the slotted Ferodo dampers fitted to the last of the R type engines, and initially scheduled for the 4.9 ltr Silver Cloud engines.

Why is this period 1953 and 1959 so important? Because when the components of the last “Bench” damper type are considered nearly every single modification to a part was completed in this period. It should go without saying that if one is not aware of the modifications and especially the reasons for the alterations, just how can a successful damper be rebuilt? More to the point, how can a substitute friction material be suggested to replace the now defunct cotton duck. A study of this period archive data shows that the company engineers were to learn for the first time what happened inside a damper compared with what is believed to happen. This makes the information important and a large slice of previous data worthless. Fortunately I have been able to locate the correspondence and all the drawings relating to this period in the history of the damper including the Ferodo dampers. The bulk of this data will appear in a following script.

Over the years an unusual R-R damper was designed and developed, and the last of this type that is the subject of this article, was fitted to the final six cylinder engines in 1959. The damper is unusual in that it is untuned and will handle crankshaft vibrations across the entire engine rpm range, whilst even current types are limited to narrow engine revolution bands. Furthermore the composite unit comprises a spring drive to cushion and maintain constant tooth loading between the camshaft and crankshaft timing gears. The damper friction components also provide damping to eliminate spring surge and over travel in the spring drive.

Often one is asked; why not fit a rubber bonded damper. Often the trade name Metalastik is quoted. These dampers are tuned for one speed range only, and there is a construction difference between rubber bonded and rubber dampers. The company used their own rubber damper on B80 / B81 commercial engines limited to 3750 rpm, and all Phantom IV engines, up to at least 1960. The type was not a bonded rubber design but a dry friction damper utilising Ferodo VM20 material in twin 5.312 inch O.D. discs mounted on rubber bobbins. This material is not to be confused with the later Ferodo VM 41 material used on the late R type dampers. Rubber dampers would not have provided all the requirements for the engine speed range of these long stroke car engines. ”Metalastik" dampers were tested between February 1958 and April 1960 as a cheaper cost option only to the R-R commercial damper. At the time they gave equal, but no better results than the R-R rubber mounted damper, which itself gave scattered results.

Sometimes doubt is cast as to whether the damper vibration periods can be felt by the driver, the answer is most certainly, yes. A particularly good example occurred at Crewe, which gives credence to this point. In this case crankshaft periods in some reconditioned eight cylinder engines were detected immediately by drivers who had no engineering experience. A number of B80 engines had been reconditioned during which the crankshaft damper type was modified, during this modification the retaining bolts for the rubber blocks should have been changed. However due to an error the old longer bolts were refitted and this had the effect of locking the dampers solid and the engines placed in stock. When the engines were fitted to vehicles the vibrations were immediately picked up. These vibrations could be and were duplicated on the company test beds. Nobody explained how the engines passed test in the first instance!


As the main objective of this current article is to illustrate certain points regarding the rebuilding of the damper the historical comments are limited and some of the procedures may appear to be radical to the reader without the full historical background. However the methods are based generally upon archive research.

They are also the result of experimenting with different engine, engine damper and chassis combinations, which may have an adverse effect on a normally sweet running car or cause combustion roughness. These include using a 1953 R type Automatic chassis B87 UL fitted with engine dampers utilising three different friction washers, carburettors fitted with needles SN,SP,SH,TA,TJ,TV and TW, ignition timing of TDC, 2 BTDC, 4 BTDC and 6 BTDC, valve timing normal and advanced, compression ratios of 6.75:1 and 7.25:1, and using a combination of axle ratios of 3.72:1, 3.42:1, 3.08:1 and 2.92:1. Five and six blade cooling fans and normal and high-speed fan pulleys were also tried, as the drive originates from the damper hub.

Enthusiasts who wish to learn of the early history of the R-R crankshaft damper should read Tom Clarke's book, which is published by the Rolls-Royce Heritage Trust, entitled 'Royce and the Vibration Damper'. This book, besides covering the history of the damper superbly, also contains a very extensive and interesting appendix on the theoretical workings of a damper, by Ken Lea, formerly Director and Chief Engineer (Power Train) at Rolls- Royce Motors. He makes a very true and interesting comment on page 116. He say's “Let me be very clear that if the 3rd order frequency approaches natural frequency (of the crankshaft), there is no damper in this world that will cope with the resulting transient energy inputs”. As we shall see this true statement has some influence on the steps taken to rebuild our damper.

Why fit a Damper?

The fired gas pressure impulses cause the crankshaft alternatively to wind up and then unwind to a very fine extent. Although the movement may be fine it is happening many times per minute. The longer and thinner the shaft, the worse becomes the problem. Adding balance weights and long piston strokes with little overlap of main bearings and journals, and high engine RPM aggravates the issue. In these circumstances the critical 3rd order frequency on a six cylinder inline engine in particular, will usually occur very close to, or within, the maximum attainable engine speed. In short the natural frequency of the crankshaft becomes low enough to equal the firing impulses at a particular engine speed.

Assuming a clean working damper, the archive shows that the 3rd order natural crankshaft frequency is 15410 cycles per minute, equivalent to 5136 engine rpm. On the other hand a solid sludged damper can reduce this 3rd order frequency to 11460 cycles or 3820 rpm. These calculations were later proven on test rigs with both new and sludged dampers. It was further confirmed that these calculations placed the node or reaction point on the crankshaft between the centre of no 6 cylinder and the flywheel.

A damper is used to counter the effects of the vibrations and I have already made mention of Ken Lea's comment that no damper will handle 3rd order frequency and that engine limit is unlikely to be reached by the normal driver. In fact in the case of the early post war 4.25 ltr Bentley MkVI engine extremely unlikely, unless the damper is unserviceable, as the maximum practical engine limit is around 4000 rpm. However overrunning conditions with a closed throttle from high engine speeds do bring dangers.

The average driver will however experience the next worst vibration period at 2500 rpm or half the engine critical speed, whilst not breaking the crankshaft this vibration can and does cause other damage. In practice it is pointless trying to protect against 3rd order frequency but the damper needs to be capable of handling the half engine period and other minor disturbances. This article aims towards providing some damper protection during these phases, which are experienced in the normal driving range.

Although I do not like presumptions, one must presume that the normal torsional forces at play at the half crank period of 2500 rpm are also affected in a similar fashion by a sludged damper, and the critical RPM band is altered accordingly.

More often than not enthusiasts tend to brush off the importance of the damper, little realising the implications in terms of damage. So the few lines below are intended to bring those implications to the forefront and explain what damage can be expected.


Running an engine in the half period condition without effective damping will hammer the timing gears and spring drive. The result will be broken springs both in the spring drive and those controlling the pressure plate. In quick time the timing gears will be damaged and gear rattle will be heard. This can be prevented.

Damage that is expected to occur if the engine reaches 3rd order frequency (5000 rpm), is substantial. It is best described by listing the damage on an engine after a company attempt to deliberately break a crankshaft on a 4.25 ltr engine. This crankshaft test was contained in the EER 1027 report of 4th April 1955, a long time after this engine had ceased to be fitted to car chassis.

The 4.25 ltr crankshaft was fitted into a 4.5 ltr block with the main bearing caps to accept the earlier thin end web shaft. The engine was run up to high speed for one minute in periods of 19, 12 and 29 seconds. Each period being caused by some hiccup such as a plug lead coming adrift, and not scientifically determined, and of course with gas load reversals!

During the examination, after the test, it was found that six sump setscrews had dropped out, all the others were loose, the S.U dashpot screws were loose and all the flywheel nuts, except for four, were loose. The real failures were much more serious. A crack had opened up 180° around the centre of No 6 crankpin and there were cracks in the radii of No 5 and No 6 crankpins. The piston on No 6 had struck the cylinder head when the B/E on that cylinder had failed, due to the cracked journal, and in the course of the failure the connecting rod had bent. Extreme fretting had occurred, on the feet joints of No 4, 5, 6 and 7 main bearing caps and between the flywheel and crankshaft flange. In the latter case local welding had occurred between the face joint. Heavy stop damage was present on the crankshaft damper spring drive dogs and the joint on the rear cotton duck washer had started to fail. The taper on the crankshaft forward end had also fretted heavily on the damper drive taper. Further investigation showed that all the main bearing shells had wiped their lead plating and a ring of metal had broken off the timing case adjacent to the acme thread.

Most owners would not be overjoyed with the bill, the moral is keep the RPM down and make sure the damper is working.

The torsional stresses existing in the node section of the crankshaft might be very illuminating to the owner enthusiast and just may serve as a warning to those who insist on taking these engines through and beyond the normal working rpm band. Notice how steeply the torsional stresses rise just between 4000 rpm and 4500 rpm in the results shown below.

Journal

Speed 4000 rpm, stress 2580 lbs sq in.
Speed 4500 rpm, stress 8180 lbs sq in.

Crankpin

Speed 4000 rpm, stress 6340 lbs sq in.
Speed 4500 rpm, stress 20400 lbs sq in.

If you are still not convinced of the possibilities and insist on being a motor mover and not a driver examine the following few failure illustrations.

Fig 2. Fretting of the spider faces on the crankshaft gear after damper failure.

Fig 3. Friction drum fretting on the crankshaft nose, neither the crankshaft nor drive springs are much good afterwards.

Fig 4. It takes but a few seconds to do this and destroy the main bearing caps!

Fig 5. The ultimate, crankshaft failure, just before parting company with the flywheel.