Probably more has been written on the subject of crankshaft dampers than any other subject on Rolls-Royce engines and hours of time must have been expended on the setting up of dampers. The whole subject is one to set alight any editor’s e-mail inbox or postbag.
With some notable exceptions, most of the previously written technical considerations are directed towards the choice of different damper friction materials or the arbitrary poundage setting of the damper. Most acknowledge the ability of the dampers to break, seize or stick, but suggest no long term cures, and even less has been said to indicate the circumstances during which a crankshaft will suffer damage, other than sheer engine overspeeding. Even less has been said on the subject of the last of the 4.5 ltr engine dampers, that had Ferodo friction washers and slotted dampers and which I covered briefly in previous articles.
One is forced to ask the questions, what happens if the damper is sludged or otherwise not working correctly and what can go wrong? What can be done to improve the situation? Possibly, more importantly, what did the company do to address the situation and what did they know? Whilst conducting research on the Ferodo dampers it became apparent that very little, if anything was known and inquiries to various specialists drew a blank. Fortunately the archive contains the answers to most of these questions and provides an interesting history, and the reasons why the original dampers with cotton duck washers were also modified.
Although this article is not intended to cover crankshafts in detail it is impossible to discuss the dampers without outlining some history of the post war six cylinder crankshafts. The results of a number of company reports, documents and memo’s on crankshafts and dampers have been paraphrased, hopefully without losing the true translation.
Of all the engineering staff who were involved in crankshaft dampers at company level, one stands out in particular. He was Stanley Bull. In my own opinion he was so important to this later history of the damper. His approach seemed down to earth and practical, to say the least. Further consideration of the damper would, in fact, be an injustice without giving some background to this very practical engineer.
Stanley Bull joined Rolls-Royce in 1919 and knew Royce very well. He worked for Harvey Bailey Snr during the last war. Stanley, known internally in R-R parlance under the initials SB, became technical manager for cars, initially in Derby and then in 1945 at Hythe Road service depot in London. In 1959 he took up the post of World Service manager at Crewe and then became Service Director. After completing 50 years service with the company he retired in 1970. Sadly he past away on 5th August 1976 aged 72 years. An engineer, who appears to have kept a level, practical, engineering head. We shall see shortly how the initials SB featured in this story.
I make no apology for reminding enthusiasts who wish to learn of the early history of the R-R crankshaft damper that they 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 by Ken Lea, formerly Director and Chief Engineer (Power Train) at Rolls- Royce Motors.
I would also remind readers to compare comments and particularly a later section of this article, called Actual Crankshaft Breakages, with a comment by Ken Lea on page 116 of this booklet. In no uncertain terms he points out that the damper, and no doubt the engine is doomed should the 3rd order frequency approach the natural frequency of the crankshaft. It is indeed educational (if you are interested in cars) to see just what parts break, and I hope that this article will leave no doubt, whilst covering a very interesting part of history.
Part 1 of this series of articles set out to explain some of the more practical aspects of the post war crankshaft dampers, whilst this article part 2 sets out to tell some of the later history. Inevitably some points are repeated to complete the story.
Reported Crankshaft Damage
I have known of two cracked 4.5 ltr crankshafts, one visual and one found by crack detection, other specialists to whom I have spoken have admitted to never witnessing a damaged post war shaft. This is not to suggest that shafts cannot be damaged, but probably the occurrences are not so common as generally believed and when they do occur are the result of sheer vandalism on the part of the driver. However there is no doubt that many owners have experienced crankshaft torsional vibration, which has spoilt the experience of driving one of the company’s products to say the least. It is not surprising that some specialists have never experienced a broken post war crankshaft. The company themselves did not face such an experience until November 1953, and it came with a vengeance!
At this particular time the company was in the middle of quite intensive experiments to improve the crankshaft liveliness and balance of the Siam engine, later to enter production as the 4.8 ltr Bentley S1 / R-R Silver Cloud 1 engine. During the course of the following tests on dampers and crankshafts and their endeavours to improve on existing crankshaft vibrations, the company had a few surprises. They found that the swash or wobble on the flywheels reached 0.020 inch, that the outer rim of a damper also would move axially 0.020 inch and that the crankshafts acted in a concertina fashion by lengthening and shortening from front to rear. To add to the troubles the concertina action was not consistent with the front and rear ends acting together, but each end could have a mind of its own.
The first engine crankshafts were not fully balanced until late in 1953. The company, at this stage, were pursuing both out of balance and torsional vibration problems in parallel and this story crosses both paths. It is normal for the crankshaft vibration to be felt around 2500 rpm but company engineers had identified particular unbalances during other RPM ranges and even engines with trembles at idle speed.
A number of noteworthy points arose from the tests and in July 1953 the Engineering Department Report EER 627 was raised. This was conducted on an inter Siam crankshaft part number UE 209. It highlighted, for the first time, three specific points when it says amongst the general conclusions:-
“The tests show the present method of balancing rotating assemblies to be unsatisfactory”
“The most satisfactory solution would be to balance the rotating masses (and possibly the reciprocating masses) in the crankcase. It is suggested that the possibility of obtaining a dynamic balancing machine as outlined in this report, should be investigated”
“In the meantime it is recommended that all clutches be balanced when assembled on the flywheel, the necessary corrections being made by drilling the pressure plate through the springs”
Although the company did possess a dynamic crankshaft balancer, this was fully engaged on balancing military crankshafts. The EER 627 report highlighted some of the problems experienced and goes on to outline crankshaft balancing methods thus:-
“The present method with hand change gear boxes is to balance the crankshaft and flywheel together on knife edges, any bias being removed by drilling on the flywheel periphery. The clutch is then added, and tried in three possible positions, the best being selected. The clutch position is then marked, no further corrections being made. The clutch driven plate is also added without further balancing”
The last comment is very important because it was found that the largest out of balance force on the crankshaft assembly was the clutch driven plate itself. Indeed on this particular test it proved hard to obtain one in the whole works, which was near enough in balance to be useful. The report recommended, amongst other points, that all clutch driven plates be henceforth balanced separately on a mandrel.
Besides highlighting that crankshafts were not dynamically balanced, this report also discusses the use of a special rig, which was later modified to accept a disused alloy crankcase and later mounted on an Avery balancer. The alloy crankcase was one of a small number, one of which was especially used for testing pistons running directly in parent alloy bores. The special rig consisted of an inverted crankcase, which rested on a rubber sheet, in which the crankshaft assembly was rotated whilst being supported in no’s 1, 4 and 7 main bearings. The balance was checked by the movement of the crankcase and was sufficient to detect differences due to an out of balance clutch drive plate. Up to a point the simplest ideas work!
Only in December 1953 was the finalised dynamic balancing rig instructed for use, initially with all engines fitted with manual gearboxes although the balancing of automatic gearbox dressed engines was also becoming a problem. The glitch here was ensuring that the torus cover was filled with oil and the air was completely expelled.
Balance problems had been a thorn in the side of engineering staff for some time. There are regular references to out of balance crankshafts due to the bowing of the shafts after nitride hardening. Very strangely, the company allows a maximum in service bow of the crankshaft of 0.010 inch, far more than they experienced on production, and yet their small production tolerance was promoting balance troubles! There is no doubt that, nitriding apart, these shafts have a tendency to bow in service, and even a third of the allowance allowed by the company will make it almost impossible to centre and regrind the shaft, unless it is straightened beforehand. This is not an impossible task, but very time consuming, and it is imperative that these shafts should checked for truth and straightened before regrinding.
Actual Crankshaft Breakages
Here Stanley Bull (SB) enters our picture and his report of 18th November 1953 was not one the engineering staff at Crewe would have been happy to receive, in fact, at the very least it would have spoilt their day! The first sentence read, “ We have to report two broken crankshafts, which have occurred on large bore Bentleys”. (Large bore in this case means the 4.5 Ltr engine.)
Both these crankshafts had failed in an identical manner, namely across no 6 crankpin adjacent to the face of the forward web on that crankpin. In simple terms the failures were at the node point, or position on the crankshaft at which the torsional forces are calculated to act in the greatest manner. One shaft had broken completely, whilst the other was in an advance state of cracking.
The chassis numbers were B226 MD and B168 SR, both fitted with 4.5 engines and having completed 24,000 and 19,000 miles respectively. The crankshafts on these 4.5 ltr engines were RE 11551 differing significantly from the earlier 4.25 ltr crankshaft, in having a thicker rear end web. Also significant was that both these cars had manual gearboxes where down gearshifts were possible at high speed with no gas pressure above the pistons. Within months B 226 MD, in the same ownership, was to break yet another engine crankshaft, now making three shafts broken in service! This latter chassis was undoubtedly being driven with little sympathy for its engine survival!
This internal memo certainly caused the consternation one would expect. By early December 1953 the stresses in the crankshaft node area had been reworked, 3rd order crankshaft frequencies rechecked and tests arranged to find out the engine speeds and conditions that would break a crankshaft. Of particular interest to this current article is that it was feared that sludged dampers might even bring the engine crankshaft critical speed down into a normal drivers operating range, and this was confirmed.
Assuming a clean working damper, archive calculation showed that the 3rd order natural crankshaft frequency was 15410 cycles per minute, equivalent to 5136 engine rpm. On the other hand a solid sludged damper reduced the 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.
Over the years warnings regarding sudden throttling back from medium to high speeds, which have astounding consequences in immediately reversing the stresses, have been few and far between. Suddenly releasing the throttle and hence gas loads allows a wound up crankshaft to immediately and uncontrollably unwind. This condition is exaggerated by the overrun driving inertia of the car, and not helped if the engine is suffering maladies such as a misfire, when the gas loads will be erratic. It is possible that during the following related company tests, that throttling back between test cycles or misplaced plug leads may have been more significant than was believed at the time.
