First Crankshaft Breakage Tests

On 26th January 1954 the first test to deliberately break an engine crankshaft was completed, this report was number EER 783. The rig engine chosen was BM134 built up as a 4.8 ltr six inlet port engine and this dress was probably chosen as it was close to the type which was to be in production within the year. The crankcase was a standard 4.5 ltr unit bored out to 3.75 inch bore and fitted with a six port head and twin S.U carburettors. The exhaust was passed through a water cooled test bed manifold to a 3.0 inch single pipe and thence to two silencers. It was fitted to a syncromesh gearbox run in neutral.

At full throttle the maximum rpm was 4800 and after a minute a plug lead detached. After shutting down and replacing the lead the engine was run again at 4800 rpm for approximately one minute. After this period the exhaust pipe and silencers were removed and the engine ran at 5000 rpm maximum speed for another minute.

The engine was then stripped and the spring drive extracted with some difficulty and, although the slipping load of the damper was 14 – 16 lbs, one friction cotton duck washer was trapped between the friction drum and damper wheel. This trapped washer was fairly significant, as we shall see later during the study of the actual dampers. A typically trapped friction washer is shown in Fig 1 but it should be pointed out that this photograph is from test number EER 1143 dated 19th October 1955. The front taper on the crankshaft had picked up; the flywheel had fidgeted on the crankshaft rear abutment and no’s 5, 6 and 7 main bearing caps had fretted. Trailing all this damage the crankshaft had cracked right around the forward end of no 6 crankpin.

All this damage occurred in the space of three minutes and, as we shall see later, this and much more can occur in only one minute or less. This is the point to remind readers of my earlier warning and that of Ken Lea’s, that even when in good condition, no damper can save an engine when its the 3rd order frequency approaches natural frequency. This situation is clearly demonstrated in this series of tests.

Further high speed testing on this same engine rig was carried out and contained in the EER 926 report of 16th August 1954, when a second crankshaft failure occurred. This second test was time restricted to see if the same amount of damage occurred in a short run period, or if the frequencies needed time to build up. Four short periods of 15 seconds each, were run at 4900 rpm.

After the first two periods, totalling 30 seconds, a damage check was instigated. Inspection revealed that No 5 main bearing cap was broken and fretting had occurred between the three rear main bearing caps and the crankcase. Fretting had also occurred between the flywheel and crankshaft flange.

After an engine rebuild, the test was resumed with a further two 15 second periods of running. A strip examination then showed that fretting had again occurred at the previous locations and this time No 7 main bearing cap had broken. In addition, on this occasion, the crankshaft had suffered cracks on the radii of No 4, 5 and 6 crankpins.

Following these previous tests the company had experienced a crankshaft failure at lower speeds on a test bed engine being run on piston duration testing. Problems occurring with vibrations at approximately 2500 rpm after the initial engine running in procedure had been carried out, with runs above this speed but not over 4500 rpm. In this instance it is very probable that sudden throttling back during the test had some significance.

Quickly following these tests was an in service failure of the crankshaft on Bentley Continental BC12C and much later on 18th December 1956 Silver Cloud LSXA 105, with the 4.8 ltr engine which failed the crankshaft across No 6 crankpin, whilst in a customers hands.

With some considerable evidence now at hand it was decided to test the durability of the 4.25 ltr crankshaft. Previous failures, at this time, had been with the 4.5 ltr crankshaft. This 4.25 ltr crankshaft test was contained in the EER 1027 report of 4th April 1955. The results of this test were discussed in the Part 1 article in these sequences.

In addition to the tests, which have been mentioned, the company were pursuing, in parallel, general crankshaft induced vibrations problems and a number of tests and experiments were conducted. These are too numerous to mention in this article but should any enthusiast wish to pursue that history, the following list may be helpful.

EER 624 on 2nd July 1953, Road tests on forged to size crankshaft UE 290.

EER 692 on 6th October 1953, Testing stiffened short throw crankshaft RE 20319.

EER 731 on 24th November 1953, Loosening of flywheel screws on slave engine no 8, chassis LWEME.

EER 897 on 14th June 1954, Evaluation of various crankshaft dampers on the boom period of Siam cars.

EER 912 on 8th July 1954, Evaluation of engine roughness of crankshafts with different balance weights.

ERR 1051 on 26th May 1955, Crankshaft balance with thin flywheel.

This crankshaft history is by no means exhaustive but sets the scene, at least a little. After completing the tests mentioned it was obvious the company knew that no crankshaft damper could save a crankshaft when it went “critical”. When the crankshaft did reach this point the damage was severe to say the least. Their tests on sludge dampers had indicated that this type of damage could be expected at least 1000 rpm below the point when damage might otherwise be likely.

An extract from a letter dated 17th May 1957, sent by S. H. Grylls, chief engineer R-R Motor Car Division to Mr. Ker Wilson, himself a brilliant engineer, of De Haviland Engine Co. Ltd, is fitting and informative to end this section.

“During the last few years we have found that engine mountings, body mountings and the behaviour of the flywheel are the three things which most determine the smoothness of an engine. The system of balance weights is chosen mostly for reasons of economics and today we use the four weight system pioneered by Jaguar and greatly favoured by the forgers.

We have made two other big contributions to smoothness recently. We now dynamically balance the entire crankshaft and flywheel assembly in a crankcase complete with bearings, in order to overcome the unbalance, which would result from straightening a crankshaft balanced in its bowed condition. Secondly, we determine by experiment the thickness of the flywheel back plate so that the inertia is least disturbed by the behaviour of the crankshaft flange. It looks as if we are on the eve of some further discoveries along this path”

History during 1953 to 1958

This specific period I find is particularly interesting, as it represents the ultimate in the development of the six cylinder damper, answers a few questions possibly hitherto unknown to most enthusiasts, but also poses the question… “What if, damper development had continued?” For reference a typical parts manual view of the damper is repeated in Fig 2 from an earlier article sequence.

Coincidental with the first reports of crankshaft breakages in service, company engineers had been working on experiments using different materials for the crankshaft damper washers. The original cotton duck washers, part number EW 935, were supplied by British Belting and Asbestos Limited of Cleckheaton in Yorkshire, better known under the trade name of Mintex. The cotton duck washers had certain shortcomings, most notably the ability to stick to the faces of the damper drums. Sludging was also a known problem and it is most probable that Stanley Bull from the service side of the company was one of the internal forces that propelled the organisation to try a different approach to the problems of sludging and the shortcomings of the cotton duck washers.

Slotted Dampers and Ferodo Friction Washers

In April 1954, a Ferodo material known as VM41 was under test along with other Ferodo products, and in fact Ferodo themselves had been supplied with equipment by R-R to check the material characteristics in dampers. A month later Ferodo themselves were in the course of rigging up an oscillating device to obtain surface bedding in. It was originally intended that the VM41 material, which was made into rigid disk washers, should ‘always’ be used with slotted dampers, a point worth remembering. By early May 1954 a similar test rig was in the course of production for British Belting, as a company memo of 12th May confirms. By the 24th May this rig, complete with a damper and a damper assembly drawing RE 6715, had been dispatched from Crewe.

The term slotted was used to describe a damper shell or casing that had slots formed around its periphery so that sludge and oil could be expelled and centrifuged out of the damper. The slotted damper and the Ferodo VM41 material was later to be used in 1955, on the very last of the 4.5 ltr engines and was also intended for the later 4.8 Ltr engines. Reference to Fig 1 will show what looks like sludge drain holes drilled in the periphery of the rear damper drum. It is possible that this method was tried to test the theory. Fig 3 shows slots machined in a front drum, this one showing fretting evidence after a crankshaft breakage test.

Production dampers had the slots cast in during the manufacturing of the drums, which no doubt was a less costly exercise than machining. The slotted damper drums and Ferodo washers are listed in the modification bulletin index as the last modification made to engines prior to the launch of the Bentley S1 and Silver Cloud engines. The respective slotted damper drums becoming RE 21657 for the 4.5 Ltr engine and RE 21656 for the later 4.8 Ltr. Each rear drum contained six equally spaced slots 0.500 inch wide and 0.040 inch deep. The Ferodo VM41 friction disk was UE 2549 of some 6.690 inch O/D and 4.040 inch I/D. UE 2559 was a slotted rear drum to suit the early Wraith type damper. All measurements of course were subject to production tolerances.

A slotted rear drum taken from a late Z series R type part no RE 21657 is shown in Fig 4 after considerable service and the damper parts were all still intact and working.