Operation of the Crankshaft Damper and Radial Spring Drive

Refer to Fig 1 and note that only the RE 6714 friction drum is fixed to the crankshaft, every other component is part of a floating inertia mass or flywheel. This 'Flywheel' is made up of many components including the crankshaft pinion, pressure plate, front hub, pulley, starter dog, front wheel and rear drum,
all driven through the friction of the EW 935 cotton duck washers. (These particular cotton duck washers have not been available for some time and will be called Friction Washers for simplicity).

As the crankshaft twists, the flywheel part of the damper attempts to follow the twist of the crankshaft nose. The flywheel mass then applies an inertia effect that damps the amplitude of the crankshaft twist as movement is reversed. The resulting very fine but rapid movement allows the energy to be dissipated in the form of heat through friction between the friction washers and their mating metal surfaces.

A spring loaded flexible pressure plate applies the load to cramp the friction washers between the pressure plate face, the friction drum and the rear drum, thus connecting the flywheel mass to the crankshaft.

The total damper movement is limited by the radial limits of the spring driven crankshaft gear, the periodic oscillations of the crankshaft are minute and only in the worst instance would these spring drive stops come into play. The radial spring drive itself also benefits from the friction damping stopping spring rebound and oscillations.

This spring drive is made up of inner and outer springs positioned radially between the four lobe spider extension of the crankshaft gear and the four internal webs of the crankshaft fixed friction drum. This arrangement allows the crankshaft pinion to be driven through a flexible drive and retains a constant tooth load between the timing gears, irrespective of load variations or reversals.

The Main Crankshaft Damper Parts

Front Hub

The front damper hub RE 4032 contains one of the bushes that allow the damper assembly to float on the crankshaft nose. The alignment and mounting of the hub together with the crankshaft pinion that contains the other mounting bush is critical to the operation of the damper, spring drive and timing gears. When the bush in this hub or that in the crankshaft gear is renewed it is imperative that the two parts be bolted together in their normal running position before the bushes are bored to size. Absolute concentricity of the bushes and crankshaft nose bearing area (shown at A and B in Fig 6), is vital.

Serrated retaining nut

A right handed serrated nut EB 3227 retains the whole assembly onto the crankshaft. It is quite important that this nut is tightened by a wrench made for the purpose, after the special EB 3226 washer and lock tab have been fitted. To those who may be inclined to just punch the nut around to tighten it, I suggest that they should read once again the previous sections outlining the potential damage and forces involved. I have personally seen the results of one of these dampers actually detaching itself from the crankshaft, due to some misguided individual who believed the nut could be punched up tight enough. The damper physically cuts itself right through and out of the front timing case before the engine comes to rest. If you punched up one of these nuts don't think you are safe because it has not happened yet…………. these dampers react to crankshaft torsionals. When the serrated nut is tightened a hardwood block will require inserting between the very foremost crankshaft web and the inside of the block, making sure that the block does not contact the No 1 connecting rod cap. Locking the flywheel or blocking the rear portion of the crankshaft to prevent rotation needs avoiding at all costs. This blocking sequence and warning also applies when the front pulley nut is tightened.

Front Wheel

The RE 4030 front wheel provides a large proportion of the weight mass and houses the pressure springs which control the pressure plate. The spring recesses also act to hold and locate any spacer washers which may be needed to increase the total pressure plate loading, but without the springs being choc-a -bloc.
Radial Drive Springs

Eight inner springs E 57758 are located inside eight outer springs E 58239, the combined springs are fitted between the crankshaft gear lobes and the webs of the friction drum. The inner springs are loaded to 10 lbs ( +/- 6 oz) when compressed to 0.525 inch, they are specified to be left hand wound. The inner spring E57758 shown in Fig 8 as incorrect is detailed on the part drawing with the opposite handed coiling. This would make this spring incorrect but some older dampers that have been stripped do have inner springs with handed coils as shown in Fig 8. The correct situation with this spring remains unclear, but it is available in the market place wound both left and right handed.

The outer springs when compressed to 0.525 inch are loaded to a minimum of 55 lbs and a maximum of 61 lbs. The maximum tolerance is preferable when late type camshafts are fitted. It is essential that these springs must be opposed matched and fitted in order that the damper runs centralised. The outer springs are drawn as left hand wound and although archive drawings are not specific it is believed that all inner and outer R-R genuine springs are now left hand wound from 2003. These springs are shown in Fig 7 and Fig 8.

A method is shown in Fig 9 of achieving mirror or opposed matching of these springs for load at a given length. The radial drive springs of the same strength need marking and eventually placing in opposing locations in the radial drive, this is extremely important in order to attain timing gear silence, spring balance and accurate running timing. In the example shown in Fig 9 the opposing spring was 0.90 inch different in length when this spring was compressed to 0.525 inch, a very good example of why the springs must be matched for load against length.

Pressure Plate Springs

The six RE 3943 pressure springs apply the load to the friction surfaces, being interposed between the front wheel and pressure plate. The interesting point about all these damper springs, including the radial drive, is that the drawings show them all as being left hand wound. These springs must be matched before fitting, by positioning them in a vice, in pairs, back to back. When one spring has been depressed 0.825 inch the other spring should match. With some judicial swapping around matched springs can be selected that will be positioned oppose each other in the final assembly. This spring matching is vital if the pressure plate is not to act like a swash plate in service, and the applied load is even across the friction washer facings. Each spring is rated at 53.5 lbs when in the fitted position compressed to 0.825 inch. The load tolerance, which gives us a problem, is +/- 3.375 lbs. A quick calculation shows that the total variation is very wide and the highest limit is really required. Free length is approximately 1.275 inch. The handing of these springs as drawn is shown in Fig 8 and Fig 10.

Pressure Plate

The RE 3942A pressure plate is secured between the front wheel and rear drum faces and when in this position the friction face must be exactly parallel to the outer joint. It is worthwhile bolting the pressure plate temporary to the rear drum and then checking the friction faces for being parallel by gauging. The outer section of this plate forms a number of fingers to allow the plate to flex when positioned under the influence of the pressure springs. The friction plate section must be a minimum of 0.150 inch thick after any re-machining operations.


It is not generally known that this pressure plate actually lifts off the friction surfaces under certain conditions. This situation was proven during company tests when axial stops were fitted into the damper front drums. The small working gap was found to take up during engine operation and the stops received heavy end wise blows, proving axial movement was taking place. Such a situation demands that the pressure plate load and hence friction slip load need to be increased, one reason for setting the slip poundage higher than usual. A good pressure plate is a major key in this damper operation.

The fingers of the spring disk take full damper torque load and are subject to some axial movement; as a consequence fatigue results and the fingers weaken and break away. A strict examination of the fingers and pressure plate surface is necessary and they must both be in first class order. It is important that the pressure plate is trapped by clamping and not drawn up on its six bolt fixing when the damper is assembled.

Friction Washers

The original cotton duck friction 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, although this was not the only failing of the damper design. A rot proofing process was carried out on these washers and they finally finished up with a 6.650 (-50) inch O.D and 4.050 (-50) inch I.D.

Fig 11 shows a set of typical worn out washers after they have been extracted from a damper. Note the square shape of these old washers. This material was in itself never adequate and always stuck on at least two friction faces, and the company never stopped pursuing an alternative. It is not generally realised but later cotton duck washers were bonded across the scarf joint to strengthen the joint, which came apart when under load.

At the end of the R type, Silver Dawn production Ferodo Washers similar to clutch lining material was used together with slotted damper wheels. These appear to have been successful in service and the writer has researched all the drawings and trials with these dampers. When this scheme was tested, so successful were the initial results that production drawings were produced for the earlier pre-war 'Wraith' to be used on early post war engines.

Incredibly at no time during any post war test bed testing of dampers was anything but absolutely clean oil used, not exactly the lubrication exposure dampers in cars were experiencing…..no wonder car and test bed trials did not agree. One reason perhaps why Ferodo material was not standardised although similar contrary results were recorded with cotton duck.

After the special machinery was scrapped at British Belting the friction washers for replacement parts supplies were made from a Tufnol type of material. Fig 12 shows parts from a test damper, in this case the friction washers are Tufnol but they have been drilled to allow oil pooling and grooved radially. This material raised some different problems from the original cotton duck washers and was not successful particularly since they were fitted with the original cotton duck poundage settings. This type of material does not attach itself to the friction surfaces but nevertheless the slip and stiction action is very varied and the slip poundage always drops after some miles. If a damper is set to the lower end of the slip poundage tolerance the drop off in performance can be quite drastic and enough to cause the timing gears to be audible at even idle speeds. Timing gear fretting, even if not audible will occur at critical crankshaft speeds and this is evident on many stripped engines. It is because of the inadequate performance of Tufnol type of materials that suppliers have pursued other materials.


In more recent times “Lamp Wick” material has been offered, although not by the company. This material tends to shred threads and if used with a drilled damper, which is essential, the loose threads pick up on the oil pump intake strainer. It also does not appear to have been passed through a rot proofing process by dipping in a solution of 1 % Sodium Di-Hydrogen Phosphate. The stick stiction with this material is good but it suffers from the original troubles of cotton duck.

Today a variety of different materials are offered, some will have doubtful performance as the dampers have still not been modified to cure other ills. In the writers opinion a number of these materials have promise providing the alterations are made, these including adapting different slip poundage, drilling the dampers, bonding the friction washers and most important rebuilding the damper and applying the correct technique to measuring the stiction and slip.

Friction Drum

This is the only component fixed or locked to the crankshaft; it is located on the crank by three woodruff keys and has two friction faces. The original thickness of the friction face when new was 0.130 inch and after machining it must still measure a minimum of 0.110 inch. The radius where the friction face meets the hub must be maintained to prevent the disc shearing. The drum is retained on the crankshaft key ways by a special washer EB 3226, lock tab EB 3228 and a right hand threaded serrated nut part number EB 3227.

Rear Drum

This component forms the main mass of the flywheel and also one of the friction faces. The depth of the friction face was 0.425 (-3) inch originally, from the outer face, it must not exceed 0.450 inch after machining.

Timing Gears

The crankshaft gear is steel while the camshaft gear is alloy, this allows the camshaft drive to have a low inertia and still retain a degree of quietness. The alloy camshaft gear compensates for the cylinder block expansion and the design is intended to retain the gear centre line positions throughout the thermal range of the engine operation. The alloy gear was originally part RE 9346 and this was fitted right into the start of the 4.9 ltr Silver Cloud engine range. It was superseded by a stronger alloy gear part RE 22149 on the car engines, only so production could be standardised across the car, commercial and military engines. The stronger gear was intended to prevent cracking around the outer rim which occurred on B80 eight cylinder engines, in practice this cracking always occurred from time to time on these B80 engines on particular duties, even when cast iron gears were used. The early RE 9346 gear will suffice on all car engines and at any time when the RE 22149 gear is used on a Bentley R type or MkVI engine with the pre-Silver Cloud timing case it is imperative that the alloy gear is checked to ensure it does not foul the timing case.

These gears are supplied by low-pressure unfiltered oil on car engines even when a full flow oil filter is fitted. A worn or faulty damper is at best likely to have left a build up of debris between the gear teeth, this debris when subject to the working tooth pressures almost becomes part of the tooth. Any gear that is to be re-used should at least be brushed across the teeth with a brass brush, which will remove the debris.

Any engine that has suffered an unserviceable crankshaft damper, particularly if the main crankshaft bearings are worn is liable to have irregular wear on the teeth of the alloy gear. Although it was intended that these gears were to be renewed in pairs, providing the damper and crankshaft mains are in good condition, it is feasible to renew the singular alloy gear. In all circumstances some gear noise may result for up to 2000 miles at idle speed after renewing any of the gears. Once a damper has been operated in an unserviceable condition renewing the camshaft gear will nearly always make the engine quieter.

The holes in the camshaft gear and front face of the camshaft are equally spaced but the tooth mesh is phased such that the alloy gear needs indexing round to match the camshaft holes when valve timing is completed.

It is wise to ensure that the camshaft thrust plate that is mounted behind the cam gear is not turned around in the mistaken belief that this will even out wear. Turning the thrust plate or renewing it will push the camshaft rearwards so that it mounts the wear ridge on each camshaft bearing, failure of the bearings is then a distinct possibility as the ridge breaks away. If the thrust plate is to be turned it is best to consider renewing the camshaft bearings.

The camshaft gear needs checking for run-out by bolting it to the camshaft and taking a clock gauge reading on the outer front face as the camshaft is turned. The run out should be expected to be around 0.001 inch. Both Crankshaft and camshaft gears require assembling in their mated positions and the tooth backlash checking at several points. This backlash measurement of 0.002 to 0.004 inch is taken on the pinion at a measured radius of 1.68 inch from the crankshaft centre.