The pursuit of an alternative to the cotton duck friction washers continued, and the initial test results of rigid washer materials from Ferodo were received at Crewe in August 1954.
The test procedures and results are shown in FIG 7 & 8. The company had conducted their own tests and were to eventually decide on the VM 41 material for production, which is shown as item 5 in the results.
Methods of friction measurement and preparation of friction washers
The test conducted by Ferodo and some remarks made by Stanley Bull bought to light some rather strange and fundamental points. To discuss these points we must deviate temporarily from the chronological date order.
Anyone who has overhauled the crankshaft damper on one of these post war six cylinder engines will not have failed to notice the Workshop manual requirement to use a spring balance to determine the slipping poundage. In various articles written on the subject, this method is stressed to say the least, and yet strangely was not introduced as a post war production technique until September 1955.
Ferodo engineers had been well versed in the procedure for testing the slip loads by using a spring balance. When they forwarded their report on 25th August 1954, the covering letter contained the following paragraph.
“ You will also note that the method of checking the pull was carried out by attaching a container to the spring balance and loading this accordingly with water, and it was considered that this was more accurate than trying to determine the pull on the spring balance by hand”
In a memo of 11th July 1955, commenting on the minutes of a Modifications Committee Meeting held on 15th June 1955, Stanley Bull had the following to say.
“ One other point on which Hythe Road differs from Crewe is that we use a spring balance to test the poundage and not a dead weight. The latter may not detect a slipper drive, which is ‘juddery’, but a spring balance will, and this does enable us to throw out any dampers, which have a variation of more than 2lbs, which we know will give trouble” (This also applies when a torque wrench is used, thankfully almost unheard of in the U.K. NWG)
“We have always believed that the rotary ironing process used for many years was better than just compressing the lining, but, of course, this is laborious and it can be overdone.
We have tried to reduce the time required to obtain a reliable friction setting, and we believe this is now possible by a very simple expedient. This consists of soaking the washers in a lubricant consisting of 75 % (Castrol) Hi-press and 25% SAE20………….The washers are soaked for no less than 24 hours, and are compressed for a further 24 hours”
It would appear that Stanley Bull’s opinion was respected as on 2nd September 1955 the following instruction was raised, the full text of which read:-
“Pre-build Treatment of Spring Drive Cotton Duck Washer EW 935
On production the treatment at present given to these washers is to soak them in SAE 20 oil followed by a period in a press. The soaking and pressing periods are supposed to be of 24 hours duration each but the procedure has apparently never been given the full status of a planned operation and the times are not adhered to. We therefore wish to regularise the treatment by making it a proper operation and also bring it into line with that which Hythe Road have found to be advantageous.
The treatment is to be as follows:-
The washers to be soaked for a minimum of 24 hours in a lubricant consisting of 75% Hi-press and 25% SAE 20 oils.
The washers to be transferred directly from soaking into a press where they are to be retained under a pressure of 75 lbs/sq.in. for a minimum of 24 hours.
In addition to this, the method of measuring the spring drive slipping load during assembly is to be carried out by means of a hand operated spring balance instead of by weights as at present. The purpose is to ensure the detection of undesirable stick/slip characteristics”
It would appear from the above evidence that no post war slipper drive was set on production with a spring balance until the Silver Cloud / Bentley S1 engine production had already commenced. Latter tests did in fact show that the dual mix of oils used for initial setting of the friction slipping loads did not give any advantage over the use of straight SAE 20 oil.
Still pursuing a slotted damper
In spite of delaying the introduction of the slotted damper in May 1954, the pursuit continued. Rejoining our slotted damper story brings us back to 21st September 1954, when EER 949 was issued. The title of which was… “Spring Drive Unit, Shortcomings and Recommended Modifications”. The conclusions of which are shown in full:-
“The principle faults and modifications required to overcome them are as follows:-
1. Increase in torque setting due to sludging can be avoided by provision of oil release slots
2. Fatigue failure of the spring plate spokes is primarily due to sludging, and this is relieved by the same
modification.
3. Unsuitability of the cotton duck friction washers in the following respects:-
Inconsistent damping performance due to the presence of a joint.
Tendency to disintegrate or rot apparently as a result of faulty inhibiting treatment --- a not uncommon occurrence.
The ease with which it can become trapped on its outer periphery on account of its soft pliable nature.
Friction disks in Ferodo VM 41 material have proved on test to be at least comparable on general performance. Being jointless and fairly rigid they avoid faults (a) and (c) and there is no indication that they are susceptible to corrosion.
We therefore recommend the introduction of slots and Ferodo washers”
The main body of the report mentions that sludging could be avoided by the use of oil release slots, and this had been proven. That axial vibrations of the pressure plate and friction drum, to surprising amplitudes, appeared be the true cause of friction washer trapping and that this was most likely to occur when the engine was overdriven, not under its own power, to around 4500 rpm. Fatigue failure of the spring pressure plate spokes had been traced to a combination of high torque loading on sludged dampers and bending through axial vibration. It was reported that, except in one instance, no failures had occurred (of spring plates) on dampers with oil release slots. The exception was a suspect case since the pressure plate had previously been used in an unslotted damper. Just after this report was published it was also decided to discontinue the separate balancing of the spring drive sub assembly before adding it to the crankshaft.
Ferodo was still conducting testing in January 1955 in respect of static and kinetic friction properties at various friction face temperatures and pressures. Initial production slotted dampers were being built in December 1954 with the Ferodo VM41 friction washers part number UE 2549. These washers having been reduced in outside diameter and the outer edges chamfered to prevent any fouling between the inside and corners of the damper drum. The slotted dampers were eventually used in the last 94 engines fitted in cars prior to the introduction of the Silver Cloud and Bentley S type.
At least by 1953, company engineers at the sharp end of the business were only too well aware of the problems arising from sludged dampers but this practical experience does not appear to have been shared by all the top division of engineering management. Stanley Bull was still a big believer in eliminating the sludge in dampers, he wrote to Grylls, chief engineer R-R Motor Car Division on 2nd February 1955, on the subject of slipper drives.
“Firstly, I think it is unquestionably a fact that we require oil circulation through the slipper flywheels to prevent sludging, which does appear to be extremely undesirable and probably is a major factor in causing breakage of the spring plate.
So far as I know, the decision to abandon slots has been a result of cases of trapping of the cotton duck washer. The theory presumably being that with better lubrication the cotton duck washer is better able to slide or move radially, and the only place it can move is the annular space between the flywheel and friction plate.
It seems to me that the flexible and deformable material like the cotton duck washer ought not to be spigoted or restricted on its outer diameter, also if it must move radially, there should be space for it to be accommodated.
On our pre-war designs there have been several different ways of avoiding this trouble. In the earlier designs the cotton duck washers were spigoted on the inner diameter and had ample space to spread radially. We had trouble with trapping on the spigot diameter until the dimensions were put right. The cotton duck actually was too small for the spigot.
On the pre-war Bentley no attempt was made to control the cotton duck washer either internally or externally. We had no trouble with trapping, and I do not think there was anything wrong with this design except that today we would probably criticise it because the cotton duck washer could assume an eccentric position and be out of balance.
I would like to add that on all pre-war Bentleys we had oil escape holes in the flywheels so there does not seem any good reason why we should not today have suitable escape holes or slots without detriment to the location of the cotton duck washers.
Summing up, it seems to me that we require oil escape holes or slots and either an internal register for cotton ducks or greater radial clearance to avoid trapping”
Grylls replied the next day, and although his day to day experience in the results of factory testing far outweighed that of Stanley Bull, the converse appeared true in regard to practical up to date experience at the sharp end. He wrote to Stanley Bull.
“……..On the other hand, I think that it is true to say that our customers have had little trouble with spring drives since the war. We have objected to the appearance of sludge and we have objected to broken window plates, (he was referring to pressure plate broken spokes) but the customer has been blissfully ignorant of what is going on. There have been just a few cases where dissembling a spring drive has been said to cure a rough engine, but it is obviously difficult to substantiate this assertion when so much of the motor car has also been disturbed.
Ninety four engines have left the factory with Ferodo washers and it will not surprise me if they all give trouble.
A combination, which proved quite unsuccessful, was the use of cotton ducks and large release slots in the periphery of the damper. FJH thinks a likely explanation is that endwise relative motion of the window plate occurs because we have lost viscous damping. The tiny release holes used before the war may not have had this effect.
It is even possible that the pre-war release holes only worked for a short time and I seem to remember that this was the case.……….”
Stanley Bull’s practical experience was obviously not going to let him rest the case at that point, on 7th February 1955 he replied.
“ ……I am afraid the Service Department has never shared your views that the owner or driver of the car is unaware of the existence of a slipper drive which has become solid or has excessively high poundage.
We have many complaints of engine periods or vibration, which are cured by attention to the slipper drive, and we must have dealt with hundreds of post war slipper drives. Therefore, we need any improvement possible which will maintain a reasonably constant poundage…”
The only other document of interest in respect of the present discussion is report EER 1162 dated 30th November 1955. It is also of interest because, although raised by the same authors, it contradicts the finding of the EER 949 report raised only just over one year before, and is an example of the contradiction of findings throughout the whole subject thread.
The use of axial vibration stops mentioned in EER 1162 had previously been used to prove that the effective damper pressure plate load does in fact substantially reduce during certain periods of damper excitement. A point, which is extremely important to the practical understanding of what happens in practice compared to what should happen according to the masses of theoretical calculation. When axial stops had been originally fitted to test axial movement, the pressure plate had contacted the stops sufficient to make very heavy contact depressions on the ends of the stop studs. In this particular instance, any effective loading on the friction washers has all but disappeared when part of the pressure plate spring load is taken off the friction washers.
EER 1162 is significant in that it appears to close off any further development of dampers. It is titled “Results of various modifications applied to the spring drive on experimental car engines”. The conclusion lists the following:-
“The damping performance of spring drives with oil escape slots has been generally most unreliable despite various modifications.
The existing unslotted unit with its known major drawbacks of sludge choking is therefore still preferable.
We believe that an efficient slotted type can be developed, but we cannot at present afford the time in view of work urgently required on new designs. Further development will be carried out as and when time allows”
The reason for raising the report says:-
“When the combination of a slotted damper and Ferodo washers was found wanting as regards damping performance it appeared that the fault lay in the friction characteristics of the Ferodo material. We therefore tried to retain the slotted unit together with cotton duck washers and the addition of axial vibration stops to overcome washer trapping which was the original objection to combining slots with this type of washer. Experiments with increased pressure spring loads and increased inertia dampers have also been conducted.”
