Comet tanks - fault

Oh my! We are on gear ratios - all addendum and dedendum, I can see an hour of excitement for us gear groupies - off to get my copy of Machinery's Handbook' from the shelf (engineers bible cost me about a months pay in the 1960s started apprenticeship on 1/9d per hour about the cost of ten fags) Right, calculator set up on screen - turn to formulas for gear ratios, come to think of it that's a nice looking ship coming up the Solent!




Here's one- just the one mind!






Gear ratio - Wikipedia, the free encyclopedia--------------------have a care it could be addictive! A tempter:


The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear train. The torque ratio of the gear train, also known as its mechanical advantage is defined by the gear ratio

 

Take a closer look at something in here for a moment...

(1) In considering the failures which form the subject of the present report account must also be taken of other failures which have occurred on machines in the Service. It was these additional failures, occurring at mileages of the order of 100, which brought about the drastic decision to change all final drive gears in Comet tanks already built and to substitute 4.5:1 ratio gears from Cromwell T.F.D.5 assemblies.

If I'm reading that and the subsequent paragraph correctly...the final drive gears were changed because of "other failures"....and THEN the Cromwell 4.5:1 clusters began to fail???

Because the SECOND paragraph seems to be a written vindication of the 4.92:1 cluster!

(2) The relative merits of the gears of the 4.92:1 and 4.5:1 ratios have been fully discussed in Transmission Branch Report No. TN.84/1. Briefly, that report showed that despite a difference in strength between the wheel and pinion of the 4.92:1 ratio gears, calculation indicated that the 64 toothed wheel should not be appreciably weaker than the 54 toothed wheel of the 4.5:1 ratio set. The pinion of the 4.92:1 ratio set should actually prove somewhat stronger.


So...I wonder -

Is this actually a case of the OP's dad remembering the shitty work involved in replacing/re-torquing the taper roller bearing...that would have had to be disturbed at best and replaced totally at worst when the gear cluster was pulled/changed?

Hence...

Ouch! We now have THREE Comet faults from three different sources


It may still just be TWO...the OP's dad may have remembered the nastiest bit of the whole job!

 

I know he doesn't remember the time with any great affection. Whilst the rest of the Regiment were living the life of Larry in Ypres, the fitters were grafting away every night for weeks on end.

Things did improve when they discovered one of the local Frenchmen, who had been employed to remove the dummy village off the workshop roof, trying to nick a barrel of oil. But that's another story......

 

I realise this has been touched on before in this thread but, on the basis of what is known in all the files and reports we have been discussing, why would the Admin History 21 AG report be so specific about replacing the front Tensioner Wheels prior to the Rhine crossing when they clearly weren't replaced with the cast steel type until post-War; no-one could get something that wrong unless there were actually two versions of the early rubber-tyred Tensioner/Idler wheel?!.......and Grimmys Dad has either forgot that bit (which I doubt because he sounds very 'with it') or wasn't involved with that particular piece of work (which again flies in the face of all the evidence. A real puzzle is this one....

 

The National Archives Reading Room was extremely busy yesterday and my main object was to look at some original card indices created by people involved in the European War Crimes process. I did not ask the staff to untag the file so that I could copy the photographs which I could see showed broken gears etc. One of the files had obviously never been requested because a new file cover was created and the wrong document put inside, which took an hour to resurface.

I have typed the remainder of the first part of my partial transcription in my last post and will post it as soon as I am happy that it contains no errors.

I think it is worth saying that the report does indicate a large number of failings.

I would have liked to have seen the names of all the suppliers and sub-contractors involved but what we get on the distribution list are the names of the companies that carried out the metallurgical analysis.

There is a marked reluctance to name suppliers.

I am certain that there would have been all sorts of civil claims arising out of the series of mishaps indicated, and I wonder what further material might lie hidden at Kew.

 

The chronology of this saga is difficult to fathom but the earliest document, although reprinted in 1951, is the repair guide, which has nothing in it about hundredths of inches tolerances on bearing adjustments ( if that is the correct terminology).

The time given to do the job which appears to be relevant seems rather mean. Hats off to anybody who could do this.


WO279/190.

"REPAIR IN THE FIELD

Guide For The Removal and Replacement of Assemblies in Comet Tank

Prepared by R.E.M.E. School

February 1945 reprinted by the War Office January 1951


"Foreword.

1. The instructions contained in this booklet have been prepared with a view to assisting R.E.M.E. Workshops in the removal and replacement of major assemblies and components.

To re-assemble or replace reverse the operation


(...)


4. More information has been put into this publication than in the previous ones in the same series issued from this Establishment, which it is hoped will add to the usefulness to R.E.M.E. "

page 15

(8) REMOVAL OF FINAL DRIVE (Operation E)

1. Remove rear section track-guard.

2. Break track at rear.

3. Remove driving muff. ( See Gearbox dismantling)

4. Secure track brake.

5. Remove slotted nut securing input coupling.

6. Remove sprocket end cap. (3 set screws)

7. Remove locking bolt for nut securing sprocket.

8. Remove nut ( Spanner 65727).

9. Remove sprocket ( sling to bar on hull side)

(page 16)

10. Remove bearing housing for outer bearing on input shaft.

11. Drain oil ( Allen Plug in base) one gall.

12. Remove set bolts securing outer case to hull side. (21 bolts)

13. Remove case (use 1/2" B.S.F. withdrawing screws) 4 bolts.

14. Remove bearing from outer end of input shaft ( withdrawing tool needed).

15. Lift large gear away ( take care of speedo drive on N.S.)

16. Remove input shaft (jack against output shaft of gearbox, or if gearbox is removed, remove drum and withdraw coupling). Care must be taken not to damage Perfect oil seal which remains in position in final drive housing. It may be advisable to pack input coupling against bolts securing brake mounting to prevent outer end of coupling fouling the oil seal previously mentioned."



In a schedule the suggested times and manpower given are:

3 hours to remove

4 hours to replace

2 fitters required.

 

The second document at Kew in date order is WO194/951 and I give here a transcription from the front cover to the end of the distribution list. This document was published before the metallurgical report and only refers to three specific final drives, whereas the metallurgical report refers to thirteen, and was published in April 1945.

This takes in the extract posted yesterday but is not the complete document, which has appendices of data and photographs firmly welded to the file in an envelope. The printer seems to have taken the printing instructions from the author fairly literally. With the usual apologies and acknowledgements:




WO194/951

Front cover.

D.D.(MAT)

129/91/201

SECRET

The information given in this document is not to be communicated either directly or indirectly to the press or to any person not holding an official position in His Majesty's Service.

FIGHTING VEHICLE PROVING ESTABLISHMENT

Report No. W.S.504

with

Department of Tank Design Cover Sheet

on

FAILURE OF FINAL DRIVE GEARS FITTED TO COMET TANK W.D. NO. T.334906

Chertsey Surrey

Copy No. 12


( end of cover )

COPY NO: SECRET

A.F.V. : Comet
File ref: 192/69/1
Project Nos: D-E.10090 & D-E.10206
Date: 9th March, 1945.

DEPARTMENT OF TANK DESIGN
COVER SHEET & JOINT D.T.D./F.V.P.E. CONCLUSIONS TO F.V.P.E. REPORT No. W.S.504
FAILURE OF FINAL DRIVE GEARS FITTED TO COMET TANK W.D.NO. T.334906

ORIGIN:

These trials were arranged in order to determine the life of the final drive gears of the Comet tank and the reasons for the premature failures which had been experienced.

COMMENTS:

(1) In considering the failures which form the subject of the present report account must also be taken of other failures which have occurred on machines in the Service. It was these additional failures, occurring at mileages of the order of 100, which brought about the drastic decision to change all final drive gears in Comet tanks already built and to substitute 4.5:1 ratio gears from Cromwell T.F.D.5 assemblies.

(2) The relative merits of the gears of the 4.92:1 and 4.5:1 ratios have been fully discussed in Transmission Branch Report No. TN.84/1. Briefly, that report showed that despite a difference in strength between the wheel and pinion of the 4.92:1 ratio gears, calculation indicated that the 64 toothed wheel should not be appreciably weaker than the 54 toothed wheel of the 4.5:1 ratio set. The pinion of the 4.92:1 ratio set should actually prove somewhat stronger.

(3) These findings are confirmed to some extent by the failures accompanying the track shredding trials with Comet T.334906, described in this report, and a failure which occurred on Cromwell T.121728.

In both cases the tracks became jammed and probably full engine torque in 1st or reverse gears was transmitted through the final drive.


In the case of the Comet the gearbox output shaft was broken and the sprocket shaft twisted.

The Cromwell gearbox output shaft and the variable speed shaft were both twisted and had all but failed.

It is reasonable to assume, therefore, that the torque transmitted had been of a similar order in each case. The condition of the final drive gears were as follows:-

Comet Main Gear. One tooth broken away. Many others cracked at root.

Cromwell Main Gear. One tooth broken away.

Comet Pinion. One tooth cracked. Faces of several others crushed.

Cromwell. All teeth broken off.


- 2 -

These results would suggest that other things being equal the Comet 4.92:1 ratio gears are not necessarily inferior to the Cromwell 4.5:1 ratio gears.


(4) It is apparent, however, that other things have not always been equal.

The metallurgical aspect has now been thoroughly investigated and D.T.D. Materials Division will be reporting on this in due course. From the evidence collected two main facts have emerged.

(i) A mix of steel having a high phosphorous content (0.06%) has been used for some of the forgings. These have yielded Izod values as low as 8-10 ft.lb.

(ii) Forgings have suffered from over-heating to a greater or lesser degree. Of the specimens examined by Leyland Motors laboratory those from one supplier showed only 20% which were completely free from over-heating while from another supplier every specimen examined had been over-heated to some extent and 60% of the specimens had been badly over-heated.

In view of this evidence it has been decided to scrap all billets and forgings at the stampers works and all forgings, partly machined and finished machined gears at machinists works or assembly plants.

(5) The forgings for the gears in the T.F.D.5 final drive assemblies, which are being used as an expedient until production is resumed, have all come from a different source. This source will in future also supply forgings to the firms manufacturing Comet final drive units.

(6) The metallurgical reports on the gears forming the subject of this report are briefly as follows:-

Description of Gears: Original standard gears.

Mileage: 1050

Conclusions from Metallurgist's Report: Analysis normal; surface hardnesses satisfactory. Core hardness of pinion low corresponding to low tensile strength ( 69.0 tons/sq.in.). No evidence of over-heating in sections of pinion examined. Very slight evidence of over-heating in teeth of wheel.


Description of Gears: Replacement standard gears.

Mileage: 95

Conclusions from Metallurgist's Report: Analysis normal; surface and core hardnesses and mechanical properties all satisfactory. No evidence of over-heating in sections examined.


Description of Gears: Tip relieved gears.

Mileage: 994

Conclusions from Metallurgist's Report: Analysis normal. Surface hardnesses, core hardnesses and mechanical properties satisfactory except the Izod value of the wheel ( 20 ft.lb.). Evidence of over-heating near tip of pinion tooth. Over-heating very evident in one tooth of wheel and slightly evident in a second tooth.


(7) From the above report it will be seen that the set of gears which failed at 95 miles under conditions of severe overload were sound gears as far as material is concerned. This evidence is consistent with the associated failures of the gearbox and sprocket shafts.


- 3 -

The pinion of the original set of gears had a low core strength which, together with some over-heating of wheel, probably accounts for the failure.

Both wheel and pinion of the tip relieved gears had been over-heated and the Izod value of the wheel material was low.

(8) Attention was drawn in Transmission Branch Report No. TH.84/1 to the higher surface stresses with the 3/4 D.F. stub teeth of the 4.5:1 ratio gears which would consequently be expected to display earlier signs of pitting.

The 4.5:1 ratio gears fitted to the left-hand final drive assembly of Comet T.334906 were inspected at 732 miles and found to be extensively pitted.

The taper roller bearings supporting the pinion, however, were found to be out of adjustment and this undoubtedly largely accounted for the condition of the gears.

Signs of pitting were also to be seen on the pinion of the right-hand drive assembly in which the bearings were in better adjustment.

The gears were refitted for further running and have now covered more than 1,000 miles.

(9) The extent to which manufacturing blemishes may have contributed to the failures experienced is rather over-shadowed by the metallurgical evidence now available.

The failed standard gears which covered a total mileage of 1539 exhibited typical fatigue characteristics and cracks visible at the roots of many other teeth. These may well have started as a result of careless grinding leaving residual tension stresses in the case.

CONCLUSIONS:

(1) From the evidence now available it would seem that the bulk of the failures experienced can be ascribed to faulty material.

(2) The absence of tip relief and the presence of other manufacturing blemishes may have been contributory factors in some of the failures.

(3) The pitting of the 4.5:1 Cromwell gears at present running in Comet T.334906 is considered to be largely due to slackness of the pinion bearings, which arose from the use of incorrectly machined pinions.

It will not be surprising, however, if pitting of these gears is experienced when they are fitted to Comet tanks, in view of the fact that slight misalignment is sufficient to produce this condition when running in Cromwell tanks and the Comet is 4 1/2 tons heavier than Cromwell.

FUTURE ACTION:

(1) In view of the faulty material which has been discovered all billets and forgings at the stampers works and all forgings, partly machined and finish machined gears at machinists works or assembly plants for 4.92:1 ratio gears will be scrapped.

(2) As an expedient all machines in service and on the production lines are being fitted with 4.5:1 ratio gears from T.F.D.5 assemblies.

(3) In view of the fact that several hundred machines will thus have 4.5:1 ratio final drive gears, production will switch over to this ratio but the face width of the gears will be increased to 3.25 inches.

(signed) G.V.Cleare
For Asst. Director (Transmission)

(signed) (?)

Director


( end of page )


Distribution:

D.G.A.F.V. 1
D.D.G.F.V.(E) 2
D.D.G.F.V.(F) 3
F.V.P.E. 4-6
B.A.F.V.(T) 7-10
DD(TC) 11
DD(MAT) 12
T.R. 13
C/DM 14
V/TN 15,16
D.D.F.V.(D) 17
D.F.V.P. 18
C.I.F.V. 19
D.R.A.C. 20
D.D.R.A.C.(T) 21
D.M.E. 22
Leyand Motors Ltd. 23
Mond Nickel Co. Ltd. ( Mr.Johnson) 24
DD(RR) 25
Spares 26-30

C.S.A.24(s) "



End of 4th page. There are further pages.

 

Brought a smile to my face, one of the apprentices bought a newish bog standard mini 67/68ish he had the block out and we fly cut a few thou off that, bored the cylinders to the new larger pistons, a few other tricks, finally two bloody great big Webber carburettors, new big bore exhaust to allow all of this out t'other end. Lasted about a week, he had just got through town when it was goodnight nurse one dead mini, when it was recovered to the workshop it was a sorry sight, he had done all this and the new found torque was introduced to a baby gearbox! Baby was not ready for the big bad world and expired in bits. Not one slide rule was used - in fact I don't recall a calculation of any sort.


Reading through the reports it would appear it is a case of - as many engineers call it -'chasing rainbows' trying to pin down a problem and as you are about to jump for joy someone says . ah but you did know this was happening too, didn't you? Looks to be a catalogue of problems, design - gear ratios, material spec, and failing to produce to that spec. One point that jumps off the page, - (The failed standard gears which covered a total mileage of 1539 exhibited typical fatigue characteristics and cracks visible at the roots of many other teeth. These may well have started as a result of careless grinding leaving residual tension stresses in the case.) If asked to explain that with the long years between and knowledge now available . I would ask were these components made under the piece work scheme quicker they come off the more you get paid - has draw backs!- 'careless grinding' at the root this sounds like a case of 'plunging' where the grinding wheel is plunged onto the surface to remove stock quickly - soon detected on aircraft components by inspection. Plunging can localize heat and change surface hardness stress and set up minute surface cracking, these will with use propagate and as they are in the root area will lead to tooth separation. Pitting - well that could be badly made forgings/castings or as is often small particles of metal eroded from wear can suspend in the lubricant and under many tons of surface pressure cause pitting.

 

The second document at Kew in date order is WO194/951 and I give here a transcription from the front cover to the end of the distribution list. This document was published before the metallurgical report and only refers to three tanks, whereas the metallurgical report refers to thirteen, and was published in April 1945.

.

Do the reports mention the 'T' census numbers of the three and thirteen Comets ?

 

"Theory is when you know how it works but it still doesn't. Practice is when it works but you don't know why. In this Department [Physics], theory and practice are joined together: nothing works and no one knows why!"

 

"Theory is when you know how it works but it still doesn't. Practice is when it works but you don't know why. In this Department [Physics], theory and practice are joined together: nothing works and no one knows why!"

Brilliant. My 'scientists' definition would be slightly different to your engineering one but the message is essentially the same.

Reminds me of the various stages of a project, which may also apply in this case:
Optimism
Pessimism
Despair
Search for scapegoats
Punishment of the Innocent
Reward for those who were never involved

 

Papiermache...I've just re-read what you typed up there. Something has leapt out at me in the second reading...


The taper roller bearings supporting the pinion, however, were found to be out of adjustment and this undoubtedly largely accounted for the condition of the gears.

Signs of pitting were also to be seen on the pinion of the right-hand drive assembly in which the bearings were in better adjustment.



...and yet...

CONCLUSIONS:

(1) From the evidence now available it would seem that the bulk of the failures experienced can be ascribed to faulty material.

(2) The absence of tip relief and the presence of other manufacturing blemishes may have been contributory factors in some of the failures.

(3) The pitting of the 4.5:1 Cromwell gears at present running in Comet T.334906 is considered to be largely due to slackness of the pinion bearings, which arose from the use of incorrectly machined pinions.

It will not be surprising, however, if pitting of these gears is experienced when they are fitted to Comet tanks, in view of the fact that slight misalignment is sufficient to produce this condition when running in Cromwell tanks and the Comet is 4 1/2 tons heavier than Cromwell.

FUTURE ACTION:

(1) In view of the faulty material which has been discovered all billets and forgings at the stampers works and all forgings, partly machined and finish machined gears at machinists works or assembly plants for 4.92:1 ratio gears will be scrapped.

(2) As an expedient all machines in service and on the production lines are being fitted with 4.5:1 ratio gears from T.F.D.5 assemblies.

(3) In view of the fact that several hundred machines will thus have 4.5:1 ratio final drive gears, production will switch over to this ratio but the face width of the gears will be increased to 3.25 inches.



Incorrect bearing adjustment is by the end of the document regarded as a MINOR contributing factor, and there's NO mention of correctly adjusting the bearings as part of the remedial actions!

This reads to me as a series of gearbox breakages, caused in the main by incorrect adjustment of the bearings....but when they THEN looked into the issue in depth, found a range of issues with the heat treatment of gerbox components! I.E. they wore faster/broke more readily because of those issues...but it was the bearings that put undue stresses on them in the first place???

 

If only some of the horrors were exposed, the other division of the company where I was doing my apprenticeship were building part of the pilots console for a prototype, instruments fitted all the wiring and pipework connected when a team arrived , in those days Harris tweed coats, grey slacks and the obligatory pipe with a handful of drawings tucked under the arm - a round of tut tutting and the decision scrap it we have come up with this! When asked if the instruments could be stored and re-used? Oh no they have serial numbers listed for that project we have new serials for the new console! Those instruments were still on a shelf when I left the company. Drain, shovel, money.

 

Papiermache...I've just re-read what you typed up there. Something has leapt out at me in the second reading...




...and yet...



Incorrect bearing adjustment is by the end of the document regarded as a MINOR contributing factor, and there's NO mention of correctly adjusting the bearings as part of the remedial actions!

This reads to me as a series of gearbox breakages, caused in the main by incorrect adjustment of the bearings....but when they THEN looked into the issue in depth, found a range of issues with the heat treatment of gerbox components! I.E. they wore faster/broke more readily because of those issues...but it was the bearings that put undue stresses on them in the first place???

This, in my mind, vindicates what Grimmy's Dad has said all along. Picture it like this. Comets develop faults with gears. Faults rectified on all Comets in construction. Comets already in theatre also need fixing (i.e. preventative maintenance). Because the taper roller bearings are the cause of the trouble and there is no point in adjusting the bearings because they might be worn, then the best action would be to simply replace the bearings in the Belgian workshops and, knowing how thorough the fitters would be, it goes without saying that they would be properly adjusted. Because there was a War on (major offensive due in a very short time i.e. the Rhine Crossing) any further action would not be accomplished in time and so what if a few tanks broke down on the way through Germany (they could always be fixed anyway) i.e. modest risk is better than not having the Comets at all.

I would still like to know, however, why the 21AG report is so definite about the problem being at the front and necessitating the replacement of the Tensioner/Idler Wheel.....

 

I would still like to know, however, why the 21AG report is so definite about the problem being at the front and necessitating the replacement of the Tensioner/Idler Wheel.....


Remember what the report says in passing about the tracks jamming...then all the torque going through the gearbox/spocket into a static track I.E. is the tensioner/idler wheel the reason the tracks jammed in the first place?

 

To give you all something more to mull over the first part of the metallurgical report and the distribution list is as the transcription after this introduction.

I have edited the introduction to the previous long transcription by substituting the words "specific final drives" for "tanks".

I had not realised that the final drives were being moved in to the same tank as a test-bed in some cases.

The Leyland Motors Limited report is not given: they have reported separately. In the body of the metallurgical report there are references to samples of gears from tanks with numbers T.334912 and T.335021.

A list of "T" numbers found on various pages:

T.334904
T.334906
T.334912 (Leyland Motors Limited)
T.334968 ( from Continent BUT T.334998 on label)
T.335009 from Aldershot
T.335010
T.335023 (from Continent)
T.335021 ( Leyland Motors Limited)

"N.S." and "O.S." are shortenings for near-side and off-side.

No dates of failure are given.

Here is the first part of the report dated "April 1945", with the usual apologies and acknowledgements:




WO194/728

DEPARTMENT OF TANK DESIGN
MATERIALS DIVISION
METALLURGY BRANCH
FINAL DRIVE FAILURES - COMET

ORIGIN

Following the occurrence of a number of failures of the final drive components in Comet tanks, arrangements were made for a detailed metallurgical examination of the failed pinions ( BC.61187) and gear wheels (BC.61188). Some components were examined by Leyland Motors Limited who have reported their findings separately; nine other failures were investigated, on behalf of D.T.D., by the Research & Development Departments of The Mond Nickel Company Limited and the detailed results of this investigation are given in the attached report.

SUMMARY OF RESULTS AND CONCLUSIONS

Full particulars of the metallurgical and mechanical properties disclosed in the examination of gear wheels and pinions from nine final drive failures and from one final drive unit ( No.3) which had not failed, are contained in Table 1 of the report.

The main features of a deleterious character which were noted, particularly in the case of the failed gear wheels, were distinct evidence of overheating and/or a phosphorous content slightly above the maximum amount permitted by the specification. Both defects seriously affected the ductility and toughness in and near the gear teeth and were likely to have contributed largely to the failures of most of the components examined.

In addition to these defects, four of the pinions and one of the gear wheels were low in core strength, the ultimate tensile strength in the worst cases being 11 t.s.i. below the specified minimum of 80 t.s.i. ( wartime relaxation).

The following summarises briefly the pertinent particulars of all the failures which have been subjected to metallurgical investigation including the nine dealt with in the attached report and three which were examined by Leyland Motors Limited; also included are one unbroken

- 2 -

pinion and gear wheel ( No.3) which survived running trials without failure:-

( NOTE: the original has columns which I have not transcribed.)

Ref. No. 1
Vehicle: T334906 (N.S.) Mileage 1023
Significant faults noted in investigation.
Only obvious deficiency in pinion was core strength 69 t.s.i. Slight over heating noted in gear wheel but probably not sufficient to affect performance. ( Note that this failure occurred at F.V.P.E. under condition of severe turning.)

Ref. No. 2
Vehicle: T334906 (N.S.) Mileage 994
Significant faults noted in investigation.
Slight overheating in pinion. Definite evidence of overheating in teeth of gear wheel, sufficient to account for failure. ( This failure occurred at F.V.P.E. on a reverse turn).

Ref. No. 3 ( not broken)
Vehicle: T334906 (O.S.) Mileage 1023
Significant faults noted in investigation
No deficiencies in either pinion or wheel. Neither component was broken; the unit was the companion to No.1.


Ref. No. 4
Vehicle: T334906 (N.S.) Mileage 95
Significant faults noted in investigation
No metallurgical fault in either pinion or wheel. This failure occurred when "everything jammed. and may therefore be attributed to severe over-stressing. Several other components in the power train failed simultaneously.

Ref. No. 5
Vehicle: T334904 (N.S.) Mileage 1070
Vehicle: T224906 (N.S.) Mileage 1539
Significant faults noted in investigation
Definite evidence of overheating in both pinion and wheel. More pronounced in latter and probably cause of failure. Core strength of pinion and wheel below specification.

Ref. No. 6
Vehicle: T335009 (O.S.) Mileage: About 100
Significant faults noted in investigation
Pinion satisfactory. Definite evidence of overheating in gear wheel which also contained 0.055% P. and had Izod value of 8 ft.lbs and low ductility.

Ref. No. 7
Vehicle: T335023 Mileage: Unknown-returned from Continent.
Significant faults noted in investigation
No evidence of overheating in pinion and wheel, but P content of wheel above specification with consequent low toughness and ductility. Core strength of pinion only 69 t.s.i.

Ref. No. 8
Vehicle: T334906 (N.S.) Mileage: 95
Significant faults noted in investigation
Pinion satisfactory.
Vehicle: T335023 (N.S.) Mileage: 711
Definite evidence of overheating in teeth of wheel; toughness and ductility affected.

Ref. No. 9
Vehicle: T334968 (O.S.) Mileage: 238(?) from Continent.
Significant faults noted in investigation
Core strength of pinion only 69 t.s.i. Definite overheating in wheel, and low ductility.

Ref. No. 10
Vehicle: T335010 (N.S.) Mileage: 263(?) from Continent.
Significant faults noted in investigation
Pinion satisfactory. No overheating in wheel but P high (0.057%) and low ductility and toughness.

R.1592 Vehicle: T334904 Mileage: -
Significant faults noted in investigation
Gear wheel slightly overheated; P content 0.06%, Izod 10 ft.lbs.

R.1594 Vehicle: T335021 Mileage: About 100
Significant faults noted in investigation
Gear wheel slightly overheated; Izod 27 to 32 ft.lbs.

R.1595 Vehicle: T334912 Mileage: - From Continent
Significant faults noted in investigation
Gear wheel overheated; P content 0.06%, Izod 10/12 ft.lbs.


In addition to the above-mentioned 12 failures, one further broken wheel and

- 3 -

pinion were brought to notice, the failure having occurred after about 150 miles running. No detailed examination was carried out.

FUTURE ACTION

It is clear that, in the case of a large proportion of the 13 failures, the inferior quality of the material must be held responsible for the inability of the gears satisfactorily to fulfill their function. In considering remedial measures it was therefore desireable to ensure that the metallurgical defects were avoided.. The steps taken comprised:-

(a) The immediate substitution of all existing Comet final drive gears by T.F.D.5 gears from stock; these latter were of different ratio and tooth form but were suitable for use in Comet from the design point of view, and since they were known to have been produced by sources of supply which had given a satisfactory product and which were different from those used for the original Comet gears, there was a reasonable chance that their behaviour in Comet would be acceptable.

Tests by Leyland Motors Limited on some of these pinions and gear wheels confirmed their suitability; there was evidence of overheating on some of the gear wheels examined but not sufficient to justify rejection.

(b) The application of strict methods of control in the future supply and manufacture of Comet gears. The Iron & Steel Control arranged for electric steel only to be used and for the forging to be undertaken by two firms who had a good reputation in the provision of high quality forgings.

Furthermore, a system of coding, testing for overheating, and general inspection and control procedure was instituted by C.I.F.V. after consultation with Leyland Motors Limited, the Forge masters (sic) concerned and D.T.D.


(signed) F. Dickinson
Deputy Director (Materials)

(signed)
( A.A.M. Durrant)
Director

D.T.D.,
Chobham Lane,
Chertsey.
April, 1945.

M.814(s)
MH.

Distribution Overleaf.

- 4 -



Distribution:

D.G.A.F.V. 1
D.D.G.F.V.(E) 2
D.D.G.F.V.(D) 3
D.F.V.P. 4
C.F.V.P.E. 5
C.I.F.V. 6
B.A.F.V.(T) 7-10
DD 11
DD(M) 12
M/MY 13-14
V/TN 15
C/DM 16
D.R.A.C. 17
D.M.E. 18
S.T.A.M. 19
C.S.A.R./S.M.R. 20
A.A.S. Australia via C/DM (T.R.) 21-22
Leyland Motors Ltd. 23
Iron & Steel Control (Mr.Muirhead) 24
Mr.H.H.Burton ( Chairman, Overheating of Steel Forgings Committee) 25

M.814(s)

(end of page)

 

The first few paragraphs of the report show that the "report" is a "review" of a report. The document consists of A4 pages and A4 sheets sellotaped to the sides to create a wide document. The trouble with this is that some of the extension cannot be unfolded without intervention from a Kew archivist.

The introduction gives the specification for the steel ( usual apologies and acknowledgements):

Project No. M.6217M/4
Report No. 1

DEPARTMENT OF TANK DESIGN
MATERIALS DIVISION
METALLURGY BRANCH

FAILED COMET FINAL DRIVE GEARS

INTRODUCTION

A considerable number of premature failures of the final drive gears in the "Comet" tank have occurred recently during trials in this country, and in service on the Continent. In general, the failures were similar in character, the teeth breaking away from the gear wheels and pinions, and in one case the gear wheel had fractured across a diameter.

The material specified for these gears is steel to specification En.39, the chemical composition requirements of which are:

Carbon 0.12/0.18%
Silicon 0.1/0.35%
Manganese 0.5% (max)
Sulphur 0.05% (max)
Phosphorus 0.05% (max)
Nickel 3.8/4.5%
Chromium 1.0/1.4%

and optional

Molybdenum 0.15/0.35%.

The gears are case hardened to a minimum hardness of 60 Rockwell 'C' (765 D.P.H.) in the following manner:-

Carburise 900/910 oC to give a case depth of 0.055/0.065 in.
Refine 850/860 oC oil quench
Harden 760/770 oC oil quench
Temper 140/150 oC

Ten pairs of gears, each consisting of one wheel and one pinion, nine of which had failed, have, at the request of D.T.D., been subjected to metallurgical examination by the Research and Development Department of The Mond Nickel Co. Ltd, in order to obtain data on the general quality of the material used for the gears and to assess the probable cause of their failure.

The results of this examination are reviewed in this report.

In addition to the investigation carried out by The Mond Nickel Co. Ltd., a number of final drive failures from "Comet" tanks have been examined by Leyland Motors Limited, who also investigated the properties of pinion and wheel forgings and unfinished components which were in current supply for "Comet" production.

The results of Leyland Motors investigation, which were closely similar to those recorded herein, have been reported separately.

( transcription ends)

 

I think I've figured what happened. NB: the early Tensioner Wheel was replaced (post-War) because it jammed due to mud/dirt buildup. The cast steel version was designed with an open structure to avoid this problem.

I've a feeling the following is what happened during the War on trials Comets and a few in service (as detailed in the reports/reviews mentioned in previous posts):
a few Tensioner Wheels jammed with mud etc but this was not spotted, instead all the powers-that-be were of the view that it was faulty gears/bearings (which were not the cause of the problem but a symptom) and everyone then got side-tracked. Eventually some bright spark spotted the real problem and the Tensioner Wheels were replaced (post-War). The 21AG report was then written (post-War), the author got the timeframe/facts mixed up (fog of War, cover-up or whatever) and that is how the 21AG report effectively re-wrote history.

If my argument is valid, the question therefore becomes; who wrote the 21AG report and when?

 

a few Tensioner Wheels jammed with mud etc but this was not spotted, instead all the powers-that-be were of the view that it was faulty gears/bearings (which were not the cause of the problem but a symptom)


Yep, I would go with that all right!

....and everyone then got side-tracked. Eventually some bright spark spotted the real problem and the Tensioner Wheels were replaced (post-War). The 21AG report was then written (post-War), the author got the timeframe/facts mixed up (fog of War, cover-up or whatever) and that is how the 21AG report effectively re-wrote history.



But does THIS mean that the failures actually continued???

Interesting that they home in on the high phosphorous content; I wonder if there are any more wartime examples of this being an issue...given how we lost access to Swedish iron ore in 1940?

 

Thanks Diane, I'll ask Dad if he knows him.

The 23H reunions are unofficial affairs these days (the last official 'do' was also 2005), but all vets are welcome. Dad went for the first time last year and there were representatives from A, B and C Squadrons, so if Reg wants to contact any of them just say the word.

Quite a few 24th Lancers transferred to the Hussars after they were disbanded - but you knew that of course!

Thanks again for the info.
Grimmy, When is the 23rd Hussars reunion? I would really like to find out if anyone remembers my uncle. I will give you the information again if it was possible to ask on the day especially from "A" Squadron. Regards Len