The Diff has been in the Marlin for 10 yrs now so I thought I had better take a look at it. All looks OK except a small area of pitting on the teeth of both planet gears and 1 sun gear all the teeth are effected, its in the centre of the polished wear mark covering about 10% of the mark . My spare diff has more crown wheel to pinion slack than the current. If I swap bits about should the planet and sun gears be kepted in sets. Does anyone know if new gears are available.
Cheers
Pete
I assume your 10 CWT Diff. is the unit that was fitted to Morris Marina cars and vans and the early Banjo axle MGB.It was also known as a "B" series axle. Over the years I have managed to break more than my fair share by abusing them in my MG J2 and have also learnt a little about what works.The first thing is that the crown wheel and pinion assembly will last and last, given clean oil and no bits of broken star wheels rattling around the axle casing. When first set up each crown wheel and pinion pair is set to a backlash marked on the crown wheel, this varies for each set so different assemblies will vary.If nothing else is wrong, once an assembly has run OK, its best left unchanged, and varying amounts of backlash between assemblies doesn't seem to cause any problems.The real problem with these units however, is the two star wheel diff which when used hard in a trials car can break. I have experienced two main types of failure. The first is caused by wheel spin on one road wheel, causing the star wheels to spin on the cross pin at high speed . Unless the unit is well worn and has lots of clearace between star wheel and shaft or is modified with extra clearance the lubrication fails, and the resulting seizure bursts the star wheel, and we all know what happens then! Now your diff has run OK for 10 years so it clearly has enough clearance, and some marking on the gears is usually OK. The second type of failure is straight overload when the car takes off and lands with full power, and I haven't managed to back off in time. Again the star wheels bursts! This problem may not occur with Marlins, I dont know.The best answer for the MG I have found is to fit a four star diff into the "B" series assembly, using four std star wheels instead of two. This has worked well for over ten years. I made mine but others have used Ford transit bits. Others have used needle rollers, or bits supplied by Julian Fack. For Marlins I don't know, but if your diff has lasted for 10 years you have obviously got a good one and leaving well alone could be a good option. The old adage " if it aint broke don't fix it " is probably as true of diff assemblies as anything else we can think of. Hope this helpsDudley
ReplyDeleteI use an American made 4 planet diff in my Beetle and have never had any trouble, so far touch wood. However, my to local buddies, Neil Bray and Simon Robson both trial Skodas and found the diff's lasted about six trials.They had the idea of getting special 4 planet set-ups made but couldn't find anyone able to do it in the small space available. Then we remembered that an old clubmate called Peter Smith had gone into business re-building Hewland racing gearboxes and Simon went to see him up in Yorkshire.Pete couldn't work out how to fit four planet wheels in a Skoda either but suggested using Red Line Synthetic gear oil. Since then the Skoda twins get at least two years (12 trials for them) from a diff. provided they remember to lift of when landing on a bad bump!Based on their experience I paid my 40 quid and put Red Line Synthetic in my box as well.With the Skodas the big problem is the pin hobbing an oval hole in the diff casing. This gets so bad the teeth come out of engagement and "bang". I understand that some of the Skoda boys weld the pins in place to prevent this happening but how I don't know.The whole question of axles and diffs is interesting. One of the Imp boys was telling me they put Ford diffs in their transaxles. He had a straight face and it wasn't 1st April so it might be true!Going back to your Marlin Pete. Have you considerd having the parts tuftrided. I understand this is a gas hardening process that doesn't cause any distortion. Perhaps this together with a sythetic oil would extend the life of your diffs.Michael
ReplyDeleteThanks Dudley & MichaelI should have said the planet gears are running on needle rollers that was the mod I did back in 1990.CheersPete
ReplyDeleteWe are using a Transit Van differential in the Marina 10cwt axle and have also found the main cause of failure to be bursting of the planet wheels.It seams to be predoninantly a lack of lubrication that causes our failures as we have even had such a bad 'pick up' on the cross shaft in the centre of the diff that the planet wheel has welded itself to its pin and has sheared the pin off.This is what actually happened on the clouds as even thought the diff didn't make any noise or cause any steering difficulties on the road on one occasion coming out of Climperwell the car seamed reluctant to turn.We continued on without difficulty and with still no other sign of diff problems and finished the trial.As it is obviously easier to check the diff in the garage than on the side of the road we pull the diff out to do a quick check which revealed that the pin had sheared!Due to the fact that there was no noise, no steering difficulties (except at the top of Climperwell) and it sailed through the diff test before Catswood section how do we know when it it actually broke & how much longer would it have lasted before the failure became terminal?
ReplyDeletePeteUsing needle rollers appears to be rather successfull. Iv'e been considering using needle rollers with my four star diff. However I did some calculations which suggest that the needles could indent the beraing surface when carring full load without rotation, it depends on how hard the surfaces are. I wonder is there any sign of this happening with your 10 year diff, and when you made the modification were the shaft and star wheel rehardened after modification. MichaelIv'e used Tufftriding very successfully on crankshafts. I used to break cranks, with fatigue cracking, about every18 months, with the T Type engine in the MG. However my current crank which is a std MG forging, not an expensive steel billet crank, has now lasted for 20 years and the main difference is Tufftriding. I looked up the information I have on Tufftriding and its mainly used to improve fatigue life with low or medium alloy steels rather than improving hardness so I am not sure the process will work so well on hardened steel gears, and pins.Dudley
ReplyDeleteDudley,I follow the logic on what you say on Tuftriding. In fact further conversations with the local "team Skoda" reveal it is the diff carriers they have had tuftrided to try to stop the holes I mentioned going out of shape and cracking up.Now I have a confession to make. These days my day job is mainly concerened with Sales and Marketing. However, in the dim and distant past I was apprenticed to Borg Warner, who later sponsored me to go to University to study Engineering. The part of Borg-Warner I was with made automatic gearboxes, overdrives and limited slip diffs!Now I was a Production engineer rather than on design and I remember that we had a lot of old American machines tools that we had to make bits for as spares weren't avaialble. I seem to remember that when we re-furbished shafts we used to have them gas hardened with a process that didn't susequently require grinding. Could this have been "Nitriding"? I'm pretty sure it was.Michael
ReplyDeleteA further piece of info - gossip really. The local team Skoda is going to be reduced by one soon. Simon Robson (Class 4 winner on the Allen) has ordered a Liege which he hopes to have ready for the Lands End.We are all a bit doubtfull that those delicate Reliant bits are going to stand his "determined" driving!Michael
ReplyDeleteDudleyThe modified pin I used was developed by the sporting trial mob. I think it was produced by Hewland and consisted of a new pin machined to take the needle rollers. The pin is not the original and is made of a very hard steel I tried to drill the locking pin hole once with no success at all. I understand if you use a std pin they wear very quickly. I have not removed it so cannot comment on your concerns of indentation. Do you think the planet gears ever actually stop. I did some calcs( No guarantee I got them right)and if you assume a difference in rolling radius of the road wheels of only 2mm at 40 mph the planet gears are doing about 5 rpm not fast but turning.Pete
ReplyDeletePeteWhen taking off from the start line using full power, when both wheels have equal grip on a reasonably grippy surface, Crooked Mustard for example, it is possible for both wheels to spin initially ( even with the modest power output of my Imp). Under those circumstances I suspect that there is no rotation within the diff whilst the load will be high. If it were my decision I would probably adopt the ' if it aint broke ......' policy. If I ever get a diff to last ten years of trialling I will have it chrome plated and stick it on the mantlepiece!Regards, Stuart
ReplyDeleteMichael Being a Classic Trial fan and following your web site for some time the following may be of interest to some triallers. Those who run Mk 1 and 2 Escorts and also Escort based kit cars, Duttons and Rickmans etc, may like to know of an alternative back axle for their machines. Being a builder of a Locost I came across the fact that the axle from the Lada ( no, don't laugh) seen in the Fiat 124 based series of models is dimensionally identical to the Mk 2 Escort. The advantage is that any available are likely to be considerably newer and have therefore covered less miles than the Escort unit and would be considerably cheaper. The ratios available are (from the Haines manual) 4.1, 4.3, 4.4 and maybe 3.9 and the prop shaft flange will fit the small Ford flange. The brakes have large finned alloy drums and whilst they are not of Ford pattern can be re-drilled to suite. Unlike the Escort, it is designed for vertical coil springs, telescopics and a Panhard rod but could be easily converted to Escort configuration with the correct brackets. Hope this may be of use to someone. Myke Pocock.
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