Broken Cush Spring

[RichardL]

I'm almost sure the answer will be "yes," but I'll ask anyway. Has anyone ever experienced a broken cush spring? About 5.5 miles from home today I started to experience what I thought was a total gearbox crash, with grindy, clanky, noises and rev increase without power. While limping home at about 10-15 MPH, I kept envisioning a gearbox full of sheared off gear teeth, or maybe something with the clutch. I did not consider a broken cush spring. By the time I got within 0.4 miles from home there was not enough forward force remaining to climb the slight uphill leading to my street, so I guess I've had my workout for today.

Anyway, since I've yet to fit a new spring and try a test ride, does anyone think that some bad activity in the gearbox could have caused the spring to break, or has the warranty on the spring just expired after 66 years?

Richard L.

[groily]

The warranty on one of mine expired very prematurely in Year 59 Richard, with the same results. Luckily my daughter was riding it so I had someone to blame. And she got it home - just. So - 'Yes'!

[KeithJ]

Had my SR for over 50 years.  Had one break probably 15 years ago and replaced it with a second hand one as new ones not available.  Still going. 

[bikerbob]

I have an A7 now but some years ago I had an A10 and the cush spring did break but I had a triumph 4 spring clutch fitted that had the rubbers fitted in the central part maybe that helped in getting the bike home as it took most of the strain and apart from some noise from the primary there was no other problem with the gearbox replaced with a second hand spring and it ran Ok till I sold it some 7-8 years later. Probably just fatigue over the bikes previous years running that caused the breakage.

[Greybeard]

I bet you were very happy it wasn't the gearbox. 👍

[RogerSB]

Oh hell! I've never considered cush spring failure - I'd hate to have to push an outfit - so there's another thing to worry about! 

[bikerbob]

I'm surprised that someone has not come up with a way of doing away with the cush spring setup and go over to the rubbers in the centre of the clutch a bit like the set up on my A65, BSA did away with the cush spring system when they went over to unit construction. Looking at it, it should not be to difficult to engineer a way of fastening the engine sprocket to the crankshaft securely maybe it has not been explored because cush spring failure is very rare and so not commercially viable.

[chaterlea25]

Hi All,
I keep going Back to this point on various threads and forums,
Even if there is no spring fitted the cush drive lobes should not ride over each other
If they do then there is some mix up in the parts fitted, severe wear or the crank nut is loose...
The Triumph 4 spring cush drive clutch hub is a straight swop for the solid type.
I had one fitted in my SR clutch ,
Although I rebuilt it when I put the bike together 20 odd years ago the clutch always would slip a bit when revs hit 4k
This year when I had the primary apart to fit the electric starter I changed the clutch hub to a solid one and fitted a 7 plate kit
No sign of slipping since

John

[KiwiGF]

I'm surprised that someone has not come up with a way of doing away with the cush spring setup and go over to the rubbers in the centre of the clutch a bit like the set up on my A65, BSA did away with the cush spring system when they went over to unit construction. Looking at it, it should not be to difficult to engineer a way of fastening the engine sprocket to the crankshaft securely maybe it has not been explored because cush spring failure is very rare and so not commercially viable.

One (expensive) alternative way is a belt drive conversion 😉 (a Bob Newby kit on mine), it does make the drive slightly harsher at very low revs but otherwise is a good upgrade.

[RichardL]

In fact, the nut was quite loose, to the point of rubbing the primary cover. So, I wonder if spring breakage was due to a loose nut, or vice-versa. I really can't envision the mechanism for either case with a nut that wss tighened to 65 ft-lb.

Richard L.

[Swarfcut]

 Spring or no spring that l'il old nut should bear directly against the drive sleeve in all cases. Drive sleeve pushes onto the inner race and clamps the race against the crank.

 Likely scenario is a loose nut and a fatigue fracture as the spring had more space to extend. Add to that extra leeway and travel and the cush now being subject to shock rather than a smooth take up of drive on the cush ramps. A broken spring has probably come as an unexpected surprise on a good few bikes, performing well but with a hint of unexplained roughness in the transmission as even with a broken spring a correctly torqued nut should support the drive, but of course this depends on the amount of wear on the ramps. I would endorse CJ's valid comments, and on the suitability of substituting the later 4 Spring  Triumph Clutch, with the rubber vaned centre.

 Look on the split pin solely as a way of preventing damage to the primary case when using the original parts, rather than it performing any sort of locking arrangement.  Plunger bikes and early S/A have no locking arrangement. Later S/A have a tab washer fitting the splines on the crank and knocking into two cut outs on the later nut. Certainly awkward to get right, access is through the spring coils and good chance only one cut out can be reached.  Early S/A nuts are plain, without these cut outs.

 Swarfy.

 

[chaterlea25]

Hi All,
Richard,
I hate to bring this up but you now need to check the crank endfloat again
The hammering from the cush drive may have caused some damage to the shims
I have always used thread lock on the crank and nut, and touch wood have not had one come loose,
In the kit that came with the electric starter there is a selection of little spacers to go inside the split pin and take up the gap usually found with the original type nut
No use with the SRM nut though.

John

[RDfella]

Apologies for batting on about a subject I've raised before, but this 65ftlb magic figure concerns me. Without removing the barrels to jamb the crankshaft, how on earth does one restrain the crank from turning? Trying to do so by restraining the sprocket means the torque reading is compromised because the cam is fighting against you. Could walk away smiling at having achieved 65ftlbs when in fact it's probably nearer 20. 65ftlbs is way too low for that size thread anyway. If it was a coarser, say a BSF, thread, I'd expect more like 400. I always use a drift to tighten mine, and stop when it won't turn anymore. Dunno what torque that is, but 'bloody tight' would be the 'torque' reading in days gone by. Never had a crank nut come loose in over 50yrs yet.

[RichardL]

RD,

I was a little leery of trying this, but I did, and it seems to have worked without objection from the engine. I sent a long 5/16" bolt into one of the inner-case hold-down holes to jam the crank. Having done so, several intelligent members are about to tell me I have breeched the first rule of A10s. As for torque, I think I recall actually setting the wrench for 68 ft-lb, but, at least 65, per SRM recommendation. Nevertheless, that would still be inadequate per your recommendation. I can't recall if I used Loctite (bad engineering practice to not keep records). If I did, it must not have been enough. After I check the end float (thanks, John), if I don't have to dismantle the engine, I will be reassembling with somewhat more torque at a figure TBD and generous blue Loctite. 

Richard L.

[Colsbeeza]

I totally agree with RDFella. The cam angle reduces the final nut torque by at least 40%. I also calculated the bottom run of the primary chain tension at a couple of hundred Kgf when using a torque wrench at 65 ft-lbs and relying on the rear brake to stop rotation, equivalent to a couple of blokes weight standing on the edge of the clutch sprocket. Anyone found a bent gearbox mainshaft? I decided to follow Muskrat's advice and used my cheap battery impact driver purchased some time ago from my local hardware supermarket. It was the cheapest and lowest rating they had - about A$100 at the time. If you don't already own one, best to purchase one to suit your brand of cordless batteries. Mine has a maximum torque rating of 180Nm, or 88 ft-lbs. Most are rated much higher. I left out the soft metal lock washer on John Chaterlea's recommendation as it will squish and release some of the tension over time, put a drop of Blue Loctite on the thread, then rattle up the SRM cush nut until I cannot see it turning any further, then give it a few seconds more. If you have the original BSA cush nut, a drift and a dolly hammer is probably the best tool. You need a means of overcoming the cams pushback, so an impact method will do that. I imagine that the inertia weight of the crankshaft should prevent any shock damage to bearings etc. I also appreciate that many swear by the original BSA method.
Col