Cush & Clutch Nut Forces

[Colsbeeza]

G'Day Chaps,
I mentioned recently that whilst torquing the Cush & Clutch Nuts, I chickened out at 55 ft-lbs. I have always been a bit soft with mechanical things, and was beginning to cringe at 55. So decided to do some calculations to test my mechanical chickenness.
I worked out that to tension the cush nut to 65 ft-lbs and with the clutch plates locked, (and with an 18 tooth engine sprocket), the tension in the lower primary chain is about 543 lbf, or 246 Kgf, ie. the equivalent of the weight of nearly 3 persons standing on one side of your clutch sprocket. With a 19 tooth or more, this tension is a little bit lower. Some complain of bent mainshafts - hmmm!  So my chickenness may be somewhat justified.
Nevertheless, most people have been locking the clutch forever and there are many ideas on how to lock the crank to do this. The rattle gun Muskrat uses avoids these stresses, but others might worry about consequential impact loads on bearings. And BVSR's propping bar (between sprockets) has a lot going for it.
There doesn't seem to be the perfect stress-free safe solution.
Anyway, I made my own tool this week to ease the sweat on my brow and then tensioned the cush nut to 65 ft-lbs with a grin on my face. Or at least until my biceps started to tear.!  ( I have my left bicep torn such that it is not visible if trying to show off the muscles, and the right one starts to pain as well). I have a 1 foot long shaft on my tension wrench and it needs to be longer.
The clutch locking tool takes all stresses away when doing up the clutch nut, so there are no worries there.
I have attached a photo of my clutch locking tool, and the Excel Stress Calculations for your interest.
I wrapped a short length of chain around the engine sprocket and bolted it to a steel bar.
The eyes of the primary chain measured at 11/64" or 4.365mm. I was able to find High-tensile bolts at 3.9 (Nominal 4mm) at 12.8 Hardness (Rockwell I think - the seller didn't know). The Pin Shear Stress was about 1/3 of the chain stress, so safety was OK.  To clear the spring, I had to fix the bolts out a bit which left the chain a bit loose, risking a slip over the teeth. There is a flattened stick of solder jammed between the steel bar and the sprocket boss to keep it all tight. I could have attached the steel bar to the sidestand, but went overboard and chained the bottom of the bar to my motorcycle lift.
I did intend to wrap a rag around the engine sprocket whilst torquing but forgot. But no worries.
Anyway, I thought I'd throw this in to the mix. The calculations might stir up some thoughts. Insert your own numbers.
I am sure you will be by now.
Colin

[duTch]

 Funny you should post this Col, I came across  an old post of mine just now while searching for something else;
 It's dead simple, all you need to do is have the crank in the right spot, and screw an appropriate strength bolt in; Bottom for tighten, top for loosen;
Have a read here; some like it some don't, is easier than it sounds
 
 https://www.a7a10.net/forum/index.php?topic=9121.msg65855#msg65855

[Greybeard]

While I totally respect your scientific methods please remember that young dudes have been doing these nuts up by standing on a long Stilson wrench for the last 60 years. How many crankshafts have broken? I have no idea, but I reckon those things are damned strong. The BSA Service sheet says something like 'Do the nut up very tight'; no torque value given.

PS, I may have to delete this post if I discover that my shaft has snapped!

[berger]

well I might be the odd one out but I still use the original crank nut and pull it up with stillys to the sprocket bearing sleeve than whack it with a punch and listen for that ime not getting any tighter sound{ I love the sound of mettalica on mettalica] then I put the homemade lock nut on that I made by turning down an old nut to fit beutifuly inside the recess of the crank nut.. not let me down yet! but the splitpin method and washers to space to the split pin hole  to the nut did and it chewed the shim up

[Colsbeeza]

Hi GB,
Not doubting the strength of the crank, and as I said - I grinned whilst torquing it to 65 ft-lbs. But the force pulling on the clutch and gearbox was my concern, not the crank strength. I certainly acknowledge that that was the method BSA used and no doubt thousands for the last 60+ years. I just didn't like it.! Just wanted to raise alarm about the huge stresses on the clutch via the primary chain. 540+ lbf is ridiculous.
The fact that so many members have developed a method to directly hold the crank shows just how many were concerned about the same issue.
Dutch - I hadn't seen that post before. That now makes about 5 or 6 methods on the Forum to hold the crank without relying on the clutch locking tool. I see that a few people worried about buggering up the thread in the case.
Berger, your whacking with a punch is a bit like a poor mans rattle gun and probably no more stress on the primary chain and clutch. I did it that way back when I was about 18.
Richards Cable Idea in Dutch's reference is also a good one.
Having made my tool, I like Dutch will stick with this method. It cost about A$10, so cannot throw it away. The only downside is that the lobes run up and lock, but don't think that causes any damage.
This Forum has been the backbone for my restoration, and I am blown away by the expert opinions here. . It is because you don't all agree on everything that I must stick my own brain into gear and think about it - thus learning from it.
Col

[BSA_54A10]

You are not going to damage the crank with a 20' long extension.
BSA cranks are forged steel.
What happens is they develop a fatigue crack at the junction of the mainshaft and the first web.
Like all fatigue cracks i grows till there is not enough cross section to hold together.
Just had a pair of cranks back from the grinder, both of them cracked at the same place.
If it was a problem caused by excessive force used on the cush drive nut they would have been breaking all through the 50's & 60's when they were daily transport and the cush drive was being torqued monthly if not weekly.

[bsa-bill]

I have a 1 foot long shaft

carefully avoiding the oh so easy replies.

Seriously as has been said you need a longer bar than a foot for many jobs around a bike, a bar twice that length will make a tremendous difference, much easier on your biceps and much greater feel thereby giving greater control.

Do like your solution for stopping rotation, will copy next time I need to

[RichardL]

Having made my tool, I like Dutch will stick with this method. It cost about A$10, so cannot throw it away. The only downside is that the lobes run up and lock, but don't think that causes any damage.

I've taken to wedging little wood pegs between the coils of the cush spring when tightening the cush nut, in order to avoid lobe lock.

Richard L.

[Colsbeeza]

Thanks RichardL,
I will try the wooden wedges next time.
Once again Trevor and Bill, I have no problem with the crank strength - NOT THE CRANK!! - THE CLUTCH, THE CLUTCH.   .
Not Sure how to extend the Tension Wrench without interfering with the function. They seem to be designed with hand on handle in mind.
Col

[duTch]

 

.......Not Sure how to extend the Tension Wrench without interfering with the function. They seem to be designed with hand on handle in mind.  ...

Maybe depending on the type of wrench, if you extend the handle outside the indicator (length of pipe), all you'll be doing is increasing your own mechanical advantage, but the reading should be the same

 

.....Dutch - I hadn't seen that post before.....

 It actually came about by accident- when the primary wasn't connected,  and I  remembered that if the bottom bolt is too long the web catches,  so figure I'd see if i can use it to my advantage,  but I think only works with the bottom bolt- and if I didn't say, it needs to catch top or bottom of the web...