Broken Cush Spring

[Greybeard]

Locking the crankshaft or the crankshaft drive sprocket will take the cam lobe ramp angles out of the mix in terms of torque reaction and lost effort.

 In the case of locking the sprocket, one is tightening a nut whilst a cam is pushing back. I have no idea how much tightness is lost through compressing the spring and fighting against the cam, but it certainly won't be insignificant. An indication would be to see how much torque is required to get the nut to just before it touches the sleeve - remembering that the more one tightens after that, the more the cam effect works against you.

I reckon the rattle gun, with its percussive action, helps overcome that lost torque from the cam.

[Colsbeeza]

BagoNails - The object jammed down from top is a stick of solder. soft enough to hammer in a bit and keep the chain tight around the sprocket. This was the best I could do to avoid imagined stress on the clutch & gearbox. However, for the reasons that RDFella has very clearly expressed, I have gone away from this method and now use the Impact Driver to ensure I get 65ft-lbs or greater on the nut. Given that the crankshaft is not built-up, I am sure of no damage.
Peter - I don't even hold the engine sprocket with using the Impact driver, but rely on the crankshaft inertia only to stop it rotating.
Biker Bob - The SRM nut allows me to get a socket onto it for the impact Driver. It also allows a degree wheel fitted for more accurate setting of ignition timing. SRM provide the degree wheel, screws and threaded holes for this - and all this to fit my plan to install my Steve McFarlane electric starter kit in the new year, when the ignition timing must be set on the more retarded side - about 30-31 degrees to minimise kickback. And to get that accuracy, I have a little electronic box with a little red light to tell me when the points are just opening. It is currently set at 32 Deg now, and no signs yet of any kickback. I know some of us are asking of the bike something that BSA never imagined. For me, it is an upgrade for a particular purpose.
Unless you want to go down this rabbit hole, a couple of hits with a hammer is fine although a drop of Loctite would help. If your approach has worked for you for so long then why change?. However, IMHO the standard dished BSA nut does not lend itself easily for a locknut. I used to have great difficulty in getting a split pin in and out as it was well below the outer nut surface, and at best it can only prevent the nut from coming loose enough to gouge the outer cover. If you can drive in a split pin and guarantee it will hold the 65ft-lbs, then you are a much better man than me. If the split pin is ever needed, then the nut has already come loose and filled the crankcase with shim . (Sorry- a bit tongue-in-cheek).
Col

[RichardL]

Well, when I started this topic, I was just trying to figure out why my cush spring broke. I didn't realize it would go into the in-depth study of cush-nut tightening, but I think it's been valuable. A long way back now, I posted this method I used a couple of times

https://www.a7a10.net/forum/index.php?topic=368.msg58817#msg58817

I thought RD's point regarding kinetic friction against the spring was interesting. I just now measured it and came up with about 43 in-lb against a dry spring. Not completely insignificant, but not huge. Oh no! I just realized that I need to measure with a greased spring to fulfill the Scientific Method.

Richard L.

[BagONails]

Locking the crankshaft or the crankshaft drive sprocket will take the cam lobe ramp angles out of the mix in terms of torque reaction and lost effort.

 In the case of locking the sprocket, one is tightening a nut whilst a cam is pushing back. I have no idea how much tightness is lost through compressing the spring and fighting against the cam, but it certainly won't be insignificant. An indication would be to see how much torque is required to get the nut to just before it touches the sleeve - remembering that the more one tightens after that, the more the cam effect works against you.

RD as Richard has alluded maybe we are moving quite far from the original point here but it is all related and an interesting sand pit to inhabit so here goes for now .

So the way I see this the axial force generated by the nut is huge in relation to the spring force, something like 6-7 tons in fact (T=cDF where T= Torque, c = coefficient of friction for steel, D= diameter of the thread and F= Force) This is a very rough calculation and you can look up and find much more complicated versions taking into account thread pitch angles and friction in the thread etc but for our purposes it serves to prove a point. The spring may fight back a little but it really is negligible in this context.  For this reason I say also the cam ramp angle can also be ignored in fact I didn't see my cam move back toward me hardly at all and it is no longer in the equation once the joint becomes hard in any case. The nut compresses the spring easily and the torque will barely start to rise until the nut hits something solid. That will be when the nut starts to compress the splined bearing and sandwiches this between the face of the nut and the steel spacer/main bearing inner/shims and the crank web. At this point the joint changes from soft to hard and the torque rises rapidly.  The nut  compresses the spring no further as it is now bearing directly against the solid steel of the splined part (also called the bearing) compressing these parts squashing them very slightly and putting the crankshaft threaded section back to the web face all in tension which stretches of course not that you would notice...  This tension 6-7 tons of it is what primarily keeps everything up tight, not withstanding the Loctite of course don't forget the Loctite!

Col, rattle guns are fine for fencing screws but you really have no idea what torque is going in and even then it depends how long you hang on to the trigger even what state of charge your battery might be in. I prefer at least to have some idea so will go with the torque wrench myself but each to their own and whatever works for you I say.  It is true that in more enlightened times on modern car engines for example you will find torque and angle being used also fasteners that go into yield. This is all to try and deal with the issues of varying clamping loads that arise from all the variables we have discussed but in this case I'm pretty happy with my method...or maybe I might just go back and give it a quick blast with the rattle gun just to be on the safe side....No just joking. 

Taking the best part of the SRM nut I also drilled and tapped my standard nut 2x M3    to allow mounting of a timing disc in future.
 

[Colsbeeza]

Hi BON,
 Yeah you're right about the Impact Driver. Does anyone have one that also indicates torque.?
I have to accept that the makers who rated it at 180Nm should know their business. To be sure (repeat if Irish) I do take it to it's capacity, theoretically 180Nm or 88 ft-lbs with a fully charged battery. That'll do me!
Col

[BagONails]

Hi BON,
 Yeah you're right about the Impact Driver. Does anyone have one that also indicates torque.?
I have to accept that the makers who rated it at 180Nm should know their business. To be sure (repeat of Irish) I do take it to it's capacity, theoretically 180Nm or 88 ft-lbs with a fully charged battery. That'll do me!
Col

Hi Col,
 
I think the correct unit of impact driver torque is given in Milliwaukees, I've never seen a conversion showing Milliwaukees to NM however

[KiwiGF]

Hi BON,
 Yeah you're right about the Impact Driver. Does anyone have one that also indicates torque.?
I have to accept that the makers who rated it at 180Nm should know their business. To be sure (repeat of Irish) I do take it to it's capacity, theoretically 180Nm or 88 ft-lbs with a fully charged battery. That'll do me!
Col

I just did a wheel nut up with my 1/2” drive impact wrench/rattle gun.....on the low setting .....the high setting would break the stud off, then checked the torque with a torque wrench.....so I can estimate it stops tightening at around 80 ft lb.....with a full battery.

I was surprised by my mates rattle gun, seeing it made me go and buy one (albeit 1/2” square drive) his used a feeble looking 1/4” drive bit, but it was rated at something like 90 ft lb and loosened the wheel nuts on my gold wing easily, I was struggling to loosen them using a ratchet.

[RichardL]

Just arrived. Isn't she beautiful in her NOS fall colors? The title of the picture is "Spring in Fall."

Richard L.

[RichardL]

Mr. RD will be interested to learn that this NOS lubricated spring presented as about 83 in-lb (or,7 ft-lb). That's 40 in-lb more than the old dry spring. I wonder if it will present as more when the nut contacts the sleeve and the friction goes from kinetic to static. Hard (not impissible) to test that.

Richard L.

[Jules]

aaaaahhhhhhhh, sooooooooo poetic Richard, Lol..

[RichardL]

Moving at my normal lumbering pace, I just got the primary cover back on, so I can now report that I torqued the cush nut to 83 ft-lb using a long bolt through the inner cover bolt hole as the stop. Also, used blue Loctite on. the crank thread. I made a point of inspecting the long bolt for any trace of thread distortion and there was none.

Richard L.

[RDfella]

Well done, Richard. As this thread has raised a number of issues, I've just ordered one of those sockets purported to fit our cush drive nuts. Plan is to compare the magic 65ftlb torque figure with a nut tightened by the old drift method. I've always expected the latter to be tighter (mainly because it avoids the cam reaction) but of course I could be wrong ....

[KiwiGF]

I reckon if the nut is properly loctited it doesn’t matter how many ft lbs it is tightened too, it takes a lot of force to undo a loctited nut of that size......way more than 65 ft lbs, even if it is a bit hot.

Just my 2 cents worth, I guess a lot of people would not trust the loctite?

[RichardL]

Just that much more force for clamping shims against the web, i suppose. Though, in my case, the gap-filling, semi-permanent Loctite on the inner race had best not move with the clamping.

Richard L.

[BagONails]

Just that much more force for clamping shims against the web, i suppose. Though, in my case, the gap-filling, semi-permanent Loctite on the inner race had best not move with the clamping.

Richard L.

If you used bearing fit between the crank journal diameter and the inner race I doubt that would see any movement due to the slightly higher torque. If it did then you would have to be distorting the inner bearing race and that would also affect the bearing functionally. Highly unlikely in my opinion and I would guess unheard of?... Please tell me I'm right guys!