SRM Dynamo Belt Drive

[Mosin]

I got given an SRM dynamo belt drive for Christmas by my Mum (well, she gave me some money and I bought it). And today, with the help of my 8 year old son, I got round to fitting it.

What should have been a very simple job has pretty much driven me up the wall. For starters I experienced all the difficulties with removing the chain sprockets which have been well documented in other threads. Eventually I got them both off and was finally ready to fit the new kit. First the pinion pully. Went on ok, but it seems that the thread on the idler pinion shaft is damaged. The SRM supplied nut will not go on at all and the original nut must also have a damaged thread as it will not lock up. (the original set up used a tab washer and didn't appear to slip).

I left this and moved onto the dynamo end. The pulley slotted on fine, but obviously as soon as I tried to tighten up the supplied sleeve nut the dynamo just rotated and I could not see any way at all of preventing it and allowing me to get the nut tight. Needless to say there is nothing in the supplied instructions about how to do this. By this point my son had wandered back into the house to play Fortnite. So, this leaves me with two clear questions:

• Does anyone know what the original thread of the idler pinion shaft is so that I can run a chaser down it and hopefully resurrect it?
• How have other people managed to prevent the dynamo from rotating while they tighten up the nut on the pulley/sprocket?

 
But right now it is 2 degrees C in my garage, damp and rapidly going dark (no heating or electricity), so I think I will give up for today. Not my best ever day of BSA ownership. 

[Greybeard]

Sometimes it's best to leave it and go back another day.

[JulianS]

The idler shaft thread was originally 3/8 x 26 tpi cycle.

When you have the large pulley secured in place and with the belt on both pulleys try gently tapping the dynamo pulley onto its taper, that  may grip enough to allow you to tighten the nut.

The dynamo shaft thread has a different thread,  3/8 x 20 tpi BSF.

The other way is to remove the dynamo and partially strip it so that the shaft remains in the drive end bearing and housing. Carefully grip the armature in a soft jawed vice and then fix the pulley.

[Mosin]

Following Greybeard's sage advice I returned to the garage today.

I managed to clean up the idler pinion thread and fit the larger pulley without too much difficulty. Having a headtorch and actually being able to see what I was doing was a big help! However nothing I do will prevent the armature spinning when I try to tighten up the small pulley on the dynamo. I notice that the armature shaft has a keyway cut into it for a Woodruff key, but the pulley doesn't have a corresponding slot (the old sprocket didn't have one either). I cannot believe that I am going to have to remove and disassemble the whole dynamo just to tighten up the pulley. Surely there must be a way? 

[Swarfcut]

  Cheat's Method.  Make sure the nut goes on easily without turning the armature. Clean both tapers to be grease free, remove the dynamo end cap to allow the armature shaft to be supported while you drive the new pulley onto the taper with a socket to spread the load from the blows.  If this fails, repeat the assembly but this time with a dose of Loctite on the tapers, drive on the pulley but leave to harden, then try the nut again.  Holding the new pulley and armature still may be a problem, maybe a few turns of so called baler twine on the pulley as a strap wrench will help.  If Loctite is used, a puller and a bit of gentle heat will be needed to get the pulley off next time.

 Plan B, strip out the armature as already noted.

 Swarfy.

[Greybeard]

Are you able to put a socket on a power tool of some sort? That may spin fast enough to do the job. I have a pneumatic wrench but I've also used my battery drill with a hex to square adaptor and 3/8" drive sockets.

[muskrat]

G'day Mosin.
Correct, there is no woodruff key. As Swarfy said, tapers need to be clean and thread needs to be in good shape so nut goes on easy. The pulley should only need a light tap to seat on the taper.
Then to tighten the nut.
This may sound a little crude.
I use a small timber wedge between the dyno pulley and the inside top of the inner cover to hold it still.
Cheers

[Mosin]

Some good suggestions here. The threads seem to be in good condition, but I will have a proper good go at getting them really clean and dry. I will also have a go at the powertool idea. I have a cordless impact driver which should do the job ok. I am also wondering about giving the inside of the pulley and the taper of the armature a rub over with very very fine wet & dry, just to slightly increase the friction while I get the nut started.

Tomorrow = day three of what should have been a one hour job. 

[Greybeard]

Some good suggestions here. The threads seem to be in good condition, but I will have a proper good go at getting them really clean and dry. I will also have a go at the powertool idea. I have a cordless impact driver which should do the job ok. I am also wondering about giving the inside of the pulley and the taper of the armature a rub over with very very fine wet & dry, just to slightly increase the friction while I get the nut started.

Tomorrow = day three of what should have been a one hour job. 

The idea of the power tool is not to hit it with high torque; you don't want to strip the threads. It's just the speed may overcome the friction of the thread. Try to use a battery drill if you have one.

[Alex kettle]

Tomorrow = day three of what should have been a one hour job. 

We’ve all been there and done that. I sometimes get disheartened when a simple task turns into a right load of hassle. But every time I fire the a10 up it’s soon forgotten . Keep at it. It’ll be worth it 👍🏻

[RDfella]

Hold the gear against the armature with one hand and hit the nut with a cordless drill on high speed. As Neil says, provided the nut is free, the fact the armature has mass and will take a second or two to accelerate to the nut's speed should ensure success.

[Mosin]

Day three, and we're on! A combination of most of the above techniques, combined with gently gripping the armature shaft (without the pulley on) and working the well greased sleeve nut up and down dozens of times, managed to reduce the friction sufficiently for me to be able to fit it with the pulley in place. All is once again well with the world, and best of all I even managed to put my hand straight on the new gasket which I had put away in that "safe place"!

Thanks for all the help and suggestions folks. It's irritating that such a simple job should turn into such an ordeal, but hopefully documenting it here might help others who find themselves in a similar situation. In the future, I think I will just stick to nice little simple jobs like complete engine rebuilds! 

[Greybeard]

... I even managed to put my hand straight on the new gasket which I had put away in that "safe place"!

Quick, rush out and buy a lottery ticket!! 

[Greybeard]

Is that a pressure feed on the crank nut? I've never seen the details of how it's done.

[Mosin]

Is that a pressure feed on the crank nut? I've never seen the details of how it's done.

It is. And if I hadn't just replaced the cover and got it all nice and oil tight I would take some more detailed photos to show you.... 
Still, I guess I will have to take it off in a hundred miles or so anyway to check the belt tension