Alton e-start, woodruff key shears

My Alton estart failed today. The starter spins but the engine didn't. On inspection I find that the alternator rotor woodruff key has sheared. Now on a standard bike this key simply ensures the alternator rotor is driven. With the Alton, it not only performs this function but it also transmits the estart drive to the crankshaft. Maybe this is just a freak failure but its made me wonder if this is a design issue i.e. this key isn't man enough. Thoughts anyone?

David

The engine sprocket key is a location tool, it does not and is not designed to transmit forces, the forces are transmitted by the taper on the end of crank so you need to check this and the sprocket mating taper to ensure they are smooth and touching on most of the surface. They may need lapping together and the crank nut torque specs adhered to.

Thats assuming the alton uses the engine sprocket to apply drive, if its using the alternator rotor then I doubt I will be buying one, keys are not driving devices.

You may have to add a means to drive the alternator rotor from the engine sprocket

Some cheap types of key are made of soft steel but if a good quality one is used it should not shear.

The clamping force of the crank nut should have helped to hold the sprag clutch alternator assembly in place.
Are you sure the nut was tight an had not come loose?

Prefect piece to experiment with nitrogen -300' F cryo tempering and may be worth while to seek out the super cyro with helium down to -400'F. I'd sure try cyro on the weak factory sprag gizmo. On the other hand could have 2nd key way cut in.

"Thats assuming the alton uses the engine sprocket to apply drive, if its using the alternator rotor then I doubt I will be buying one, keys are not driving devices."

I agree that keys are not driving devices in normal use but the Alton E start sprag/alternator rotor are attached at the standard rotor location so there is no taper involved as there is with the triplex sprocket at the crankshaft. So the key, in this case, probably takes a fair bit of the load. OTOH, keys are used in various applications as the drive with the idea that that are specifically designed to sheer with sufficient load/impact. Of course, this is not one of those applications!

I haven't had any problem with the key on my Alton so far and it's been in place for about a year. As asked in a previous post, was the rotor nut tightened to proper torque?

The nut was slack when I took it apart. I think it was tight when assembled, but I'm prepared to accept that I could be wrong about that. This seems to be the most likely cause, thanks for your input gents.
David

Alton have recently changed the backfire shock absorption device is yours one of the new types??

Th old type such as fitted to mine has a nylon hexagonal piece driving the sprocket that turns the chain an the newest version has small round nylon dowels.

The nut may of course have come loose after key sheared but no reassembly a spot of thread lock may be good idea.

Check carefully the sprag clutch bore an the thin area round the keyway in it for cracking. Any problems with it or the rest of the assembly contact Alton they will no hesitate to help an are friendly an understanding they replace parts without fuss unlike some notable suppliers.

Thanks Toppy. I have the newer backfire device as my original one failed. This is really why I was unsure about the integrity of the key. It initially seemed like I'd removed a weakness in the backfire device and moved it along to the key. But as you say, it's not clear if the key broke and loosened the nut or if a loose nut was the cause. One clue is that I couldn't see any trace of thread lock compound and I would normally put some on there so I suspect I may have not done the job properly. I will also check the sprag unit for damage as you suggest.

Two of my friends also broke the key in 2015, the first time with an ignition Boyer, the second time with an ignition Tri-spark and the more solid steel key.
It is the last version of the starter Alton with the new backfire shock absorption.
They are testing a new system of training of the starter sprag, that of Alton seems too fragile,like says Kommando.

Your problem almost happened to me. I did not use the full torque on the nut and discovered (in time) that the key had halfway sheared. From then on I used the full torque pressure on the nut and problem was solved.

Much to Alton's credit, they, and their USA outlet were very, very helpful. A very good group to deal with.