The reason why the sprag went over.

My sprag clutch jammed over last week (as I mention in Tim_S's thread), and it, the sprocket and gear wheel were new 10 months ago. Tonight I have dismantled the offending items, and have found the reason for this happening. I was a bit perplexed with this happening with new parts, but when it jammed the engine had backfired, and so I thought that I would check the backfire device.

What I found was that it was jammed with the ball bearings in mid channel, as it were, and not sitting in their respective static holesl, and when I put the unit in the vice to check it was releasing at 50 LB FT it was jammed solid. I couldn't move it! I had to undo the tensioning nut to release it and that was very tight. When I set this up four years ago, I didn't depress the collar on the tensioning nut into the channel on the shaft to lock the nut, but locktited it on.

Could this nut have rotated in a clockwise direction and increase the pressure on the backfire device? The primary has been running in oil for the last two years, so it isn't as if it hasn't had oil splash and seized up.

The bike has run for about 30 miles with the sprag "jammed" and on examination, there are several (I suppose 18) detents or lines on the sprocket sprag face and the the gear wheel sprag interface. The sprag has been corrected and seems to work OK, but I am slightly concerned about the lines/indents whether they are likely to jam the sprag clutch or whether I would be better dressing the marks out?

Any words of wisdom?

I still can't understand why or how the backfire device has jammed :?

Reggie, I think mine jumped because I got thumb happy with the starter button at the same time the backfire device was trying to work. Just lucky we have the kicker or we would have to find a big hill or do some wrenchin on the side of the road.

Phil

Old Brits has a good article about how to set up the anti-backfire clutch. When this came up on the forum some 2+ years ago I was in the process of assembling my primary on a MK III and decided to give the anti-backfire clutch a hard look, even though it had only 6300 miles on it and it was driven by the "Kick-Assist" OE starter. I found my anti-backfire clutch totally bound up and quite scored when I finally got it apart, made no sense. I also noticed that it's fixing nut was staked by a fold up washer so no additional torque had been applied by the engine or outside agents (no molestation on this example).

I plan to visit this area again and check the break-away torque on the clutch when I helicoil the primary drain plug.

RS

It is very common for the anti-backfire device to tighten itself after it has been moved. It is hard to get it locked in place effectively.
I have been known to put a spot on it with my MIG welder to keep it in place. Jim

comnoz wrote;

It is very common for the anti-backfire device to tighten itself after it has been moved.

Click to expand...

So what happens? When the backfiredevice operates to save the engine, it jumps anti clockwise which I would have thought would have if anything loosen the tightening nut? I'm going to pay more attention to this nut today, as not only will I "stake it," I'll bearing loctite it. And it was all new 10 months ago and now it's slightly damaged :cry: :cry: :cry:

I am not sure why they get tighter. Obviously has something to do with the harmonics or pulsing of the drive. I just know they do get tighter if they are not adequately staked and locktighted. Jim

comnoz said:

I am not sure why they get tighter. Obviously has something to do with the harmonics or pulsing of the drive. I just know they do get tighter if they are not adequately staked and locktighted.

Click to expand...

"To every action there is an equal and opposite reaction"!

The A/B shaft assembly rotates clockwise in order to drive the sprag gear and clutch and thus turns the crank in the normal anticlockwise direction (when viewed from the left) for starting. When a kickback occurs, the gear on the A/B shaft will be forced to turn the opposite way to the A/B shaft - anticlockwise.

The 6 balls contained within the A/B device are preloaded against the face of the gear by the disc springs and thrust washer, so the balls are effectively trapped under pressure between two metal discs (the gear and thrust washer) and held static in relation to the shaft by a third disc (the carrier) and under normal circumstances the balls sit in the gear indents which are spaced to match the ball positions in the carrier and the complete assembly rotates together.
However, in the event of a kickback occurring, the A/B gear will be forced to turn anticlockwise in relation to the shaft, and as the gear slips, the balls will be forced out of their indents and start rolling along the groove in the face of the gear, as the balls roll over the surface of the gear, because they are held captive by the carrier and under preload, the rotation of the balls will be trying to force the thrust washer, disc springs and nut to rotate in the opposite (clockwise) direction to the gear-thus pulling the nut ever tighter if it's not locked securely to the shaft.

The Old Britts info mentions that the nut loosened every time the A/B slipped which would happen if the A/B was made to slip in the wrong direction.
http://www.oldbritts.com/e_start_backfire.html

LAB wrote;

However, in the event of a kickback occurring, the A/B gear will be forced to turn anticlockwise in relation to the shaft, and as the gear slips, the balls will be forced out of their indents and start rolling along the groove in the face of the gear, as the balls roll over the surface of the gear, because they are held captive by the carrier and under preload, the rotation of the balls will be trying to force the thrust washer, disc springs and nut to rotate in the opposite (clockwise) direction to the gear-thus pulling the nut ever tighter if it's not locked securely to the shaft.

Click to expand...

That makes sense :idea: :idea: :idea:
Thank you for taking the time to explain it. I'd have never worked it out. To be honest my Commando hardly ever kicks back, but as I'm running my 920 barrels in at the moment, I haven't got round to timing it up with the strobe yet, so I suspect that the timing might be a bit advanced?

That makes sense. I really never have taken the time to look closely.
Something much better is in the works. Jim