Dynamo chain

[pjm01]

Investigating zero output from the dynamo (1959 A7 SS), I discovered a broken drive chain. This got me thinking about why it should break, as it lives in a nice clean, protected environment and does not transmit much power (60 Watt dyno, allowing 50% conversion efficiency = 120 Watts = 0.16 Horsepower).
Looking back through the forum it seems the chains do break and indeed it's even mentioned in the Haynes manual.

Could it be that the chains are just not up to the job ??? or is something else going on here ???

Fortunately the broken chain did not do any damage and just dropped neatly out of the way into bottom of the chaincase. In a funny way, by failing it did me a favour, as the split pin was missing from the castellated nut on the end of the dynamo shaft which could have led to future problems.

I tested the dynamo by driving it with an electric drill and it rotates freely and gives a good output.

Peter M

[bsa-bill]

as the split pin was missing from the castellated nut on the end of the dynamo shaft which could have led to future problems.

neither of my bikes had castellated nuts on the dynamo.
I was always breaking chains in my youth, probably overtightening ?

[Billybream]

Hi Peter.
You are correct there is a castelated nut, but no split pin, you should have a lock washer with tab ends to bend over the nut.
Was there any evidence of grease in the housing, as this is required to lube the chain. Also correct chain tension should be applied on reassembly.

[pjm01]

That's all very interesting ....... there was plenty of grease all around AND some evidence the chain had been loose as there was a worn area on the inside of the chaincase where the chain had been rubbing.

There is no lockwasher under the castellated nut BUT there is a hole through the end of the shaft through which a split pin can be passed, fitting neatly between the 'castellated bits' of the nut. I have now done the nut up firmly and put a split pin through, which I guess should hold it in place as well as a lockwasher.

Just a thought though ...... wonder if the sprockets will NOT be correctly aligned now as the 'dynamo sprocket' will be 'out' by the thickness of a lockwasher ?

Peter M

[pjm01]

On reflection .... please ignore that last sentence ..... it will make no difference to the sprocket alignment ...
Peter M

[Rich]

I would guess that any chain wears out no mater how it is lubricated, so there may be no underlying cause as to why it broke. A good time to fit a belt drive, no lubrication required, quieter and if you use one of Steve McFarlane's it gives a better output at lower revs, slightly more than SRM's.

[pjm01]

Before removing the dynamo I measured resistance between the dynamo casing and earth (+ve terminal of battery) and was surprised to see 0.1 ohm. This would give a volts drop of 1.0V at full dyno output of 10A @ 6 ish Volts.  Resistance was measured using a good quality analogue meter and confirmed with a digital meter. Measuring resistances down the wiring between the battery (-ve terminal/earth) and the dyno casing ..... most of the resistance was 'contact resistance' between the dyno casing and it's mounting on the engine. I have now thoroughly cleaned up that contact area but am considering running in an earth wire directly to the dyno. Peter M

[duTch]

 I ran an earth wire from the earth inside the dyny (genny) to the main auxillary earth wire I added in the loom- seems to work..?

[beezermacc]

The reason most dynamo chains snap is because the drive gear is very poorly manufactured. The drive gear seems to be stamped out then riveted onto a boss which is attached to the idle gear pinion. Nearly every A10 I've worked on has a very conspicuous tight spot and slack spot which is caused by the drive gear running eccentrically. You can prove this by making a mark on the drive gear and noticing when the tight spot occurs - always when the drive gear is in the same place. Most owners aren't quite careful enough in setting the chain tension when it is at its tightest, consequently the chains stretch and snap. Conversely, this is also the reason why most A10 timing cases have witness marks from dynamo chain rub, i.e. because the chain was allowed to run a bit slack even at the tight spot so when it hits the slack spot it's flapping about. Belt drives are OK as long as they do not put too much tension on the dynamo bearing, particularly if you are still using the original exposed bearings.

[Peter in Aus]

Good point about the belt drive loading up  generator bearing. my A7 has been fitted with Alton 12V AC generator and was supplied with a sprocket for the generator, I fitted a new chain and the chain has a very loose spot in it, so was considering replacing the chain with a belt kit. Not sure if it will fit so have E-mailed Alton to see if I can fit a belt kit.
Keep you posted.
Peter   

[pjm01]

Replacement drive chain arrived today and I could not bring myself to install it as quality was so poor. Loose rollers with the cheeks not fitting snugly against them ...... very poor and inconsistent rivetting. Whole chain flexed sideways and I'm sure woulld have failed very shortly ...... sent straight back to supplier. Another ordered from a different source  ..... shall wait to see. Weather is improving so anxious to get back onto the road. Peter M

[tombeau]

strange...My Alton was supplied with a belt drive. I seem to remember he recommended not using a chain.

[trevinoz]

Peter M.
                  How did you measure the resistance between the dynamo case and the battery terminal?
If I used a multi-meter and got a reading that low, I would be very happy.
When you are measuring very low resistance, bear in mind that you are also reading the resistance of the meter leads as well as any contact resistance.
With a clean dynamo body and a clean crankcase surface you should get a better current path than a piece of wire.

Trev.

[pjm01]

Trevinoz ......  used a very good quality analogue meter accurately set to zero ohms with the test leads/probes shorted out (this will compensate for any lead/probe resistance). Measured resistance at a number of positions on the dyno body and averaged the result. Measurements were later confirmed with a digital meter ( zero checked with a copper 'dead short').
I was not happy with 0.1 ohm contact resistance as that would drop 1 volt at full dyno output (10A on a VERY good day !)....... that's a lot when you only have circa 7 or so volts to start with.
Having cleaned up the dyno body and the mounting position in front of the engine, I now can not measure any contact resistance ie it is as close to zero as it can get and as close as I can conveniently measure without using something like a Wheatstone Bridge. I may however run in an earth lead just to make absolutely shure. Peter M

[pjm01]

Trimmed up the commutator, fitted new brushes etc and carefully reassembled ...... to my absolute delight, dyno is supplying full lighting load AND putting a little into the battery ..... that's remarkable in my view for a 60Watt dynamo. What is even better, I'm not measuring any volts drop between the dyno body and frame earth ie virtually zero resistance. Next job will be' oil seals + bushes' in the forks but maybe wait until next Winter for that.Thanks all for your useful comments/help. Peter M