Stuart-Turner marine

Am I missing something here? If the ratio isn't 1:1, the same teeth will not mesh on consecutive rotations!

Lets say we have 22 and 20 teeth on the gears, here's what will happen:

Drive tooth, driven tooth; 1,1; 2,2; 3,3; . . . . 20,20; 21,1 ; 22,2;

1,3; 2,4; etc. 1,1 does not occur again until the eleventh rotation. All of the "evens" mesh with each other and all of the "odds" mesh with each other, but mixing odds and evens will not change the wear significantly will it?

Dan

I think the idea is each tooth on one wheel will mesh with all the teeth on the other over a sufficiently large number of revolutions. In this way any error in one wheel will not transfer itself by uneven wear to the other. I'm not that hot at maths but I would guess for this to happen the two should not have a common denominator.

I agree that 20/22 isn't too much of a problem. However, 40/20 would be as the teeth on the smaller gear always meet the same teeth on the larger one. This is where a hunting tooth comes in. If you use 41/20, the ratio is very close but the gears do not mate tooth to tooth as described before. If you are driving a pump, this ratio may be acceptable. However, an engine camshaft drive wouldn't work too well with a hunting tooth. The problem is that the teeth wear into each other uniquely in a 2:1 gear set. Gear teeth are a sliding bearing. If you strip the machine and do not assemble exactly as original, the bearing surfaces that have worn together are now working somewhere else with other bearing surfaces.

John

John, Far from it as a SC 4 Jaw will be about as inaccurate as a 3 Jaw and also a bit pointless to me :-)) hence the need for an adjustable 4 jaw and TDI to set it up.

Martin P

I can see that a high-speed gas turbine (or steam turbine for that matter) reduction box would perhaps be critical for noise and wear, but for your 'average' bit of hardware it surely isn't that critical ?

I had a quick swot in the very good Gears book I bought last year, and it doesn't go into huge detail, being more concerned with tooth form and production, but going back to engines I never noticed that teeth on timing gears on engines were ever marked as needing specific assembly other than for timing and they HAD to be 2:1 for obvious reasons.

Engines that I can think of that didn't, were the Bedford 381 and 466 diesels which had an intermediate gear and so did the Scania's IIRC, mainly because the camshaft was too far away from the crankshaft to do it in one step. The 330 and

220 may have been one step, I cannot remember just now.

Listers also had an intermediate gear on most of their engines, probably for the same reason.

Peter

-- Peter & Rita Forbes snipped-for-privacy@easynet.co.uk Engine pages for preservation info:

Timing gear sets are all marked, of course, so are necessarily re-assembled as they were taken apart. Magneto pinions are usually fibre, nylon or aluminium, each of which will adapt to changes of mating teeth as they are softer than the steel gear that drives them.

At least on motorcycles, the gears all have hardened teeth (cyanide hardened, IIRC) and as they are lightly loaded for their service and are well lubricated, do not wear much anyway. Nonetheless, Vincent timing gears were notoriously noisy after reasonable service and what with worn rocker bushes too, used to sound like two old fashioned gas stoves being dragged over cobbles.

Regards,

Kim Siddorn,

Agreed, but I guess it is still good practice in less critical applications, and who would expect anything less of S-T.

I agree, but having seen some of the pretty poor quality gearing on a few engines, they were probably amongst the minority!

Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk