gearing 101 in a Shoebox

acotrel said:

Brad, A thing which really confuses me is the hunting gear use in Triumph timing chests. I cannot get my head around how the 2 to 1 ratio is maintained while the contact position changes. About square four Ariels - I almost killed myself on one in about 1960. I wound it up to top speed across an airstrip and found myself running out of room to stop before the chain wire fence at the end. I finished up riding the bike in a U curve very fast. Brakes were almost non-existent, and the clatter from the crank gears was really bad. Once something gets within your braking distance the experience becomes very bad.
About those women - never sell yourself short. Guys like ourselves who have skills are not so common. If a woman is turned off by your ugly face, she is not worth knowing. I was in my first marriage for 32 years, it was a soul-destroying experience. You might believe in that situation you can shag around - I could not do that to my three kids. These days I am remarried and spend a lot of time looking after my step-daughters two babies due to a family violence situation. It is all part of life, however never worry about where your next root is coming from, it will find you. - quicker if you wear a wedding ring.

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Gidday Alan. Alan, this is an area where I do not have a lot of knowledge.

This is my definition of a hunting ratio. I may be wrong here. I dont get into this too much.

anyone is welcome to chime in.

A hunting ratio has no common factors in either gear of the pair.

so 14/28 : 2 divides into both 14 and 28 so to me this is a non hunting ratio. 17/28. no common factors here so this is a hunting ratio.

Old work (Ichimaru, ASME 1980), showed in some cases more load carrying capacity was obtained by using a NON HUNTING ratio.

Also the pair wore in and developed the proper oil film more quickly with a NON HUNTING ratio than a hunting ratio.

So there you go. Whether or not to use hunting ratios is a very very complex subject.
The science and applications are really beyond my knowledge. Hope this helps bradley

I do not get a chance to discuss anything that I am a "qualified expert" on (maybe an expert, depends on who you talk to LOL). Gears are something that I live with everyday.

First: Brad, quit giving away all the secrets!!!!!!!!!!!!! I have been machining for almost 50 years. In any group of machinist, only about 3% know how to cut gears correctly. And of that 3%, only about half that amount know gear theory.

Brad is right on in his statements, and I guess it is good info for the very technically sophisticated crowd on Access Norton.

The only thing I would add, would be a quick comment on lash. Lash is important so that only the drive side of the gear is in contact with the driven side of the associated gear. Not enough lash (for a given grade gear or how true the TIF is / TIF=True Involute Form) will cause contact on both sides of the gear face and generate friction and heat, a lot of heat, red hot kind of heat. Too much lash give a smaller cross section on the gear tooth, and hence makes the gear tooth weaker. So too much, or too little lash is bad, and is proportional to the DP of the gear and class of the gear (class=TIF among other factors like indexing)

Brad can most likely comment further, as he is the only person I have talked to in a while that really understands gears. (but, Brad, please quit giving away secrets as I cannot speak with intellectual superiority anymore when you have educated all the Norton guys. LOL)

acotrel said:

Brad, A thing which really confuses me is the hunting gear use in Triumph timing chests. I cannot get my head around how the 2 to 1 ratio is maintained

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There is nothing tricky about hunting gears - but it ALWAYS has to involve 3 gears at least.
It cannot be done with only 2 gears in the set, for example.
What the ratios gain on one gearing they have to lose on the 2nd gearing/ratio.
Distributes wear all around all the teeth...

When Velos introduced (slightly) different tooth counts on some of the timing chest gears, just postwar,
it introduced the possibility of accidentally adding a hunting tooth, if you weren't careful with selecting the right components.
Apparently this meant that the spark timing would only be correct every 348 (was it !?) revolutions of the crank.
Thats a long time between bangs...

G81 Can Cycle said:

I do not get a chance to discuss anything that I am a "qualified expert" on (maybe an expert, depends on who you talk to LOL). Gears are something that I live with everyday.

First: Brad, quit giving away all the secrets!!!!!!!!!!!!! I have been machining for almost 50 years. In any group of machinist, only about 3% know how to cut gears correctly. And of that 3%, only about half that amount know gear theory.
Brad is right on in his statements, and I guess it is good info for the very technically sophisticated crowd on Access Norton.
Brad can most likely comment further, as he is the only person I have talked to in a while that really understands gears. (but, Brad, please quit giving away secrets as I cannot speak with intellectual superiority anymore when you have educated all the Norton guys. LOL)

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gidday G81 Can Cycle. how welcome it is to have you chime in.

mate, you actually run a gear shop so you do speak with authority. if I stuff up please chime in and correct me. Your comments on lash are very true and relevant. Your comments are valued.


I think I get the joke regarding secrets mate. but this is all public domain stuff.
available in machinery handbooks, trade journals, etc.

If this helps one person make a better informed decision when buying/replacing gears, this is then a good thing.

There is too much mystery about when looking at buying proprietory branded gears.
if you sell a quality product then there is nothing to hide. So a potential buyer can sort out the "marketing claims" from good engineering practices. that is a good thing.

So G81Can Cycle, your views are most welcome here mate, may you contribute more often. best wishes bradley

B.Rad said:

I think I get the joke regarding secrets mate. but this is all public domain stuff.
available in machinery handbooks, trade journals, etc.

If this helps one person make a better informed decision when buying/replacing gears, this is then a good thing.

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Has anyone actually had problems with buying replacement gears for a Norton ?
They do seem very interchangeable, and non-fussy with it.

This reminds too of someone who had a noisy diff (car = 4 wheels) and non-adjustable diff. (or so it was said).
Placed in the hands of one of these diff lapping mobs.
Who charged a fair bit for it, and had strict instructions for running it in (more so than for a new car).
It was noiser than before...

Don't recall if he sold it, or binned it and found another.

Rohan said:

acotrel said:

Brad, A thing which really confuses me is the hunting gear use in Triumph timing chests. I cannot get my head around how the 2 to 1 ratio is maintained

Click to expand...

There is nothing tricky about hunting gears - but it ALWAYS has to involve 3 gears at least.
It cannot be done with only 2 gears in the set, for example.
What the ratios gain on one gearing they have to lose on the 2nd gearing/ratio.
Distributes wear all around all the teeth...

When Velos introduced (slightly) different tooth counts on some of the timing chest gears, just postwar,
it introduced the possibility of accidentally adding a hunting tooth, if you weren't careful with selecting the right components.
Apparently this meant that the spark timing would only be correct every 348 (was it !?) revolutions of the crank.
Thats a long time between bangs...

Click to expand...

A CORRECTION: The claim that a hunting ratio ALWAYS must include 3 gears is Wrong. A box may only have 2 gears, driver and driven. this pair may or may not be a hunting ratio. depending on what is wanted. there is no such principle of 'gaining and losing".

As for the decision on whether to use a hunting ratio or not, this is indeed a very very complex subject.

the debate on this , by its nature, gets very very technical. ( Ishibachi, Ichimaru, ASME, 1980. )

" the decision on whether or not to use a hunting ratio becomes much more complex. Several pros and cons need to be considered." Dudley, Darle, W. handbook of practical gear design. 1984. pp3.9 This reference gives a table of considerations.

This correction is not meant to argumentative. however information that is not correct is not a good thing. bradley

Its durned hard to drive a camshaft or ignition with a gearset of only 2 gears that is not a fixed relationship between drive and driven !!!
Gearboxes need no such relationship, but ignitions and cams etc most certainly do.
Talk sense, not nonsense, and distinguish the 2 ?

Enquiring minds would also like to know why j*p*n*s* machinery has gearboxes that are near bulletproof ?

OK, we know that they are mostly all indirect drive, and have clutches and drive sprockets with very liitle to none overhang from their bearings,
and the bearings in many cases have individual oil feeds to each one, but still.
Is the steel better, the design better in some way, the shafts beefier, ???

Sample pic, selected at random.
Some of the bigger roadbikes have boxes that cope with hundreds of hp, and more if turboed or supercharged.

What to look for in a good gear suitable to run in a Commando?.

A fine example of bad gears that are unsuitable, I refer to the post "Quality of replacement gears" by ggryder.
TimeWarp in the same thread gave photos of a vey well made set.

there are a few simple questions that can be asked by a potential buyer.

There are no secrets, or there should not be. reluctance to get satisfactory answers should at least start alarm bells ringing.

here are some examples provided by ggryder in his post of unacceptable gears.




so here are a few questions that may be asked.
This stuff is all public domain .

Q. What accuracy standards are the gears made to?.
A. medium to high accuracy, class B or C. The teeth may be used as cut, or processed further by grinding. American Gear Manufacturers Association no's 8 to 10 ( 9 is a good number), Euorpean DIN no's 6 to 8. The main factor to look for is the pitch circle runout is about 1&1/2 thou max. if that is right the rest is likely to be correct also.

Q. What are the gears made from?
A. A low carbon, low alloy, case hardening steel. under .25% carbon Nickel/Chrome/Moly ( moly may be optional).
Examples are
American 8620, British EN36A, European, Bohler E110. I like 8620 myself as it distorts very little during heat treatment. 8620 aircraft composition is a very clean steel.

Q. What heat treatment has been applied?.
A. depends on the steel used and the stability during hardening. For steels of proven stability, single quench. For the maybe tougher steels , double quench. EG: 8620 or EN 36A, single quench is fine.
A double quench gives better metallurgy, but distortion usually means double quench gears should be ground. For the class of gears used in these boxes, a single quench is satisfactory.

Q. What hardness of the case and core?
A. Core 34/36 Rockwell C, case 60/62 Rockwell C. case thickness 20 to 25 thou.

Q. What is the surface finish of the teeth flanks and bottoms radius.?.
A. Between 2 and 8 microns(metric ). 8 microns (less than 1/2 thou)is at the upper limit, any more than this can play havoc with a case hardened gear.
Especially at the bottom of the tooth in the fillet. Tooth finish is very important.
A scratch 2 thou deep can cause unacceptable stress concentrations in the bending and compressive stresses.
if the gear is running close to the design limit, tooth breakage is most likely from a 2 thou scratch on a case hardened gear..

these answers will indicate that the gear is a good quality, and will stand up to the load of a Commando box.

Though there are a lot of other considerations, of course. The intention is to provide some general guidance to the non engineering people that the gear is not complete rubbish made from chewing gum.
Even used under racing conditions. The teeth are unlikely to break.

Blokes like Quaife and TTI Industries do not use standard proportion gears of 20 degree pressure angle.
these are well proven designs with proven performance and a well deserved reputation..

Other gear offerings may indeed be very very good with proven reputations as well. In Field proven designs the quality is self evident. But if you have never heard of a particular offering, it will do no harm to ask.
if you see an offering similar to the above photos, lock the wallet and run.
A comment from G81 Can Cycle would be most welcome. bradley

Did we ever find out who supplied those dodgy Norton gears ?
Not naming the supplier of shoddy stuff defeats the porpoise of this forum...

Timewarp should have gone with a Kwaka KLR650 !, those things are as tough as old nails.
That was the j*p gearbox internals that I showed.
The US Army bought them for military use, and some were even converted for diesel operation.
Have to be tough to withstand that...

Thanks for the thoughts on the gear making/cutting/quality etc.