Commando gear teeth thread.

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Gidday . hearty good wishes to All.

This subject would probably hijack the gearbox identification thread so I need a seperate thread.. This is a thread about Commando gear teeth . Not shells, shafts or bearings. mainly focusing on interchangeability and strength and design changes..

I will endeavour to be as factual as my knowledge allows. By nature, the subjects of strength and durability will get reasonably technical. Cant avoid this. so I will try and keep the technical jargon to a minimum though this may sometimes be necessary to speak jargon.

it is also intended to inform and educate lay people who may have an interest in the technical side of gears.
My experience is at trade level only. So any qualified engineers and others are welcome to chime in and correct me where necessary. Please do. I do not know it all and it is inevitable that I will stuff up somewhere.

where technical terms are necessary, I will try and give a definition of these terms and what they mean. Another purpose is to provide factual information that is relevant and accurate.

claims will be justified as best I can. the onus of proof is always with the person making the claims.

Some of my claims may challenge "conventional wisdom".
just like the "conventional wisdom" that claims line boring was used in crank case production.

gear design changes can be similar to head porting. yes, changes can be easily made.
but a design change may have negative consequences as well. so not all changes have to have a positive effect.

Stephen Hill, this thread may answer your question about increased load carrying capacity of early and late changes and the consequences of such change. i agree with you that there is no signifiant strength changes between early and late. though it may get a bit convoluted.

So a bit of basics for those interested.

A pinion will be the driving gear. A gear will be the driven gear.

the teeth of the pinion and gear touch at special diameters, roughly in the middle of the tooth... these diameters are known as the pitch circle. If the teeth were removed from gear and pinion, the imaginery point of contact will be where the pitch circles touch.

the pitch circle is very important.it has a dimension and can be calculated.
for any 2 gears to mesh, the spacing of the teeth on both gears must be the same.

but because the teeth are at an angle to each other, and the pitch circles differ in diameter, measurement of the spacing must be at a particular point. So where do we measure the tooth spacing. At the outside of the tooth, the bottom of the tooth or somewhere in between. the answer is somewhere in between. the pitch circle.

The center distance between pinion and gear is the distance in inches between the centres of the pitch circles.

So the pitch circle circumference divided by the number of teeth will give whats known as the circular pitch. or tooth spacing.
this tooth spacing must be the same for any meshing driver and driven gear.

But there are other factors also, such as the tooth width and depth. these must also be the same for any pair of meshing gears.

So we need something like a standard tooth size. so the size of teeth like thread pitches is standardised.

this standardised sizes is known as DP, or Diameteral Pitch. it has no measureable dimension and basically refers to a certain tooth size. it is expressed as a number only.

the numbers mean a 20DP gear has a very small tooth, 5 DP has a lot larger tooth, 1 DP is a very large tooth.
The bigger the number the smaller the tooth. 1 DP is the biggest standard tooth.

The tooth size of all AMC boxes since 1956 including our Commando's and Dommies etc is 10 DP. before that I dont know and this thread is only relevant to Post 1956 boxes.

Bigger teeth than 1 DP use a different system entirely that is not relevant here.

if any gear or pinion is of the same DP, the circular pitch or tooth spacing is the same, tooth thickness is the same and depth of tooth is the same. and they will mesh.
A 20 DP tooth will not mesh with a 5 DP size tooth.

end of sermon. a bit more basics to come before we get down to the nitty gritty. i do reckon this basic stuff is very necessary to follow what is to come.

If some kind person would post a drawing of gear teeth terminology that shows this stuff, especially pitch circle and tooth spacing, addendum, dedendum etc, it would be appreciated.

It is easier to look at a drawing to understand this. More to come . best wishes Aussie bradley
 
Did you know that Norton gearboxes gear pairs tooth count always sums to 42, more or less.
(So = 42, but 41 and 43 may be possible).

This is the quick-n-dirty method for pairing up the gears in a dismantled box, without doing anything more than a cursory tooth count.

We need to find that short discussion of the change of tooth form in AMC boxes, discussed somewhere relatively recently.
Or some blueprints/drawings detailing same.
 
Rohan said:
Did you know that Norton gearboxes gear pairs tooth count always sums to 42, more or less.
(So = 42, but 41 and 43 may be possible).

This is the quick-n-dirty method for pairing up the gears in a dismantled box, without doing anything more than a cursory tooth count.

We need to find that short discussion of the change of tooth form in AMC boxes, discussed somewhere relatively recently.
Or some blueprints/drawings detailing same.

Gidday Rohan.
Thank you for that. No I did not know that. I have learned something.

Mate, if you could find that discussion about change of form, it really be appreciated.

I just read that Hemmings stuff, could there have been a change back as far as 1956?. I really dont know. perhaps thats when they changed the pressure angle. if they ever did.

But that change of form discussion could be very very relevant.

I missed that cause I been away from here for a couple of months.

Could some one else guide us to this discussion please.

hey thanks again very much. Aussie bradley
 
Apparently, the tooth form of the Commando 4th gear pair was modified from serial 128646 (INOA Tech. Digest).
This seems to tie-in with the '68-'70 parts book, as both 4th gear part numbers changed for 1969.
 
L.A.B. said:
Apparently, the tooth form of the Commando 4th gear pair was modified from serial 128646 (INOA Tech. Digest).
This seems to tie-in with the '68-'70 parts book, as both 4th gear part numbers changed for 1969.

Gidday Les.

Thank you very much for that. this is exactly the kind of info we need to help get to a conclusion .

If I may be bold, can you please advise if the tooth numbers involved in this change. any change in ratio would indicate a profile change or shift. so if I know the tooth numbers before and after I can work out what type of modification took place.

4th gear pair:: if the tooth numbers changed for 4th gear pair, that is one type of mod. if tooth numbers stayed the same, and part no's changed, that indicates maybe another type of modification

2nd gear pair:: I do know the profile changed in the seventies for the second gear pair only. this had something to do with noise emissions. slows the engine RPM down, so not so loud.

so we had 2 changes, the fourth and second gear pair. things are progressing nicely

What would be very handy is to know if the 1st gear pair tooth numbers ever changed after the Commando was introduced. or any change in the part no,s for The Commando 1st gear pair. this also would comfirm any change. if the tooth numbers did not change, but the part no's did, that indicates what type of change occurred. but if the tooth numbers changed, that may be a different type of modification
clear as mud mate. to summarise, if tooth numbers change, that indicates what type of mod took place. if part no's changed but tooth numbers stayed the same, I can still work out what was going on. this data can then be tabulated and analysed and a conclusion reached as to how much increase in strength and durability took place.

tooth numbers and part no changes can indicate exactly what was going on.

Thanks again mate, Aussie bradley
 
B.Rad said:
If I may be bold, can you please advise if the tooth numbers involved in this change.

It seems the 4th gear tooth numbers remained the same at 18/23. This appears to have been the standard AMC road gearbox 4th gear tooth count from 1960. Before that, it was 18/24 according to my information.

B.Rad said:
What would be very handy is to know if the 1st gear pair tooth numbers ever changed after the Commando was introduced. or any change in the part no,s for The Commando 1st gear pair.

As there doesn't appear to have been any change of 1st gear part numbers from '68-on, it would seem that no significant changes were made to the Commando (28/14) 1st gear pair.
 
B.Rad said:
2nd gear pair:: I do know the profile changed in the seventies for the second gear pair only. this had something to do with noise emissions. slows the engine RPM down, so not so loud.

The 2nd gear profile didn't change, it was the tooth count.
The gearing was raised, in 2nd gear, so the bike couldn't accelerate quite so hard - so the noise emissions were met.

This is a somewhat lateral thinking method of a reduction in noise. !!
Like oiling the chain beforehand......
 
Rohan said:
Enquiring minds could comment that 4th gear is a 1:1 direct ratio.

However, I think enquiring minds would also comment that changing the '4th' gear 'ratio' alters the overall ratios of 1st, 2nd and 3rd.
 
4th gears aren't really 4th in a Brit 4 speed box, they are the second pair for 1st, 2nd and 3rd, once you are in 4th the sleeve gear is locked to the mainshaft and yes you get direct drive with all the other gears and shafts spinning but not having any drive through them. But they are called 4th so 4th they are ;).
 
Rohan said:
B.Rad said:
2nd gear pair:: I do know the profile changed in the seventies for the second gear pair only. this had something to do with noise emissions. slows the engine RPM down, so not so loud.

The 2nd gear profile didn't change, it was the tooth count.
quote]

Gidday Rohan. as I am focusing on the tooth profile, I may have not stated myself as clearly as I should have.

But mate, what you miss here is the PROFILE CHANGED. if the tooth count changes, the tooth profile changes. It must.
this is a given and not open for interpretation or argument..

the whole point here is to analyse the tooth profile changes to determine how much benifet the changes were to strength and durability. hence my focus on profiles. tooth counts just tell me what the factory did.

Rohan said:
Enquiring minds could comment that 4th gear is a 1:1 direct ratio.

Of courese. elementary. but again "4th" just directly locks the input (mainshaft) to the output (sleeve gear). when input speed = output speed there is very little option but 1:1.
Stay focused here please mate. It's all about the consequences of tooth profile changes. Not red herrings. But thanks for your contribution. I sincerely hope you continue.

quote="kommando"]4th gears aren't really 4th in a Brit 4 speed box, they are the second pair for 1st, 2nd and 3rd, once you are in 4th the sleeve gear is locked to the mainshaft and yes you get direct drive with all the other gears and shafts spinning but not having any drive through them. But they are called 4th so 4th they are ;).

Kommando: thank you for that. you are correct of course. a lot seem to miss this point.
one pair at a time in this box does not provide all the drive.

Les . as usual you provide accurate relevant info for this purpose. thank you. I am building a good picture now as to what was done.
all the best Aussie bradley
 
I suspect that the actual relative ratios of all the gear pairs in the box affect durability. With a close box the exposure to torque of individual gear pairs is less once the bike is moving, unless you raise the overall gearing which mainly only adversely affects first gear and the main shaft.
 
I think the idea of changing the internal gear ratios to moderate noise for environmental reasons is hilarious. How bloody futile ?
 
If the British couldn't fit a decent 6 speed box into their bikes as the Japanese did, their bikes such as the commando should have been used in line with their design intention - BOAT ANCHORS !
 
acotrel said:
I think the idea of changing the internal gear ratios to moderate noise for environmental reasons is hilarious. How bloody futile ?
it was the only way they could get the noise levels down ,IE the test had to be done riding past in second gear so the ratio was raised in second gear to get the revs lower ,along with huge air filter boxes balance pipes and black cap silencers etc ,,,,,,,,,,,,,,,,,,,,,,,,,baz
 
acotrel said:
I suspect that the actual relative ratios of all the gear pairs in the box affect durability. With a close box the exposure to torque of individual gear pairs is less once the bike is moving, unless you raise the overall gearing which mainly only adversely affects first gear and the main shaft.
gidday Alan.

mate ratios have very very little to do with durability. first choose the ratios and then design around the ratios. Taking ALL things into consideration. very often design changes are made as something comes up and needs attention.

Gear design is not a linear process where steps are followed in a certain order. and then you reach a result.

it is applied power or torque that determines contact stress on the tooth. the limit here is about 200,000 newtons per square millimeter contact stress. This is a bit on the optimistic side. 180,000 may be more realistic. this figure may also apply to sub surface stresses as well.The ability of the tooth to handle the contact stress is a big part of durability. hardness therefore plays a big part. lots of other considerations too.

Durability usually means the ability of a tooth to not break or become unacceptably noisy or rough for the number of cycles that is the design life. expected failure rate comes into this now as well.

teeth need no more than 50,000 to 100,000 psi tensile strength to have an infinite life regardless of what steel they are made from.. But hardness will affect this figure. this is the endurance limit.

To get infinite life at 50,000 lbs tensile you need around 300 brinell hardness , 100,000 lbs you need 450 brinell. anything over this the life becomes finite and this is why good design is so critical. Most automotive gears have a finite life of around 10 to the 6th or 7th cycles at maximum torque.

without having reference to factory info, I suspect our boxes have a design life of somewhere around 10 to the fifth cycles AT MAXIMUM TORQUE. if less torque is applied the life increases.

Run the AMC box at maximum torque and watch the life expectancy plummet. This is why they last a bit. they run at far less than the maximum torque for a given number of cycles. even racers do not operate at maximum torque from start to finish. May go very close but not 100% maximum.

most gears have a designed life of a given number of cycles. after this failure can be expected.

Durability has a lot to do with tooth surface hardness . to get good durability ( with particular reference to our boxes) as the contact stress goes up, so should the hardness.
Hope this helps Aussie bradley
 
The small 1st gear on the layshaft is almost always rather worn in Norton boxes.
Can't imagine it actually does too many miles, so the torque kills the surface....

I'd also suspect that LABs mention of the tooth count changing on '4th' gear from
even numbers to odd numbers of tooth counts would be to spread the wear all around the gears.
ie not to have the same teeth always meeting the same teeth....
 
acotrel said:
If the British couldn't fit a decent 6 speed box into their bikes as the Japanese did, their bikes such as the commando should have been used in line with their design intention - BOAT ANCHORS !

Gidday Alan.

Thanks for that contribution.

This is indeed a fine example of how to hijack a thread and get off topic.

I took the bloody trouble to try and explain to you why ratios are not a big consideration in durability.

and you write this rubbish.

Please provide documentation supporting your claim that the design intention was for Commando's to be boat anchors.

perhaps you are in the wrong forum.

best wishes Aussie bradley
 
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