gearing 101 in a Shoebox

Gidday All.
I am starting a new thread here that is intended to be closely related to my Commando gear teeth thread.

I realise that this is such a big subject I need to seperate the subject into 2 parts, discussion directly related to the teeth and factory changes relating to the AMC box, and a information/educational session relating to the theory behind what went on inside the AMC box.

So this will be the adjunct to the Commando gear teeth thread.

The title of this thread says it all. Soon there will be an introduction here with pictures. the delay has been because, to me, the forthcoming picture presentation has been a bloody big job. But also a labor of love.

Hopefully the pictures will be self explanatory.

Please ask any questions you like relating to this thread. I will answer all but replies may not be on the same day.






































for all those following the gear theory part on the other thread, I really do thank you for your patience. There is light at the end of the tunnel.
Best wishes to all Aussie bradley.

B.Rad said:

There is light at the end of the tunnel.

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So Bradley, what exactly are we waiting for and more importantly.... why?
Gidday Mark

Labors of love need no reason to express so ignoring the noise Bradley what's the deal on backlash & measure?

I am enjoying reading this discussion. I am an interested non-engineer and have gotten a few "Ahaaa...." moments from the above illustrations. Thanks B-Rad
Lance

Keep up the good work Bradley. When you get to the correct area of your article, perhaps you can explain why straight cut gears are noisier the helical cut gears.
Dereck

Mark said:

B.Rad said:

There is light at the end of the tunnel.

Click to expand...

So Bradley, what exactly are we waiting for and more importantly.... why?
Gidday Mark

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Gidday Mark.
the purpose here is to explain the theory of gearing to non engineering persons that may be interested .

The other thread will examine the nature of changes made to the box by the factory to improve strength and life.

Also examine issues of interchangeability, eg, there seems to be a change from the Atlas 4th gear pair ratio that was fitted to the Commando box. This change improved strength and gave longer life.
Some persons may not know this.

And also identify potential improvements that may still may be made.

Some people may then wish to fit the Commando pair to the Atlas for example. thus getting the benifets.

For the non engineering persons, this thread attempts to explain the basic theory of gearing. Without this explanation, non engineering persons would definitly think that the other thread is just incomprehensible rantings.
best wishes Aussie bradley

hobot said:

Labors of love need no reason to express so ignoring the noise Bradley what's the deal on backlash & measure?

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gidday Steve. BACKLASH.
imagine a shaft fitting into a bush. if there is none or not much clearence, the shaft will not fit or even seize when in use. backlash is the equivalent of this cleartence applied to gearing.

Backlash is the correct amount of clearence between the gear tooth and the space into which it fits. with zero backlash in the AMC box, under power the gears will not operate properly, the lubrication between teeth is affected, and tooth danage will result.
Zero backlash at speed under load will run very hot.

there is no one correct amount of backlash for all applications. small teeth need smaller backlash. larger teeth need more backlash than smaller teeth. especially when under maximum load.

Too much backlash is not necessarily a bad thing. Too much will result in noisy operation and lost motion under reversal. This can be acceptable or not acceptable. it depends on what the gear is used for.

For example a naval gun needs to be positioned very accuratly, and held in this position positively. any changes in gun elevation need to be done quickly and without " lost motion". so they will run a very small backlash. the same applies to the gearing used to position large radio telescopes.

however in a large industrial power gearing used to drive a crushing mill, operating at relatively low speed backlash dont really matter. can be a 1/4 inch or even more.

Some control gearing needs zero backlash. then the cost of these gears will be very expensive, and the making of these gears will require the very best in machine tools. the usage of these gears is very rare.

So in most cases, there must be backlash. The amount depends on the application. For an example of where close control of backlash is demonstrated, Truimph timing chest gears are a good example.

If the backlash here is excessive, the cams, especially if fierce, will have a tendency to apply constant reversing load to the gears . they will be noisy and suffer other types of damage. lost motion here is not a good thing.

A good figure for the commando box is 4 to 6 thou cold.
Best wishes Aussie bradley

hobot said:

Labors of love need no reason to express so ignoring the noise Bradley what's the deal on backlash & measure?

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gidday Steve.
MEASUREMENT OF BACKLASH.

when making a gear pair, to obtain backlash, there must be some thinning of the teeth from the theoretical tooth thickness where the tooth and the space into which it fits are the same..

This thinning of the teeth may be applied just to one tooth, or both teeth . if applied to one tooth, the tooth is thinned by the total amount of backlash required. if applied to both teeth, the amount of thinning required of each tooth is half the amount of total backlash required.

the measurement of tooth thickness takes place on the pitch circle. therefore the expressed measurement is the length of the circumference of the curved portion of the pitch circle.. This is the CIRCULAR TOOTH THICKNESS. this is very difficult to measure.

It is a lot easier to measure in a straight line

this straight line forms a chord of the pitch circle. ( refer drawing 6&8) it is therefore the chord length that is required. This is known as the Chordal Tooth Thickness.
so to measure this dimension it is necessary to first know how far down the tooth is the pitch circle.

the amount the tooth sticks up from the pitch circle is known as the addendum. therefore it is easy to calculate the addendum by subtracting the pitch circle diameter from the outside diameter..
this measurement will give the radial depth of the addendum.

because of the curvature of the pitch circle, the actual radial depth of the chord will be more than the difference between the outside diameter and the pitch circle (addendum).

so to find the correct depth,the radial dimension of the addendum is added to the height of the chord. (refer drawing6&8) this is known as the Corrected Addendum.

to calculate all this refer to drawing " Formulas for calculating chordal thickness" ,drawing no6&8.

To measure all this, a special vernier is used. (drawing9)This is known as a gear tooth vernier. it is basically 2 verniers in one frame at 90 degrees to each other with a sliding stop on the vertical scale that rests on the outside diameter of the gear..

So first set the vertical scale of the gear tooth vernier to the CORRECTED ADDENDUM. Then place the stop on the outside diameter of the gear.

then move the horizontal scale until contact is made with the sides of the gear teeth. lock the horizontal scale and remove imstrument. the reading of the horizontal scale gives the Chordal Tooth Thickness.
best wishes Aussie Bradley

alrighty Merlin told me to learn something to break deep depression so a good pick em up for me Bradley. Ok then, acklash in gears is like crank bearing clearnaces which makes me wonder now about the cush drive element on backlash desired.

kerinorton said:

Keep up the good work Bradley. When you get to the correct area of your article, perhaps you can explain why straight cut gears are noisier the helical cut gears.
Dereck

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Gidday Dereck

straight cut or spur gears are noiser than helicals because there are less teeth in contact at one time, so there is a less smooth transition between teeth when engaging. therefore the loading impacts are more abrupt and noisier.

Noise factors also depend on speed and accuracy as well and these can be significant. Big helicals running fast can also get quite a whine up. heavy or light oils can also significantly contribute to noise levels.So the term noiser is really realtive to a lot of factors.. it can be objective as noise can be measured . For a spur and helical set of the same size running at the same speed on the same oil, the spur will show slightly more when measured with a DB meter.

In a spur gear there is a portion where 1 tooth only is carrying the load however brief.
but for most of the time 2 teeth share the load.

therefore the number of teeth carrying the load is an average of the number of teeth in contact at one point in time. This number must vary between 1 & 2.

This average number is known as the CONTACT RATIO. This can be calculated but this is beyond the scope of this post.

For straight gears, 1.7/1.8 is a very good number. This means that there is an average number of 1.7 teeth in contact at any time.
1.4 is about the bottom number for a tooth expected to carry a relatively heavy load for the tooth size (DP).
a contact ratio of 2 is not possible for spur gears..

Of course there cannot be 1.7 teeth in contact. this is just a mathematical average.

This figure gives an indication of not only smoothness of engagement but also one factor indicating load carrying capacity. helical teeth get up to a contact ratio of over 2 and because of this the transition is smoother therefore the loading transition is quieter.
best wishes Aussie bradley

Brad
While you mentioned Triumph timing chest gears as requiring minimal backlash, a more critical application was the Ariel square four crankshaft coupling gears. The square 4 ran two 180 degree crankshafts coupled by a pair of straight cut gears. It is not hard to imagine the noise that would be generated, if there was excessive backlash to the coupling gears when first a cylinder to one crankshaft fires and then a cylinder to the second crankshaft fires. So in effect first one crankshaft is driving and then it is being driven. Clearly not an application where excess backlash could be tolerated.
As backlash had to be kept to a minimum the square4 coupling gear teeth were ground to finished size.
Ando

ando said:

Brad
While you mentioned Triumph timing chest gears as requiring minimal backlash, a more critical application was the Ariel square four crankshaft coupling gears. The square 4 ran two 180 degree crankshafts coupled by a pair of straight cut gears. It is not hard to imagine the noise that would be generated, if there was excessive backlash to the coupling gears when first a cylinder to one crankshaft fires and then a cylinder to the second crankshaft fires. So in effect first one crankshaft is driving and then it is being driven. Clearly not an application where excess backlash could be tolerated.
As backlash had to be kept to a minimum the square4 coupling gear teeth were ground to finished size.
Ando

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Gidday Ando. thanks for your informative reply. Much appreciated and a fine example.

mate, I have never seen a square 4, but I can imagine the gears now you have brought it up.

that is a fine example where there is more to those gears than meets the eye.
Hardness and toughness would also have to have been spot on. Plus a lot of other stuff.

But accuracy would have been beyond what a just hardened gear was capable of. I have no doubt they were ground.

As you are aware, but for the non engineering persons following this, , zero or very little backlash means the pitch circle runout also has to probably kept below 1 to 1/2 a thou, tooth thickness 1 thou or less for this application. Most probably less.

imagine the work involved in making that pair. pre CNC too. would have had to have some nice form dressing gear just to do the right shape of the grinding wheel.

for those interested, drawing #7 gives an idea of accuracy levels. these were probably around 10/12 AGMA standard.

Ando, thanks for very good practical example. if you got any more stuff like this, please dont hesitate to chime in. this thread is after all, intended to educate and inform. and I certainly dont know it all.

best wishes Aussie bradley.

There a heck of a lot of info to read through but it's gold mine for us non-engineers.

Thanks

I remember drawing gear teeth in drafting class. This all makes my head hurt.

I too semi enjoyed the detailed focus of drafting class. Typing class more as only girls and gave more used skill.

Bradley are you set up to cut improved profiles - just for the love of it?

pommie john said:

There a heck of a lot of info to read through but it's gold mine for us non-engineers.

Thanks

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Gidday John.
If this thread helps one person make a better informed decision when buying/replacing gears, it will have served its purpose.
especially racing lads like yourself.
Best wishes Aussie Bradley

hobot said:

I too semi enjoyed the detailed focus of drafting class. Typing class more as only girls and gave more used skill.

Bradley are you set up to cut improved profiles - just for the love of it?

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Steve, if I had more brains I would have gone after the girls too. but with a face like mine, well???.

No Steve. I dont do commercial work. I only have a very small gear hobbing machine and no heat treating facilities at home.
best wishes Aussie bradley

Ugh I had to trick gals to pay any attention to me either. Someone told me if ya can mess with their minds ya can mess with the rest of em. For some reason all gear primary popped to mind thinking about the all gear cam drives of ole Chevy V8's which come in quite or whiny gear froms. I guess Norton power units are too elastic and thermally swelling to get away with gears there. 1st time I opened the gear box it felt like opening up rog guts - all fitted so nicely together but little idea of what the parts were.

bwolfie said:

I remember drawing gear teeth in drafting class. This all makes my head hurt.

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I forgot all about that. Flashback...

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.