# gear building questions



## SE18 (Feb 21, 2008)

I purchased a 6-speed Tamiya gearbox/motor. I put it together a couple of ways and it's interesting to see how to wratchet the gearing up or down. Good learning experience and I plan to eventually try and hook it up to a scratchbuilt locomotive axle (it is a bit noisy).

The question I have, is, instead of having a very fast motor and gearing it down, wouldn't it be more efficient to use a slower motor and less gearing (stepper motor?)? Also, might it not be a bit quieter?

Lastly, are there any cool sites on gear building for beginners. Thanks.


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## Don Gage (Jan 4, 2008)

SE18, 
By using the high speed motor and gearing it down you increase torque and diminish the effort against the motor. While steppers are great motors, in order to get enough torque to perform the way you wish, will require a larger motor. 
Just some thoughts, 
Don


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## Les (Feb 11, 2008)

SE,

Gee, you didn't get many replies.

Stepper motors are (to my experience) rated Peak to Peak for torque values. On a scope, you usually see a triangular waveform. A fairer rating would be to multiply the advertised torque by 0.707, roughly the RMS of a sine wave, to see what the effective power output is. That'll put you in the ballpark.

And no, the reason small motors drive big heavy stuff is gearing. Look at the very earliest steam engines: the ones that pumped water out of mines. Huge bores & strokes. They usually drove a walking beam, the mechanical advantages of which didn't seem to be considered in those days. To get power in a small size, you need a torque converter, be it a transmission, gear train, or a string of pulleys/cogwheels--a thing not to be overlooked with models in this hobby.

Les


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## SE18 (Feb 21, 2008)

thanks; I did wonder how a high-speed, low torque motor could power a fairly heavy train; guess it is the mechanical advantage then from the gearing. I still don't know why some gear boxes make so much noise compared with others. The Tamiya is a screamer.


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## Totalwrecker (Feb 26, 2009)

Noise comes from the mesh of the gears, grease may help. 
Are they machined or cast gears....


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## Les (Feb 11, 2008)

Some gear boxes are noiser than others for a variety of reasons. First and most common, the cheaper the gears, the louder the gearbox. Take a look at a pic of a well-designed gear. Just Google one up. You'll note right off the teeth are not simple triangles, but have sloped faces where they mesh. This reduces friction and power loss. Gear design is a whole field in itself. Also, bearings on gearshafts help, too--something you rarely see. Go scarf up a few garage-sale R/C trucks and take 'em apart. You'll learn a good deal just by looking.

It does not follow that you have to have state of the art gears for your RR. Whatever works satisfactorily to _you_ is where you want to be, pricewise.

Don't overlook the infinite possibilites of sprocket and chain drives for model puposes. While that might not fit your era, you can mechanize all sorts of neat things that way.

And, you can 'design' your own gear train by pretending the gears are pulleys, and use the OD for your calculations. You'll be off a few percent, but it won't matter, overall.

Les


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## SE18 (Feb 21, 2008)

Hi Les, 

They're cast; I'll try and put some white lith on them; Thanks. I also decided to try my hand at building my own gearboxes. I've been ripping apart appliances and pulling gears and casting them the 2nd photo shows the castings before they're cleaned up. Actually, some new castings are drying as we speak. In the first photo, I found that a spring-loaded punch helps in punching out the axles from the chassis. However, it quickly dulls the point for steel axles. 

Getting the pitch and proper teeth mesh seems a bit tricky. I'm going to use spacers so the gears mesh just right. The gear box might be wood, steel or even putty; I'll try a variety of ways until something works. I'm trying to reverse engineer every gearbox I see and check on the ratios and efficiencies. 

This weekend, I'm going to pour some molten metal ones as well. The sprocket and chain sounds like a swell idea. I'll have to find a toy one and make copies. Much thanks; I'll try and up date on projects but I'm juggling a bunch right now.


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## Les (Feb 11, 2008)

There you go: you're off and running. Somewhere on Google is an animated site concerning gears, ratios, etc. Might look for it.

Cheap triangle gears work okay, how long they'd last is an open question. I suspect, no long.

Gearboxes need to be made of metal, or a stout polymer. I'd 'bush' every gear axle if I used a metal box.

My train 'experimenting' came to a stop when I got this humongous load of firewood dropped on me last month.

Suggestion: try something simple, like a mocked-up turntable, but use the gearing you select for the purpose. It's so simple you can't miss, and you'll learn a lot about shafting, bearings, pillow blocks, etc. Gear lash, too.

Les


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## SE18 (Feb 21, 2008)

bushings, turntable, perhaps a geared crane; got me thinking in many directions 

thanks for your experienced wisdom; I hope you recover soon!


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## RimfireJim (Mar 25, 2009)

Pittman Motor has a really nice technical paper on the characteristics of DC motors, but I see that you now have to register on their website to download it. 
For a simple answer to your question, power equals torque times speed for any rotating machinery. To get high torque at low speed requires a big package (think diesel engine) whereas the same power can be generated at high speed with low torque in a small package (think Formula 1 car). Which choice is best depends not only on the size limitations but also on the practicality of using a speed reducer between the prime mover and the output shaft, i.e., how many gears (or pulleys) you can use, how much torque they have to transmit, what speed they will be running at, to name a few considerations. Running an electrical motor at a speed slow enough not to require a gearbox for a locomotive, real or model, is just not practical. You might find the following interesting: How a diesel locomotive works


As for gearing, the torque multiplication is N2/N1, where N2 is the product of the number of teeth on driven gears and N1 is the product of the number of teeth on driving gears. Speed is the inverse of that. Idler gears don't play a role in the calculation, they just reverse direction. Regardless of whether you are gearing up or down, there is also an inefficiency term which reflects the power lost to friction.


-Jim


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## SE18 (Feb 21, 2008)

1. Jim: "Running an electrical motor at a speed slow enough not to require a gearbox for a locomotive, real or model, is just not practical." 

one exception might be my worm-driven critter I'm building: 

you will notice the worm is directly connected to the gear on the axle (or is, since I've put it together) 










It currently is just on a chassis but several days ago I mounted a camera on it and ran it around my Phase1 track. Here's the Youtube of it running: 

http://www.youtube.com/watch?v=iL6FUkobhmI 

it is currently onboard battery driven only (4 AAs); and admitedly with a gearbox, it could go much slower; but the torque is incredible and it can push around a train 

2. I'm currently planning a geared down, high-torque critter capable of climbing steep grades with a train in tow. If the experiements work, I'll be adding more track later this year that will be very steep in gradient. I'll pay attention to wheel slippage and so on and run full tests and publish results here. I'll check curves on grade as well. I'm starting with a tomiya gearbox geared down to about 1:500 or 1:1000; I'll see which is better. The tradeoff will, of course, be slow speed


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## ralphbrades (Jan 3, 2008)

Eeeerrrrmmmmm........

Your worm and spur gear set up is actually a reduction gear. The step down is 1 tooth from the worm to the number of teeth on the spur gear. I am not sure but I would hazard a guess at 1:20(?) The secret of successful towing is how you get the traction down to the rail. There is a simple formula that will tell you the weight to torque ratio that you have 


For a Steel to Brass interface this is 4.3 :1 -or for 1 Kg of weight on the wheel 0.230 Kg of it can be used at the point of contact for motion. This means that the force at the wheel is rated at: 230 Grammes per Centimetre

Developing more than this at the point of contact will result in wheel slip.

regards


ralph


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## Paradise (Jan 9, 2008)

Ralph... interesting, I can see this is where you need to start -T R A C T I O N. 

So on a single axle with two wheels would you divide the total weight on that axle by two to get force of a single wheel ? 
Is that correct ? 

Andrew


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## Les (Feb 11, 2008)

Posted By Paradise on 06/08/2009 6:35 AM
Ralph... interesting, I can see this is where you need to start -T R A C T I O N. 

So on a single axle with two wheels would you divide the total weight on that axle by two to get force of a single wheel ? 
Is that correct ? 

Andrew 






I suggest you wait until them as knows answers (Ralph). Don't forget, the weight of the vehicle will be dispersed across two axles and four wheels, which is why balance or center-of-gravity is also something not to be ignored. If you have an 0-2-2, for instance, the weight wants to be on the driver pair, mostly. If you have say, a x-6-x, then the center of gravity, if too far aft, will 'unload' the front drivers to the point where traction will suffer.

Les


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## Paradise (Jan 9, 2008)

Yes Les, 

I was keeping the question simple to one axle but of course that wouldn't stay upright without other axles being involved. 
Unless of course, the wheels were square but they have different issues... 

Andrew


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## ralphbrades (Jan 3, 2008)

Andrew, 

Sorry for the delay in getting back to you, (I am in Switzerland at the moment). The formula gives the tractive force of an AXLE i.e. two wheels. So, if I have an 0-4-0 that weighs 2Kg, 230grammes is the tractive force on each axle. If we take the case of a Price 16 Wheeler with 8 driven axles, then the potential tractive force -is quite large and the potential for wheel slip is also quite large... In the case of a 4-4-0 the front bogie is classed as being an axle and the total weight available for tractive force is thus 2/3 of the weight of the loco. Similarly in the case of a 4-6-4 3/5 of the weight is available. Pony trucks and radial axles count as 1/2 an axle in the calculation -thus a Pacific is 2/3 again . 

The physical centre of gravity should be between the driving axles for optimum tractive force. 

I hope this helps! 

regards 

ralph


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## BarrysBigTrains (Sep 4, 2008)

Perhaps I can lend a few ideas which helped me build a good power combination.

One rule which I had to learn the hard way. I prefer worm drives, but they must be at least a double thread worm and worm gear or the the loco will surge with a heavy train on a down hill. From a design standpoint, triple thread and quadruple thread worms will also work, but then the problem is a bit more difficult trying to get to a correct gear ratio (final drive). 


Once this problem, the surging, appeared many solutions were tried before finding the answer. When the surging was solved then it was time to re-determine the proper gear ratios.


I use a custom wound Pittman 8000 series motor rated at 10,000 rpm no load. I have custom brass worms, I use Acetal wormgears with brass hubs. The countershaft has an custom aluminum spur gear which engages the axle, with it's acetal/brass spurgear. The overall gear ratio is just under 30:1. The countershafts and axles are all in ballbearings, the motor is also in ballbearings. 


It took about five design iterations (the same as the Bachmann big hauler). Since the last change, I have not lost a gear. This is the most durable drive yet.

If anyone has a direct question I will be happy to answer. I will try to monitor this site each evening. 


Barry - BBT


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## Les (Feb 11, 2008)

Barry,

I have a couple of questions. First, the Eternal One: "How Much?" Second, a teknikul one, I haven't done the math on the numbers you gave (the morning's supply of coffee hasn't gotten to my brain yet) but, what is the _output shaft _RPM, under average load? I ask because I intend to build a series of critters and 0-4-0's and slow operational speeds are envisioned. Torque is relatively unimportant for my apps for the forseeable future. What is the diameter of the output shaft, and roughly how long is it? Does its axis of rotation lay parallel with the driven axle of the engine, or is the unit small enough to be mounted transversly? You see how unfamiliar I am with the actual application in the scale world. I can think of a couple of ways to get power to an engine's axle, but I need to know how the 'Big Boys' (pun intended) do it.

Thanks,
Les


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## Les (Feb 11, 2008)

Posted By Paradise on 06/08/2009 8:06 PM
Yes Les, 

I was keeping the question simple to one axle but of course that wouldn't stay upright without other axles being involved. 
Unless of course, the wheels were square but they have different issues... 

Andrew 








Duh, me!







I sure wasn't awake on that one. Yeah, square wheels will make a bit of difference. 

Les


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## RimfireJim (Mar 25, 2009)

Posted By SE18 on 06/03/2009 6:43 AM
1. Jim: "Running an electrical motor at a speed slow enough not to require a gearbox for a locomotive, real or model, is just not practical." 

one exception might be my worm-driven critter I'm building: 

you will notice the worm is directly connected to the gear on the axle (or is, since I've put it together) 




I guess I should have been more careful in my wording, and said "Running an electrical motor at a speed slow enough not to require a _speed reduction device_ . . . is just not practical." That device could be a worm/worm gear combination such as on your critter and found on a multitude of small scale model locomotives, a more complex gear train as found on most large scale model locomotives, or a chain and sprocket system, or a belt and pulley system, or some other system or combination of the above.


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## BarrysBigTrains (Sep 4, 2008)

Les,

Thanks for the question. I'll try and cover what you are asking. I don't know the output speed, but have an optical tachometer which could tell me.

The motor output shaft is .125", the worm is a double thread 32P brass, with two set screws, 90 degrees apart. This engages a 30T 32P wormgear on a shaft shared with an aluminum 16T 32P spurgear. The spur gear engages the 30T 32P axle gear, mounted on the axle. The motor is mounted vertically.


The drive can be run so slow so as not to be seen in motion, and you could count the motor shaft revs on the top of the motor. The speed could cause a derail in most locos.

If you are really interested in more, send me an email [email protected] and I can forward pictures of anything you would like to see.


The motor and gear parts are not for sale separately, but I can guide you to the part numbers from Stock Drive, Boca Bearing, etc. The gear parts are modified for my application, so the hubs maybe narrowed, the set screws enlarged, etc.

Barry - BBT


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## Paradise (Jan 9, 2008)

Duh, me! I sure wasn't awake on that one. Yeah, square wheels will make a bit of difference. 

Les, 

I was thinking of a load on a single axle then you got me thinking the load would be affected by other axles so my example would just flop over ! 
I thought square wheels would fix that ! LOL 

Andrew


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## Paradise (Jan 9, 2008)

Ralph, 

Thanks for the post. 

As I am trying to take this all in, a larger wheel would seem to provide more traction but if I think about it, a large wheel would still have a theroretical infinate small surface contact with the rail no matter how big it is. 
An infinite size wheel would still be round but its surface would be 'almost' parallel whith the rail. 
So what is going on here ? I'm lost ! 

Switzerland WOW !
I have been sudying the RhB etc lately and ...
just ordered a 4/5 Rhb Brawa and cars.
I did a search on google for 'SSB Railway' got a google maps link and was on top of the world looking at one of the most pretty google map views I have seen.
I want to go there one day.

Andrew


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## ralphbrades (Jan 3, 2008)

Andrew, 

The interface between the wheel and the rail is considered to be a "line contact" one. So, the contact area is (theoretically) infinately thin. However normal things like the plastic deformation / reformation of the rail and the wheel provide more than adequate surface area for the wheel to grip the rail. 

I work for a Swiss company -so being in Switzerland is quite normal (and boringly so!) for me... 

regards 

ralph


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## Paradise (Jan 9, 2008)

Ralph, 

Does your formula example relate directly to the coefficients of static friction for brass and steel ? 
Being approximate value of .51 (Wikipedia) (depending on test factors) halved because the load is shared by two wheels ? 

I can see that the surface area of contact has no effect as a big brick will slide at the same time as a small brick down a sloped board. 
all relating to static friction which needs to be overcome to slip. 

I'm not sure if I have good grip on the subject.
Am I going up the wrong track spinning my wheels ? 

Andrew


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## ralphbrades (Jan 3, 2008)

Andrew, 

You are getting a little more than lost. The Co-Efficent of Friction (COF) remains the same regardless of the area of contact. What you are confusing is pressure and friction. *The formula is for an axle -thus it is pre-equated for two wheels.* The COF of steel to steel is 0.43 -so you would get a slight advantage on a steel to brass interface. 

regards 

ralph


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## Paradise (Jan 9, 2008)

Ralph, 

The formula is for an axle -thus it is pre-equated for two wheels.

Your formula produces a figure that seems to be aproximatly half the COF values published about the web. 
Is that because your formula has two wheels 

I don't question the accuracy of your data but I'm not sure if I understand the relationship between the two. 

Wipipedia: 
*Approximate coefficients of friction Materials Static friction, Dry & clean *
Brass Steel 0.51 
Steel Steel 0.80[9] 0.16[9] 
Aluminum Steel 0.61 

Regards 

Andrew


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## ralphbrades (Jan 3, 2008)

Andrew.....

The formula gives the tractive resultant of the weight on ONE AXLE and thus TWO WHEELS. I don't understand where you are going with this argument?


regards

ralph


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## Paradise (Jan 9, 2008)

Ralph, 

I'm not arguing, I will get to the point. 
Would I use a value of aprox .3 for steel~aluminimum for an axle? 

Thanks for being patient with me. 

Andrew


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## Les (Feb 11, 2008)

Posted By Paradise on 06/12/2009 7:48 AM
Ralph, 

I'm not arguing, I will get to the point. 
Would I use a value of aprox .3 for steel~aluminimum for an axle? 

Thanks for being patient with me. 

Andrew









Andrew,

Feel free to tell me to butt out. I think you're making something hard out of something hard--which isn't of critical importance to a model engine.

The reason I jumped in is, I got wound up in the same sort of confusion when I started out. Not being highly educated but with a lot of practical experience, it finally dawned on me that tractive effort on a model isn't of the final importance. What is very important is: will your proposed engine pull the rolling weight in cars up the grade you want w/o the motor drawing enough current to overheat it? And will it do it and not overly shorten the life of the motor?

Tie a string onto your heaviest proposed consist. (Or a brick on a flatcar, if you don't actually know, or the consist doesn't exist yet.) Or some suitable weight. Then use a spring scale to pull it, and there's your effective rolling weight. (Not your starting weight, however, that's another story.) 

Next, mock up a frame with your motor and drive assy. Put an ammeter on the line between the power supply and the track (positive side). Hook up your spring scale and turn on the juice. See what the current draw of the motor is at wheel spin on your engine. Now add some weight to the engine and see what the current tops out at at stall. It'll be higher, almost surely. Compare the two readings so you have some numbers. If you want to go even simpler, just hook your mocked-up engine to the flatcar w. brick and read the current to the motor as it pulls along.

Now here's the clarifier: if the current draw of a motor is less than the rated draw, you're in Fat City. (Good shape. Sorry, sometimes slang doesn't translate well.)

As a rule of thumb: Tractive Effort is the amount needed to get a stopped train started. It (the Drawbar power) has to be higher, so if you pour the coals to it, you'll get wheel spin. This is loss of Adhesion, Adhesion being the 'stickiness' of the wheel to the track. More wheels, more adhesion--but also you need to spread the weight out in the Real World, so more drivers were used to spread the load because the pounding of the pistions (energy shoved into the wheel) --a portion of which also got absorbed by the track--and the track broke. Or wore it out faster.

I'm sure Ralph will correct any blatant misstatements on my part, or refine most of 'em. But I was a machinist, and he's edumacated in these here things.

If you want to understand how power gets from the coal pile in the tender to the last sound of a disappearing train, well, that's a long story best told with numbers for precision. I like to know how things are supposed to work in the real world too, so I'm not in any way, shape or form insulting you or trying to correct you. But that's a whole bunch of numbers--why, so many they hadda put 'em in a lotta books.

I'm happy you're interested in building your own. If you have any machinist-related questions, I'll do my best to help you. Including saying, "I don't know," when I don't.

Les


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## ralphbrades (Jan 3, 2008)

Les has got it almost right!!! 

As I have said before -you are confusing *pressure* with *friction*. The two are different. If I have an 0-4-0 loco that exerts 10kg pressure on the track (i.e. 5Kg per axle) then the maximum tractive force that this will give me 2.3Kg of tractive effort. The CoF of the wheel rail interface does not come into this part of the equation. 

It comes in later... 

I have a motor that develops 10W at 100Kg per Metre torque -or 100 Grammes per Centimetre torque. If the wheels of the loco are 10 cm diameter this gives a peak torque of 230 Grammes per Centimetre. Thus If I powered the axle with 2 motors I would never get wheel slip, if I added a third -then I would. The maximum power I could develop to the rails would be 40Watts for an 0-4-0. 

I now decide to use an 0-6-0 configuration, the weight is still 10Kg, the tractive force is still 2.3Kg, the power to the rails is now 60Watts. 

If I work it out for a Price 16Wheeler then (deep breath) The weight is still 10Kg, the tractive force is still 2.3Kg, the power to the rails is now 160Watts(!) 

This means that a Price 16 Wheeler could pull 4 times the load of an 0-4-0 whilst still having the *same* tractive force. 

This assumes the world is flat... 

The 0-4-0 and the Price now climbs a hill, the friction between the wheel and the rail now comes into play. The slope is 1:10 the force pushing the loco back is 1,000 Grammes. The Price puts 1.25 Kg of pressure on the rail per axle -it has a CoF of 0.51 thus it can develop 8 x 637 Grammes of forward thrust (1,629 Grammes) -it climbs the hill. The 0-4-0 puts 5Kg of pressure on the rail per axle -it can thus develop 2,550 Grammes and so it too climbs the hill! 

The tractive force developed by a loco is a direct result of its pressure on the rail. The coefficient of friction between the wheel and the rail is only applicable during changes of state. SO, if your loco irrespective of the rail material (stainless steel, brass, aluminium or wood!) a 10Kg loco will develop 2.3Kg of tractive effort. 

I hope this has all helped!!! 

regards 

ralph


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