# How to lap gears



## rideinstile

Hi there, I'm new to the forum and starting to get into the hobby with my 9 and 11 year old sons, I've noticed it being metioned about lapping the gears in the T-Jet with toothpaste. What exactly is involved in doing this. I honestly looked in the search part but couldn't find anything on how to do it. Thank you very much. Dave


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## Bill Hall

Dave, Everyone has their own method. Using various compounds in conjunction with some form of lubricant.

Brasso/other polishes, Comet/other cleansers, Toothpaste.... whatever.

The jist of which is to make a lubricate slurry of lubricant and mild abrasive, then drizzle it onto the gears, shafts, axles while running at moderate speed. Clean it out and start over if it heats up or binds. Some guys even just dunk the running chassis upside down in a tupperware of slurry so the plate is submerged but the comm doesnt foul. I do both the drizzle and the dunk; as well as the quickee toothpaste trick, just depends on the individual car/setup and what I think I'm trying to prove.  

The idea is to allow the abrasive to cut gently and gradually without ruining the motor by either excessive friction or riduculous RPMs. Really the whole deal hinges on decent setup to start with. Proper clearances on shafts and gears are the true starting point. Bound or boogered gears, and warped chassis and tweaked gearplates cant be lapped straight.

THE most important thing is to go easy on your motor so you dont do irreversable winding damage; AND be absolutely scrupulous when cleaning up afterwords. 

HO World has a comprehensive tutorial.

Go easy! Be Clean!


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## [email protected]

Simichrome chrome polish is a favorite of many. mj


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## martybauer31

I chuck the rear axle into a dremel (make sure it is spinning the gears in the correct direction), pack in the simichrome and fire it up at a low rpm. Make sure to remove the motor brushes, and you don't really need the magnets in either as the dremel does all of the work. I'll repack the gears with simichrome a few times and then when it sounds like it has quieted down, I will finish it up with brasso. 

Doing this on top of following Bill's advice on gear clearance will get you a much faster car.


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## afxgns

Watch for the "gears" section of Fray car tuning.

The simi-chrome thing is going in the right direction.

Tim


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## Franko

Interesting thread. Shouldn't you run it *backwards* if powering from a dremel on the axle to get the gear tooth faces lapped right?


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## martybauer31

Like I said in my post....


martybauer31 said:


> I chuck the rear axle into a dremel (make sure it is spinning the gears in the *correct* direction)


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## Bill Hall

Franko said:


> Interesting thread. Shouldn't you run it *backwards* if powering from a dremel on the axle to get the gear tooth faces lapped right?


Both directions!...but finish in the forward direction. A smooth backlash makes for a smooth car when your off the juice.


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## [email protected]

Bill Hall said:


> Both directions!


Bill do you think that that is true of all HO cars including G3 type cars. thx mj


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## vaBcHRog

I run them both ways. For standard TJETS I have a Junk chassis with an extra lon axel. I chuck it up in my reversible drill add the semi chrome or mothers mag polish and let her rip in both directions.. I have a bowl with denatured alcohol that I place the top plate in and run the drill to clean everything up then hit it with some compressed air and I'm ready to go. For slim lines they are a different animal. I have a TJET plastc box lid with to posts epoxied to it so I can place the slim-line chassis in the box upside down and screw it in place. The posts are set to jusl lift the the gear plate off the bottom a hair. I then pour Brasso in the box even with the gear plate the hook up two alligator clips and let the slim-line run fro 10 mins in one direction the 10 mins in the other. When I am in a hurry with a slim-line I will put it on my break-in box and take a plastic cup with Brasso and a paint brush and add a little Brasso at a time to the gear plate.

Roger Corrie


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## Bill Hall

[email protected] said:


> Bill do you think that that is true of all HO cars including G3 type cars. thx mj


All things being somewhat relative IMHO; Smooth gear mesh in both load and unload conditions seems like the ticket to me whether its a modern inline or the venerable T-jet monkey motion.

It's just like the ring and pinion in a big car where toothloading patterns are critical. Certainly the forces are smaller in slot cars but no less important. Nowadays in competition the highend guys are fighting for small fractions in lap times. It is truly a marvel from whence we've come. 

That being said the common theme seems to be that a smooth car with good throttle input manners trumps a twitchy rocket that is a chore to drive.

The variables car setup are many. Comm pressure, motor magnet strength, down force relative to magnet strength and tire profile, pickup geometry and spring strength. All factors of friction to a simpleton like me. A fine line that can never be drawn in black and white. In other words some stuff works in certain combinations and some stuff just doesnt.

I feel gear mesh is more of a yes or no proposition. It's either right or wrong. As we do spend a certain amount of time off throttle the backlash should be addressed. The total smoothness of a car is a factor of how the geartrain responds in both load and unload condtions providing all the other setup variables are reasonably correct. 

Conversly, at some point, too much lap creates a scenario where the tooth contact points actualy slap or hammer against their mate. The tooth geometry is worn to the point that they are clubbing and not meshing smoothly. The clue here is visual to the naked eye, one gear moves excessively before the other engages. A trained ear can hear this on the test pack when other noises are not present.

Some folks consider gear selection tedious but is just a fact of life unless mated sets are purchased. I just keep trying till I get something that feels right and lap from there. No guarantees. I've got a few hundred gears for this and that, both used and new. As I go through the process they are sorted into simple piles - those that are fat/tight - those that are thin/loose - and of course the trash which is tossed immediatly. Ya just never know what's gonna work til ya get there. Cuz I'm old and blind it helps me to use the ole magnifiying glass to spot "tooth decay or irregularities so I dont waste time on a turd gear.

A complete lap provides one less thing to wonder about.


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## Hornet

City watch the pitch count on your mag car gears,some are 64 and some are 48,and they don't interchange.
Wizz's are usually 64 and BSRT's are usually 48,not sure what Slottech's are.
You can visually check whether the pinion and crown are going to match up,by meshing the pinions and crowns in your fingers,and looking very closely at the bottom of the teeth,if you can see daylight through the bottom of the teeth,they won't work together,but if you can't see any daylight through the teeth,then it's a worthwhile test. :thumbsup:


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## [email protected]

Hornet said:


> City watch the pitch count


Thanks for that Hornet. One more nugget, so much to learn. I'm having better success with my gearing. As Bill said sometimes i have to try a few to find the two that mesh the best. Then lap them. mj


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## Bill Hall

vaBcHRog said:


> ...snip... When I am in a hurry with a slim-line I will put it on my break-in box and take a plastic cup with Brasso and a paint brush and add a little Brasso at a time to the gear plate.
> 
> Roger Corrie


Hey Roger, How about a pic of this contraption...something ya made?


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## AfxToo

Tom Bowman's article has some great pics of a gear lapping rig. 

http://www.bat-jet.com/webspot/fraycar06.html

By the way, Tom's article is an excellent How-To reference for building up a competitive Fray ready racer. Great advice from a great source. His Fray compatible resin bodies are second to none too. Highly recommended.


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## bobhch

AfxToo said:


> Tom Bowman's article has some great pics of a gear lapping rig.
> 
> http://www.bat-jet.com/webspot/fraycar06.html
> 
> By the way, Tom's article is an excellent How-To reference for building up a competitive Fray ready racer. Great advice from a great source. His Fray compatible resin bodies are second to none too. Highly recommended.


Thank you for posting this link as these pics and steps for tunning a car up right.. Maybe some day I can build a fast car. Seem to be more of a body man lately. 

Having a small handfull of cars built by some of the HT profesionals on the boards makes me want to build better cars. Zoooooooooom baby!  

Bob...zilla


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## SCJ

Bill Hall said:


> All things being somewhat relative IMHO; Smooth gear mesh in both load and unload conditions seems like the ticket to me whether its a modern inline or the venerable T-jet monkey motion.
> 
> It's just like the ring and pinion in a big car where toothloading patterns are critical. Certainly the forces are smaller in slot cars but no less important. Nowadays in competition the highend guys are fighting for small fractions in lap times. It is truly a marvel from whence we've come.
> 
> That being said the common theme seems to be that a smooth car with good throttle input manners trumps a twitchy rocket that is a chore to drive.
> 
> The variables car setup are many. Comm pressure, motor magnet strength, down force relative to magnet strength and tire profile, pickup geometry and spring strength. All factors of friction to a simpleton like me. A fine line that can never be drawn in black and white. In other words some stuff works in certain combinations and some stuff just doesnt.
> 
> I feel gear mesh is more of a yes or no proposition. It's either right or wrong. As we do spend a certain amount of time off throttle the backlash should be addressed. The total smoothness of a car is a factor of how the geartrain responds in both load and unload condtions providing all the other setup variables are reasonably correct.
> 
> Conversly, at some point, too much lap creates a scenario where the tooth contact points actualy slap or hammer against their mate. The tooth geometry is worn to the point that they are clubbing and not meshing smoothly. The clue here is visual to the naked eye, one gear moves excessively before the other engages. A trained ear can hear this on the test pack when other noises are not present.
> 
> Some folks consider gear selection tedious but is just a fact of life unless mated sets are purchased. I just keep trying till I get something that feels right and lap from there. No guarantees. I've got a few hundred gears for this and that, both used and new. As I go through the process they are sorted into simple piles - those that are fat/tight - those that are thin/loose - and of course the trash which is tossed immediatly. Ya just never know what's gonna work til ya get there. Cuz I'm old and blind it helps me to use the ole magnifiying glass to spot "tooth decay or irregularities so I dont waste time on a turd gear.
> 
> A complete lap provides one less thing to wonder about.


 
I agree with everything Bill (BH) has stated here and personally use Semichrome as my abrasive medium...it also works great for removing scratches from original (Molded in color) Tjets and or body polishing....but have not seen anyone (including BH  ) mention what I think is the most important thing with gearing when taking the time to "run in" a set of gears.


*Marking your gears.......yes, I said it, you need to mark them!?! It's simple really, if you take the time to "run a set of gears in" so that they mesh smoothly with one another, you have essentially "Matched" one tooth on gear A with another tooth on gear B so that they "mesh" together smoothly. So....when you've finished "running them in" but before you remove them from the gear plate for cleaning, mark the gears (I place a small scratch across them). What this allows you to do, is reassemble the car with the same teeth on gear A touching the same tooth on gear B just as it was when you bothered to "run them in", same as the 1:1 racers do. In theory this is the smoothest mesh one can achieve.

* 
 :freak: 

Did that make since?


-------------------------
www.SlotCarJohnnies.com


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## Hornet

HUH???
I don't see how that would make any differance,correct me if i'm wrong,but the average slotcar or 1:1 car,doesn't run with a 1:1 gear ratio,so marking and lining up the teeth doesn't do much good,the first timer it turns over,the gears are in a differant spot.
Been around lots of 1:1 SS cars in my life,never heard of anybody matching the gears by teeth,matched sets of gears yes,but not matched teeth.
The theory sounds good on paper,but is unrealistic in real life unless you run a 1:1 gear ratio


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## SCJ

I have seen a few hundred 1:1 car rearend gear sets (my cousin works at a transmission and rearend shop) and they "mark" every set of gears they put in or take out of a rear end so that the "match"........all the newer aftermarket type gears (That I have seen) come already marked from the factory when you take them out of the box. Maybe not for the same reason, but marked for alignment non the less.

-------------------------
www.SlotCarJohnnies.com


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## Hornet

Not marked for alignment,marked as a matched set.
Think about it John,you have say a 3:1 gear ratio,which means the pinion spins 3 times faster then the crown gear,how do line that up .
Which turn of the gears do you pick to be the line-up of your gears,nigh onto impossible to match line them,you can have matched sets,but you can't line them up and have them stay inline,not if the car turns a wheel.
Your buddy is feeding you some wrong info if he's telling you they line them up,and expect them to stay on that plane,when they don't match teeth wise.
A 9 tooth pinion turns alot more R's compared to say a 27 tooth crown,so each revolution of the gears throws your theory out the window.
They're marked to give you a starting point to base your backlash off,that's about all they're marked for.
BTW:i use dykem to mark the 1:1 gears i've set-up,and you always rotate through a complete turn when setting up a rearend,so your idea is already out to lunch,after one turn of the pinion,the pinion and crown isn't gonna line up on your marks
If you can back your theory up with a laptime improvement,i'll believe there's some benefit,but till then,i think it's a waste of time,lap them in and go,that's all i'd worry about,mark em if you want,but i highly doubt you'll see a laptime improvement :wave: 
Rick


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## afxgns

AfxToo said:


> Tom Bowman's article has some great pics of a gear lapping rig.
> 
> http://www.bat-jet.com/webspot/fraycar06.html
> 
> By the way, Tom's article is an excellent How-To reference for building up a competitive Fray ready racer. Great advice from a great source. His Fray compatible resin bodies are second to none too. Highly recommended.


Don't get me wrong here, I love racing with Tom. But "second to none" is just not correct.
Jack Rutherford at Fandango makes a pretty good fray body too!


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## afxgns

Marking the Driven(24) and the idler(24) is a good thing to do. They will always be meshed.
Marking any others is moot.


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## Hornet

I'd even wonder about that Tim,even if they are a 1:1 ratio :thumbsup:


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## TK Solver

Marking gears is important for matched gears with integer gear ratios because the same teeth do mesh repeatedly and any alignment issues will be accounted for during the break-in period. So for a 1:1 or a 3:1 gear ratio, this would be important. If you have a 9-tooth driver paired with a 25-tooth driven, it's not an issue.

Regarding lapping/lubrication of plastic gears... be very careful not to use anything too abrasive as the molding process creates a smooth skin that you want to keep intact. Break that skin and the friction and heat will increase greatly. In most cases, lapping provides very little benefit.

The best lubricants for plastic gears are those which can actually embed themselves into the plastic skin. Our gears expert here at UTS is also an avid model railroader and he recommends "Dry Moly Lube" (molybdenum disulfide). He knows a few things about gears and works with Dupont and Ticona to integrate plastics properties into our software. See http://www.uts.com/ProductTutorialsExamples.asp?ItemGroupID=600 for a listing of our gear design programs. In particular, program 60-610 (listed on the third page of programs) focuses on plastic gear issues. Having said all this, I'm NOT a gear expert but it's great to have access to one.


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## Hornet

Man somebody in your org gets paid to write/type,i quit reading at pg 66  .
Wouldn't the interger gears have to be identical in size and ratio,before they'd match on every turn,if they're 2 differant sizes and tooth counts,they aren't gonna match on every revolution.Isn't that the idea behind speeding up or slowing down a driven gear,you can split the ratio differances between 2 sets of gears instead of 1.
Been around drilling rigs a fair bit of my life,so i've taken the odd gear reduction box apart,and i've always marked the gears,but not for wear patterns,more to keep them in time.It was my impression that was why you marked the extra gears,not to keep the wear patterns,but to keep the timing in sync,maybe you could class both as the same,but i wonder if you could
TK,if you have access to your gear guy,ask him if he'd reconmend boiling or baking a delrin gear,and if he'd boil or bake it in anything. :thumbsup: 
Thanks
Rick


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## TK Solver

Boiling or baking a molded delrin gear will likely cause undesirable shape changes as the stress in the gear is relieved by the heating process. The stress is initially introduced during the molding process as the skin cools faster than the interior. In some processes, such shape changes are ok but not with a precise component such as a gear tooth.

When you have an integer gear ratio, the two gears repeatedly cycle through the same teeth but more than one tooth on the driven gear will be matched with each tooth on the driver. Take a 9-tooth driver and a 27-tooth driven for example. Each tooth on the driver will contact with the same three teeth on the driven gear. Tooth 1 on the driver will contact tooth 1, tooth 10, and tooth 19 every time. So a slight imperfection on tooth 1 of the driver will impact those three teeth only on the driven gear. The same concept would then be applied to the other 8 matched groups of teeth.


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## tjd241

*I go an entirely different route.....*

Not as scientific. I take a saucer or small bowl (preferably a ceramic bowl). Next I carefully place the chassis into the bowl (gear side up, minus tires, hubs, axels, brushes, springs, shoes, and magnets). _I prefer to work with one chassis at a time, in about a 4oz bowl._ I myself don't worry about marking the gears, but I do keep them together as a set (plate and all gears) after the lapping process is over...

Anyhow... I pour in enough whole milk (never skim) to just about cover the gears, but I leave just the barest hint of the post for the idler gear showing. Now... here's where I call in my cat Cheddy. Once he see's he's got some work to do (milk in the bowl)... I sit back and watch as he starts with the lapping process. Takes only a few minutes.

Usually the one treatment (w/ whole milk) is sufficient when you factor in the abrasive nature of his tongue, but with older abused chassis, I opt for a first treatment with half-and-half, followed by a second treatment using a 1% or even 2% lowfat milk. :roll: :woohoo: :roll: :woohoo: 

nuther dave


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## SCJ

According to the American Gear Manufactures Association (AGMA)

"......taking into consideration the transverse, circular, axial, base and chordial thickness as well as the span measurement and dependent upon if each tooth is a stub, full, long or short tooth and of equal addendum it is certainly plausible that two gears of unequal ratios would work best if aligned (marked as you stated) consistently in the same fashion......"

As if I weren't lost enough already, he then went WAY TO FAR above my head talking about:  

*Surface of action

*The surface of action is the imaginary surface in which contact occurs between two engaging tooth surfaces. It is the summation of the Paths of Action in all sections of the engaging teeth.

*Path of action

*The path of action is the locus of successive contact points between a pair of gear teeth, during the phase of engagement. For conjugate gear teeth, the path of action passes through the pitch point. It is the trace of the surface of action in the plane of rotation.

*Zone of action (contact zone)

*Zone of action (contact zone) for involute, parallel-axis gears with either spur or helical teeth, is the rectangular area in the plane of action bounded by the length of action and the effective face width. 

*Arc of action* 

Arc of action is the arc of the pitch circle through which a tooth profile moves from the beginning to the end of contact with a mating profile. 

*Arc of approach* 

Arc of approach is the arc of the pitch circle through which a tooth profile moves from its beginning of contact until the point of contact arrives at the pitch point. 

*Arc of recess

*Arc of recess is the arc of the pitch circle through which a tooth profile moves from contact at the pitch point until contact ends.

........who knew!?! 

I just race little toy cars!  

-------------------------
www.SlotCarJohnnies.com


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## Bill Hall

SCJ said:


> big snip... I just race little toy cars!
> 
> -------------------------
> www.SlotCarJohnnies.com


Me too John! ...Ugh me makum lil car go!  

I gotta go with Hornet. Unless it's a 1:1 setup. Indexing seems superfulous to me. 

However marking used sets for re-assembly in the 1:1 world is good practice and the mark of someone who is planning ahead. The "Hansel and Gretel" approach...always leave a trail of bread crumbs so you get your timing and ballance shafts right the first time! The old boys also tought that a certain harmonic or frequency can be upset by flipping or mis-indexing a used gear within the set. Hear that Howl? Did ya mark your gears and put them back in the same order? Doesnt sound like it!

Does it make a huge difference with T-jet Monkey motion? 

I just lap for best possible smoothness...and leave the complex science to more qualified individuals like Nuther Dave and his crew chief Cheddy.


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## AfxToo

> Jack Rutherford at Fandango makes a pretty good fray body too!


Exactly. No complaints from me. All great stuff.


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## Hornet

So if i take a gear reduction box apart that has a 30 tooth driver with a 10 tooth idler ,driving a 60 tooth driven gear,you're telling me the same teeth will contact on every rotation,now that one i'm gonna disagree on,the same teeth will contact on every 3rd and 6th rotation of the idler,by my figuring,not every turn,but hey i'm only a dummy here.Yes the same teeth will contact on every turn of either the drive or driven gear,but in the meantime the idler has to turn either 3 turns or 6 turns before it'll match back-up with either the drive or driven gear
Like i said mark em if you want,but if you can prove a faster laptime by doing this,i'll eat my shorts,good thing i don't wear any,huh :thumbsup:


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## TK Solver

Hornet, 

You're no dummy. I never said that the same teeth on every gear would contact on every rotation -- but if you're talking about a rotation of the driven gear that would be true. In your example, each tooth on the 60-tooth driven gear will always contact with the same tooth on the idler. Each tooth on the idler will contact the same six teeth on the driven gear. That tooth will never contact the other 54 teeth. Likewise, each tooth on the driver will always contact the same tooth on the idler and each tooth on the idler will only contact the same three teeth on the driver.

I've never lapped or marked a slot car gear and probably never will -- with or without my shorts. If I were a competitive racer, I'd skip the lapping and use dry moly during the break-in period instead to permanently condition the gear teeth surfaces. But it's not cheap and can be dangerous to the lungs if inhaled so only serious racers should carefully try it.


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## Hornet

LOL,hey TK i knew what you were describing,and i also know that every turn of the driver or driven will match up,just pointing out some obvious facts for other guys.
Thanks for the complement,just gotta get the better half to agree,she don't think i'm that smart,just ask her :wave: :woohoo: 
You did make me dig out some of my old "JetLube 550" antiseize compound,it's a molybdenum disulfide compound,i've never thought of trying it on my gears till you pointed it out,but it does sound like a good idea,so i'm gonna give it a whirl,thanks for the heads up on that TK.
Rick


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## vaBcHRog

With the new CNC'ed gear sets you don't have to lap anymore 

Roger Corrie


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