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Lapping gears advice please...

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Hi all,

So after tinkering many evenings during lock down I’ve decided to repeat my steps over and over, learning more and trying to achieve more with each time I build, looking at the finer details I’m interested in lapping differential gears and shimming the gearbox.

At the moment I’m not worried too much about destroying something if I learn from what happened to it, and so I decided to add liquid diamond polish to the diff gears generously, do it up shove half a dog bone into a dremel and listen to see if the sound reduced over a few moments... It did and it also the metal parts got rather warm 😅 (I do not recommend spinning the diff with a dremel btw, it spins very fast and is dangerous).

The outcome is that all the top surfaces of the gears are shiny and the side profile of the gear teeth are much more rounded compared to brand new gear teeth.

What am I trying to achieve you might be asking? From my understanding if I get the gears running smoothly with new clean grease the diffs will last longer, they will be quieter and more efficient. I’m aiming to get everything running perfectly in the kit.

Any thoughts from you all would be most welcome, I have different grades of diamond polish and could run from harsh the fine thinking I would achieve very smooth mirror like gears.

Thanks for you time,

Axle

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One of the guys I know from the LHS told me once how they used to fill buggy gear cases with tooth paste and run them overnight to polish and break in all the gears.  He claimed it reduced the gear friction quite a bit and provided some benefits in competitive racing.  So, your idea sounds consistent with what I've heard before.  The trick is to do just enough, but not overdo it.

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9 minutes ago, speedy_w_beans said:

One of the guys I know from the LHS told me once how they used to fill buggy gear cases with tooth paste and run them overnight to polish and break in all the gears.  He claimed it reduced the gear friction quite a bit and provided some benefits in competitive racing.  So, your idea sounds consistent with what I've heard before.  The trick is to do just enough, but not overdo it.

Thanks for this, I think this is what I’m asking, is there a certain sound you get from spinning the gears that tells you it’s done, or is it after inspecting the teeth that you’ll know not to push it any further?

I’m presuming that after destroying a few diffs you end up with a sixth sense.

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I have to assume the same thing -- it probably takes some trial and error to figure it out.  You have a few variables to juggle, such as drill/motor speed, the equivalent grit number of the abrasive, how much loading is on the gears opposing the motor, the gear material, the cut of the teeth, etc.  I don't think the LHS employee ran his gearbox under load; all the gears simply idled and squeezed the toothpaste around the teeth at a low rate of speed.  I think the idea was to give everything a light polishing but not try to fundamentally reshape the tooth profile, as gear tooth profiles are generally designed to have a single line of contact to another tooth through the gears' rotation.  Therefore, I would err on the side of being conservative and not get too aggressive with the polish for fear of affecting the gear tooth geometry in funny ways.

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I've heard of using Brasso in full-sized auto diffs, both as a lapping compound and all-around miracle cure.

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I find Brasso or toothpaste works pretty well for plastic gears. For metal gears, lapping compound seems to work fine.

If the gears are too rough though then I find they will need some actual physical intervention which can seem a bit scary but if you have really tight gears then there isn't much to lose. Metal gears, especially diff ones seem to respond quite quickly to lapping compound. Plastic ones can be a bit more time consuming.

I had a Lunchbox last year that had an awful tight spot and it seems the spur gear had some warping on it, almost making a ticking noise each time the bad spot met the pinion. In the end I put a drill on one axle and held the other one to make the spur turn then carefully offered up some sand paper just close enough that only the warped part of the gear came into contact (just going by sound) I did this carefully until I couldn't get any closer without also touching the rest of the gear. This yielded a dramatic improvement. I then put the motor back and applied a little brasso between the motor and pinion and span it at a low rpm for a few hours, stopping to re apply brasso now and then. After a few hours the gears were way smoother and quiet. I guess the brasso would have had an effect on the pinion too but the mesh was still decent without too much backlash. I know it seems scary using an abrasive let alone using something like sandpaper directly on the gear but don't forget that where you have a tight spot, the mesh will be way too close or even bottoming out completely so you can remove some material and still end up with a safe amount of mesh. 

As an example of metal diff gears, I own a "skeleton king" which is a cheap Chinese E-maxx clone and the rear diff had a point in it's rotation where it was pretty much seized solid. I put some lapping compound inside and use some shims where the screws go so I could close the diff but not tight enough for it to totally seize. I just put the diff input in a vice carefully and attached a drill to one of the diff outputs and left it spinning. After about an hour or so it was spinning freely so I opened it and cleaned out the compound which now had a semi metallic look to it from all the tiny metal particles in it. Now I was able to add more compound and reassemble the diff without shims so it was fully closed. Now the diff could turn without seizing but still had a defined tight spot so I gave it another hour or so and after that it would spin nice a freely with no more tight spot. With this particular one, I tried to avoid using too much compound because I didn't want it to get between the spider and the inside of the bevel gears because obviously it could take material away from there too and make things loose and wobbly. In such an instance you can try using a thick grease applied selectively to form a barrier. Obviously over time the compound would just mix with the grease but for the relatively short time periods needed to lap the gears it should be enough to keep it away from places you don't want it to go.

One thing I would not recommended though is filling the entire gearbox with toothpaste, especially if it's an older model that has gears riding direct on shafts without bearings. I currently have an old Yonezawa Land Dash buggy that I'm trying to get up and running and the gearbox is nice and smooth with the cover off but as soon as you put it on it's almost seized solid. But because some of the gears have no bearings, if I filled the box with tooth paste, it could get in between the gears and the shafts and end up boring the gears out and wearing the shafts down and just introducing a load of play. I don't know if the gears have warped or expanded with age or what but I'm pretty sure it wasn't this tight when it was new. My aim is to get it to spin with almost no resistance with no motor attached. 
 

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So I've also heard about the toothpaste trick. If I was going to try it I think the key would be to put a very thin layering on the main gear teeth, and as speedy said, run at low rpm for a fairly long period of time to polish the surface of the gear teeth without wearing them down.

The other things I would note (And this is just my own view):

1. Tamiya metal diffs are mostly made of some fairly awful chineseium, and are fairly soft. They are moulded gears rather than machined gears. Most of the roughness I've experienced with them is where there is still a little flash where the gears have been broken off the mould, or very small amounts of flash round the edges of the gear teeth where the mould is worn. I've removed these with a needle file, and then the gears have operated fine. Always worth checking for this before running them through a power drill to try and remove roughness.

2.Diff gears only spin a lot when you lose traction, so unless you're getting stuck a lot the diff gears aren't going to see a lot of rpm. They mainly have to stay in position and not slip over each other when you a apply lot of instant torque, so it's not really that important for them to be smooth and quiet. From a performance point of view it would be better to reduce the friction on the main transmission gears.

3. I've found the most quiet gears to be machined steel gears. Not cheap but the profiles are perfect and you get a great surface finish from the machining process.

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Do you guys think this would work out for the full case bodies? Specifically the GF-01 & G6-01? Would you fill the whole body? Then how do you run the motor for that long? Attached it to a DC supply?

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Posted (edited)
2 hours ago, isomer1 said:

Do you guys think this would work out for the full case bodies? Specifically the GF-01 & G6-01? Would you fill the whole body? Then how do you run the motor for that long? Attached it to a DC supply?

I wouldn't fill the whole body. I'd open the gear case and turn the gears over by hand and then slowly add toothpaste where the gears mesh and just watch it until everything has a slight covering. The run on a DC supply so the motor is spinning slowly, I would use 3volts max. And don't forget not to use the kit aluminium pinion.........

Edited by MadInventor
missed out 'mesh'
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On 7/23/2020 at 7:27 AM, •Axle said:

https://gearsolutions.com/features/polish-grinding-of-gears-for-higher-transmission-efficiency/

This is an interesting read, what I’m taking from it is they did a two step process, gear grinding and gear polishing, also somewhere in there they noted that keeping the lower parts of the gears pitted was beneficial as it kept the lubricant in the right place.

Thats a very scientific read. I liked that. 

When it come to polishing, I can relate to what the mean about retaining some 'scratches' to distribute lubricant. When i polish & wax my 1:1 cars, the better the finish (I aim for zero swirls and holograming) the more a liquid will bead on the surface. Not an ideal scenario when you want lubrication. I should post some pics of the work i have done on my car(s) :)

Years ago, there was a display in a local motor factors of a gearbox additive that enabled the gear oil to stay on the gears, and climb up through the stack. Ideally we would want an oil like that in our gearboxes. They aren't oil tight, and a hydrocarbon based oil will normally not be the right lubricant for the plastic cases. I would love to be able to use oil in my diff's and gearboxes

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