Lapping in bushings

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JohnR
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Lapping in bushings

Post by JohnR »

I have just finished the trucks for my 7 1/2 gauge Mich Cal style shay.
I want to drag it around the track and loosen up the bushings using garnett
compound with oil.
Does the garnett break down to the point that I can flush out with oil or
do I have to take apart and clean ?

Thank you
John R
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Harold_V
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Re: Lapping in bushings

Post by Harold_V »

Do keep in mind, any soft bushing (or shaft) is likely to be imbedded with lapping compound, so even dismantling for cleaning may not eliminate continued wear.

I subscribe to the theory of making proper fits instead. That's the process used today in modern engine building.

Unless you have some fits that refuse to work, I think you'd be better served by simply keeping the bearings lubricated and allow them to seat.

H
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apm
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Re: Lapping in bushings

Post by apm »

Isn't this the purpose of timesaver lapping compound?https://www.newmantools.com/lapping/time.htm
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Bill Shields
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Re: Lapping in bushings

Post by Bill Shields »

With Harold on this

Do not do that...Garnett does not break down and will eventually eat everything up...not just bushings but pins and axles as well.

Keep in mind that this is the stuff they put in water jet machines and is not water or oil soluble.

Just run in.

If it cannot make it around without binding then I recommend hand scraping bearings...or maybe a bit of oversize reaming or careful drilling to loosen things up.

Knowing how much is not an exact process since you really do not know the waviness of the track or how much the suspension is going to work once you have a boiler full of water on top

Remember the builder's adage...just about worn out is just about right.
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cbrew
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Re: Lapping in bushings

Post by cbrew »

I also never use any kind of break in compound. I personally shoot for .002 slip fit. Assemble with grease on run it. It will not take long to wear in.
If it is not live steam. its not worth it.
STRR
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Re: Lapping in bushings

Post by STRR »

+1 on Timesaver. Model T folks use it all the time on bushings and more on babbitt bearings. If Timesaver is good enough for use on soft babbitt, it should be fine on regular brass or bronze bushings.
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NP317
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Re: Lapping in bushings

Post by NP317 »

I agree with CBrew: Machine for a "proper fit." Our locomotives always run better with loose bearings than with tight ones.
After a test run of my new Mikado, it is obvious I need to loosen up all the front suspension joints so it will track properly. Oops.

I am also a fan of Timesaver compound for fitting tight rotating bearings. It polishes in the surfaces, and then removes itself with use and flushing with normal oiling.
I have successfully used to on full-sized steam locomotive driver axle bearings, after experiencing a hot bearing when "on the road."

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Steggy
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Re: Lapping in bushings

Post by Steggy »

Harold_V wrote: Tue Oct 12, 2021 4:48 pm Do keep in mind, any soft bushing (or shaft) is likely to be imbedded with lapping compound, so even dismantling for cleaning may not eliminate continued wear.

I subscribe to the theory of making proper fits instead. That's the process used today in modern engine building.

Unless you have some fits that refuse to work, I think you'd be better served by simply keeping the bearings lubricated and allow them to seat.

I concur with Harold. Machine the parts to the correct fit.
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tetramachine
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Re: Lapping in bushings

Post by tetramachine »

The word is burnishing the bearings, for bronze, brass type sleeve, a hardened steel ball is forced through bearing and expands it just aa might. A tapered and hardened drive pin can to the same.
Time s may have worked on model T's, but 10K miles was the life of a T.
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Bill Shields
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Re: Lapping in bushings

Post by Bill Shields »

In reality it is a lot easier to just build in the necessary clearance. Everyone who has not been through it wants to make suspension tolerances too tight...and ends up drilling and filing to make things work on real world track.

Connecting rod clearances have to be enough so that if one wheel us at the top of spring travel while the other wheel is at the bottom of travel...the rod bearings have enough clearance that it will not bind.

It is simple trig to work out that you need to DRILL bushings oversize...

Forget lapping for clearances in this area...you are building a locomotive not a watch.

There are times and areas where being a horologist applies...this is an area where being a blacksmith is closer to being appropriate. This is one of them

If you need a hand with the math..there are many of us who have built multiple locomotives who will be happy to help.
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Harold_V
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Re: Lapping in bushings

Post by Harold_V »

Bill Shields wrote: Thu Oct 14, 2021 9:27 pm In reality it is a lot easier to just build in the necessary clearance. Everyone who has not been through it wants to make suspension tolerances too tight
Uhhh---that's not exactly true. Tight tolerance is NEVER the cause of improper fitting. Poor dimensioning is. Look at it this way. If you dimension properly and hold a tight tolerance, the net result is a guarantee of a proper fit. Loosen the tolerance and that fades away quickly, often resulting in components that are either too loose, or too tight.

The first sentence is correct. Build with the required dimensions, and hold close tolerances.

I make mention because the myth of "tight tolerance" is being repeated time and again, and it's not the right message to send to those trying to learn.

H
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Kimball McGinley
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Re: Lapping in bushings

Post by Kimball McGinley »

Don't confuse tolerances with clearances. They are not the same thing.

Our locos need plenty of clearance in these areas. That may allow more tolerance.
When a device needs tight clearances, wide tolerances are clearly unsuitable.
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