Brass and steel

tigertc · 25-12-2006

Merry Christmas all!!!!
Now down to business!

I recently purchased a job lot of Tamiya gear boxes for down gearing the boxes in my tanks. The original Tamiya motor gear cog misses the first gear in the add on gear down box by about 2mm.

The cog on the motor shaft is brass and i would guess it is fixed by heat expansion to the steel shaft. My question is simple, If i apply heat to the brass cog, will it expand more than the steel shaft allowing me to move it to the correct position?

Mucho respect TigerTC-crashed out after mammoth Xmas food overload.

Phoenix · 25-12-2006

i think it will just be a interferance fit onto the shaft so it should just prise off

giving the brass a little heat at the same time to make it expand a little wouldnt hurt though

**GEEDUBBYA** · 25-12-2006

Howdy Tiger,

Ok, being a machinist for most of my life, and having worked with all forms of "affixing" one type metal to another, you should be able to heat the brass enough to make it swell and be able to remove it from the steel shaft. The trick is not to heat the steel shaft at the same time.
One way to do this (depending on the size of the parts) would be to heat the brass with something like a propane torch while holding the part with pliers, then cool the steel shaft (not the brass) by quenching the shaft in water or in ice, thus causing the shaft to shrink while the brass is still expanded. the part should just about fall off.
Now if this fails to work, you might buy a puch set that could be used to "knock" the shaft out of the gear.
Or, if you have access to a drill press and small "wire" drill bits, you could possibly drill out the shaft from the gear.
But, my first choice would be heat, hopefully the gear will just fall off of the shaft.

Have a good day, Merry Christmas,

Greg

tigertc · 25-12-2006

Thanks for the help guys, tricky one this. If the shaft wasnt such an integral part of the motor i would have gone for the water and heat solution. I need the shaft intact, so drilling it out is off the board.
I think i will go for the straight heat and hope it will push down the shaft with help from some implement.

**Bunkerbarge** · 26-12-2006

I would say it is nearly always worth making some sort of purpose built pulling tool for jobs like this. I would definately apply a bit of heat to the gear wheel, but have some sort of jacking arrangement which applys pressure to the gear wheel ready to hand when it gets hot.

I have seen a number of press fit gear that have a 'blade' pinched onto the shaft that cuts a nice slot into the gear wheel as it is pushed on.

Heat and some sort of home made jack will do it for you. Just think about what you are pushing/pulling against to ensure you are not going to damage the motor. i.e. to put the new one on you need to push against the end of the shaft and not the motor housing.

**Bunkerbarge** · 26-12-2006

Actually Alan, coefficients of expansion aside, which play a part here, if you can't cool the shaft the danger is that the shaft may expabnd more than the gear wheel and you could make it tighter.

I think it may be worth jacking it off first and if you can't shift it try a bit of heat. Another important consideration is that if you can't remove it from the motor you stand a chance of damaging the bearings with heat as well.

tigertc · 26-12-2006

I have scoured the web bunk mate and i cannot find any info on whether brass expands more than steel when heated. Gonna attempt it tomorrow anyway i'm guessing with brass being softer than steel, my silly logic tells me it should expand more or faster than the steel. If i knack the motor its £10 down the drain (RS380 motor).

There is a valid and important reason for me doing this modification.
After putting metal tracks and all the other metal upgrades on my original King Tiger, it has now been run for a couple of miles. Yesterday it stripped a gear whilst attempting an incline (Either that or it had been steadily wearing down) I then checked the other original Tamiya gear boxes and they were all in a dreadfull state. With all the metal upgrades they couldnt manage much of an incline anyway!
This has forced me to gear down both KT's.
This will give less speed (More accurate scale speed by luck) with the bonus of more torque.
So for anyone putting metal tracks on their Tamiya 1/16 tanks, down gearing is an absolute must.

**Bunkerbarge** · 26-12-2006

Tiger, check out here:

http://www.engineeringtoolbox.com/li...ents-d_95.html

Now they are coefficients of linear expansion which are differrent to coefficients of volumetric expension, however you can see than Brass is greater than Steel.

The trouble is, of course there is more to it than that, as usual! If you imagine a huge diameter shaft with a very thin brass ring shrunk onto it although the coefficient for the brass may be greater the larger mass of the steel shaft will actually expand more as it is all as a percentage of the volume.

In your case I would expect the brass gear wheel to expand more than the steel shaft although I would try to avoid getting too much heat into it.

All interesting stuff, let us know how it goes.

alan2525 · 26-12-2006

If you have a gear thats fixed to a motor shaft, I wouldn't fancy trying to apply heat to the gear as it'd probably conduct along the shaft and not do the motor workings much good either.

I think the best idea is to make some kind of gear puller from a couple of pieces of angle iron and a bolt although as Bunker said you dont want to be pulling against the motor housing though incase you damage it.

I made a gear puller to remove the steering wheel from my old escort to change a speedo cable one time. It took a while to make but made the job much easier!

IainM · 27-12-2006

You could try modifying a bicycle chain link extractor by either turning down or filing down the diameter of the drive pin. If the gear is smaller than the slot in the puller, try using one of the small spring links as a backing plate.
They are generally not expensive so worth experimenting!

Worked for me in the past.
Best of luck.