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Gary Paine
03-02-2015, 01:59 PM
Ok, I'm thinking of a shortcut here and thought I'd see what others suggest.
I've finally worn the teeth on a 20 tooth tumbler gear on my Atlas/Craftsman 12 inch. It gets a lot of use in power feeding and is now worn to the point that the noise it is making is objectionable.
I could just buy a replacement, or, of course, make one.

Aluminum is what I am thinking about for a replacement gear. It should be as durable as the zamak gear it will replace. I just happen to have the right tooth cutter and an index head, so, I really just need to make an arbor and go to work. However, the gear has a 3/4 inch hole with two opposed keyways in the center. It fits over a keyed bushing with a smooth bore to free spin on the stud. The bushing appears to be steel and the stud is of the shoulder variety. If I reproduce the gear, I will have to employ the shaper to cut the keyways (or abuse the carriage on my lathe a bit) and that makes more work. The arbor would probably just use a woodruff key on one side.

All that seems a lot of trouble. I have an arbor already for a smooth bore the size I need to fit the stud. If I just made the whole gear of aluminum, the teeth would end up perfectly concentric with the bore and I wouldn't need to cut keyways. I could machine an inch and half or so up and just slice off completed gears. That way if the aluminum bearing surface on the ground stud begins to wear out, I have spares to throw on.

If this were an oil bath environment I wouldn't hesitate. The gear will spin at nearly lathe rpm when I am using the leadscrew and is an open gear. The only lubrication it will have is what it can hold. Would I likely be happy or unhappy that I used such a dodge?

If I do go the keyway route, I'm thinking of cutting the gear from acetal. If it held up, it would be nice and quiet. I can't imagine getting away with a delrin bearing at 2000 rpm, though, even with a bit of oil.
And, of course, I could probably use an Oilite bearing in either blank, but that complicates my arbor.

MikeL46
03-02-2015, 02:21 PM
I'd probably make a larger hole in the gear and press in an oilite bushing with the correct ID.

Mike

Carm
03-02-2015, 03:32 PM
I think that could be OK, given a little grease. (straight alum) Depends on alloy though.
But I don't understand why the OEM had the double key?
Cutting the keyways on the lathe if you don't have the broach is N.B.D.
Yer just in for the labor right?

Gary Paine
03-02-2015, 03:51 PM
I think that could be OK, given a little grease. (straight alum) Depends on alloy though.
But I don't understand why the OEM had the double key?
Cutting the keyways on the lathe if you don't have the broach is N.B.D.
Yer just in for the labor right?

Thanks for the input. The double key allows the gear to be one of their standard change gears which need the keyed bush to hold them in pairs. I tried a magnet on the keyed bushing and it doesn't attract. It looks cast, so it is probably zamak as well and not steel. I suppose if zamak will work as a bushing, aluminum would. The bore doesn't appear worn much, just the teeth. Sound right?

Bob Fisher
03-02-2015, 03:55 PM
Aluminum running on steel, especially with oil, should be OK. Again, alloy might make a difference, and longevity could be a consideration. I like the idea of a press fitted bronze bushing. Otherwise, aluminum as a gear should be at least as good as Zamack. Bob.

Carm
03-02-2015, 04:25 PM
Hope your alooneyum is a high number series (6xxx) and not soft.
Had to look up Zamak.
Zinc, aluminum,magnesium and copper (und kopfer for those who know dwarf fart)
Not much to lose if you make sausage and cut a slice when you need it.

Toolguy
03-02-2015, 04:49 PM
You probably want 2024 or 7075 for that. One of those in a T-4 to T-6 temper would probably run for quite a while.

Gary Paine
03-02-2015, 08:47 PM
Thanks for the input guys. I used 7075 aluminum. I reamed the hole to a close fit with the stud, turned and bored a blank, and cut the teeth. It's installed and working fine. My visions of a sausage went by the wayside when I discovered I didn't have a bar of 1 1/2 inch aluminum, so I just chucked a scrap and made one off.
I've been turning a lot of steel lately and had almost forgotten the joy of machining aluminum. Start to finish about 3 hours including time out for dinner. It looks good, works fine, and is quieter. I'm happy.

CCWKen
03-02-2015, 09:09 PM
The gear is used as either an idler or as a drive gear depending on where you use it. The 20-T is used often. That's why all the gears may or may not use the bushing.

I set up several banjos so it would make changing the gears easier for threading. That lasted a few times then I put my feed set up on and it's been there ever since. I switched to dies and never looked back. :)

peekaboobus
03-03-2015, 01:32 AM
aluminum is not ideal for this. Too soft. Likes to friction weld to surfaces.

atty
03-03-2015, 02:08 AM
Lay the open gear lube to it. Been running 7075 for years. Works fine.

Black_Moons
03-03-2015, 03:44 AM
aluminum is not ideal for this. Too soft. Likes to friction weld to surfaces.

Agreed, likes to gall and make aluminum oxide everywhere, a wonderful abrasive.

Willy
03-03-2015, 09:16 AM
aluminum is not ideal for this. Too soft. Likes to friction weld to surfaces.


Agreed, likes to gall and make aluminum oxide everywhere, a wonderful abrasive.


It all depends on the alloy, look at all the sliding action going on within a steel/iron cylinder mated to an aluminum piston.
Piston to cylinder surface speeds up to 4,000 ft./min. Billions of examples doing very well.
Also most overhead camshafts run directly in aluminum heads for hundreds of thousands of miles with no issue.
Too many other examples to mention.

strokersix
03-03-2015, 09:19 AM
Acetal is a great material for gears and probably ok for the bore as a bearing material too. Cut one from acetal or perhaps use the friction melting technique to form teeth.

firbikrhd1
03-03-2015, 09:28 AM
In addition to the uses Willy posted, aluminum main bearings have become more common in vehicles and small engines such as Briggs and Stratton have used just the aluminum housing as a bearing in their non industrial series engines for years.
I came across this site which speaks to materials used for aluminum bearings.
http://www.keytometals.com/Article106.htm

Alistair Hosie
03-03-2015, 12:50 PM
What about making one.Perhaps from a cast iron weight from an older dumbell set.I used one for a handle on a drawbar I made it turned our pretty nicely too. Alistair

Black_Moons
03-03-2015, 05:12 PM
It all depends on the alloy, look at all the sliding action going on within a steel/iron cylinder mated to an aluminum piston.
Piston to cylinder surface speeds up to 4,000 ft./min. Billions of examples doing very well.
Also most overhead camshafts run directly in aluminum heads for hundreds of thousands of miles with no issue.
Too many other examples to mention.

Agreed, IIRC most cast alloys of aluminum have significant amounts of silicon in them. Very hard grain structure iirc. You might want to think differently about standard machining alloys however.

Also, Aluminum pistons are 0.002~ undersized of the cylinder and are an alloy engineered to do that job. I'll also note that aluminum pistons running in aluminum bores always have the bore chromed.

Also, Pistons/cams/etc in engines get LOTS of oil, even on a 2 stroke the walls stay pretty well oiled.

Paul Alciatore
03-04-2015, 12:39 AM
I have purchased several used change gears for my SB9. They are all some form of cast iron, probably the allow that South Bend advertised. They are 3/8" thick and have a simple 9/16" bore with a 1/8" key way. One thing I noticed as the most common problem with these used change gears was that the bore had far more wear than any of the teeth. My conclusion is that the bore is where you should concentrate your efforts at limiting the wear.

I like the pressed in bushing idea. When it wears, it can be replaced.

For lubrication I use way oil on the teeth. It has a lot of stickion so it stays put. Once a day is usually OK. Spindle oil on the hub is good if you stick to the once a day schedule.




I'd probably make a larger hole in the gear and press in an oilite bushing with the correct ID.

Mike

Willy
03-04-2015, 01:13 AM
................

Also, Aluminum pistons are 0.002~ undersized of the cylinder and are an alloy engineered to do that job. I'll also note that aluminum pistons running in aluminum bores always have the bore chromed.

Also, Pistons/cams/etc in engines get LOTS of oil, even on a 2 stroke the walls stay pretty well oiled.

True, automotive pistons are now made with high silicon content alloys to minimize wear. They were not always built with this alloy though. New tighter tolerance engine designs with very little oil to lube the pistons due to tougher emissions regulations has pretty well mandated the use of the newer piston alloys in order to ensure longer engine life. All the while operating under some extremely hot and dry conditions. Also they utilize a much tighter piston/cylinder wall clearences than .002 at present. Even a new Harley will run .0006-.0015 clearence and that's on an air cooled motor.

But yes not quite the same alloy as 7075, but then the gear as mentioned in the OP is also operating under a much lower stress level than the other applications mentioned.
And yes similar metals usually make poor bearing/bushing companions, whether it's aluminum and aluminum or steel mated with steel, life expectancy is usually short.

I'm sure a dab of oil or grease will allow the gear to enjoy a very long life as is. A bronze bushing would be ideal but I don't think it will wear at an alarming rate without one.

http://www.californiametal.com/Aluminum_7075_Sheet_Plate_Pipe_Tube_Rod_Bar.htm


[QUOTE]Applications:
Due to its high strength-to-weight ratio, 7075 is used for highly stressed structural parts. Applications include aircraft fittings, gears and shafts, fuse parts, meter shafts and gears, missile parts, regulating valve parts, worm gears, keys, and various other commercial aircraft, aerospace and defense equipment and components.



[/QUOTE

peekaboobus
03-04-2015, 03:58 AM
Having a functional hard coating on the aluminum for the bearing surface is different than relying on native oxide. Its a surface that is in mechanical contact and subject to friction. So when you change a surface property, you change everything. Aluminum engine bores can have the bores coated. So its different to a home shop guy boring out a hole in aluminum and using it as a bearing surface. It also depends on the other mechanical contact part what is its material.

Aluminum rubbing on aluminum has one of the worst friction coefficients. Aluminum on steel is better. So it depends on the other part is. But you can always get better, lower friction coefficient with another material pair where aluminum is avoided.