7075 aluminum

[JPSAXNC]

I've built two small compressed air motors for a model plane. The crank shafts are made in two pieces, on the first motor the front and rear crank shafts were made of 6061 alum. and rode in brass bushings. I gave all the moving parts a small drop of 30 weight oil, this motor worked well. On the second motor the front half of the crank was made from 7075 alum. riding in a brass bushing, the rear half of the crank was made from 304 SS. riding in a 7075 alum. bushing, all the moving parts got a small drop of 30 weight oil. I ran the motor for about one minute it seemed to work ok, I put it aside for a while and later when I tried to turn it over I felt resistance. I took the motor apart and there was a thick black gunk on the 7075 alum. shaft in front and the 7075 alum. bushing in the back. Maybe the 7075 alum. and oil were incompatible or the metals used or both. I cleaned everything up and tried again with the same result.

[sb327]

I believe your answer lies more in your successful bushings. Brass is what some describe as ‘self lubricating’ and is why it is commonly used as bushing material.

I do not recall seeing aluminum or ss as a bushing material.

Got a picture of the motor? Sounds neat.

Dave

[Motorhead]

If a low rpm model engine & virtually no heat created  ... a Bushing/ Bearing machined from AF grade acetal, MDS nylon, Teflon or even PEEK are IMO the better suited.
None of the above would require addition lubrication ( Which is drag )

[KWK]

Is your motor a wobbler design? Oscillating engine?

[JPSAXNC]

Thanks for the replies. I thought of using peek for the crankshaft, but I need more weight up front to balance the model so I'll use steel. I don't have any pics yet, but if you google ( Pachasa compressed air engine), It's a twin, mine is a single operating cylinder, built around a crank case. It's supposed to be a sort of scale engine for a Guillows Stearman PT 17 model. On the Pachasa engine the air is fed in from the front side, I wanted my air supply coming in behind  the engine. That's why the crank is in two pieces, the back half of the crank is the valve, air inlet and exaust. It's been a real can of worms.

[sb327]

Interesting.

A single cylinder of this?

Dave

[JuryRigger]

I seem to recall reading somewhere that the single biggest factor that would make-or-break  a plain bearing was the difference in hardness between the shaft and the journal-had to be at least so many points on one of the hardness scales difference.. Can't remember what those numbers were, and they'd be absolutely meaningless to me and likely you as well, but the concept stuck in my head 
In other words, the same material on the same material at the same hardness will result in quick wearing, and or galling of the mating surfaces...
I believe there is an exception to the rule once you get hard enough, something about the necessary difference growing smaller as the general hardness level went up?.... Some precision lathe spindles were made with a hardened spindle running in a hardened bush, and last a very long time...
Dunno, too much packed into my brain-attic, and the filing system is atrocious... 

Might have something to do with it, also seems to me that 6061 feels 'slicker' than 7075, so there's that, I suppose...
Jesse

[sb327]

You are spot on. You should always design a softer/harder bearing surface interface. Obviously the softer being your sacrificial side (inserts in an engine).

It has been studied quite a bit

https://www.bing.com/search?q=hardness+difference+to+prevent+galling&form=APIPH1&PC=APPL

Dave

[KWK]

Interesting.

A single cylinder of this?

Dave

I made a very similar single cylinder rotary valve engine like that when I was 14. The exhaust came out through the center of the crankshaft. I still have parts from it out in my shop. I made a better version when I was 15 but lost it in my early 20's.
Here is a wobbler type engine that runs real nice

[sb327]

I’ve made a few myself. A  couple wobblers and a dual acting standard (I run it real slow on an aquarium pump).

My favorite I built is a vacuum ( flame eater).

Dave

[KWK]

The rear axle is the crankshaft. Double acting single cylinder wobbler. The whole car is only 5 inches long. Not painted yet here. The whole thing is soldered together including the engine.  Only machine used to make the engine was my drill press. Lathe not necessary. Wheels and tires are model aircraft. Sheet metal from a metal 5 gallon paint bucket. One single peice was folded up and soldered.