Friction of materials

[Wannabe2]

I'm in the process of building one of Jan Ridders LTD Coffee Cup Stirling engines. This engine has certainly been a challenge but I knew it wouldn't be easy before I started it. They are so touchy the slightest increase in friction kills them.
I've currently got the engine to run off the heat of a settop box on top of the TV but haven't been able to achieve running it off the heat of my hand as yet and I'm now looking at ways to further reduce friction.
The engine currently has a 3mm dia (1/8") displacer shaft with a stroke of 12mm (1/2") running through a brass bush however I was wondering since I shouldn't be using oil to help reduce friction if Delrin would produce less friction than the dry brass. I've also considered using graphite as the bush as Jerry Howell does on his Miser which would definately reduce the friction but I'm worried the wear rate of the graphite would be too great.

 

[PhillyVa]

th_wwp

Philly

 

[bearcar1]

Say Wannabe, along those same lines, it would seem to me that a reduction of surface contact would play a bigger roll than type of material used, although that most certainly could play a major roll as well. I'm not at all familiar with the operating characteristics of the stirling types of engines but basic laws of physics apply no matter what. Quality of finish is another factor as well. Perhaps you could use a ceramic bushing?

BC1
Jim

 

[Bluechip]

Wannabe2

Delrin would be better, PTFE bushes better still.
If you can (?) obtain PTFE bushes , one point, you need to observe the fit of the outer. I had some dealings with them in the past, they appear to be quite oversize on the ID. They need a specific interference fit on the OD to squash them to correct ID. Not something I knew until I got the application data .. ???

Or the ones I had did anyway, It may be general.

There is some info. on this site for data for engineering plastics, and other stuff ..

http://www.matweb.com/

Best of luck, you'll get there ..

Dave

 

[Wannabe2]

Thanks guys. I though Delrin would have a lower Coefficient of Friction. Based on Bluechip's link and google it appears Delrin has a Coefficient of 0.25 and Brass appears to be 0.35. I don't have any PTFE but I do have some Delrin so I'll give it a go and see how it goes. What I was planning on doing was drilling out the brass bush, turn up a piece of Delrin to fit tightly within the bush then drill/ream the delrin for the shaft with the Delrin inplace. That way the compression of the hole should be minimized.
PhillyVa here's a couple of pics if I can figure out how to post them. It's still an ongoing Work in Progress so it hasn't been tidied up too much yet and I need to get some better screws.

 

[Mainer]

I built Jerry Howell's "Miser" with the graphite bushing. I haven't found wear to be an issue. THe rod needs to be polished, of course.

You're right about needing to eliminate all possible friction.

 

[Wannabe2]

I might give the graphite a go first then. The way I'm going to bush the brass it's a simple job to drill it out and replace it anyway if it does wear.
I've got Jerry Howells "Miser" lined ready to go if I ever get this one going to my satisfaction. I suppose I should be happy enough with the way it's running now because it flies on a cup of hot water. From what I've read a lot of people don't get these things to run at all.
These stirlings have got me hooked. I'm fascinated with them. Whenever anyone comes to the house and asks what I make I show them some of my engines and they say "what does it do". The idea of this stirling is to have it sitting on top of the TV just chugging along. It intrigues them (and me) that these things run on a bit of heat.

 

[DavesWimshurst]

Wannabe,
I've built a few low temp difference Stirlings to J. Senft's design. They use a brass sleeve of 1/16" bore and displacer piston rod of 1/16" drill rod. The tubing is just the kind sold in hobby stores. I use graphite powder for lubricant just brushing it all over the surface of the rod. These engines use small ball bearings, two as crank shaft support and one each for the connecting rod big ends. The little ends are plain bearing, wire through a hole. In this design the weight of the pistons are counter balanced.
It is essential to eliminate any binding due to misalignment or roughness of sliding surfaces. I used graphite for the power pistons and on two engines a lapped brass cylinder. On two others I used glass cylinders modified from commercially made dashpots. All have successfully run on hands, one is regularly used to demo to public groups.

Demo engine

My prototype on a TV. This one has a brass cylinder.



I have built a Ringbom Stirling that uses a graphite bushing for the displacer piston rod which has worked well, I'll try to take some pictures of it soon.

I hope some of this is helpful.
Dave

 

[Wannabe2]

Just an update on how things went.
I drilled out the brass sleeve and turned up a piece of graphite to fit tightly inside then drilled/reamed the graphite out for the shaft. I'd say I'm now getting another 10 RPM out of it due to the reduced friction. It's been running almost continuously now for two weeks and the graphite seems to be holding up well.
I'm currently running a Delrin piston so I think the next step might be to see if I can track down a large enough piece of graphite and turn up a new piston.
I think my top bearings could also be an issue. The bearings came from an LG video recorder head and I can hear a bit of noise coming from them. I was going to buy some replacements but trouble is I've since discovered they are an odd size so I'll have to mod the bearing housing before I can do that. Guess that will teach me for not getting the correct stuff to start with. Never mind, it's all a learning experience. :

 

[DavesWimshurst]

Wannabe,
You might have some luck carefully cleaning the bearings in mineral spirits. I've had some small bearings from salvage smooth up after flushing them out and re-lubing them with very thin oil.
I have also used Delrin pistons in some compressed air motors and found that they changed in size on me. Not sure if it was temperature change or if they absorbed oil but they went from a nice fit to too tight to move. For LTD Stirlings a graphite piston run dry in a highly finished cylinder is hard to beat for low friction.
Good luck in tracking down the last bits of friction.
Dave