Keeping eccentric strap from "wandering".

[Brian Rupnow]

I have been rather intrigued with the way Gail from New Mexico makes his eccentrics in two peices bolted together, with a flange on the eccentric to keep the eccentric strap from wandering off the eccentric. I didn't want to do the extra work involved with making 2 peice eccentrics, but as I was looking at the twin horizontal I am currently building, I noticed that I had made the eccentric about .15" longer than the eccentric strap itself. Suddenly a light came on!! I am going to make up a ring 0.100 thick from mild steel and bore it a couple of thou smaller than the eccentric diameter, then after I have the eccentric strap "fitted" properly to the eccentric, I will heat the ring cherry red and place it over the outboard end of the eccentric. Theory is, it should cool and shrink in place, effectively creating a flange to hold the eccentric strap in place. Whatcha think???

 

[GailInNM]

Brian,
That should work, but it will make any disassembly difficult. If you do go this way, I would make the ring out of brass or alumninum as they have a much higher coefficient of expansion than steel and will require less heat or could be pressed on cold easily.

I have also used "E" clips for such purposes and have seen washers retained by cotter keys.

Since you have so much room, I would probably just put a 2-56 set screw in the retaining spacer. There will be very little force on the retainer so a single small screw would hold the retainer if it is a close fit on the eccentric.

A little Loctite retaining compound or even CA (superglue) would also work and could be disassembled by heating to 350 degrees F where it will lose it's strength.

Gail in NM,USA

 

[Brian Rupnow]

Thanks Gail--I will probably go with steel, because I have a peice the right diameter in my scrap bin. It will only be 0.100 wide x 0.100 in the other plane, and if I have to remove it later I can buzz thru it with a diamond wheel on my air tool.

 

[GailInNM]

Brian,
One last thought and I will leave you be.
Making it out of steel, but bore it undersize, say by 0.005 or so, and then split it like a piston ring. It should have plenty of grip when sprung open by sliding on the eccentric.
Gail in NM,USA

 

[Mcgyver]

I will heat the ring cherry red and place it over the outboard end of the eccentric. Theory is, it should cool and shrink in place, effectively creating a flange to hold the eccentric strap in place. Whatcha think???

that it'll be really hard to accurately place a piece of cherry red steel in assembly it. all the eccentrics I've made had a ring in the middle of them that fit into a corresponding groove in the strap - there's an example below (it was a tricky one as the eccentric itself had to be split). I know that means extra bother in splitting the eccentric straps before turning and some extra turning on the eccentrics; but what the heck, making an engine is extra bother compared to watching the boob tube and we're supposed to enjoy this, right? I proffer that its such a common way to make them because its such good way.

if you had to work with a solid strap, try to devise something that bolts or threads on to hold things in place because Gail makes a good point that part of good design is the ability to get it apart again.

here's some pics of what i meant by the ring in the eccentrics

here it is with straps

 

[Mcgyver]

the option to affix a ring, instead of a shrink fit, is just loctite it - dis-assembly is just a matter of warming it up

 

[Brian Rupnow]

I am in the final stqges of assembling my twin horizontal engine, and simce I had to tear everything down after a preliminary fitting, I decided to go with my original idea of turning a 0.060 thick steel ring, heating it cherry red, and shrinking it onto the eccentric to keep the strap in place. It calls for having 3 hands at once, one to hold the torch, one to hold the pliers with the hot ring in it, and one to hold a small hammer to "persuade" the ring into place, but it seems to work very well.