Opinion wanted on engine design

[edan]

Hello,
I would appreciate any opinions/comments on an engine design that I have in mind please.
The engine in question is an opposed two cylinder that will run on compressed air. This will not have a lot of run time normally, just run occasionally to show the concept works.

I would like to have a splash lubricating system that will oil the crankshaft bearings, cam, pushrods, and gears.
On the flywheel end there will be a double ball bearing setup and I would like to put two o-rings to seal the crankcase from leakage around the crankshaft.

I know the o-rings would wear quickly down to the shaft size, but if the o-rings only wore slightly larger than the shaft (.001 - .002), would this keep the oil from seeping out on short runs?

Again, this engine would only run for short periods and not subject to any heat to speak of. The bearings are excellent quality and do not have any run out so I do not anticipate any abnormal wear on the o-rings from crankshaft “wobble”. The crankcase has a large breather directly above the crankshaft/cam and inline with the cylinders. The oil level would be approximately .38 from the bottom of the crankshaft hole. The o-rings that I have have a relaxed ID of .487 and the crankshaft diameter is .500. The rendering is a little dark, but I hope it will show what I tried to explain.

I am not shy about getting opinions so please feel free to be direct if this is a poor idea.

Thank you in advance.

 

[Deanofid]

Hello edan;

First off, let me say that is a very nice rendering. Very realistic.

You didn't mention your expected RPM range, or how much air pressure you expect the engine will require. If you intend for it to run on fairly low pressure, the o-rings may not be suitable. They do involve some friction if they are expected to work well. If the o-rings will end up as much as .002" over the size of the crank, you would do just as well using a plain bronze or brass sleeve bushings. That type of bushing, for a length as long as appears to be available, will let very little oil by. It also presents very little friction.

You might consider a felt wiper in place of the o-rings. It looks like there is a fairly wide area for one, and they work well for this type of application, and present a minimal friction loss. As long as the crankcase is not pressurized oil seepage will little to none.

If air pressure is not a consideration, in other words, if you can use as much as you want, the o-rings will probably be fine. In that case, you could also use a spring seal, similar to the type used for automotive wheel bearings.

As an aside, air engines need very little lubrication when compared to I.C. engines. You may not need any kind of oil reservoir at all, unless the crankcase is completely sealed. That's just meant as a comment. I understand this is to prove a concept.

It looks like an interesting project. I hope you will share the build with us in the Work in Progress section.

Dean

 

[Omnimill]

Edan, this type of question has been asked before somewhere regarding O Rings. I stand to be corrected, but I'm sure the answer was that O Rings are not normally designed to work with rotating elements in this way? If correct then perhaps the best answer you could expect is, maybe, it depends ... There are lots of different O Ring materials out there so I expect selecting the right one will make the world of difference.

Good luck with the project, and lets th_wwp see some pics when it's done!

Vic.

 

[PaulG]

Edan, I must admit I had to study this a bit to my OLD brain a working, (don't laugh BigB)

I would replace the outboard bearing with one of the following, and leave out the "O" rings.

See ENK-000-HP http://www.bocabearings.com/RC-Engine-Bearing-Guide.htm

Paul

 

[Bernd]

Why not use an oil seal. Our modern cars have these to seal the ends of the cam shaft and crankshaft. Has seemed to work for over 100 years.

Bernd

 

[bearcar1]

I do not have enough experience with oil seals to offer any suggestions in that area, however a thought (yes I do get them on occasion) did occur to me that could possibly assist in keeping oil away from whatever sealing devices you ultimately end up using. Could a shallow helix be machined into the shaft in question that would be in a counter direction of shaft rotation? This helix would act as a passive type pump of sorts and any oil that would tend to accumulate would be held in check by the augering effect of the helix. This is merely an initial design idea and thought it may be practical given your operational situation.

BC1
Jim

 

[GOOFY063]

wEc1 depending on the size of the shaft, automatic transmissions have small seals such as the speed-o-shaft seal or shift shaft seal in a GM turbo 350
goofy063

 

[Jared]

You can get seals that are the same dimensions of a lot of bearings. You could replace the outer bearing with a seal without altering the design or if you really want the two bearings you could make a seal housing to bolt on outside.

Something like this: http://www.applied.com/apps/commerce/catalog/catalog.do?e=10&s=1435965&r=0&type=a&mp=4912
This is just one, there are a few thousand others to chose from.

Just take a piece of metal about 1" square or so and approximately the same thickness as the seal, bore a hole to press fit the seal in the center and drill bolt holes in the corners wherever you need to mount it, make a gasket to put behind it or goop it up with RTV silicone and side it over the shaft.

 

[edan]

Thank you everyone who replied to my question.
By reading all the comments I can now see that my original idea was probably not the best design. I was using the o-rings for something they were not designed for.

I am researching the felt wiper as this seems to be the easiest/most cost effective solution to use with my current design.

I am also looking into using automotive seals. I mistakenly though they would apply too much pressure and did not include them in my design. I have gone down to the local Chevy/Honda dealer and explained my situation to an understanding parts person who showed my a few that would work with only a slight modification to my design.

I need the double bearings because the crankshaft is a overhung design with a follower crank to drive the cam gear. The basic idea for the engine is from Edgar T Westbury’s prototype called “The Venturer”. I don’t have very much information on this engine but it looked like an interesting design and I thought I would like to build one. I have only seen a few drawings of the basic idea so I have adapted my ideas to the design.

I really appreciate the comments. It’s funny how if you don’t have anyone to bounce an idea off of you get “stuck thinking” a certain way and can not see your mistakes.
Thanks again.