Shaft sealing

[stevehuckss396]

I am thinking of building a couple of these ridiculous looking things but I need to get something figured out. I am going to suck fuel thru the block to lube the crankshaft. The cam and crank exit the front of the motor.

Is putting a sealed bearing on the shafts at the front enough to seal the crankcase or do I need an o-ring somewhere?

How are the RC engines sealed up?

View attachment aaaaSingle.pdf

 

[Mosey]

...utilizing the new virtual camshaft is said to eliminate most of the frictional losses...

Go Steve!

 

[Mosey]

I am thinking of building a couple of these ridiculous looking things but I need to get something figured out. I am going to suck fuel thru the block to lube the crankshaft. The cam and crank exit the front of the motor.

Is putting a sealed bearing on the shafts at the front enough to seal the crankcase or do I need an o-ring somewhere?

How are the RC engines sealed up?

My experience is limited to the lower end of R/C engines (OS LA 40 etc.) and they are sealed at the crank end by close-fitted metal to metal, no O rings. I understand that is the secret of why they run so well, the fitting of parts, particularly piston to cylinder, is so well done that they have very little loss. And that is probably why we mortal home builders work so hard for good running engines. The factory is able to machine to much better tighter fit than we (me).

 

[mu38&Bg#]

On the crank I'd just use a sealed bearing. That is, one with rubber contact seals. You'll have to talk to a bearing supplier to get exactly what you want. The bearing manufacturer offers both contact and non-contact rubber sealed bearings. Otherwise you could use a short tight fitting bore on the shaft and accept the small leakage you might see.

Why not just build the cam bearing as a blind hole so you don't have to worry about it? if using a strong enough magnet, the hall sensor will pick it up through the aluminum housing anyway.

Are you using a reed or other timed valve on the carb inlet into the crankcase? You'll have flow in both directions equal to displacement if you don't.

Greg

 

[stevehuckss396]

Are you using a reed or other timed valve on the carb inlet into the crankcase? You'll have flow in both directions equal to displacement if you don't.

Greg

I have already scraped the idea. I am going to go with the carb on the head. The more I think about it this is not a good idea. I might just put a little oil in the crankcase and forget the flow thru idea.

I should have thought about it more before I posted this but if I valve the intake and the crank pressure builds it will most likely force the intake valve open.

OK Never mind!!! I i'm taking a mulligan!!

 

[doc1955]

I

OK Never mind!!! I i'm taking a mulligan!!

:big: :big: :big: Mulligans usually cost a buck :big: :big: just kidding

 

[gbritnell]

Hi Steve,
For future reference a 2 cycle engine has the same situation. The fuel is flowing through the crankcase to lubricate the crank, rod and cylinder. To seal the crankcase they use a neoprene lip seal, the type that have the spring around the inside of them. These engines will go many hundreds of yours without the seals leaking. The only thing is I don't know how small they make them. It's possible that an O ring would work. I've never tried it so I don't know.
George

 

[jpeter]

Hey Steve, I've fooled with a lot of model airplane engines. They generally have a sealed front ball bearing but also they don't have any oil in the crankcase. The 2 stroke sleeve type engine has no seal but for a good fit. As one wears it'll start to throw oil out the front. The 4 stroke ones have vents in the crank case but they run on premix and only seal the crank with a sealed front bearing too. My 4 stroke weed wacker, which has oil in the case, seems to have more than that, maybe an o-ring. What would be wrong with a standard shaft seal?

Check this site: http://www.mcmaster.com/#rotary-seals/=fniiwq

Hey, how did you seal your v8 water pump shaft?

 

[mu38&Bg#]

Many model aircraft two stroke gasoline engines rely on the bearing seals for crankcase sealing.

Steve, I somewhat assumed you were going to put a bearing in anyway. If not, shaft seals can be purchased quite small. Mcmaster has some that go down to 3/8" shafts.

 

[MachineTom]

An O-ring is awful as a seal on a rotating shaft, you may see them on very slow osolating shafts. But not on rotating shafts. My OS and Saito 4 stroke engine do not have crank seals as they are vented to atm. Based on your drawing it looks like a 4 stroke, so why do you ask about 2stroke engines ??? The cylinder blowby is what lubes the crank and cam bearings. Years ago YS had an engine, 4 stroke with crankcase induction, this was used together with a rotary valve as a supercharger.

 

[Ken I]

Try looking in a needle bearing catalogue - they supply very small shaft seals for use with needle bearings.

IIRC down to about 6mm shafts.

I'm currently using them on my elbow engine

IKO OS10173 - thats 10mm shaft, 17mm bore and only 3mm width.
Shaded red in the section below - it is drawn as a true cross section of the actual seal.

Looks good to me.

Ken

Update - I just consulted the IKO catalogue - seals come in
6 x 10 x 2.5
7 x 11 x 2.5
8 x 12 x 3
8 x 15 x 3
9 x 13 x 3
9 x 16 x 3
10 x 14 x 3
10 x 17 x 3
12 x 16 x 3
12 x 18 x 3
12 x 19 x 3
13 x 19 x 3

This is a properly moulded wiper lip (without gator spring at this size) in a steel ring housing - whilst meant for gravity pressures I used a 10 on 2000 psi (as act of idiocy) it lasted about an hour, on 600 psi it lasted about 2 weeks - so I would guess (and I emphasise guess) that they will probably perform up to 100 psi - smaller more so, bigger less so.

I would imagine imperial sizes similar to these are also available.

 

[stevehuckss396]

It was just a dumb Idea.

I am thinking about doing it in an opposed or inline 2 cylinder. That way, would the crank pressure would be somewhat normal?

The only thing is I don't know how small they make them. It's possible that an O ring would work. I've never tried it so I don't know.

Hi George. I dont think I will be able to find a seal in .1875 ID. Smallest I have seen is .250 ID.

 

[stevehuckss396]

Many model aircraft two stroke gasoline engines rely on the bearing seals for crankcase sealing.

I somewhat assumed you were going to put a bearing in anyway.

Mcmaster has some that go down to 3/8" shafts.

I have designed for a sealed bearing. My shafts are small (3/16) so I was hoping a bearing would be enough. I have already deleted the flow thru design and put the carb on the head. Now I have to put oil in the sump.

 

[jpeter]

Hey Steve, check that link to mcmaster-carr I posted earlier for the ptfe shaft seals. Smallest is 1/8 id. 3/16 is also available. Kinda pricey. Never-the-less...

 

[stevehuckss396]

Hey Steve, check that link to mcmaster-carr I posted earlier for the ptfe shaft seals. Smallest is 1/8 id. 3/16 is also available. Kinda pricey. Never-the-less...

I see them! OOOOUUUCCCHHH!!!

 

[stevehuckss396]

I'm going to try the sealed bearings and see how it goes. Scrapped the idea and moved the carb up to the head. It's an ugly bugger but I might make one just because.

 

[kcmillin]

I like this engine design Steve.

how big is this going to be?

I am curious as to the placement of the rockers being perpendicular to the cam lobes. Is there any funny stuff involved in the design of the cam and or tappets to make it work at the angle? I have seen this before on motorbikes but have always wonders about it.

Kel

 

[stevehuckss396]

how big is this going to be?

I am curious as to the placement of the rockers being perpendicular to the cam lobes. Is there any funny stuff involved in the design of the cam and or tappets to make it work at the angle? I have seen this before on motorbikes but have always wonders about it.

The block is 1-1/4 square stock. It's about 3-3/4 tall. The fuel tank is 5/8 OD tubing. It's small.

No magic going on with the rockers. The pushrods will be canted. The rockers might wear funny if it's run alot but I dont see that happening. You see it alot on the radials.

 

[jpeter]

How about designing a hemi head so the valves and rockers can be angled to match the pushrods. The tappets could then too be angled to align with the pushrods. That'd be kinda cute. Just a suggestion cuz its pretty cute the way it is.

 

[mu38&Bg#]

I wouldn't worry about the pushrod angles at all. It's fairly common to see in model airplane engines that get a lot of run time. Hardness of the parts will be a bigger concern.