Author Topic: 30cc Flat Four (Puma)  (Read 25336 times)

Online Vixen

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Re: 30cc Flat Four (Puma)
« Reply #90 on: August 04, 2019, 05:44:44 PM »
Thanks Jason,

I have found the groove dimensions you refer to in Section Eleven of 'Model Engineers Handbook' by Tubal Cain, published by Argus Press. I do not have the Reeves catalogue to compare.

Mike
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Sometimes, it can be a long and winding road

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #91 on: August 04, 2019, 08:52:36 PM »
Thanks for the validation. I was expecting some negative responses however this area of model engineering doesn’t suffer from that habbit as much as some.

As the grove for 1x20 O rings is similar to that required for CI rings I can afford to experiment. I had expected to need to make two sets of pistons but that’s now not the case. My supplier is only 10 mins away from the house and they have stock of them in Viton. I’ll be over there this morning once the lemmings have gone to work.

My understanding of the grove width is that it needs to be slightly wider than cross section of the O ring. That then enables the pressure to get behind the whole face of the ring and distort it against the base of the groove and the wall of the cylinder. With a 4 stroke that will also reduce friction, to some degree, for75% of the time.

https://www.fluidpowerworld.com/how-does-an-o-ring-seal/

Also, for that reason, only one ring should be required. If the first ring is doing its job the second one becomes redundant.

Anyway, nothing ventured nothing gained. There is an element of experimentation and certainly learning in this hobby.

Regards

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Brian Rupnow

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Re: 30cc Flat Four (Puma)
« Reply #92 on: August 05, 2019, 01:16:31 AM »
I don't get too deeply into metric world, but I have great success with Viton O-rings as piston rings on my i.c. engines.--So--Here is what works for me. I use a "nominal" 1/16" (1.57mm) cross section o-ring. The REAL cross section is actually 0.070" (1.78mm). I cut the piston groove 0.094" (2.39mm) wide x 0.056" (1.42mm) deep. The only thing to be aware of, is that there will be more "drag" as the piston moves in the cylinder than if you use cast iron rings. (This is irrelevant unless you are making a "hit and miss" i.c. engine). That is why I have purchased cast iron rings for use on the current vertical hit and miss engine that I am building. I have about 5 or 6 "hit and miss" engines that I have built, and they all have viton rings, and I have never been able to get any of them to "hit" once, then go thru a number of "miss" cycles before "hitting" again.  I deeply envy all those who have built model hit and miss engines which actually run the way there full sized brothers do. (My engines all have to "hit" three or four times before building up enough speed to go into "miss cycles", and when they do, the "miss" cycles never last very long because of the "drag" effect.

Online Vixen

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Re: 30cc Flat Four (Puma)
« Reply #93 on: August 05, 2019, 11:16:27 AM »
Thanks Brian,

Those numbers give a real live working design case.

Actual O ring diameter = 0.070".      Grove width = 0.094",       Groove depth = 0.056".

We can make those measurements dimensionless (expressed as a percentage) so that other sizes of O ring groove may be designed from your data.

Based on your information:  Grove width is 34% wider than the measured O ring diameter. Groove depth is 80% of the measured O ring Diameter. The cross section Area of the ring groove is 36% larger than the O ring cross section area, which gives the O ring plenty of room to compress and move about.

If we allow say 0.002" clearance for the piston in the cylinder, the O ring will be squeezed down to ( 0.058")  73% of the O ring diameter, thats a 17% nip.

My old company had a 'rule of thumb' for O ring fits. They recommended 20% for a static seal and only 10% for a dynamic seal. But that was for water pressure applications, NOT I/C engines.

If that holds true for I/C engines, then you should be able to use a groove 0.005" deeper and gain the benefit of lower static friction (lower drag). 

However Brian, your dimensions are proven to work, so why fix them?    :stir:    Just saying

Cheers

Mike


« Last Edit: August 05, 2019, 04:47:37 PM by Vixen »
It is the journey that matters, not the destination

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Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #94 on: August 08, 2019, 08:30:23 AM »
Pistons done and assembled on their conrods. The piston on the right has the O rings fitted. I decided to use 2. They are pretty small.



Installed in the crankcase



Time to hone the cylinder bores. Rather than risk distorting the liners in the chuck I've clamped them in the cylinders with washers



And cylinders installed





[youtube1]https://youtu.be/JdfKjWFKrdk[/youtube1]

Next up is the rockers, valves and all the other fiddly bits that go with that. I've made a start on the rocker supports.

As an aside; I showed engine at the club night last night and mentioned the lathe tool as a fly cutter. I made a video of it today

A skim cut and then 0.5mm cut at 1800RPM using CCGT0 60204 Aluminium cutter blade insert in a lathe tool.

[youtube1]https://www.youtube.com/watch?v=roKUtbEgzpM[/youtube1]

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Jasonb

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Re: 30cc Flat Four (Puma)
« Reply #95 on: August 08, 2019, 01:06:02 PM »
I think you need to modify the top of the crankcase to incorporate a viewing window otherwise those red conrods will be lost to most :LittleDevil:

Offline Brian Rupnow

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Re: 30cc Flat Four (Puma)
« Reply #96 on: August 08, 2019, 01:08:47 PM »
Absolutely stunning work. I am enjoying this build.---Brian

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #97 on: August 08, 2019, 09:34:40 PM »
Thanks Jason and Brian

A window has crossed my mind a number of times, including last night. I have some polycarbonate sheet that would be perfect. I have even considered replacing the sump with a polycarbonate sheet and running it upside down.

Lubrication would be the issue as the window would soon get covered in oil.  A dry sump and pump was suggested by Bruce but without any oilways built in it would need to be spray lubrication and back to being messy.  Then it could be completely dry and lubed up before running like we do with our locos and Brian’s  twin.

However, this should run at a minimum of 1000RPM so there wouldn’t be much to see when it is running.  It could be a case of ‘never say never’, as generally with these engines after the fun of getting them built and running they tend to go on a shelf and rarely, if ever, get run again. That’s what happens to mine.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Jo

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Re: 30cc Flat Four (Puma)
« Reply #98 on: August 09, 2019, 07:45:03 AM »
Why did you paint (I assume it is paint) the con rods Pete? And what did you paint them with?

I am worrying about the paint coming off, as I saw this happen many years ago when one of our forum members was measuring the speed of one of our torpedo engines :-X

Jo
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Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #99 on: August 09, 2019, 10:47:09 PM »
Why did you paint (I assume it is paint) the con rods Pete? And what did you paint them with?

I am worrying about the paint coming off, as I saw this happen many years ago when one of our forum members was measuring the speed of one of our torpedo engines :-X

Jo

Jo

Good question. I took the view that I'd do a bit more with this engine rather than leave it 'naked'. They look great when they are first done but quickly oxidise and don't look to good. I use epoxy enamel. I dries hard and is durable. It seems to be as close to powder coating you can get without powder coating.

http://www.wattyl.co.nz/en/find-the-right-product/exterior/Metal/Topcoat/WattylKillrustEpoxyGlossEnamel.html

Using the rattle cans I've done all my locos with it and on steel it doesn't need primer, so lazy painting. On brass, copper, aluminium the etch primer is good. The crankcase is currently just primer.

As for the paint coming off of a torpedo motor, I'd guess they run at far higher speeds I'd ever expect to achieve. I don't think its going to be a problem here.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Laurentic

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Re: 30cc Flat Four (Puma)
« Reply #100 on: August 10, 2019, 01:06:59 PM »
Going back to Brians post No.92, the figures/dimensions Brain gives for O ring grooves are almost exactly as detailed on this website https://www.polymax.co.uk/media/documents/Polymax_O-ringen_Guide.pdf by a supplier, which shows Brian is spot on with what works! :ThumbsUp:

Chris

PS I would like to try epoxy gloss enamel, but Wattyl stuff is not available it seems in the UK - anyone have an alternative?

Offline Brian Rupnow

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Re: 30cc Flat Four (Puma)
« Reply #101 on: August 10, 2019, 01:30:30 PM »
I have built glass viewing panes into engine crankcases. You can see fine until you start the engine. With no oil in the crankcase, all you see is a blur when the engine is running. With oil, you can't even see the blur.

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #102 on: August 10, 2019, 10:48:57 PM »

PS I would like to try epoxy gloss enamel, but Wattyl stuff is not available it seems in the UK - anyone have an alternative?


Laurentic

Wattyl are part of the Valspar Group maybe there is a similar product but under another brand in the UK.

Dulux do something similar, but again AU/NZ https://www.dulux.com.au/products/31A



I have built glass viewing panes into engine crankcases. You can see fine until you start the engine. With no oil in the crankcase, all you see is a blur when the engine is running. With oil, you can't even see the blur.


Brian

Much as I expected would happen.  With oil you can't see the blur but with the engine stopped can you see anything? Was it worth the effort to include a window?

Pete
« Last Edit: August 10, 2019, 11:14:24 PM by doubletop »
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Brian Rupnow

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Re: 30cc Flat Four (Puma)
« Reply #103 on: August 10, 2019, 11:46:41 PM »
No

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #104 on: August 11, 2019, 07:43:06 AM »

No


Well that’s one job off the list then. No window.

When I was showing progress at the club monthly meeting somebody commented that the engine was smaller than they expected it to be. After this weekend’s activity I have to agree with them. The rockers are tiny.



When I did the Bobcat I was used to doing small stuff, having worked on a 3.5” gauge loco. I then went to a 5” guage loco and more recently have been working on 7.25” gauge projects. You don’t realise that you get conditioned to the size of your work.

I guess it the valves and springs are next.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

 

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