Info on piston ring pressure requirements?

[Naiveambition]

I'm looking for info regarding how to calculate the pin size for rings in heat treating according to the Trimble method.
My measurements are
Piston. 1.499 inch
Cylinder. 1.5
Ring size. 1.5 od .065 thick .285 wide

Trimbal states 2/3 of the gap will be left after treatment
Somewhere I picked up the min ring gap should be .006 according to bore size. .006 x 3= .018 pin size.? Their math was .0045 x bore size.
On the Kerzel plans he states the ring gap is selected for sidewall pressure. And is about two strokes of the file, so I'm guessing..001 -.002. So .002x3 is.006 pin size.? Is this anywhere near correct?
I'm doubling the scale, and wandering if this changes accordingly?

 

[stevehuckss396]

Here is what I get for a 1.5 inch bore according to the Trimble method.

 

[Gordon]

Here is what I get for a 1.5 inch bore according to the Trimble method.


View attachment 109045

Is that chart or calculation something available or is it something you generated? I purchased the copies of the series on the Trimble method but I am still having a problem trying to calculate the various values for different bore/ring sizes. I have been just assuming that it is proportional for other sizes. For example a 1.5 dia would be 50% larger than a 1" dia.

 

[Naiveambition]

Thanks for the reply Steve, that is very helpful. I'm pretty close to all these dimensions, But my question lies more torward how to know the pin size to create the gap needed. I did run across a post somewhere that stated a lot of hit and miss use no rings to reduce friction. So on that thought maybe a higher gap would create unneeded pressure, which is why I'm looking for this particular answer.

 

[stevehuckss396]

But my question lies more torward how to know the pin size to create the gap needed.

If by pin size you mean the diameter of the pin in the fixture, that is the same as the dowel diameter. Dowel offset is the dimension of the pin off center.

 

[stevehuckss396]

Is that chart or calculation something available or is it something you generated?

It is a small program I wrote to do the math laid out by Mr. Trimble.

 

[stevehuckss396]

I have been just assuming that it is proportional for other sizes. For example a 1.5 dia would be 50% larger than a 1" dia.

I would think you are correct. Here is data for a 1 inch bore.

 

[Gordon]

I would think you are correct. Here is data for a 1 inch bore.

View attachment 109052

Thanks. That pretty much verifies my calculations.
Is there any reason for making the rings that narrow? .03375 and .0225 are really narrow and smaller than any cut off blade so it requires a special ground bit. The rings I have made have been 1/16 or 3/32 wide. A .0225 wide ring would seem to be pretty fragile.

 

[stevehuckss396]

Is there any reason for making the rings that narrow? .03375 and .0225 are really narrow and smaller than any cut off blade so it requires a special ground bit. The rings I have made have been 1/16 or 3/32 wide. A .0225 wide ring would seem to be pretty fragile.

That width and thickness are what Trimble recommends. Going wider does not make for a better seal and going thicker would probably give greater wall pressure and wear the cylinder faster. Just my opinion so take it for what its worth. I have cutoff blades as small as .019 width so a ring of this size I would use the dimensions given or bump it to the next cutoff size or two which is .025 or .031. The bump would be to a .0235 or a .0285 wide ring.

If 1/16 or 3/32 is what you are comfortable with and seem to work well then I see no problem doing that. The numbers Trimble cites are more inline with a conventional engine. We all know that there is not a lot about model engines that is conventional so go with what works.

Remember that his method is supported by lots of math and research that seems to make a lot of sense to me. Also the fact that many builders over the years have made successful rings if they follow the method closely. That's why I use it and my Rings have worked very well the last few times I have used it.

If you try to make rings, other than the width of the ring, follow the method exactly as laid out. It's kind of a pain in the rump but easier than making rings 2 or 3 times.

Use the right cast iron
Machine everything to the correct dimension
Stress relieve them correctly
Fit them in the ring correctly.

Do all that and you will be quite happy.

 

[mu38&Bg#]

Almost all production model engines have rings of 1x1mm in cross section bores under 30mm.

 

[Gordon]

That width and thickness are what Trimble recommends. Going wider does not make for a better seal and going thicker would probably give greater wall pressure and wear the cylinder faster. Just my opinion so take it for what its worth. I have cutoff blades as small as .019 width so a ring of this size I would use the dimensions given or bump it to the next cutoff size or two which is .025 or .031. The bump would be to a .0235 or a .0285 wide ring.

If 1/16 or 3/32 is what you are comfortable with and seem to work well then I see no problem doing that. The numbers Trimble cites are more inline with a conventional engine. We all know that there is not a lot about model engines that is conventional so go with what works.

Remember that his method is supported by lots of math and research that seems to make a lot of sense to me. Also the fact that many builders over the years have made successful rings if they follow the method closely. That's why I use it and my Rings have worked very well the last few times I have used it.

If you try to make rings, other than the width of the ring, follow the method exactly as laid out. It's kind of a pain in the rump but easier than making rings 2 or 3 times.

Use the right cast iron
Machine everything to the correct dimension
Stress relieve them correctly
Fit them in the ring correctly.

Do all that and you will be quite happy.

You are right. I missed the width specification probably because I was trying to make rings for an existing engine where the width was shown at 1/16 or 3/32. I do not find any cutoff blades smaller than .040. I guess that the solution would be to grind them to the proper width. I no longer have a surface grinder but I could resize them with the tool post grinder. It may even be an excuse to make a tool grinder.

 

[stevehuckss396]

https://www.kbctools.com/itemdetail/1-293T-001

Insert is about 17 bucks. Holder can be expensive but I have made 2 to fit the sherline lathe. Wasnt that bad. They have .019, .027, .031, .043, .051, .063 wide

 

[kuhncw]

AR Warner, Latrobe, Pennsylvania, sells the same kind of grooving/cutoff tool in HSS.

https://www.arwarnerco.com/Cutoff-and-Grooving-s/169.htm

Little Machine Shop sells the Warner tooling also https://www.arwarnerco.com/Cutoff-and-Grooving-s/169.htm

Steve, that is a nice ring parameter program you wrote. What language/software did you use?

Chuck

 

[stevehuckss396]

Visual Basic

 

[petertha]

I get similar numbers as Steve (for the nominal 1.000" bore). As it should be, hopefully we read the same article LOL. Its been a while since I looked at this but the ring width is expressed as a Width/Bore ratio by Trimble. The default I have in there is one that he suggested, but looks like there is a range. If you start with the cutter width like the Nikcole & size the ring width to that + allowance, I think thats a good way to go. (Or at least the way I intend to go LOL). Those cutters work well & btw they are metric with a tolerance even though they are often referenced as IMP to varying degrees of decimal places & round-off.

I took the calculation one step further & show the Trimble chart with constraint curves so that one can enter arbitrary ring parameters & see how it compares. ie whether its within the constraint curves or outside & if so likely implications. Red dot is in the bullseye, black triangle (justthis particular example) is outside where exceeding installation stress of CI might be an issue.

 

[Gordon]

Is there any advantage or disadvantage in using a narrow ring? Would a .025 ring seal any better than a .060 ring? None of the plans that I have used have anything narrower than 1/16". .040 cutoff blades are easily obtained so would it be worth the trouble to find a .025 blade?

 

[stevehuckss396]

Not an engineer but you might get better wall pressure with the narrow ring as far as psi goes. I think skinnier or thicker equates to more wall pressure. Need that wall pressure. I put .031 rings in the pacifier with a .750 bore and no smoke on the second start. First start had lots of wd40 in the gas. Second start just a little wd40 in the gas and almost no smoke even with the wd40. Not sure if .060 would help or hurt or be the same. Try it and see.

The spread sheet Petertha might be able to compare the two senarios.

 

[Gordon]

This is interesting to me because I have been fighting with an Atkinson Differential engine for way too long. I have made at least three sets of pistons and six sets of rings using different methods and I still have a compression problem. Due to the design there is a very short compression stroke and the back of the piston is outside the cylinder at the firing position. There is leakage at the end of the stroke no matter what I do. Just before it reaches the firing position there are soap or oil bubbles around the piston. It would seem like I should get a good piston/ring setup just on the blind pig theory. Now I have to decide if it is worth the effort to make another set of pistons and rings. The original Gingery design calls for 3/32 ring width and I never even thought to question that.

 

[bluejets]

This is interesting to me because I have been fighting with an Atkinson Differential engine for way too long.

There is heaps of comments around about these particular engines (2 piston type)and reference to the flaw in the design of the model engine.
Basically the timing of the cycles is way off.
Mate of mine was in the process of redesigning it as he can watch the process in a cad "open cage" view, just other projects have taken over.
We have both built the cycle engine with success.

 

[stevehuckss396]

I have scoured the interweb looking for a calculator with zero luck. I was going to see what the wall pressure (PSI) is for a .043 X .0225 ring is, and then do it again for a .043 X .060 ring and see what the PSI does.

We need a guru.