Making Piston Rings

[Steamer5]

Hi All,

After seeing quite a few builds here & the issues we all seen to have making them I thought I would dig out this article that had been sent to me by a friend in the UK some 14 years ago.

A link to what  looked to be part of this article was posted in Brian’s build of the Jaguar, & when I saw that some stuff had been missed I had to post it!

 Credit for this article goes to Mick Collins of Taunton UK. As you will see he has complied it from other article while he was making piston rings for his hot air & other engines & as such gets a lot of information in one place that most likely will be of help, I hope, to those making piston rings of all sizes. Mick had a website which since his death several years ago is no longer available, so by putting this info here I hope will preserve it for others.
  If you have any questions I’m sorry I can’t answer them, I’m new to this game too! Although if you have any additional information please feel free to add it!!
 
 It is quite long & wordy so please bear with me!

The following is an introduction to a suggested method for making small piston rings, based on articles by Prof. Chaddock & Tom Walshaw (Tubal Cain), But I must admit that I’ve NOT yet attempted the final machining operation.
 In these explanatory notes the following abbreviations will be used:-
 D = cylinder bore,   
t = radial thickness of ring
w = axial width of ring   
p = wall pressure
Fdd = force required to close ring, applied at 90 degrees to the gap
and “  “ = actual quotations from the articles

In an article in the Model Engineer (21/4/67 p.396) Prof. Chaddock describes an efficient 5 cc Four stroke I/C engine, built for an attempt on the model aeroplane duration record & he details his method of making piston rings by “ heat forming”.

“The rings are turned to a diameter D + 0.002” & a thickness not more than 1/25 or less than 1/30 of the bore diameter. The modern tendency is to make width equal to or even less than their thickness so the same rule can be applied to width. The ring must now be cut with the thinnest possible saw or (preferred method) broken by holding in the fingers & snapping it. A tiny gap piece, of width t x 4, is now inserted into the gap to expand the ring before clamping it, with several others if required, between two metal plates by a center bolt & heating the whole assembly to annealing temperature in a gas flame or muffle”

 Prof Chaddock does not give the temperature required but says that the practical test is that if, after cooling, the ring springs inwards from t x 4 gap then it is not fully relieved of stress & the process must be repeated,( see later).
 He says” Finishing the rings once they have been “heat formed” can follow conventional practice, that is, they can be closed in a tube & clamped between mandrel plates ——– for finishing the periphery & the sides lapping freehand on stone or emery paper. Very little, one or two thou’ only need be taken off in these final operations to bring them to dead size.”

….” they hold compression as well or better than a ground & lapped piston & cylinder”
  He doesn’t mention any ‘working’ gap being required & the rings he actually made were 0.025” x 0.025” cross section to fit an 18 mm (0.709”) bore.

The essay by Tom Walshaw (Tubal Cain) in the SMEE journal (Dec 1992)
Is largely concerned with designing rings with the correct wall pressure for their particular application & he refers to an article by Michael Smart (M.E. 16/8/1974 p.824) in which is explained how a properly fitted ring is expanded by the pressure behind it to augment the initial wall pressure.  Tom’s conclusions being that the majority of model engines when running with excessive wall pressures & he included a table of wall pressures he had measured on a selection of commercially made ‘model’ rings.
 He then describes a simple method of measuring it by closing the ring to its bore diameter with a force, Fdd, applied at diametrically opposite points, 90 deg’s from the gap. Then:-
         p = Fdd / 1.135 x w x D

(Please note , Mick put a caveat on this “ Due to a possible error in this part of the article, this formula may be wrong if anybody who has an understanding of these things could they please contact me so that I can  correct info…… many thanks Kerrin)


An expression for calculating “p” is given as: -

            p = g x E / 7.06 x D x (D/t –1)³

Where g = difference between the free & closed gaps, in inches
           E = Young’s Modulus, lbs. / sq.in.
 
“ Values of E can vary between 12 to 24 million lbs. / sq.in. & reference to the supplier is advisable. A figure of 18 million is typical for 20 –22 ton UTS, & between 15 – 17 million for the 17 ton UTS centrifugally cast iron normally available for home made rings.”
“To complete this exploration of the arithmetic, the stress in the ring can be calculated in the usual way from bending moments  -- & simplified to: -

            f = 3 x p x (D/t)² lbs. / sq.in.”
 
After a long dissertation on model piston ring practice in which Tom draws on every article he could find in the model engineering press he discusses the criteria for design & comes to the following conclusions: -
“Wall pressure for models
(a) Steam plant. It is clear that wall pressures used in our models are higher than need be, even with plain Ramsbottom type; still more so with modern heat formed rings. For stationary, marine & road locomotive engines, all of which can be motored for initial bedding down, 6 – 8 lbs. / sq.in should be adequate. However, there is a small problem; the radial thickness is so small with ”classical” gaps that parting off may be difficult !!! For this reason I set a minimum value of “t” at 0.035” & use a smaller gap when necessary.
Fixing on g = D/10 instead of 4 x t simplifies the pressure formula to: -

            D/t = ³ E / (70.6 x p) + 1

But this ratio should not be used for wall pressures above about 12 lbs. / sq.in. without checking the “fitting” stress. (see latter)
For rail locomotives it may be prudent to use slightly higher pressures. The low end of the  “ Chaddock standard” provides about 13 lbs. / sq.in. with “E” at 17 million (i.e. Dt = 30 & g = 4 x t), but this will vary pro rata with the value of E”
 
!!! It isn’t – see the method
 
I.C. Engines. For the classical horizontal gas / petrol engine model I have substituted rings at 6 ½ lbf / sq.in. with no problems, but where the designer has called for two rings I have used three. This pressure lies outside the “Chaddock standard” (t = D / 33 – 35) so that, again ring gaps of D /10 are used. Many published designs use rings which are too wide, & I suggest
w = 0.03 x B as a guide, with a minimum of 0.04”.

For all other I.C. engines it is no surprise to find that adherence to Prof. Chaddocks rules will give satisfaction, though at the high end (D/25) it is important to check stresses, as stresses are higher than the usually available 17 ton iron can carry.
 
For the benefit of those not familiar with these rules the figures are: -

            Radial thickness, t = D/25 to D/30
            Ring Gap       g = 4 x t

If “E” is 17 million lbs./sq.in these rules offer wall pressures from 28 down to 13 lbs./sq.in. The wall pressure will of course vary with the value of E in direct proportion. The lower the wall pressure, the less friction, of course!
For very high performance engines it goes without saying that experiment is always necessary, & although it may add to the time & cost a single cylinder prototype, arranged for measuring both oil consumption & blow- by (as well as output & fuel consumption) is well worth the effort. Even here I would not expect to find more than 25 lbf/sq.in. to be necessary & that only when a single pressure ring is supported by a scraper between the top rings & the main oil controller.”

[Steamer5]

Hi,
 As I said its a wordy article so heres the next part.....

Ring Fitting

(a) Running gap After an illustrated explanation of the comparative insignificance of this gap, Tom recommends:-
“The minimum gap – for steam or I.C. – should be 0.002” & a guide might well be an installed gap equal to
            0.001” + 0.001” / inch of cylinder bore.”
(b) Fitting gap. This is the dimension G when the ring is sprung over the OD of the piston when fitting. If this is excessive the ring may be overstressed, but it is the dimension G-g which is significant, so that, as already remarked, the risk of overstressing increases as the free gap is reduced. Unfortunately “the books” all seem to assume that the ring clasps the piston closely when fitting but this is not the case, & stresses based on this assumption will be too low. Geometric analysis is almost impossible, as the ring does not assume the shape of a pair of half circles, & in any case the actual direction of the loads holding the rings apart are indeterminate.
Experiments with a number of rings of various D/t ratios & values of D show that G varies from 6.6t to 7.5t. If g=4 as under Chaddocks rules, then the ring will not be overstressed when installed- provided the working stress is safe, of course. As a very rough approximation, the installing stress can be estimated by writing, -

 Fi = fw x (7xt-g)/g 
when fi = installing stress,
fw = max. working stress,
t & g, as before
 
This estimate is by no means exact, but a check on the “risk” can be made by comparing the value of “g” before & after a trial fitting. If there is a marked & permanent increase in “g” then the ring is very near the limit.
 
Ring Manufacture

   “The original Ramsbottom rings were plain circles from which a gap was cut so that the closed ring fitted the cylinder with no more than a working gap. It was realised that such a ring would not fit properly and so could not exert an even pressure, even after hours of running. The necessary shape to achieve this was known, of course, and requires the free ring to have a radius at any point which varies as the sine of the angle of the section from a point directly opposite from the gap. Lanchester devised a machine, which would turn rings to this shape, but it is doubtful whether any model engineer would undertake to make one. However, with modern CNC machines the process is much easier, and many large engine rings are so manufactured.”
 
Tom then describes how the inside of a plain ring can be peened to produce the required effect – and considers that it is too difficult with a small ring.
He next considers a tapered ring, produced by boring the ID eccentric to the OD, but rules this out as, to get the correct characteristics, requires “t” to be reduced to zero at the gap. Then : -
 
“A near perfect ring can be achieved by “heat forming”, a process which is adaptable to high volume production, and which can be used for very small rings indeed. Here, a circular ring of uniform thickness is cut with a very small gap. It is then forced into or onto a shaped former, and stress relived so that, when cooled, the ring is of the correct form to provide both true circularity and a uniform wall pressure when fitted into the cylinder, though in almost all cases a final machining operation is carried out to “skim” the OD to allow for the inevitable tolerances. The shape of the former is, of course, the obverse of the shape used by Lanchester years ago. It is an adaptation of this method, which was described by Prof. Chaddock
 
Unfortunately there seems to have been a misunderstanding of the nature of the process by some, including Mr Trimble & Mr Tulloch. First the process is NOT a copy of that used in industry & cannot form the “perfect” ring. Whilst it relies on the relationship in expression (2) in that the wedge exerts the “tangential force” there referred to, the process does not & cannot produce exactly the correct shape. This force may induce the correct bending moment in the ring, but it does not reproduce the correct deflection, for the tangential force introduces an additional compressive stress. This is small but has a devastating effect on the shape of the rings adjacent to the gap”.
There is no way of correcting the stresses by altering the nature of the wedge, (e.g. by applying the force at an angle instead of tangentially). The fact that the wedge may fall out after clamping up the parcel of rings makes no difference, nor is there much point in “fitting” the wedge to the angle of the gap. However as Prof. Chaddock realised, the effect can be mitigated by carrying out a final skimming operation on the OD after heat treatment. This ensures true circularity & reduces the deviation from the uniform pressure condition to negligible proportions. This final machining operation is essential. However it need be no more than a skim, & the removal of 0.001” of metal should suffice for a 1” ring & pro rata for larger sizes. For models true circularity is more important than a uniform pressure.

Heat Treatment.

It appears there has also been some misunderstanding about the stress relieving process, possibly because the original article also referred to “annealing”. Most subsequent writers have quoted a “good red heat”, though Mr Trimble quotes an actual figure of 800º C. as does Mr Tulloch. This is a mistake, 800º C. or “good red” lies above the critical temperature & a metallurgical change will occur. The Brinell hardness will be reduced as will the U.T.S. & the valve ore & some grain growth will occur – just the wrong requirements for a piston ring. If the material is an alloyed iron the results may be even more serious. There is a further fact that scaling may be also caused.
It is not unlikely that the use of this high temperature has resulted in users going to stiffer rings than were needed, simply because the heat treatment caused a reduction in the valve of “E” with consequent loss of wall pressure.
The “correct” temperature is 480 – 520 º C. with slow heating, the temperature being held for 1 hour per inch of thickness, but with at least 10 minutes for very thin rings. The stack may be air cooled from this temperature, though no harm seems to arise from oil quenching. The metal has no colour at this temperature. Use can be made of Thermomelt crayons of the appropriate range to indicate this temperature. A mark with one of these will turn glossy at the indicated temperature & they are available from 100 º C up to 1200 º C . Alternatively, very little degradation of properties will result from heating to 500 – 600 º C when the metal will be just visible in a dim light, but on no account must the temperature be allowed to rise any higher. (The critical temperature is 720º C) It is preferable to use the lower temperature for the full time rather than to try to speed things up by going higher. Incidentally, scaling at these temperatures is minimal – it will come off with metal polish.
Tom also devotes a couple of pages to oil control & scraper rings before concluding that a stepped scraper with a wall pressure of between 20 & 30 lbs/sq.in. & with adequate oil escape holes should be adequate - & that provided the plugs don’t oil up, it is wiser to tolerate a high oil consumption in a model. (Dennis Chaddock doesn’t use any on his engine)
 
Bronze Rings
Rings of bronze are mentioned fairly often generally for use with gunmetal cylinders. I have had very little experience of these for model, but have no doubt that they can be satisfactory. However, certain points should be born in mind. First, the working stress must be kept below the yield (or 0.1% proof) figure – typically about 20 tonf/sq.in. – especially during installation. Young’s Modulus is of the same order as for iron – 15 million lbf/sq.in. Second, the stress relieving temperature lies very close to the annealing temperature, & great care is needed when heat forming. On no account must 350 º C be exceeded & a temperature of 300º C should be aimed at. This is the temperature at which bright steel turns blue – a useful guide!
 
Tom’s final conclusion are that for steam engines & for the classical model I.C. gas / petrol engines, which can be run in on the bench 6 – 8 lbs/sq.in. should be adequate. Locomotive rings may need higher pressures to speed bedding down, but no more than 12 lbf/sq.in. Lower pressures should be possible with properly formed rings, with consequent freer running. The workhorse type of I.C. engine may need 16 – 18 lbf/sq.in. unless the duty is severe, but even then the “Chaddock rules” should not be exceeded; 20 – 22 lbf/sq.in. is recommended as a maximum, but experiment may be needed at speeds over 12,000 rpm.
Finally & to forestall any questions I must add that Tom did not quote any ‘worked examples’ for rings designed to fit these conclusions.

[Steamer5]

And even more....

Suggested method for making small piston rings
 
First, finish the cylinder bore – to diameter D.
Decide the cross section of the ring required & if you are using a conventional, non demountable, piston then finish it, making the ring grooves deep enough to give 0.004” clearance behind the rings.
Make the steel sleeve for the machining fixture, bored to D + 0.002” & approx. D long.
Chuck a length of centrifugally cast iron & drill it to within 1/16” of the inside diameter of the rings for sufficient depth.
Turn outside diameter to D +0.002” (use sleeve as a gauge) for a length equal to: -

         (w + parting tool width) x number of rings required plus a few spares.

Use a narrow parting tool to make a series of grooves. Depth of grooves to be exactly equal to t + 0.001”. Spacing to be exactly w + parting tool width + 0.001” (lapping allowance).

Now use a sharp fine boring tool to open up the hole to inside ring diameter. As you approach this, reduce the cut to 0.001” & “lean” on the tool to prevent it cutting ‘on the way out’. When you reach the final cut you will be rewarded by a little bunch of rings on the neck of the tool.
Use a Swiss file to break the sharp corners inside the rings so that they will move freely to the bottom of their grooves.
Make a tiny nick on the inside of each ring with a very fine triangular needle file & break it between finger & thumb, or by using the thumb to press it down onto a piece of wire on a flat surface. Very carefully dress the broken surface with a Swiss file (No 6 cut).
Make the wedge to hold the gaps open.
Stack the wedged rings around a bolt, clamp them between two steel plate & heat evenly to 550 - 600 º C, i.e. just visible in the dark, NO HOTTER! Hold at this temperature for 10 mins & allow to cool – rings should not have scaled. (I’ve done this successfully on an  electric hot plate, which can be set to the correct temperature first & then the rings laid on it & left for 30 minutes.)
 
Next make the machining fixture / clamp: -

Drawing 1 is as per Prof. Chaddock’s version

[Steamer5]

Pack the rings into the sleeve & slide the sleeve on to the rod. Fit the disc & clamp the rings securely with the nut. Slide off the sleeve & use the engine cylinder as a gauge to take one thou’ cut off the rings & reduce their diameter to D.
Finally, lap the sides of the rings to fit their grooves. They should be perfectly free but with clearance of only 0.001” or less.
 
Notes.
   This drawing is copied from the one accompanying Tom’s article. Prof. Chaddock’s drawing showed the D – 0.002” dimension as D, i.e. the required ring diameter, & also only a single ring clamped in it.
I’ve successfully made, & used, C.I. rings down to ¼” x 0.010” x 0.010” (accepting 25% breakage’s) using this method but must repeat that I’ve not attempted the final skimming operation. Examination of the rings on my Stuart 10, after very heavy use, shows a perfectly even polish, but near microscopic examination of the two 0.016” square rings on my miniature marine engine, after only a few hours running, shows minute high spots at the gaps with a few degrees of darker metal extending round from them.
Nevertheless, compression / performance is excellent with very low friction – several thousand rpm at 15 psi.
 
Since writing the foregoing, I must admit that I’ve had serious misgivings about my ability to machine the last thou’ from my miniature rings. No problem with a tool post grinder or rings where “t” is over 30 thou’ but for really small rings, & with tools of dubious accuracy, I would prefer a safer method.
I have therefore taken the liberty of amending the Chaddock / Walshaw fixture to make the ring I/D minus 0.010” diameter into a spigot, fitting closely into a register machined in the clamping disc, an arrangement that I would find easier to make with the required accuracy, & I would also keep to Prof. Chaddock’s illustration & mount the rings singly.
The requirement for extreme accuracy when chucking the fixture could be overcome by making it the last object to be machined before machining the rings - & leave it in the chuck.
However my own preferred method would be to make the fixture from silver steel, harden it before use & (purists need read no further) take off the last thou” with a fine diamond file, using the fixture as a pair of filling buttons & with the lath running at about 200 rpm.
It would still require caution, these files would cut the very narrow ring very rapidly but with the much larger area of the fixture to act as both a witness & a check, it shouldn’t be too difficult to stop at exactly the right size.
Ok if you have kept up so far please check out the drawings on how to make the fixtures.
  Drawing 2 is the modified version as per the text:

[Steamer5]

Last part ......

Effects of the wedge.
The effect is shown in the figure below where the stress condition within the ring is shown.
 
      A 1” ring where t = 0.035”, b = 0.042”, g = 4t = 0.160”,
      Tangential Force @ the gap = 0.3125 lbf.
      Stress due to Ft = 14.3 lbf/sq.in.
      Max stress due to bending  = 24600 lb/sq.in.
      Mean wall pressure P = 10 lbf/sq.in.
 
See Attached Effects of wedge


Now for the graphs.
At (a) below is shown the situation in a perfect ring – I.e. without the compressive load due to the tangential force. The stress is tensile at one face & compressive at the other, with a “neutral axis” where the stress is zero at the centre section. These stresses rise from zero at the gap & increase proportionately to Sin(sqd) theta to a maximum at 180 degs, through only the first 20 degs are shown.
See Attached Graph 1 Stress in a Perfect ring


At (b) below is the situation with the compressive stress due to the wedging force added.
See attached graph 2, stress in Wedged ring


For the first 8 – 10 degs the stress is compressive only, right across the section– there is no “neutral axis” Thereafter the tensile stress  slowly increases, but the neutral axis lies well away from the geometric centre of the section, & even at 30 degs it is still some 6% of the half – thickness away. The result is that the ends of the ring will, when installed, bear more heavily at the gap & so exert a higher wall pressure in that region. Further, the reaction to this higher pressure at the ends causes a further high area about 120 degs away on either side.

Well thats it if you made it this far, well Done! I hope this is useful to those that make rings, it would be great if you do use this nethod to let us know how you get on!

[gbritnell]

Although I have read this treatise several times in the past I thank you for reposting it. I personally have only used this method once and had good luck with it. That being said I have used the Trimble method more times and have also had good luck with it. Not having the math or metallurgical background to prove or disprove any of the writings I have to take the author's writings as fact but I'm thinking that the distortion and expansion numbers he states are almost negligible in the sizes that we make. Add to this I have always used an anti-scaling powder when making my rings so the surface comes out as nice as before heat treating.
As with a lot of things in this hobby it seems everyone has their favorite way of doing things so the bottom line is whatever works for you then go for it.
Thanks again for posting.
gbritnell

[TerryWerm]

For the benefit of others, I have compiled the entire article into a single Word document, with drawings included. It is nine pages worth, as I chose a font that was a little larger for the reading pleasure of those that no longer read the fine print as easily as we used to.

[Steamer5]

Hi Terry,
 Thanks for that! Should of done it myself as I had to extract it from the various copies of the club mag that I had put it into, after I got it off the 3 1/2 discs, into a word doc to make sure I had most of it right! Then scan out the drawings! Luckily I still had the graphs!

Cheers Kerrin

PS hi George, glad that it is of use. As I said in part 1 I would hate to have this info lost to somebody who may find it solves a problem for them.