Cylinder to piston clearance

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: cbrew, Harold_V

JohnR
Posts: 59
Joined: Wed Feb 19, 2003 8:08 pm
Location: Northern Ohio USA

Cylinder to piston clearance

Post by JohnR »

Is there a formula or rule of thumb for establishing clearance between
cylinder wall and piston.
Bore of cylinder is 1.500/1.499.
I am going to hone out .

Thank you
john r
User avatar
Bill Shields
Posts: 10460
Joined: Fri Dec 21, 2007 4:57 am
Location: 39.367, -75.765
Contact:

Re: Cylinder to piston clearance

Post by Bill Shields »

It depends on the material of construction, which controls temperature growth when steam hits it.

I just assume that at some point in the running process (with steam) that the piston will be at steam temperature and the cylinders will be exactly the size they are bored - which is worst case scenario and is where it will stick if it is going go.

you also have to think about wear on the crosshead causing the piston to rock on the packing as it slides back and forth, which is cause to again increase the clearance (irrespective of the temperature).

my loco with cylinders of that size has .020 diametrical clearance and it runs just fine with cast iron rings.

I have known locos of similar size to run just fine with 1/2 of that clearance.
Too many things going on to bother listing them.
blff cty lcmtv wrks
Posts: 95
Joined: Sun Mar 10, 2013 11:56 am

Re: Cylinder to piston clearance

Post by blff cty lcmtv wrks »

i once built a two cylinder stationary steam engine using aluminum for the pistons and cast iron for the cylinders. i machined the pistons to what i thought was a good running clearance. it ran great on air, but then we hooked it up to steam, it ran for about 20 seconds and then stopped real fast. once it cooled down, it turned freely. we did it again. same results. i turned some clearance on the pistons. it worked great on steam, but would barely run on air. it made all kinds of funny whistling sounds when it was run on air.
User avatar
SteveR
Posts: 372
Joined: Thu Jun 01, 2006 4:17 pm
Location: Los Angeles

Re: Cylinder to piston clearance

Post by SteveR »

My 1-1/2" Allen Consolidation specifies 2mil difference between the cast iron bore and the bronze piston OD (2"). Cast iron rings.

My 1" LE 060 specifies the cylinder bore at 1.500+.002/-.000 and the piston diameter at 1.500+.000/-.005. Both are CI. I forget what the ring material is.
SteveR
12x36 Enco Lathe, 9x42 Bridgeport, SMAW, O/A, Miller MIG w/gas, plasma
Not enough measuring tools...
1.5" Allen Models Consolidation on air.
1" FEF in progress
1" & 3/4" LE Projects
Measure twice, cut once, wait - it was supposed to be brass! :)
10 Wheeler Rob
Posts: 1546
Joined: Tue Mar 21, 2006 2:44 pm
Location: East Hartford, CT

Re: Cylinder to piston clearance

Post by 10 Wheeler Rob »

My Allen 10 wheeler called for 0.005" clearance for cast iron cylinders and bronze (red brass?) pistons.
Soot n' Cinders
Posts: 983
Joined: Wed Jun 27, 2012 5:24 pm
Location: Marietta, Georgia

Re: Cylinder to piston clearance

Post by Soot n' Cinders »

I think I did something like 0.02-0.03in clearance. The piston ring does the sealing anyway and I figured a bit of extra clearance would help keep the piston away from the bore.
-Tristan

Projects
-2.5" scale Class A 20 Ton Shay

Steam Siphon: https://www.shapeways.com/shops/leavitt ... tive-works
User avatar
Bill Shields
Posts: 10460
Joined: Fri Dec 21, 2007 4:57 am
Location: 39.367, -75.765
Contact:

Re: Cylinder to piston clearance

Post by Bill Shields »

Soot n' Cinders wrote: Tue May 10, 2022 6:45 pm I think I did something like 0.02-0.03in clearance. The piston ring does the sealing anyway and I figured a bit of extra clearance would help keep the piston away from the bore.
:D
Too many things going on to bother listing them.
User avatar
gwrdriver
Posts: 3439
Joined: Mon Jan 06, 2003 10:31 am
Location: Nashville Tennessee

Re: Cylinder to piston clearance

Post by gwrdriver »

Here's another question to add to this thread . . . what are you using for clearance (float?) between the ring ID and bottom of the ring groove? I can't recall what I've used in the past, and I no longer have the drawings anyway. The pistons in this case are 1.625" OD cast iron, w/cast iron rings. Based on previous answers I'm sure there'll be a good deal of variation but that's OK, that will at least define a range.
GWRdriver
Nashville TN
James Powell
Posts: 504
Joined: Fri Oct 28, 2011 10:42 pm

Re: Cylinder to piston clearance

Post by James Powell »

It depends on what you are using for rings. If you are using something hard (like, for example, cast iron...or PTFE, or...) then a fairly large clearance will work fine. If you are using soft packing, then a rather much tighter clearance is needed.

Remember that in the smaller sizes, it gets much harder to get rings which do an appropriate job of sealing without stealing too much power. It's entirely possible to have pistons (and piston valves) which will stick if the steam temperature gets too high- my Caribou suffers this with the piston valves. They are just about perfect at 1/2-2/3rds a glass of water, but get a bit less happy as the water level goes below 1/2 glass, or if I use the 600 w oil instead of the 1200 w oil...

We're probably into the tenth's department here for "too tight" to "too loose", as the piston valves in question are brass in CI sleeves. Problem happens after 1 1/2 mile or more of running...
David Powell
Posts: 523
Joined: Thu Mar 13, 2003 10:38 pm
Location: Pickering Ontario Canada.

Re: Cylinder to piston clearance

Post by David Powell »

Those piston valves were made to fit , I heated the cylinders to the temperature of steam at 150 lbs pressure, measured the lapped liner bores and made the valves from a piece of some special bronze alloy which is as hard as H---- that I bought at my local metal merchants from their oddments bin. I seem to remember that i allowed just one thous total clearance with both valves and cylinders heated.
James has been trying to wear them out for 15 yrs or so and has not succeeded yet!.
Regards David Powell
User avatar
gwrdriver
Posts: 3439
Joined: Mon Jan 06, 2003 10:31 am
Location: Nashville Tennessee

Re: Cylinder to piston clearance

Post by gwrdriver »

I've just been sent a photo of a note from the Mfg of my rings (Clupet, UK), and for a ring identical to mine they quote a .071"-.074" deep groove for a 1/16" thick ring, which is gives an .009"-.012" clearance. That seems quite a lot to me.

I can't judge whether this is too much or too little, or what it does for the ring, but it would seem to me that if the groove allows the ring OD to become flush with the piston OD, then the ring's ability to center the piston or keep it from contacting the bore, will be lost. To accomplish that it appears to me that whatever clearance is allowed in the ring groove should should be less, by a few thou', than the piston to bore clearance.
GWRdriver
Nashville TN
Rwilliams
Posts: 1048
Joined: Thu Dec 22, 2005 2:45 pm
Location: Central California

Re: Cylinder to piston clearance

Post by Rwilliams »

Plenty of good discussion here in regards to fitting of piston rings. But, what is the ideal end gap for the rings when used with steam?
Post Reply