Cylinder to piston clearance

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Harold_V
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Re: Cylinder to piston clearance

Post by Harold_V »

gwrdriver wrote: Sun Mar 12, 2023 10:09 am 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.
Rings in a steam engine should NOT attempt to center the piston. If they must, the engine has not been built properly. Built as desired, the bushing in the head and the cross head are the components that center the piston in the bore. The harsh reality is that the rings should have enough clearance at the root diameter that they can NOT influence the piston.

H
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Bill Shields
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Re: Cylinder to piston clearance

Post by Bill Shields »

Amen....been saying that for a long time
Too many things going on to bother listing them.
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gwrdriver
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Re: Cylinder to piston clearance

Post by gwrdriver »

Harold_V wrote: Sun Mar 12, 2023 4:48 pmRings in a steam engine should NOT attempt to center the piston. [snip] rings should have enough clearance at the root diameter that they can NOT influence the piston. H
Re: my question . . I learned yesterday that it doesn't matter greatly if the ring can be pressed below the surface of the piston as long as there is depth clearance in the groove, and that clears a few things up.

In a conversation with an experienced builder in the UK, he learned years ago that properly fitted metal rings, of the type I have, used with adequate clearances, whether intended or not, will tend to center the piston by simply working the way they're designed. To escape confinement (or expand - however you want to state it) on the power stroke the steam will find its way into the ring groove where it will exert equalized pressure under the ring, expanding it against the bore, thus acting to help center the piston. His experiences, and what I saw in the Mfg's notes, were that side clearance is tightly specified, but groove clearance no so much (although within reason.)

No, of course this effect should never be expected to compensate for wear, mechanical defect, or poor workmanship. That goes without saying, usually.
GWRdriver
Nashville TN
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Harold_V
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Re: Cylinder to piston clearance

Post by Harold_V »

gwrdriver wrote: Mon Mar 13, 2023 12:13 am <snip> steam will find its way into the ring groove where it will exert equalized pressure under the ring, expanding it against the bore, thus acting to help center the piston.
Likely a small contributing factor to the position of the piston, but it should not be relied upon for centering, and, assuming a tight bushing and crosshead, it wouldn't alter the position of the piston. I agree that if there is abundant wear, that may not be true, but the forces of the properly aligned cross head and the proper fit of the rod in the bushing would readily overcome any small contribution made by the steam/ring/piston relationship.

Bottom line is proper workmanship is demanded. It's really quite important that the bushing and bore of the cylinder by concentric and the cross head in proper alignment (concentric and parallel) with the centerline of the cylinder. When those conditions are met, there is no need for the rings to attempt to center the piston. In such a case, wear on the components would be compensated by the piston's ability to move within the rings, which remain in the center of the cylinder.

H
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Bill Shields
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Re: Cylinder to piston clearance

Post by Bill Shields »

Amen 🙂

Ever seen the technique of extending the front end of the piston rod into a pocket in the cylinder head to be used as a guide bearing?

Most every large German loco I have seen uses this technique.

Some of the "guides" were also used as water pumps
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Bruce_Mowbray
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Re: Cylinder to piston clearance

Post by Bruce_Mowbray »

Additional $.02
Crosshead guides should be closely aligned with the piston bore, and piston rods should be almost dead parallel to the interface surfaces of the crosshead in order to prevent the piston from wandering from the center of the bore. Otherwice, the piston is likely to be centered on one end of the bore, and scuffing the bore at the opposite end.
I use .015 - .020 for piston clearance. Which isn't much. I try to keep the guides aligned within a thousandth or so aligned with the bore. This is typically done with height gauge and indicator with the frame and cylinders sitting on, or clamped to a surface plate or machine table. Shimming the guides is the way to get that final tweeking.
Bruce Mowbray
Springville & Southern RR
TMB Manufacturing & Locomotive Works
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