Piston Ring Drag

[daves1459]

I just finished the restoration of a Disney 4-4-0. The original cylinders are cast bronze with 2 1/8" bore making the loco very slippery. During the restoration I sleeved the cylinders down to 1 3/4" diameter using Boston solid cast bronze bearings pressed in place with .002" diametral interference and Loctite high temperature bearing retainer. The pistons are 304 stainless steel with .006" diametral clearance. The rings are standard phosphate coated cast iron, 1/8" wide, and 3/32 wall. The piston rod packing is virgin PTFE and the crosshead guides adjusted to .002" min. per side piston to bore clearance along the entire stroke.

My problem is the loco works very hard on grades. When the throttle is open wide the loco really barks. But, will only run slowly consuming a lot of steam. Close the throttle and the loco stops dead. Wait a minute and open the throttle and the loco moves several feet then begins to lug again. Crest a hill, close the throttle some and the loco is free and runs free and as normal as can be expected.

At first I thought the problem to simply be cylinder lubrication. So increased the feed a notch on the mechanical lubricator and primed the system with several turns on the ratchet. I'm sure the loco is lubricating as the oil level in the reservoir goes down as the loco is operated and there are no visible leaks in the oil feed lines.

A friend who was helping me ( Who knows absolutely nothing about steam engines.) suggested we block up the drivers and let the loco run to break in. Which we did and got some improvement on hills. But, the loco still lugs some as described.

I've made several steam engines with cast iron rings on bronze bores with similar materials and clearances with out this drag or lug problem. Any insight into what the problem may be and/or a solution will be welcome. One option is to simply let it wear in.

Dave

[Bill Shields]

off the top, I will say that your piston clearance is too tight.

that may not be the problem...but I would start there...

[Fender]

Suggest running the engine upgrade with compressed air rather than steam. If it runs ok on air, then the problem is likely differential expansion at steam temperature. When you say there is 0.006” clearance between the piston and cylinder, does this allow for the reduction of bore diameter from pressing the sleeve into the cylinder?

[daves1459]

Suggest running the engine upgrade with compressed air rather than steam. If it runs ok on air, then the problem is likely differential expansion at steam temperature. When you say there is 0.006” clearance between the piston and cylinder, does this allow for the reduction of bore diameter from pressing the sleeve into the cylinder?

Yes, the .006" clearance uses the bore of the sleeve after it is pressed into position.

[Bill Shields]

it does not have to be temperature differential drag 0.003 / side is very close....given ANY play in the crosshead or head mis-alignment, the pistons can (will) drag.

[FLSTEAM]

One other possibility is the ring grove is not deep enough i.e. not allowing the ring to fully compress.

John B.

[ALCOSTEAM]

Have you taken a cylinder cover off to see if there are spots of heavy wear or scraping on the liners. Did you finish the liners before you installed them or did you finish bore after they were installed. It does not take much interference fit to keep a liner in place and I have never had to use loctite to retain a liner. On a set of cylinders I am doing now they are 3.250 on the outside of the liners and I am only figuring having a .001 to .0015 i/f fit then will finish bore.

Something else to consider is with the larger cylinder bores you were getting more energy with less total steam pressure to the cylinder. Have you ever tried to move a slide valve by hand with even a bit of pressure on them, its amazing how much effort it takes. Now that you have reduced the piston surface area by 1/3 its going to take a bit more pressure to do the same work as before which is increasing the effort it takes to move the slide valve. Its quite possible that the original design took into account the piston surface area in relation to those factors.

You might have multiple factors at play causing your issues.

[Fred_V]

do you have enough gap in the rings? are the rings automobile rings? if so they will have way too much pressure against the bore. you can ease that some by grinding the ID of the ring to thin the wall down.

[rkcarguy]

I agree with Fred and Alcosteam, pull one of the cylinder covers and see what is going on. Could be just underpowered now with the reduction in bore, or it could be that the piston or ring is expanding too much with the full throttle load and corresponding heat, and is tightening up in the bore.
While .006" is tight on the piston to bore clearance it's a stainless piston and won't grow very much, I'm going to guess that the ring is getting hot and growing to the point its end gap closes up and its getting tight in the bore. If either part is dragging though, your new bronze bore is going to be short lived and should be looked at immediately before running it anymore.
The exact same thing happens in engines that people add turbochargers to, non-turbo rings are gapped tightly, and the extra heat from the increase in HP will expand the rings to the point the gap closes and then it breaks the ring land out of the top of the piston.

[Rwilliams]

Many times in this hobby, well intended tight clearances end up giving trouble. The locomotive or water pump will work just fine on air. As soon as steam is used to operate the locomotive or water pump, the performance goes downhill fast and often times stalling out is the end result. In one case of a pump being built to the highest standards but much too tight, clearance issues were the resulting problem. Everyone had a different idea and ultimate cure for the work of art pump. Once it was taken apart, examined for the wear and clearances reduced to allow for slight misalignment and steam heat expansion, it worked perfectly.

Conduct an air test and take a good look inside both of the cylinders to see what is going on.

Have also seen where steam leakage past worn rings would act as back pressure against the piston severely reducing the power output. Opening of the cylinder cocks will reduce the back pressure and help diagnose the problem in that case.

Since it is a new rebuild, things are probably a bit on the tight side. Might want to consider at least .010 clearance instead of .006 just to be on the safe side.

[daves1459]

Given what I've observed and the opinions it appears likely there Is a heat expansion issue some where in the cylinder. The next step is to remove the front cylinder covers and examine the bores for evidence. I'm laid up this week tendonitis in my right leg. But, will try to get to it next week and will provide a report. I'll include pictures if something significant shows up.

Thank you to you all for your input, Dave

[Doug_Edwards]

I'd check the piston rod packing first. PTFE has a high coefficient of expansion that will expand quickly enough to grab onto the piston rod in the time frame described.

Regards,

Doug