ModelWorks Pride of Penrhyn Overtype Steam Lorry

Pistons


Lighting Up

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3rd Oct:  Since the 1st of September I have steamed POP a couple of times.  As the last seaming session was coming to an end I noticed that POP would not work in reverse eventually the valve gear jammed.

On returning home I quickly tracked the problem to lose bolts on the valve guide bracket. Since I was not sure what damage had been done I thought it was a good time to check the valve timing .  I also hoped that I could find out why despite my modification to the regulator valve POP would not run slowly.

Whilst checking over the valve gear using compressed air I noticed that when I held the crank stationary air was hissing through the engine.  At first I though this was caused by irregularities on the valve ports allowing air to pass from inlet to exhaust.  I stripped off the valve chests and improved the surface finish using the same method I had previously used on the Burrell.  On reassembly things were no better but I was now sure there was a serious problem.  The final clue came when I blocked off the right hand cylinder by stopping up the passage from the regulator to the right hand valve chest.  I found that at two points of the stroke the hissing stopped.  The penny dropped: air was escaping round the piston.

After major disassembly I got a piston out and all was revealed:-

The 'O' ring instead of being round was 'D' shaped, with not enough material left to make a good seal.

Oct 5th: I received a couple of replacement rings from ModelWorks today, thanks Ian.  After fitting them I realised how worn the old ones were.

With the old rings fitted their diameter was 1.232, that's 28 though less than the bore.  With the new fitted the diameter increases to between 1.270 to1.280,  ie 10 to 20 though greater than the bore. (The POP bore is 32mm or 1.260 inches)  

When inserted into the POP bore each chord will be compressed or pinched by between 5 to 10 thou. I am concerned that this pinch is too much so I consulted my model engineer's handbook to find out the recommended geometry.  I deduce, based on a 3/32 chord , that a pinch of between 5 to 10 thou is in order depending whether Model Engineering or SAE recommendations are followed.  I conclude that the pinch is in the correct range and is therefore not the source of the problem.

More significantly the handbook goes on to mention that surface finish must be the highest attainable or rapid wear will result.  The finish on the bore is not good so before refitting the new rings, I need to find a way to improve the surface finish.  I also plan to wash out the cylinder block to ensure that no grit remains from the emery paper I used to polish the valve ports.  I hope this will prevent a reoccurrence.

(31/10/04) Surface finish is important and rust may have formed inside the cylinder causing a poor finish.  Cast iron rings are more able to cope and are a better combination with the POP's cast cylinders. When dismantled there was a lot of orange gunge the piston so perhaps this is what did in the original "O" rings..

October 16th: I have a nagging doubt about "O" rings so I investigated the possibility of making new pistons complete with cast iron rings.  I found two useful web pages that tell how to make piston rings. Feeney Construction Log Page 8: Piston Rings and  Making Piston Rings I used a hybrid of the techniques described in these two articles.

I acquired a length of continuous cast iron bar from my local supplier.  Making the pistons was a simple turning job. After parting off I fitted the piston to the piston rod and holding the piston rod in a collet, finished the piston a tad under size.    With the easy bit complete the hard bit was to make a set of piston rings.  Before reading the articles I assumed that all I would have to do is make them oversize - wrong.
The rings need to be perfectly round when compressed by the bore.   If they were turned oversize the ends would turn in when compressed.  The technique is to make the rings dead on bore size, distort them, fixing them in the distorted shape by heat treatment. Using the same cast iron bar, I turned and finished it to exactly the bore of the POP.  I bored this out to leave a wall thickness of about 50 thou.  I then parted off a number of rings using a slim parting tool.
I made the seven rings to allow for the unforeseen.  I expected it to be difficult to cut the rings. I tried various method, all successful. The easiest and least stressful method was using bolt croppers.
The next task is to prise the rings open using a spacer of carefully chosen dimensions (see articles).  This distorts the rings oval such that when compressed by the bore they become circular again. This distortion needs to be set into the rings by heat treatment for which a jig is needed to hold them flat and secure.
The rings have to be heated to somewhere around 550 at which temperature a lot of scale will form.  To prevent this scale forming on the rings air has to be excluded.   The first article mentions Borax, something I don't have. However the second article suggest winding with steal wire and sealing with soap. I wound two layers of MIG welding wire over the rings and sealed with household soap. I just had to hope that there was not something "extra" in either of them!
I adjourned outside, and heated the lump, maintaining the temperature for about 15 minutes. I left the package to cool naturally before opening it up to discover all was well.  There were one or two marks on the rings but no serious scale.
The next task was to polish and gap the rings. I made a jig out of the last piece of cast iron bar to hold the ring. This enabled me to polish the side faces of the rings on a sheet of emery paper resting on a flat surface. As the jig compressed the rings to the POP's bore I was also able to use it to access the gap in the rings.
I careful filled the gap in the rings, opening them up to a few thou (measured when compress in the jig).  During this step seven rings became six - I was not careful enough! The picture right should the result of my days work.
To aid fitting I made a ring to compress the rings to bore size.  This had a bell mouth and a large slot to enable removal. In the picture right the piston is about to enter the bore.  The ring compressing the piston rings can be seen around the piston. It went in a dream and I could immediately feel that all was well.  There was much less friction and signs of a good seal.
I prefer to fit the piston, truck guide and connecting rod as an assembly directly to the engine. I do not remove the boiler or block from the engine. While this saves a lot of time it makes doing up the bolts tricky.  I have an 1/4 inch 8mm socket that I have turned down reserved for the purpose. The picture right shows the 8mm socket at the end of an extension tackling to most tricky bolt.

The new rings are excellent but I will never know how long the replacement "O" rings would have lasted in a clean cylinder.

(31/10/04) I have recently brought a copy of Scale Model Traction Engine Design and Construction by Edward George (see). On page 136, at the start of the cylinder chapter, It warns against using "O" rings in cast iron cylinders.  Apparently if (when!) rust forms in the cylinder it is likely to shred the "O" ring.  When I opened up my POP there were signs of rust in the cylinder - I wonder if this is what happened?

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