The Steamboating Forum

Hi all,

I am having a duplex feed pump rebuilt. The water end has a 1 1/4" bore with brass pistions with water grooves. What clearance would you suggest be held for the new pistons to the water end bore.

Thank you

Since the pistons operate at near ambient temperature, I would think a sliding fit or a few tenths (i.e. .0002")clearance should be adequate.

-Ron

With no seal material, such as a leather cup seal or synthetic seal, then clearance must be very small, as Ron suggests.

My reciprocating pumps have at 0.003-0.002 inches diameter clearance between piston and cylinder bore, and are fitted with common rubber "O" rings. Two O-rings, with waterproof grease between the two rings keeps the O-rings in good condition. I have a 1.375 bore x 6 inch stroke feedwater hand pump, all bronze/brass, that has not been opened up for 5 years now, and never a drop of water leak past the O-rings.

I think here the intention is to use labyrinth grooves (water grooves?). These do not wear out, and whilst needing fairly minimal clearance, are not too critical as regards this. I built a pump some years ago that wouldn't pump well at all, due to poor boring of the cylinder. As soon as labyrinth grooves were added to the piston, it pumped more than adequately to boiler pressure (100 psi). They have been used in many places over the years the most notable in my world was in S.U. carburettors.

I am looking for a way to repair the pump. The original did not have any seals of the water end pistons. From what I have gathered I have three options. 1) keep it original and make a new pistons with tight tolerances 2) have a piston fitted with rubber o rings or 3) have a piston that can take graphite packing. I may go the rubber o ring route as was suggested. Looking to see what other people have done. It has better than 0.012" slop right now, not making it very useful as a feed pump.

Alex

A plain piston in a plain bore will work well enough when new. Unfortunately the wear rate will be fairly rapid. Piston rings work because they wear, not the piston, the same with "O" rings. With labyrinth grooves the turbulence created within the grooves not only stops the fluid passing but helps keep the piston from touching the walls and so reduces wear to a minimum, ensuring a long service life.

Machining labyrinth grooves requires nothing special. No sizes seem to be critical, nor do shapes, apart from sharpish edges. A minimum of three seems to be the norm, though I have never seen more than four. The distances between them varies but double the groove width works well.

Removed due to double posting.

I would go with your second option. New pistons with O-rings. And 2 or 3 thou clearance and about 10 - 15 thousandths crush on the O-ring. Polish the bore well and use an O-ring with about 75 durometer. Neoprene is fine. They have served me well too.

They have X-rings too for hydraulic service, I heard they work even better.

-Ron