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Check
Valve Designs I'm into the design of the a 1.6" scale model of the 8 1/2" Westinghouse Cross Compound Air Compressor for the Heisler. The plan is to use the compressor for air brakes. One of the design challenges is to make very small high quality check valves; a total of 6 are required. A second project is the lubricator for the Heisler. Bad experience with the check valve on the Shay lubricator has also stirred interest in high quality check valves Piston Check Valves from McMaster-Carr: The first thought was to try to cut down some small check valves sold by McMaster-Carr. These valves cost less than $10 each and are very reliable. After examining one of the valves it looked like machining the valve to a smaller size was not practical. The next step was to try to figure out how to disassemble the valve. Close examination revealed that the piston is held in place by a small retaining ring. I took out the ring and found a really interesting design that I could easily reproduce. I also found that I might be able to cut down the valve cylinder and use it in the air compressor. The nice part is that after the piston and O-Ring have been removed the cylinder can be silver soldered to the compressor heads -- and then the piston and O-Ring installed after everything cools. The particular valve I disassembled is the McMaster-Carr # 7768K15 Brass Spring Loaded Piston Check Valve with the following characteristics:
The check valves are available with male or female ends or a combination and in 1/4", 3/8" and 1/2" NPT sizes in addition to the 1/8" size examined here. My guess is that the 180 degree temperature specification is for the Buna-N O-Ring. The Viton O-Ring will probably work at a much higher temperature, probably about 400 degrees.
The photo above shows the first step in disassembling the unit. The direction of flow is left to right as shown by the arrows on the side of the cylinder. The retaining ring and piston were extracted through the input side (left side in photo).
This photo shows the piston with the spring off.
And the piston with the O-Ring off.
The sketch above shows how the piston is positioned when the valve is closed and also when it is open. The retaining ring is the stop for the piston when closed. The narrow section of the cylinder keeps the spring and piston from falling (or flying) out the output side. As mentioned previously, I think the standard valves could be cut down for the air compressor application. However, there might be other cases where I want to make a similar valve from scratch so thought it best to document the piston dimensions, especially the groove.
The cylinder diameter around the O-Ring when the valve is closed is 5/32" (0.156"). The O-Ring appears to be size 002 which has a 3/64" nominal ID, 9/64" nominal OD and a 0.050" cross section. Both the piston groove OD (0.072") the cylinder ID (0.0156") are greater than the nominal dimensions of the 002 O-Ring. It's difficult to measure the O-Ring OD & ID but I could get a pretty good measure of the cross section and the 0.050" seems to be accurate and the 002 is the only standard O-Ring with that cross section the next size smaller is 0.040" and the next size larger is 0.60". The space between the groove and cylinder is 0.156"-0.072" = 0.084" or 0.042" on each side. That gives a 0.008" squeeze on the O-Ring if the stretching of the O-Ring is ignored. The actual squeeze is probably about 0.005" which seems ideal for this application. The spring is ~ 0.33" free length with ~ 0.205 OD. Wire diameter is 0.008" and there are 10 turns. McMaster doesn't seem to carry an exact match They do carry 0.18" OD stainless steel springs with 0.016" wire size. Maybe one could use the 9/16" length of that spring and cut it in half. The spring would be a little stiffer and the cracking pressure would be a little higher but it should work. MCV-1 series Poppet Check Valves from Clippard: I used the Clippard MJCV-1 series poppet check valves with 1/8" NPT ends on the Heisler axel pump. They have smaller MCV-1 series poppet valves with 10-32 ends that looked like they might be a better choice than a piston check valve for the compressor and the lubricator. They are rated for 300 psi and come with male or female ends as well as male one end and female the other. They are available with either Buna-N or Viton O-Rings and cost a little over $4 each. The valve with the shortest length is the one with two female ends. I purchases some of the MCV-1BB valves which has 2 female ends and a Buna-N O-Ring. The next step was of course to take one apart.
After tearing apart the MCV-1 valve I assumed the MCV-2 valve was also held together with sealer. I griped the round end very firmly in the lathe 3-jaw check and used a wrench to turn the hex end. Once the seal broke it was fairly easy to pull the two pieces apart. Photo above shows the three pieces of the MCV-2 valve. The duckbill seal fits into the one end as shown in the photo below.
I may use the MCV-2 in the lubricating oil lines on the lubricator since the output (round) end can easily be threaded 5/15-24 for a 1/8" tube compression fitting. However, I don't think I'll use it for steam oil lines that feed into high pressure steam lines; I don't want to risk the mess should the check valve leak and allow steam back into the lubricator. After seeing all these parts I may never make another check valve.
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