Beginner's Locomotive Safety Valve

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Beginner's Locomotive Safety Valve

by Cliff Blackstaffe

Victoria Society of Model Engineers

The Miniature Locomotive, November-December 1954

CliffBlackstaffe SafetyValve.jpg

Boiler fittings consist of water gauge, safety valve, blower valve, injector valve, check valve and blow down valve. We may as well start with the safety valve as it is a plain turning job. No special tools are needed in this design. The seat is removable which allows an easier close examination. The valve and stem are all turned in one piece and the spring is bronze.

The body is turned from 3/4 inch brass rod. Chuck, face, turn down 3/8 inch of it to 1/2 inch diameter and thread 20 threads to fit boiler bush. Center, drill 9/32 inch to 1-1/4 inch deep, top 1/2 inch deep with 5/16" x 24 thread. Part off 1-1/8 inch long. Chuck an S.A.E. nut or make a tapped bush to take the body and turn down 3/8 inch of it to 1/2 inch diameter and run the cap in for 1/2 inch at the top end for the pressure screw. File two flats on the lower part to take a 9/16 inch spanner. The seat is from 3/8 inch brass. Chuck, face, turn down 1/4 inch of it to 5/16 inch diameter and thread 24 to fit body. Center, drill 1/16 inch for 1/2 inch depth, open out to 1/8 inch for 1/4 inch depth and open out again to 3/16 inch for 1/8 inch depth squaring the bottom with a D bit or boring tool.

The pressure screw is a 1/8 inch thick slice of 5/16 inch brass which has first been threaded 24 to fit body and drilled 7/64 inch to clear valve stem. Cut two notches one each side to allow steam to escape. By a two prong tube wrench the pressure is adjusted. The spring is would from No. 20 bronze spring wire, standards wire gauge (.032 inch).

The valve is turned from a bit of 3/16 inch bronze welding rod with a half round ball end for the valve, above which is a little piston portion which should enter the 3/16 inch enlargement in the seat to a depth of about 3/64 inch and should be about 1/64 inch loose in the hole. The rest of the stem is turned down to 3/32 inch. Turn another ball ended rod to lap seat with. By eye is close enough to form the ball ends.

Now with some fine valve grinding compound on the lap and holding the seat in the fingers lap the seat with gentle pressure and allow the seat to wobble around. This will give a true spherical seat. Now assemble the valve, all but the spring, and give the valve a light grind. Wash off and give a final rub with metal polish and the job is done. The setting is done by pumping air into the boiler. Set at 90 pounds. If it lifts and doesn't close soon enough, chuck valve by stem and file a trifle off the piston. if it blows but won't pop, piston is too small. The reason for the 1/16 inch hole is to prevent priming or high water, as a 1/16 inch hole will let the steam away fast enough away.

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