Boiler Backhead Fittings
The machining of the various boiler related castings was started and some of the boiler fittings fabricated while waiting for the boiler to arrive. The fittings for the backhead are the water gauge, the firebox door and the throttle lever. A water gauge had already been purchased from Coles so the only water gauge question was the best location for the lower connection. The firebox door castings had been purchased from Kenneth so all that was required there was some minor machining. The challenge was to fabricate a lever that resembled a real shay throttle lever.
The shay throttle lever is similar to other throttles in that a toothed sector plate and paw on the lever are used to hold the throttle position. On some locomotives the sector plate is held stationary similar to the reversing gear lever (see Reversing Gear page). However, on the shay throttle lever, the sector plate is part of the throttle rod head and moves with the throttle rod ---- to the rear and the front as the throttle is opened and closed. The next photo shows this arrangement on Cass No 5.
The throttle lever is the slightly inclined long bar. There are two brake valves, steam on the left and air on the right. The top of the reverse gear lever is between the two brake valves.
The next photo, also of Cass No 5 shows a close up of the throttle rod head with sector plate and the pivot link to the left of the head. The head and rod have a matched taper fit that is held by a taper pin. The lever rotates around a pivot pin in the head. The link straps pivot at both ends so that they can rotate in the same plane as the lever. Note the tape measure from the end of the throttle lever ~ 44 inches to the pivot point at the rod head.
This next graphic is part of a Lima Throttle Lever Drawing.
The last piece of data is that the throttle valve specified by Kenneth is a common ball valve that rotates 90 degrees and has a one-inch lever handle that connects to the throttle rod. The rod will move 1.4" to rotate the valve handle 90 degrees. From the drawing above one can see that the lever rotates about 45 degrees from off to full throttle. This leads to the first conclusion --- the distance between the rod head pin and the link pin on the throttle lever must be 2 inches (twice the valve lever length) to make the throttle lever rotate 45 degrees (from off to maximum throttle) as the valve handle rotates 90 degrees.
The next step was to cut a cardboard lever and try various sector radii. The radius on the drawing above is 3.95" that would scale to 1/2". That radius is just too small to have a reasonable number of usable teeth. Based on the teeth from the reversing lever (0.64" wide and 0.070" long) it looked like 0.032" wide teeth 0.040" long were doable and should be usable and a 1.5" radius would not look too out of scale. A bit of calculation yields 3.5 degrees per tooth for cutting teeth using a 0.032" slitting saw on a 1.5" radius disk.
This next photo shows completed parts of the throttle lever. The pivot point on the upper left is made from 5/16" hex brass and threaded 1/16" NPT (5/16" MTP). The little links are 1/16" thick steel plate. The throttle rod gland is made from a 3/16" compression to 1/4" NPT brass fitting. The compression part was cut off and a solid brass rod was silver solder into the 1/4" NPT side. The inside was then drilled #12 as specified by Kenneth. The packing area was drilled 5/16" with an allowance for 1/8" thick packing. The little white piece is a Teflon packing piece. If that doesn't work, O-Rings will be the next option. The little disk rather than a nut is used to compress the packing, matching the prototype. The studs are 2-56 stainless.
The three partially threaded screws are 5-40 made from 3/16" hex stainless stock. They will be retained with locking nuts. The photo shows the underside of the rod head, lever and paw. The paw is attached to the lifting block by a couple 1-72 button head screws. The lifting rod is 1/16" brass threaded 0-80 on each end. The pivot on the trigger are more of the 1-72 button head screws as are the screws in the spring retaining block. The spring is out of the scrap box and too large diameter. A 1/8" OD spring will be secured on the next McMaster order.
The next photo shows the top view of the assemble lever. It works slick!
A photo of the assembled backhead is in the Plumbing VIII page.