Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch
It may be helpful if we determine the names of the various parts of the valve gear and its operation. As the wheels rotate, a return crank, (mounted on the crankpin at 180º to the main crank) rotates. A vibrating lever moves with a circular motion at its lower end, where it is pivoted to the return crank and an up/down movement at its upper end where it is fixed to a die block which in turn is retained in a slider channel. A valve lever, pivoted on the vibrating lever thus moves backwards/forwards as the crank revolves. The movement of the valve rod is 180º opposed to movement of the piston. Tilting the slider imparts greater forwards/backwards motion to the valve lever and thus increases the valve travel. Reversing the tilt of the slider changes the direction of the engine. The slider is mounted on a weighshaft crossing the frames from one side to the other. The tilt of the slider is controlled by a reversing arm, connected to the reversing lever in the cab by a reach rod. The length of the vibrating lever is such that when the engine on one side is at full forward (or reverse) dead centre the die block is at the pivot point of the slide. Movement of the reversing lever at this position should produce no movement of the valve on this side of the loco.
Making the valve components.
1. Return Crank, Vibrating Lever and Valve Lever.
The return crank, vibrating lever and valve lever are fairly simple to make from mild steel of appropriate thickness. However, it is important that dimensions are equal on both sides, so all components must be made as a pair. Clamp two overlong pieces of mild steel strip together, mark and drill both components before separating. Try to get the dimensions as accurate as possible. File to shape and trim off any excess metal. The knuckles on the valve lever are best made from steel because of the increased wear compared to the knuckle on the valve rod. Use 10 BA hexagon head screws for the pivots. I also found it useful to mark all components Left or Right to ensure that a) they were kept together when fitted/dismantled, and b) they could also be mounted the correct way up each time. Silver steel pins are secured in the return crank with Loctite adhesive.
I made my slides from some brass channel that was near the size I wanted. This was trimmed to dimension and a cover strip silver-soldered to it. Mark the slides Left and Right. I then cut a slot along the length of this cover slip for the pin of the die block to slide within. I smoothed the inside of the resultant channel with square and triangular needle files to remove any excess solder and make a parallel sided channel. A 2 mm hole was drilled centrally in the rear of the channel. The pivot shank was made from some 3 mm brass rod, drilled and tapped 10 BA. A stub was turned to fit the hole in the slider, and the whole silver soldered together taking care to ensure the shank was perpendicular to the slide.
3. Die Block
The die block was made from gunmetal. A piece was sawn to slightly over the internal dimensions of the slide, then carefully filed to fit closely but slide freely, with as little shake as possible within the slide. This was probably the trickiest part of the job. I eventually made 4 die blocks before I was happy with the fit. After drilling and tapping 10 BA, a silver steel pin was secured with Loctite. Check the fit of the die block in the slide and mark the blocks (at the top end) Left and Right.
4. Trunnion Blocks
The trunnion blocks for the weighshaft are fabricated from brass, silver soldered together. They are mounted on pieces of angle secured to the frames. Note the position carefully. The engines are inclined at an angle of 5º to the frames. It is important that the weighshaft centre is similarly inclined in respect of a perpendicular from the driving axle. The weighshaft is made from 3/32" (or preferably 7/64") silver steel. The reversing arm is made from 1.5 mm mild steel. It may be convenient to make this 3 mm longer than the drawing and to drill three holes for the upper pivot point so as to allow some adjustment in the pivot travel should this later be appropriate. The sliders and the reversing arm are all silver soldered to the weighshaft ensuring that they are all in the same plane. (NB. Place the trunnions on the weighshaft before silver soldering the other components in place.
5. Reversing Lever Assembly.
The stand of the reversing lever is made from 1.5 mm mild steel, cut to shape. An arc of mild steel is filed to the same radius by clamping the two parts together. Before separating them, drill 1.4 mm holes through both pieces. Open out the holes in the arc to 1.8 mm, and thread all holes in the stand 10 BA.
7. The Reversing Lever.
The lever is made from a piece of 3 mm (1/8") square steel. I find it easiest to work with a piece a few inches long, do all the shaping and drilling before cutting to finished length. Mount this in the chuck and turn the handle portion. (Tip. To grip a piece of square bar in a 3-jaw chuck, drill a hole through a piece of scrap material which is just large enough for the bar to fit through. Mark the position of number one jaw on the piece. Remove from the chuck and saw along the length into the central hole. Insert the bar, replace in the chuck and tighten.) Mark the position of the holes for the central pivot and the reach rod pivot. Drill and tap the lower hole 10 BA and drill the central one 1/16". A 1.05 mm hole, tapped 12 BA is required for the screw to attach the spring. Wind the spring from 30 Gauge phosphor bronze wire. (To wind the spring, take about 6 inches of wire. Clamp a 4" length of 1/16" silver steel in the chuck just nipping about 1/2" or wire with it. Loosely grip the distal end of the silver steel in the tailstock chuck so that it is free to rotate. Pull the other end of the wire with pliers and slowly turn the chuck by hand. Feed the wire along at a pitch of about 1/32". When you have about 3/8" of spring, remove from the chuck. Cut a tail about 1" from the spring turnings. With practice you can make springs for all sorts of applications quite quickly by hand.)
The latch is sawn and filed from a piece of 6 mm (1/4") mild steel. A 0.5 mm hole is drilled for the spring, and a 1 mm hole for the pivot. The pivot is made from a short length of 1 mm silver steel. The ends are riveted over to retain the latch on the lever/
Two spacers are required between the stand and the arc piece. These can be made from 3 mm brass or steel just sufficient in length to prevent the lever binding in the frame. The pivots are made from 1/16" silver steel, threaded 10 BA as appropriate.
Assemble the stand, place the lever in mid (vertical) position, mark and file a notch for the latch but leave the other notches until you have had a trial fitting to the loco and noted the amount of travel required. Too little and the wheels will not turn, too much and the valve gear will bind or lock-up completely.
If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.
Copyright © 1998, Keith Bucklitch
Last Edited - November 1998