Design Part VI
The photo above shows the upper part of the valve linkage on Cass 6. The eccentrics and associated straps and links are off the bottom of the photo. The upper part of the valve linkage on MRSR 91 is identical to Cass 6. The modeling of the base for the rocker arm and tumbling arm base, the rocker arm, valve stem head, reversing link and link block are described here. The remaining parts of the valve linkage are described in the Heisler Engine Part VII and Part VIII pages.
The bearing surfaces for the valve linkage are shafts, pins and the sliding surface between the link block and the reversing link. Linkage wear introduces slack into the valve operation that can significantly reduce the efficiency of an engine, especially a scale model engine. To minimize wear, slightly over scale bearing surfaces are used and inexpensive standard sleeve bearing inserts are used on the shafts and pins. All bearing surfaces are equipped with lubrication fittings which hopefully will lead to minimum wear.
Rocker Arm Base Drawing: The drawing above shows the model rocker arm base. The piece is very complex --- a collection of many different shapes. For me, the easiest way to make a part like that is to make the individual shapes and then screw/silver solder the pieces together.
Reversing Link: The model reversing link is shown above. The link will be machined from mild steel or cast iron. The arcs are made using the rotary table on the mill. The first design had the length of the slot = 2.5". That proved to be longer than direct scale and longer than needed so the slot length was reduced to 2". The centerline is at a 3.75" radius. The link moves along the 3.75" radius arc as the link is shifted between forward and reverse. The movement is a total of about 1.2519" along this arc.
Reversing Link Block: The link block shown in the drawing above slides in the slot in the reversing link. The block rear plate screws to the link block clamping the link block in the slot in the reversing link. Both the block and the rear plate will be machined from bearing bronze. The link block pin runs through the link block, through the rocker arm and through the valve stem head. The pin essentially links the valve stem head to the reversing link. The pin is held in place by the setscrew in the rocker arm. The link pin will be made of mild steel. The pin head fits in a recess in the block to keep the front side of the block smooth so that it will side under the saddle ( saddle shown in the next page - Heisler Engine Part VII).
Rocker Arm Linkage: The drawing above shows the front view of the rocker arm linkage. The linkage between the rocker arm and the valve stem is shown on the left side. The reversing link with link block are pasted over the rocker arm and valve stem head on the right side. The key reference points on this view are the positions of the rocker arm pivot points relative to the crankshaft. .
The drawings above and below show the pieces of the rocker arm linkage pasted together. This was done to verify everything fits together correctly. The bearings and connection pins are shown in cross section to improve clarity on the top view above. The key reference points are the crankcase centerline and the valve stem centerline. (The rocker arm and valve stem head were drawn as if they were positioned horizontally in the top view to simplify the drawing construction. Both the rocker arm and valve stem head would appear shorter in an accurate drawing.)
The tumbling arm, lever and link hanger are described in the next part.