Heisler Engine Design Part VI 
Rocker Arm & Reversing Link
Nelson Riedel, Nelson@NelsonsLocomotive.com
11/15/2004, last updated 03/25/2005

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: This shows the Cass 6 rocker arm base as viewed from the right front side.   The curved rib links the upper part of the base which supports the rocker arms with the lower part of the base that supports the tumbling arm and tumbling lever.   The pipe sticking up through the hole in the rocker arm provides lubrication to the front main engine bearing.
This photo taken from the left rear shows how the base straddles the flanges between the two haves of the crankcase. The  model crankcase is cast as a single piece and there is a flat pad under the rocker arm base which should simplify the base casting design.
This view of the left side shows 2 of the 4 bolts which attach the base to the top of the crankcase.

Rocker Arm Base Drawing: The drawing above shows the model rocker arm base.  The initial plan was to make it a casting.  However, when I got around to making the patterns for this section I decided that the base could be fairly easily build up from pieces of various size steel or aluminum bar stock.  Since there is only one base per locomotive, the effort to make the mold now seems unwarranted.  I may change my mind after trying to make the base.  One simplification might be to split the base into two pieces, one for the rockers arms and the other for the tumbling arm.   

Rocker Arm:   This is a spare rocker arm they have at Cass that I was able to measure  in detail.

This is a photo of the right  valve stem head on Cass 6.   This particular valve stem head is an early design.  The left side is different as are the valve stem heads on MRSR 91.   The purpose of this photo is to show the set screw that clamps the shaft which runs through the valve stem head, the rocker arm and the link block.   The shaft is clamped to the rocker arm but is free to rotate within the valve stem head and the link block.

Rocker Arm Drawing:   The scale rocker arm is shown on the right.  The shape has been modified slightly to simplify machining of the pattern.  Note the use of the sleeve bearings on the base end of the arm.  The only machining required on the finished casting will be reaming of the holes, tapping the hole for the set screw and smoothing the ends of the cylinders that hold the shafts.
This shows the rocker arm pattern made from brass bar and rods silver soldered together. Soft solder was used to make small fillets--- sort of like the body lead your grandfather might have used on his auto.. Cores will be used in the holes so that they are accurately reproduced in the casting.

Valve Stem Head: This is a photo of the Cass 6 left valve stem head.   The valve stem heads on MRSR 91 appear to be identical. There are adjustment nuts on the threaded end of the valve stem that allow the position of the head on the stem to be adjusted.

Reversing Link: The model reversing link is shown in the drawing above.   The link will be cast in iron or steel and require a minimum of machining such as reaming the holes and smoothing the inside of the slot.  Note that 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.             

Making The Reversing Link Pattern: I thought I'd include some photos showing how I machined the reversing link.  This photo shows a rotary table mounted on the mill table.  The reversing link is being machined from a piece of 1/2" X 3" aluminum bar stock.    The pattern thickness is 00386" (0.375" + 3%) so the cuts were made about 0.425" deep, not all the way through the 1/2" block.
The notebook computer was setup next to the mill so I could easily obtain dimensions from the drawing.  The drawing had been scaled up by the 3% to allow for shrinkage.  This CAD program made it easy to account for the end mill diameter when machining edges.  It also made it easy to determine the rotary table angle settings.
This shows the block after all the edges had been machined.  The next stem was to mill the thickness down to 0.386" and then round the outside corners.
This is the finished pattern.  Cores will be used in the four holes.  The inside surface of the casting will require machining.  The plan is to mount the casting to a bar that is arranged to rotate under the milling head --- a technique used by Kozo Hiraoka in Building the Heisler.

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.