Heisler Engine Design Part VI 
Rocker Arm & Reversing Link
Nelson Riedel, Nelson@NelsonsLocomotive.com
11/15/2004, last updated 09/12/2009

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.  For simplicity I cut the flanges off and milled the area flat under the rocker arm base.   That simplified the rocker arm base somewhat.   
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 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.

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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 :   The model 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.  I find it easiest to make this type part in three pieces - the cylinder for the setscrew, the 3/4" diameter cylinder, and the remainder.   The final step is to silver solder the three pieces together.   This is one of the parts that was successfully cast using the investment casting process.   It appears that additional parts will not be available so we might as well get used to fabricating the part.  
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.    

    

Valve Stem Head Drawing:  This drawing shows the design of the model valve stem head that was cast in steel.   The design is somewhat different than the prototype to simply making the pattern and to make it easier to adjust the valves.   The only machining required on the casting will be reaming the holes and smoothing the ends of the cylinders.   

The HS-105  flanged sleeve bearings are 5/16" OD, 3/16" ID and 1/2" long.  The length will have to be cut to 5/16" and the inside bored to 1/4" before pressing into the head.  Initially I'd planned to use  the standard 1/4" ID 3/8" OD bearing but decided that would make the casting wall thickness too thin.   

Unfortunately, additional castings are not available so the design was revised as shown next to make it easier to fabricate.

This valve stem head is functionally identical to the one above and can be easily machined from a 1/2" thick piece of steel.   Once one has this design made, it is easy to round the corners using a rotary table and the mill to make the the version shown above.           

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.

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