Heisler Engine Design Part I
Crankshaft, Eccentrics and Main Bearings

Nelson Riedel, Nelson@NelsonsLocomotive.com
10/6/2004, last updated

 It was a tossup whether to start on the trucks or the engine.  However, as with the Shay, I want to get something moving as soon as possible so decided to start with the engine    This page describes the start of the design of the  model engine and covers the design of the lower engine components.

Update: This page was updated after the engine was successfully fabricated in the fall of 2009.  The updates include showing that most the parts are machined from bar stock rather than cast as planned earlier.  The changes were so massive that all the parts were renumbered.       

The first decision was to use 1.6" scale.  The local folks all use 7.5" gauge and the 1.6" scale is a little closer match than 1.5" scale.    (Bigger is better as long as I can get it in my small SUV.)   The scale factor is 1.6"/12" or  0.13333..........  which I rounded to 0.134.   It's possible to scale all the full size drawings directly in  TurboCAD.  That is what I plan to do and then adjust most dimensions to match available stock or common inch dimensions such as multiples of 1/16 inch.   

The next step was to take a look at the engine parameters and compare to the Schroeder Shay with which I'm experienced.  

Parameter MRSR 91 MRSR 91 Scale Schroeder Shay Heisler Model
# Cylinders 2 2 3 2
Cylinder Diameter 18.25" 2.45" 1.375"  2.125
Stroke 16" 2.14" 1.5"  2
Gear Ratio 2:1 2:1 2:1 2:1
Wheel Diameter 40" 5.36" 4" 5.25 
Working Pressure 200 26.6 100 100
Tractive Force  lbs @ Working Pressure 45,296 108 197 292
Minimum Starting Tractive Force (lbs)     133 190

The tractive forces shown in the table were computed using a web page that is no longer available.

The Heisler minimum starting tractive force was computed using the same technique as done for the Shay at Shay Tractive Force & Power .   

The graph above shows the normalized starting tractive force as the crank rotates for the Heisler. Note the expected two peaks, one from each piston.  The force ranges from a normalized level of  0.7 to 1.4 as the crank angle changes. The minimum starting tractive force can be calculated using the equation below.

Tractive Force (min) =  0.7 X (Boiler Pressure) X ( Piston Area) X (Stroke) x (Gear Ratio) / (Wheel Diameter)     

This calculates to 190 lbs.

The  Schroeder Shay has plenty of tractive force ----- it can pull an amazing load.  The major limitation is the ability to generate steam     The Heisler at 90 tons is about 50% bigger than the Shay so I want about 50% more tractive force.  Any additional tractive force beyond a 50% increase would likely be of little value since I'll probably not be able to generate enough steam to make use of it.  

Note the that 190 lbs min starting tractive force for the Heisler is only 43% greater than the 133 lbs starting tractive force for the Shay. However, the Heisler marketing information says the Heisler is more efficient since the drive train has half as many gears and bearings.  If that is true, the friction should be less and maybe I'll get a net increase of 50%.  

It's impossible to scale the full sized locomotive exactly because of minimum thickness of walls, etc.   The engine bore and stroke shown on the Heisler Model column seem to be a good compromise giving sufficient wall thickness and a the 40%+ increase in tractive force over the Shay.

Part Numbering:  After completing the engine design I decided that an organized part numbering scheme would be useful.    The Heisler factory must have used a numbering scheme as evidenced by the numbers that were cast into many parts .  The numbers were of the form HLXXXX, where the Xs were numbers.   I assume the HL signified that the parts were for a Heisler locomotive.

The scheme I selected is of the form: H (Type letter) (Number) (Revision letter)  Where:

Type letter =  C for castings, M for machined parts, S for stock parts such bearings.

Number =  Three digit number where the first number represents the locomotive subsystem.  The 1xx numbers will be for the engine subsystem, 2xx for trucks, etc.  The xx digits are arbitrary.

Revision letter = The initial version of the parts will have no revision letters.   If a part is later revised, the letter A will be added for the first revision, B for the second revision, etc.   

Examples of part numbers:

HC100 Crankcase casting

HM100 Crankshaft

HS100A   Engine main bearing, first revision    

Note that part numbers are not assigned to standard hardware items such as nuts, bolts and screws and these items are not included in the parts list.  In most cases the specific type of fastener will be listed in the fabrication pages.

The parts are listed on spreadsheets in the Parts List page     

Crankshaft:  The crankshaft  was the first part scaled.   The drawing on the right shows the Cass 6 crankshaft crank scaled by 0.134.  I decided to use drill rod and bar stock to make the crank.  The shay used 0.5" diameter rod so 0.75" diameter is more than adequate for the Heisler.  The shay used 7/16" thick crank webs.  A 5/8" thickness is a bit oversize for the scale dimension of 0.536" but seems reasonable.  The rod thickness also scaled to 0.536" so 9/16" was selected for the rod thickness.  This compares to 3/8" for the Shay rod thickness.   
This drawing shows the resulting crank dimensions.   Note that the offset is 1" giving a stroke of 2".  The rod bearing flanges work out to be 3/16".  The offset between the two cylinders is 15/16"   

The main bearing length is 1-9/16" including thrust surfaces on the ends.  The additional 1" length on the front side of the crank is for the valve eccentrics.

The crankshaft will be fabricated from 3/4"" diameter drill rod and 5/8" X 1-1/4" mild steel rectangular bar stock.  

Crankshaft Counterweight: This shows a counterweight which matches with the crankshaft above. The 0.75" overall thickness is slightly less than the direct scale of Cass 6 to compensate for the slightly wider rod thickness.     A pair of counterweights can be turned from a mild steel or cast iron disk. 

Counterweight Attachment: This sketch shows how the counterweight is attached to the crank.  The attachment strap (HM102) is a bent piece of 3/16" diameter HRS rod threaded 10-32 on each end.  The sketch shows  the approximate location of the holes in the counterweight for the strap.   The attachment pin is an off-the-shelf 10-32 alloy steel SHCS.
Valve Eccentric: This shows the dimensions of the valve eccentrics that fit on the forward end of the crankshaft.   The two identical eccentrics are positioned so that the smaller diameter is next to the seam between the two eccentrics.   The eccentric angular position is fixed by the key.   The computation of the 25 degree angle is described in Engine Design Part Part IV.    The eccentrics can be  turned from mild steel, cast iron or stainless steel.    


Eccentric Timing: This sketch  shows the position of the two eccentrics as viewed from the front of the engine.  When viewed from this perspective, the shaft turns counterclockwise for forward locomotive motion.   
Main Bearing: The main bearings are off-the-shelf oil impregnated (SAE 841) bronze bearings. The  flanged sleeve bearing (HS100) has a flange width of  0.187".   The flange is positioned on the inner side of the crankcase and, if everything aligns perfectly, will have to be thinned ~0.015" to match the crank and crankcase.    A grease hole will be drilled up through the bearing cap and through the bearing.  The 1/16" wide thrust bearing (HS101) will be used a necessary to position the crankshaft.
Main Bearing Cap: The bearing cap is not symmetrical front-to-back as is the cap on Cass 6.   The smooth side faces the inside of the crankcase.

The hole for the bearing should be drilled/bored after the cap has been attached to the crankcase with the two 10-32 screws & nuts

There will be a pin (6-32 SHCS) through the cap and the bearing to prevent the bearing from rotating.  A hole will also be drilled through the cap and bearing for a grease filling.    

The caps can be fabricated from mild steel.

The crankcase and associated crosshead guides are next and the subject of the Engine Design II page


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