Heisler Engine Design Part I
Lower Engine
Nelson Riedel, Nelson@NelsonsLocomotive.com
10/6/2004, last updated 05/24/2006

 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.

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.  

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

Update 5/24/06: 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. (End update).

Crankshaft Crank:  The crankshaft crank 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 crank will be fabricated from 3/4"" diameter drill rod and 5/8" X 1-1/4" mild steel rectangular bar stock.  Drawing updated 4/15/06.
Crankshaft Counterweight: This shows a counterweight which matches with the crankshaft crank above. The 0.75" overall thickness is slightly less than the direct scale of Cass 6 to compensate for the slightly wider rod thickness.
Counterweight Attachment: This sketch shows how the counterweight is attached to the crank.  The attachment strap (HM101) 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 (HM102) is another piece of 3/16" diameter HRS rod threaded 10-32 on each end.
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 initial plan was to use a setscrew to secure the eccentrics.  Later it was decided to not drill through the outer surface. The eccentrics are keyed and trapped between the crankcase and universal so the only use for a setscrew is to take out any slack.  The current plan is to use a few drops of Loctite to take up any slack.  The computation of the 25 degree angle is described in Engine Design Part Part IV.    The eccentrics will be turned from mild steel.    Updated 4/24/06.

Diamond Frame:  The diamond frame cross section dimensions were fixed before starting on the crankcase.  The inside distance between the frame sides on Cass 6 is 62" which scales to 8.3".  I want as much room possible for the boiler so the inside spacing was increased to 8.5".  That will permit a boiler diameter of 8.0" with 0.125" insulation and a thin jacket.  The frame bars scale to 0.536" which was increased to 9/16" square bar stock .

Crankcase & Crosshead Guides:  The next three drawings show the overall dimensions of the combined crankcase and crosshead guides.  These drawings incorporate the above decisions on the crankshaft and the frame

Recall that Heisler split the crankcase front-to-back and combined a crosshead guide with each crankcase half.  I intend to make the crankcase as one piece and bolt the crosshead guides to the crankcase.   This is similar to the technique used by Hiraoka.  

The flanges used to bolt the two haves together are not required since the crankcase is one piece.   A fake flange is provided in the rear half.  The flange on the front half would be hidden under the rocker arm base so it was eliminated.  The rocker arm base will mount directly to the front and the pad on  the top.

The front-to-back measurement of 5.75" gives an allowance of 1/8" for a main bearing flange at each end and match up with the crankshaft which has 6" between the outer ends of the bearings.

Crankcase:  The next three drawings show the crankcase. The plan is to have the crankcase cast using the investment casting process.  The pattern was made 3% larger than this drawing to compensate for shrinkage.   The curved section of the pattern was made from a piece of 6" schedule 40 pipe.  The dimensions shown on the drawing are 97% of the pipe size.  The recesses on the upper side of the front  provide clearance for the rocker arms
The 0.125" deep slots in the side plates mate with the flat part on the bottom of the crosshead guides. These slots weaken the slide plates so 1/8" stiffening plates identified in the Front View were added.
There are two axis of symmetry  for crankcase.    The hollow part of the crankcase and the bearings and flange where the two halves join are symmetrical around the centerline that runs front -to-back.    The holes and slots for the crosshead guides and the outer part of side plates are also symmetrical around the centerline that runs side -to-side.

Crosshead Guide:  The drawing above shows the right side crosshead guide.  The left side is identical except for the access slot in the side.  Both guides have the slot in the front and a round opening in the back.  The little box on the upper side is for lubrication.  The flat area on the bottom mates with the slot in the crankcase.   The initial plan was to solder or braze the crosshead guide to the crankcase.  However, the current thought is to run screws through that crankcase stiffening plate into the flat area on the bottom of  crosshead guide,   An additional screw can be run through the tapered shark fin flange into the rocker arm base.   The plan is to cast the crosshead guides.         

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 will have to be thinned ~0.015" to match the crank and crankcase.    A lubrication hole will be drilled down though the crankcase and on through the bearing.  The 1/8" wide thrust bearing (HS102) is used on the outer side of the crankcase, primarily for decoration .  Drawing updated 4/15/06.

Main Bearing Cap: The bearing cap in not symmetrical front-to-back as is the cap on Cass 6.   The smooth side faces the inside of the crankcase. The cap is attached to the crankcase with two 10-32 screws & nuts There will be a pin (3/32" expansion pin?) through the bearing flange into the cap  to prevent the bearing from rotating and thus keep the bearing and crankcase lubrication holes aligned.        The Plan is to cast the bearing caps.

Lower Engine Mount Bracket: There are one of these brackets on each side.  They clamp the 9/16" square lower frame bars to the sides of the crankcase.  The Cass 6 bolt size is ~ 1" so 6-32" hex head screws should be a good match.   Thirteen of the holes (those closest to the edges) match up with threaded holes in the crankcase.  The other 4 match up with holes up through the frame bar and the crankcase.   These four have screws with heads on the bottom and nuts on the top.   The brackets are aligned with the crosshead guide centerlines.  The brackets will be made from 3/16" thick HRS  angle.

The next step is to make the patterns and secure the castings of these parts.