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Heisler Engine Design
Part I
Crankshaft, Eccentrics and Main Bearings
Nelson Riedel, Nelson@NelsonsLocomotive.com
10/6/2004, last updated
09/12/2009
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
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| 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.
|
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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The crankcase and associated crosshead guides are next and the
subject of the Engine
Design II page
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