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Heisler Engine
Design Part V
Piston, Crosshead, Connecting Rod & Bearing
Nelson Riedel,
Nelson@NelsonsLocomotive.com
10/12/2004, last updated
05/17/2006
| Piston: The photo on the right and the one below
taken at the Cass shops shows a Heisler piston.
The upper end of the piston rod is tapered with the end threaded. There is a
matching tapered hole in the piston. The rod is retained with
a washer and nut.
The rod is part of the crosshead so it must be removed from the
piston before either the rod-crosshead or the piston can be
removed. A cylindrical puller tool is used to force
the rod out of the piston. The tool screws onto the the
treaded stub at the top of the piston. A rod that is
threaded into the top of the tool is then tightened against the
piston rod to force it out of the piston. |
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| The underside of the piston is hollow to make room for the
packing gland that sticks up into the lower part of the cylinder.
Because the lower part is hollow, the upper part must be domed.
The use of a slightly longer cylinder combined with a shorter
stroke and shorter packing gland on the model obviates the need
for the hollow bottom making the piston design straightforward as
shown in the drawing below. |
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| The drawing on right shows the piston design.
It can be made from brass, bronze or stainless steel. The hole
through the center is bored 5/16" half was through and the other
half tapped 5/16"-24. The piston will thread onto the end of
the rod and be secured by a jam nut. A non-threaded part of
the rod will be a tight fit into the non-threaded part of the piston
and hopefully keeping the piston and rod aligned.
There should be a 0.0015" gap between the sides of the
rings and the grooves in the piston. The HS108 rings are
standard cast iron rings for a 2.125" cylinder and 3/32" groove obtained
from Otto Gas Engine
Works. |
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| Piston Rod: The piston rod is a length of
5/16" diameter stainless steel rod with each end threaded
5/16-24. Care should be taken to make the axis of the threads
are on the rod axis so the piston, rod and crosshead will all be
in alignment. |
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| Crosshead: This photo shows one of the Cass 6
crossheads. Note that the piston rod and crosshead are one
piece ---- maybe forged or possibly cast steel. There are thin
brass colored sliding bearings between the crosshead and crosshead
guide. |
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| This shows the cross head on a slightly smaller junk Heisler.
Note the use of adjustment wedges between the crosshead and the
sliding bearings. The connecting rod pin goes through the
center hole. Note the use of a splint bearing in that hole.
The rectangular hole in the upper part of the crosshead is for a
wedge to maintain the adjustment of the split bearing. |
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The drawing above shows the crosshead which will
be fabricated from bronze bearing stock. The hole for the
connecting rod pin will likely be the most wear prone point so I chose to
line it with an off-the-shelf bronze sleeve bearing so that it can be
replaced without replacing the entire crosshead. Grease lubrication
will be provided to the bearing via a hole through the pin. No adjustment mechanism
was made for the bearing surfaces that rub against the
crosshead guide. It seems that machining new crossheads every few
years or tens of years would be easier. The shoulder will be
turned and rod hole will be
drilled and threaded first. The rod will then be screwed into
place and secured with high temperature Loctite. After the Loctite sets,
the 1/16" hole in the shoulder will be extended through the rod and a
tension in driven into the hole. The rod will then be chucked in the
lathe and the outer round surface finished to dimension. This
technique should assure that the rod and crosshead are perfectly
aligned.
The sketch above shows how the piston, rod and crosshead
all fit together.
| Connecting Rod: I was not able to photograph a Heisler
connecting rod out of the engine. However, I was able to
take this and the following three photos of a partially installed
rod on MSRS91. This first photo is of the rear side of the
upper part of the right rod. The rod pin had just been
inserted but wasn't in quite all the way. It is retained
in place by a large washer. The washer is held in place by
three studs and nuts. Two holes for the studs are visible in
the photo. That is a grease fitting on the end of the
pin.
The bolt with just the end visible toward the
upper right is used to tighten the adjustment wedge against the
split bearing around the rod pin.
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| This shows the parts associated with the pin. The large
washer on the right is held in place by the three ~ 1/4" studs on
the left. The large nut and smaller washer on the right go
on the forward side of the pin. At least one of the other nuts
goes on that bearing adjustment wedge bolt. Maybe the other
parts are just extras ( I also sometimes end up with extra
unneeded parts after doing a maintenance task.). |
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| This shows the front side of the connecting rod pin. Note that
the pin diameter is much larger that the part that goes through this side of
the rod ---- there is a shoulder on the pin that rests against the
inside surface of the rod. . The nut and washer that go on this side
hold the pin shoulder against the rod to thus keeping the pin from
rotating relative to the rod.
The three smaller holes on the rod secure an oil hose & tube
that lubricate the rod bearing. |
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| This is a good photo of the lower end of the two rods on
MRSR91. The rod on the left side of the photo has the
rod strap in place and secured by two bolts. The rod on the
right side of the photo has the strap off. (The engine was
being reassembled and the next step was to install the lower
bearing half and the strap.) Note the shape of the upper
bearing and the two holes for the bolts that secure the bearing
strap to the rod. The rod has a slot for a key (and
another on the opposite side ) that takes the shear load off the
bolts.
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| The drawing on the right shows the model connecting rod and pin.
The rod will be cast steel and require little in the way of
machining except reaming the holes, threading where required and
smoothing the surfaces next to the crosshead and the strap.
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The pin will be machined from 7/16" hex steel stock. The
pin is passed through one side
of the rod and screwed into the other side. A
1/4" jam nut will be added to the end threaded into the rod to
lock the bolt in position. A grease nipple is screwed
into hex end. Grease flows through the nipple and then
through a 3/32" passage in the center of the pin and then out the
side of the pin near the midpoint of the sleeve bearing in the
crosshead.
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The drawing above shows the design for both the bearing
and the strap. I choose to not use the the wedge bearing adjust
arrangement but instead used a 1/4"-28 bolt and lock nut up through the bottom of the
strap pressing against a 1/8" thick steel plate. This bolt will
look very similar to the grease fitting on MRSR 91.
The bearing is two identical halves that will be cast in bronze.
The strap will be cast in steel.
Before wrapping up the connecting rod, the rod and
strap were pasted on the crankcase drawing shown above to see how everything fits
together. The rod on the left is
positioned all the way up to check that the rod pin lines up with the
access hole in the crosshead guide. The rod on the right is
positioned at the point where the rod side is closest to the hitting the
side of the crosshead --- it clears with a little margin.
The ground clearance was also checked and found to be a little over 1.5"
above the rail head when the rod is position at the lowest point
The valve linkages are started in the the next part.
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