Bar & Associated Links
2/15/2010, last updated
The design and fabrication of the Johnson Bar and
associated links that control the engine valves are the subject of this
|The photo above shows the Johnson Bar
on Heisler No 6 at Cass. This is an especially
useful photo because the cab is off and the entire linkage along
the side of the frame is visible.
Photo at right shows
more detail on the MRSR91 Johnson Bar which appears to be
identical to the one on Cass 6.
The pair of arms located under the right cylinder transfer the motion of
the long link from the Johnson Bar to the ball joints which link to the
Tumbling Lever (see Engine Design VII page). I've chosen to call
the long bar to the Johnson Bar the Jonson Bar Link and the two
arms the Outer Valve Control Arm and the Inner Valve Control Arm.
The bearing block connecting the two arms is called the Valve Control
Arm Bearing Block.
Photo above shows the finished arms and bearing block installed on the
model. (The Johnson Bar is in the reverse position.)
The bearing block is centered under the right cylinder and secured to
the right engine mount bracket with couple #6 FH screws. The
screws are threaded down through the engine bracket and secured with
Loctite so that they function as studs. Note the use
of a 3/32 key locking the outer arm to the shaft between the arms.
A similar key is used between the inner arm and the shaft.
This photo shows the arms with the Johnson Bar in the forward position.
The ball joints are McMaster #6072K81(male) and 6072K71(female).
These ball joints have 10-32 threads. The threaded
ends were shortened slightly to fit.
|Valve Control Arms: This drawing shows
the two valve control arms which are fabricated from 1/8" CFS
flat bar stock and short pieces of round bar stock for the hubs.
The hub pieces are silver soldered to the pieces of flat stock.
The hubs for the 3/8" shaft are broached for 3/32" keys
and tapped for 10-32 setscrews.
The small end 0.25" hole
is fitted with a 1/4" OD - 3/16" ID bronze sleeve bearing
HM147 Control Arm Bearing Block: The
Bearing Block is machined from a piece if 1" square bar stock.
The slight recesses in the sides are to give clearance for a 1/4"
nut driver. The 1/2" hole is fitted with a pair
of 1/2" OD, 3/8" ID 1/2" long flanged sleeve bearings (McMaster
HM148 Control Arm Shaft: The control arm shaft is a length
of 3/8" diameter CFS bar stock. 3/32" key slots are
milled in each end of the shaft to match the slots in the control
arms. The angle between the two key slots (and arms)
is a littler greater than 90 degrees. The exact angle
between the slots was determined with a test fitting. (The
ball joints provide a limited adjustment.)
HM149 Johnson Bar Link: This link is sawed from a length of 1/8" X
1" CFS flat stock. The length is only approximate and may
vary up to several inches depending on the exact location of the Johnson
Bar and the Control Arm Bearing Block.
The 3/16" shaft on the right end is silver soldered into
the bar. The hole in the left end is fitted with a 3/16" ID - 1/4"
OD bronze sleeve bearing.
A 3/16" offset was bent in the link near the Johnson bar to make
the link line up with the side of the Johnson Bar. The need for
such an offset will depend on the exact location of the Johnson Bar.
|Johnson Bar: Photos above
and on the right show the finished Johnson Bar.
The offset in the Johnson Bar link mention previously is visible
in the photo above.
One of the unique things about the
Heisler Johnson Bar is the hole in the bar for the gear rack.
I failed to measure the prototype Johnson Bar so had to
settle for estimates made from photos.
key dimension was to make the gear rack have a 5" radius.
Rather than machine a rack, a 2 foot length of 24 pitch
steel rack was purchased from McMaster (#6295K12). This
rack is 1/4" square.
|Rack: The photo shows using a 10" diameter
lathe faceplate as a form to bend the gear rack. A ~
1.5" length of 3/4" square steel tube was cut in half lengthwise
to make a fixture to hold the end of the rack. This
fixture was clamped to the outside edge of the faceplate as seen
in the photo. A section of rack was heated red hot with a
propane torch, the end of the rack was slide under the fixture
and the red hot part of the rack was then bent around the
faceplate. This process was repeated until a length of ~
6" was bent to the 5" radius.
|A length containing 40 teeth was cut
from the middle of the bent part. The last two teeth on
each end were milled off leaving 36 teeth. Two 5/8"
lengths of 1/4" square steel bar were notched to match the ends
of the rack and then attached to the ends of the rack with 4-40
FH screws and then silver soldered in place. These
extensions were drilled for 4-40 screws. The photo on right
shows the extension on one end of the rack.
|Johnson Bar: Photo at
right shows the parts of the Johnson Bar. The
drawings below describe the individual components.
The spring fits inside the rectangular hole in the bar. The
spring holds the paw down against the rack. The
spring is 1/4" diameter with a 5/8" free length using
0.030" wire. It was purchased from McMaster-Carr a half
dozen years ago. I couldn't find the exact spring on the
current website. However, the 1" long spring McMaster
#1986K3 could be cut to the 5/8" length.
|The paw that fits into the gear rack is shown
on the right. It is composed of pieces of scrap bar stock
and a piece of the gear rack. A brass #2 screw was used to
hold the three pieces together while everything was silver
soldered together. After the soldering the brass screw was
drilled out and the hole used for one of the screws that hold
the cover in place.
HM150 Johnson Bar: The drawing above shows the
details of the Johnson Bar. The basic bar is machined from 3/8" X
1/2" CFS bar stock. The 3/16" pin is silver soldered
into the main bar. That is were the Johnson Bar Link connects.
The trigger is made by sliver soldering a pair of 1/8" square bars to a
1/8" X 1/4" bar. The Paw was described earlier.