Heisler Johnson Bar & Associated Links
Nelson Riedel, Nelson@NelsonsLocomotive.com
2/15/2010, last updated 02/15/2010

The design and fabrication of the Johnson Bar and associated links that control the engine valves are the subject of this page.

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 (McMaster #2868T34).

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 #1677K5)      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.  The first 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.