Heisler Middle Truck Gear Construction
Nelson Riedel, Nelson@NelsonsLocomotive.com
8/27/2005, last updated 04/08/2006

The middle truck uses the same gear case and end cap as the outside truck.  The big difference with the middle truck is the addition of the spur gears, the gear box for the spurs gears, a longer pinion tube and the addition of a line shaft.   This page focuses on these added parts used only on the middle truck.  Refer to the Middle Truck Gear Design page for design information.

First Model:  Unlike the gear case, the first model of the gear box shown above was unsatisfactory.  The gear arrangement worked but the gear box was way out of proportion to the rest of the assembly.

 

I was unable to take a photo of the gearbox portion of a middle truck but was able to take the photo of the Cass 6 cover shown on the right.  Note that the high point is rounded over the line shaft spur gear and the top slopes down to over the pinion shaft spur gear. I had hoped that the rounded corners of the square tube would soften the appearance of the square box, but it is still way out of proportion.

Oval Gear Box:   The next step was to go back to the design stage and dig out an earlier gearbox version using tubes (see Outside Truck Gear Design for drawings) in an oval configuration.  The oval gear box is fabricated from two short lengths a 2.25" OD - 1.5" ID steel tube.  This is the same type tube used in the gearbox end cap.

The first step was to cut two tubes to the correct (2.062") length.   The end of the pinion tube is  1.5" diameter which matches  the ID of these tubes.  The pinion tube slides in 0.406".  The remaining length of that tube was bored to 1.75" to provide clearance for the spur gear.  The other tube was bored 1.75' the entire length and then a 0.406" long  1.75" OD - 1.25" ID sleeve  (the same stock as used for the pinion tubes) was silver soldered into one end.  A 1.375" diameter 0.312" deep recess was then bored in the sleeve for the line shaft bearing.  Photo at right shows the two tubes
About 3/8" had to be machined off the side of each tube to achieve the ~1.5" distance between the center of the tubes required for the spur gears. The first step was to rough saw about 1/4" off each tube.
The next step was to clean up the sides and increase the depth of the cut using the mill.  (In hindsight, this operation might have went quicker if the done on the lathe, one tube at a time.)  It is not necessary that the cut depth be the same on both tubes.  In fact,  the depth of cut on the tube that connects with the pinion tube was only about 0.350" to avoid cutting though the side of 1.5" diameter end that mates with the pinion tube.   The photo shows some of the first milling cuts on the side of the tubes.
Once the cuts got close (each at about 0.350" depth) the tubes were test fit together as shown on the right.  Everything was looking good.  The remaining cut was then made on the tube for the line shaft.  I left a little excess material so that the centers were about 1.51" instead of 1.50" to avoid gear binding should the two shafts not be exactly parallel.
The next step was to hold the two tubes together and mill a 1.5" wide 0.125" deep slot in each side for the side plates. I did one side, mounted  the plate with two 4-40 FH screws and then did the other side.
The next step was to remove the side plates and clean the four pieces in a pickling solution.   All mating surfaces were coated with flux and the tubes and plates then reassembled.  The parts were then silver soldered together. Photo at right shows the resulting oval tube.
Yes, this second design was worth the effort!
The machining of the pinion tube was straight forward. The end that fits into the gear case was bored to provide clearance for the pinion gear hub and then a recess further in for the bearing just like for the outside truck pinion tube.    The end that mates with the gear box was  turned 1.5" on the outside and bored 1.375" on the inside for the bearing.    The gear box and pinion tube were then cleaned in the pickling solution, the mating surfaces fluxed and the two pieces silver soldered together.   The photo shows the soldering operation.
This photo shows the gear box end with the two bearings in place.  (The parts were cleaned in the pickling solution after the soldering.)
This shows the two spur gears (from McMaster) in place.  Note the compact design ---- barely enough room for some grease.  The pinion gear was installed first and the set screw tightened before the line shaft was installed.  The pinion shaft bearings are retained in their recesses by the spur gear on one end and the pinion gear on the other.  The other spur gear was mounted to the line shaft before the line shaft was slid into position. The line shaft is held in position by the spur gear which is captured between two line shaft bearings, one in the gear box and the other in the gearbox end.       Holes for screws that hold the end to the gear box were drilled and tapped after the end was made so that it could be used as a template.
Gearbox End:   The gear box end was made of 0.062" steel plate and a 1.375" ID sleeve which holds the front line shaft bearing.  The sleeve was made from a piece of the tube stock used for the pinion tubes.  The sleeve was silver soldered to the plate.  The plate was rough cut on the band saw and then  finished with a file after it had been screwed to the gear box.
This shows the gear box end in place on the front of the gear box.
Line Shaft Bearing Block: The photo shows the bearing block which is made from steel tube and 0.125" thick steel plate base silver soldered together.    The block will be secured to the gear case with a couple screws through the base..
This photo shows the block in position on the line shaft.  I found that the block held the shaft too far away from the gear case.  In this position the shaft was nearly touching the axel.  The base was reduced to 0.094" thickness which allowed the gearbox to rotate and raise the line shaft some.  I may want to move it 0.062" closer which will require milling a recess in the gear case for the base.  I'm deferring that decision until the trucks are assembled.
This photo shows how the universal fits on the line shaft.  The fit is as planned with the U clearing the gear case by about  0.125".  The universal is the aluminum pattern for the castings.

Swivel Shaft: The swivel shaft screws to the bottom of the pinion tube.  A slot must be cut in the pinion tube to provide clearance for the swivel bearing block.  I'm deferring this task as well as the screw that holds the pinion tube in position until the trucks are assembled so that I can verify that everything fits properly.  

The photo above shows the finished middle truck gears.  I'm happy with the results.   The photo at right shows the gears positioned with the truck casting patterns.
Another view of middle truck gears positioned with the truck patterns.  Everything seems to fit together as planned.

This pretty well wraps up the construction of the gear system.   Guess it's back to the design