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Heisler
Middle Truck Gear Design
The middle truck uses the same gear case as the outside trucks. A line shaft is added to the middle truck to connect between the engine and the tender truck. The line shaft connects to the middle truck pinion shaft via a pair of spur gears contained in the added gear box. (Note: The bevel gears are in the Gear Case and the spur gears are in the Gear Box.) The following two drawings show the organization of the middle truck gear system. (Note that the tender is on the left side of the drawing.) Update August 2005: The initial design used a piece of 3.5" square tube for the gear box. After I built that design I decided the gear box was way out of proportion (photo in the construction section). I then redid this page using an oval design that I previously thought would be too difficult to fabricate. It turned out it was much easier to fabricate (using round tubes) than the square tube version because the tubes make it fairly easy to keep the shafts, bearings and gears all in alignment.
Pinion Tube: The pinion tube is a length of 1.75" OD 1.25" ID tube, the same as for the outside trucks. The pinion tube is longer in the middle truck than in the outside trucks. It is horizontal in the middle truck whereas it angles up in the outside trucks. The pinion tube is silver soldered to the front end of the gear box. The pinion tube slides into the gear case small tube and is held in place by a screw through the gear case small tube into the pinion tube. A notch is cut into the bottom of the pinion tube for the swivel shaft bearing block. Gear Box: The Gear Box contains the spur gears that link the line shaft to the pinion shaft. The gear box is fabricated from a pair of 2.25" diameter steel tubes and 0.125" thick bar stock. Pinion Shaft: The pinion shaft is a length of 5/8" diameter mild steel rod. The only machining required is cutting key slots in each end. Pinion Bearings: The pinion shaft bearings are R10-2RS double sealed ball bearings. This bearing has a 5/8" ID and 1 3/8" OD and 11/32" thick. Line Shaft: The line shaft is a length of 5/8" diameter mild steel rod. The only machining required is cutting key slots in each end. Line Shaft Bearings: The pinion shaft bearings are R10-2RS double sealed ball bearings, the same as used on the pinion shaft.. This bearing has a 5/8" ID and 1 3/8" OD and 11/32" thick. Line Shaft Bearing Block: The line shaft bearing block steadies the rear of the line shaft and attaches to the closed end of the gear case. Spur Gears: The pair of spur gears in the gear box are 14.5 degree pressure angel 18 tooth 12 pitch with 1.5" pitch diameter and 1.67" OD unhardened steel. The gears have a 5/8" bore, 3/4" face and 1.25" overall length. Both Boston Gear ND18-B and Martin S1218 match these specifications. The gears need to be broached for a 3/16" key and tapped for a 10-32 setscrew. Swivel Shaft: The swivel shaft for the middle truck is attached to the pinion tube whereas it screws to the small tube of the gear case on the outside trucks. The same swivel shaft bearing block is used for all three trucks. In the middle truck, the pinion tube is notched to make room for the bearing block.
The sketch above is a section taken through the center of the line and pinion shafts showing the gear and and bearing configuration. Since the line shaft is positioned above the pinion shaft, the plane of the drawing is not horizontal. There is a much smaller gap between the gears and the bearings than show in the drawing ---- shaft end float will be about 0.01". If necessary, shim washers can be used to reduce the shaft end float to 0.10".
Pinion & Line Shafts: The two drawings above show the pinion and line shafts made from 5/8" mild steel rod. The only machining required will be to cut the keyway slots. Drawings and a Parts List for the Gears & Shafts may be downloaded from the Drawings and Parts List pages. This wraps up the universal, shaft and gear design. Guess it's on to the frames next.
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