Arch Bar Trucks
N
elson Riedel, Nelson@NelsonsLocomotive.com
10/31/2012, last updated 11/09/2012

The equipment described on these web pages is intended for a 1920s era logging operation named Raccoon Valley Lumber.  Such a company would save money wherever possible by purchasing used equipment and fabricating as much as possible from local materials. The trucks are assumed to be made in the company shops using wood, steel bar stock and castings from a local foundry.

I've had problems navigating the Mill Creek Central Railroad spring switches with very light cars.  Tests several years ago  revealed that the total weight of a truck with load needs to be about 35 lbs to reliably operate the switches. These trucks were made as heavy as possible so that they could be used as part of disconnected log trucks that would reliably navigate the spring switches with no load.  The truck pictured below ended up weighing 20 pounds.

The first step in the truck design process was to research prototype designs and make rough drawings.   The drawings then evolved as each component was designed.  The photo above and following drawings represent the last trucks built using Ken McCauley's journal, journal lid and arch bar support castings.    The journal boxes and arch bar supports can also be fabricated from bar stock.     

 The truck side drawing above shows the signature view of the truck and the most important to get the proportions correct.    The wheels are the standard 33" - 4 1/8" with an axel spacing of 7 1/2".   The spring planks are 1 1/2" wide ---- the lower plank 3/8" thick and the upper plank 1" thick.   The separation between the bars at the spring planks is 2 1/8".   These planks would have been hardwood on the backwoods prototype being modeled.   Steel is used on the model to increase the weight.  The journal boxes are 1 1/4" high and the width of the rectangular part is also 1 1/4"; the boxes are wider when the tabs and posts are included.  

The bars are all 1/2" wide, the bottom bar is 1/16" thick, the middle bar 1/8" thick and the upper bar 3/32" thick.  Three different thicknesses were usually used on the prototype and with the smallest about half the thickness of the largest.  Bending the bars was a real pain on past projects.  This time I fabricated a fixture that makes it easy to bend the bars to the exact shape required (correction ---- I made 4 fixtures before I got it exactly right).  

 The rods are 4-40 at the journal boxes and 6-32 at the spring planks. Four off-the -shelf coil springs are used on each side.    

Ken McCauley designed the journals to use 3/8" ID - 7/8" OD ball bearings.  The journal box castings have an undersize hole that is to be machined to 7/8" diameter and slightly elongated at top and bottom to enable the axel to move at a slight up and down angle with respect to the truck side.   I chose to used 5/8" OD - 3/8" ID flanged sleeve bearings similar to what Ken Schroeder used on his disconnected log trucks.   Flats are milled in the bearing side to keep them from rotating.   The bearing is held in position in the journal box by the three 5/16" diameter screws shown in the cutaway view of the left journal box.          

The top view above is not very interesting and included for completeness.

 

Machining the wheels is a pain --- it gets boring after a half dozen and really boring after a couple dozen.   Also, the cast iron is nice to machine but the dust makes the supervision very unhappy.    This time I bought machined wheels from Tom Bee.   His wheels are bored 17 mm - 0.669" and are 13/16" wide. 

  

    The drawing above shows the spring planks and arch bar supports. The arch bar supports are attached to the lower spring plank  with a couple screws each.    The lower spring plank has a swivel in the center to allow the two sides to rotate with respect to each other.  This concept was copied from the Heisler trucks and works quite well.  The swivel is made by welding a 2" length of 1/2" square steel bar stock with a 3/16" hole in the center to the bottom center of the spring plank.  The spring plank is then sawed in half and a 10-32 screw inserted in the hole in the square bar and secured with a lock nut to form the swivel.  

The upper steel spring plank has a 3/4" hole fitted with a 3/4" OD 1/2" ID flanged sleeve bearing.  The 1/2" diameter car attachment pin feeds through this bearing.  A shim is fitted under the bearing flange to raise the top of the flange to 3 7/8" above the rail head.  This will make the coupler centerline of a typical car fitted with a 1" X 2" center tube at the standard 4 3/8" above the rail head.   

The ends of the upper spring plank are fitted with car support blocks matching the height of the center bearing.

The lower spring plank has studs (short lengths of 3/8" aluminum rods held in place by 6-32 screws) that fit inside of the innermost pair of springs on each side to hold the springs in position.  These inter springs in turn hold the upper spring plank in position.   The upper spring plank has small notches opposite the arch bar supports to permit free movement up and down as well as a slight rotation.    

 Details on the basic truck components are at the following  pages:

Bars

Journal Boxes

Arch Bar Supports

Spring Planks

Wheels & Axels

  

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