Heisler Truck Design III
Nelson Riedel, Nelson@NelsonsLocomotive.com
/2005, last updated 01/15/2012

Update: This page was updated 1/29/2010 to show changes to use sand castings instead of investment castings.

The lower cross, upper cross, bolster and bolster cap are the subject of this part. 

Lower Cross: Before starting the design of the lower cross the following photos were reviewed.

This is a lower cross with the one side shaft broken off.   This is for a middle truck because the  mounts for the swivel bearing are offset to the left side.

This is the under side of the lower cross in the first photo

This photo shows a lower cross mounted to the truck sides.  The swivel bearing and bearing block are still mounted.  The swivel bearing is offset so this is also a middle truck.  Note that the bearing block is retained in the slot in the lower cross by straps.   The straps don't tighten against block so it can slide left-to-right as necessary to keep the block aligned with the gear case.  Note the very short heads on the left clamp which is under the line shaft.  Apparently there was only enough room for short bolt heads.    

This shows another lower cross that has been removed from the truck but is still attached to the gear case. This is an outside truck gear case because there is no arrangement for the line shaft and also the pinion shaft is at an upward angle so that the drive shaft will pass over the non geared axel.  

The smudges on the photo were probably due to rain drops on the camera lens --- it was a very damp day.  


HC205 - Lower Cross: The drawing above shows the machined dimensions of the lower cross casting.  The same design will be used for both the middle and the outside trucks.  The swivel bearing block will be centered on the outside trucks and about 5/8" off the center position on the middle truck.  The holes for the swivel bearing block clamps will be positioned to match the position of the block.   More information about the swivel are provided in the pages dealing with the gearing.   

 The first set of lower cross  castings are shown above.  The disks at the ends of the shaft were sawed off.  The pattern maker will modify the pattern so that future castings won't have the disks. 

Lower Cross End HM212:  The disks at the end of the lower cross are replaced by the machined ends shown on the right.  This makes it easier to turn the shafts at the ends of the lower casting.  A pair of 1" long 3/4" OD - 5/8" ID bronze sleeve bearings (HS205) are slipped over the lower cross shafts before the ends are screwed on the shafts.   Grease nipples are screwed  into the 1/4"-28 holes.   


Truck Bolster: The photo above shows the truck bolster from the Cass 6 middle truck.  

The photo above is of the truck bolster of a Heisler that is slightly smaller than Cass 6 & MRSR91.   The bolster caps are in place in this photo   The springs are also in place holding the bolster up against the caps. 


The design of the truck bolster casting is shown in the drawing above.  The design is very close to the prototype. However, the prototype casting is hollow while the model is solid.  There is a pin (HM214) from the frame bolster through the truck bolster which holds the truck to the frame if the locomotive rolls over.  This pin screws into the 3/8" hole in the center of the bolster. The prototype seemed to have no mechanism such as pins or recesses to hold the springs in position.  In this design there are tapered pins on the underside of the bolster which fit into the springs to keep them aligned.    The original plan was to cast the roller base as part of the bolster.  However, after receiving the first castings it was decided that the rollers could be positioned more accurately by making the roller base as a separate part. 

The photo above shows the first three truck bolster castings.   The supports for the rollers were cutoff; the roller bases were made as separate units as discussed above.  The pattern will be modified so that future castings won't have the roller supports.   

HM214 Bolster Pin:  The bolster pin is machined from 5/8" diameter CFS rod stock.  The upper end is cross drilled for a retaining pin on the front truck and grooved for a C clip on the other two trucks.  The exact location of the retaining device is determined after the pin is screwed into the bolster and slid into the frame bolster.  After all the machining is finished the lower end of the threaded part is pounded over to keep it from unscrewing.     
Bolster Rollers:  The photo on the right is a closeup of a roller on Cass 6.  Note that the roller extends below the top surface of the bolster into a cavity in the casting.   The roller pin is retained by a cotter pin through the inside support.    The outer end of the pin is equipped with a grease fitting. I assume the pin is hollow with a cross drilled hole near the middle of the roller.  The grease probably exits the pin and lubricates the roller-pin interface with the excess grease exiting between the roller and the supports.
This is a photo of a later Heisler design and is identical to the design used on MRSR91. Note that the pin is held in place by a bolt through the front support.  There appears to be no way to lubricate the pin-roller interface.  One can speculate as to the reason for this improvement. 


Roller Base HM213: The roller base is made from mild steel.  The bottom and the  3/16" thick vertical part can be length of  3/16" angle iron.  The recess can be made with a 7/8" diameter woodruff key slot cutter before the 1/4" vertical part is added (screw & silver solder)    The base fits into a 3/16" deep 15/16" wide slot milled in the top of the bolster.   

The base can be positioned correctly and clamped in position and then a 10-32 hole drilled and tapped  in the bolster using the base as a drill guide.

Roller Pin HM206: The design of the roller and pin are shown on the right.  The pin will be fitted with the oil cup shown below.   The pin is held in the roller base by a 10-32 set screw.

Roller HM205:  The roller will be turned from bronze bearing stock.   

Oil Cup - HS202: The oil cup shown on the right is pressed into the hole in the roller pin. These are available from McMaster-Carr- Part #1214K1 with  A=3/16", B=1/4" and C=9/32


Truck Springs(HS205)- Updated 1/16/2012: The springs selected for the trucks are standard parts from McMaster-Carr - #96570K24 shown in the drawing above. (This drawing was downloaded from the McMaster website---- note that the number on the drawing has a zero missing.) The spring rate is 138 lbs/inch.  The free length is 1.875" and the fully compressed length is 1.46".  A force of  57 lbs is required to fully compress the spring.    If 8 springs are used, the load to bottom the truck bolster is 456 lbs.   If 6 springs are used,  the load to bottom the truck bolster is 342 lbs.    The greatest load will be on the tender.  The tank will hold  about 11 gallons of water weighing about 100 lbs.   The tender frame and tank will weigh maybe 50 lbs. So, a 180 lb engineer and a full tank of water will exert a force of about  330 lbs which will bottom a set of six springs but not a set of 8 springs.   Hence 8 springs should be used on the tender.    The load on the two tucks under the engine is less so I'll start with 6 springs on the front and middle trucks.   (Note that the trucks themselves which are a significant part of the locomotive weight are not part of the spring load.)     


Bolster Cap: This photo shows the outside side view of a bolster cap.  The top of the cap has recesses to either increase strength or reduce weight or both.   I'm not sure how much effort I'll make to reproduce the recesses.
This is the inside view of a Cass 6 bolster cap. Note that the cap extends down well below the bolts to keep the bolster and side aligned even when the springs are fully compressed.
Bolster Cap HC207: The drawing on right shows the design of the bolster cap.  Recesses in the top are not shown to simplify the drawing.
The photo shows a couple bolster cap castings.  The under sides that mate with the truck side have been machined square. 
The photo on right shows the cap in position on the bolster and truck side.  Once it get a bit grease smeared around it'll look just like the prototype. 


Upper Cross: Photo above is a repeat of an earlier photo this time highlighting the upper cross piece. This is the later design that is also used on MRSR91.  The earlier design used on Cass 6 has a clevis on each side and a flat bar pinned between the clevises.  I decided to use a design similar to that shown above but will position the upper crosses over the axels.    The purpose of the upper cross pieces is to fix the separation of the top of the truck sides.  The separation of the bottom of the truck sides is fixed by the lower cross.    

 The drawing on right shows the design of the upper cross which is fabricated from 3/8" X 1/8" cold formed steel flat bar stock.    

The sketch above  shows how all the parts fit together on a truck.  

This wraps up the design of the trucks except for the brake components that are described in Truck Design IV.   

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