Heisler Truck Design III
Nelson Riedel, Nelson@NelsonsLocomotive.com
4/15/2005, last updated 04/09/2006

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 held in position by clamps.    Before the clamps are tightened the block can slide from left-to-right as necessary to align the block 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. .

The drawing above shows the design of the lower cross casting for the model.  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.  The sides of the slot will be cut (milled) under the center of the block to provide clearance for the swivel shaft.  More information about the swivel will be provided in the pages dealing with the gearing.   

The lower cross pattern is shown in photo above.   The structural part of the pattern was assembled from steel parts which were screwed together and then silver soldered.   A single rod was used to assure the two ends were in alignment, The center of the rod was sawed out after assembly.   Brass pieces and body filler were added to complete the pattern.  

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 model 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 from the frame bolster through the truck bolster which holds the truck to the frame if the locomotive rolls over.  This pin will be described in the frame design WebPages (i.e. I haven't figured out how to make a pin that can be removed easily).  The prototype seemed to have no mechanism such as pins or recesses to hold the springs in position.  In this design there are pins on the underside of the bolster which fit into the springs to keep them aligned. 

Photos above and below show the finished bolster pattern.  The rollers were installed to verify everything fits together properly and are not part of the pattern.   This pattern was made entirely of aluminum.

Bolster Rollers:  The photo on the right is a close up 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.
The design of the model roller and pin are shown on the right.   I'm undecided at this point as to whether to arrange the pin for lubrication.  However, if I go to the effort to make non skid tapered rollers, I should probably drill a hole in the center of the pin for lubrication. Aluminum models of the rollers are shown in are in the previous photo of the bolster pattern.

Truck Springs: The springs selected for trucks are standard parts from McMaster-Carr - #96570K24 shown in the drawing above. (This drawing was downloaded from the McMaster website.) The spring rate is 172 lbs/inch.  The free length is 1.625" and the fully compressed length is 1.28".  A force of  59 lbs is required to fully compress the spring.    Each truck has 8 springs so the load to bottom the truck bolster is 472 lbs.  The springs are compressed 0.225" by the bolster cap so the no load upward force on the bolster is 310 lbs.  The springs will compress further if more than 310 lbs is on the bolster   The maximum additional compression is 0.12" which requires an additional 160 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 160 lb engineer and a full tank of water will balance the spring holding the bolster against the caps.  A larger weight will move the bolster off the caps and a 320 lb engineer with a full tank of water will fully compress the springs.  That seems to be about right.   The springs may be a little too stiff for the trucks under the locomotive.  If so, two springs can be left off 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.
The drawing on right shows the design of the bolster cap for the model.  Recesses in the top are not shown.  I consider those recesses more decoration than structural and will sketch them out directly on the pattern after I have the basic shape fabricated.

This is the finished bolster cap pattern that is a pretty good match for the prototype. .
The photo on right shows the cap pattern in position on the bolster and truck side patterns.  Everything seems to fit.

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 the late design as shown above.   The purposes 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" hot rolled steel flat bar stock.  Hot rolled rather than cold rolled steel will be used because it is easier to bend without breaking.   The 5.375" dimension is somewhat critical and will be matched as closely as possible.

The sketch above and the one on the right are composites made by pasting the drawings of the various truck parts together to make sure everything matches.  This was useful since there were several minor errors.  Fortunately the errors were discovered before the patterns were fabricated.

This wraps up the design of the trucks except for the brakes and gearing.  The gearing is covered in the Model Gears & Shafts Design section.   The current plan is to add the design of the brake components that mount to the truck to this section at some later date.