Truck Design III
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.
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.
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.
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.)
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 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.