EMD F7 in SCALE
The full-size F7 was fitted with Blomberg "B" trucks, so-named after their designer, Martin Blomberg. This truck is characterized by a springing system that utilizes packs of leafs that support the locomotive's weight through hinged hangers attached to the truck frame. This design imparts good riding qualities and takes advantage of gravity to dampen lateral movement when the locomotive enters or exits a curve. The locomotive carbody is able to laterally shift relative to the truck bolster center to compensate for changes in direction or when encountering "wavy" track, with gravity causing the carbody to return to center after movement. The result is a reduction in the lateral forces applied to the track and a truck that behaves well at high speeds—consider that the F7 with passenger gearing was rated for a maximum speed of 102 MPH, and could safely operate at that speed on the typical bolted joint mainline track of the early 1950s.
The wheelsets are independently sprung within the one-piece cast truck frame, further improving the truck's ability to negotiate uneven track. The traction motors are supported on one side by a clevis that is cast into the truck's bolster beam, and on the other side by the gear-case, which in turn, is supported on the axle by some plain bearings. This arrangement allows the relationship between the traction motor and axle to be accurately maintained, while imparting plenty of allowance for movement.
As my unit was going to be hydrostatically propelled, using one traction motor per truck, the prototype design could not be exactly modeled. The plan was to rigidly support the traction motor from the underside of the truck bolster and couple the motor to the axles via an opposing roller chain drive. "Opposing" in this sense means that a separate chain transfers power from the motor to each axle, which causes the radial loads generated from chain tension to be largely cancelled out and not applied directly to the motor's PTO end bearing. The end result would be reduced bearing and seal wear. The traction motor mounting bracket would be made so that some lateral movement of the motor was possible to achieve proper chain slack.
The trucks were the first major assembly I acquired when I embarked on this project and were produced by a well-known builder
. The cast steel side frames were originally a Tom Bee product (still available, I believe), as were the bearing boxes. As this project progressed and I got to a point where I could mount the frame to the trucks I encountered quality control and design problems in them that ultimately lead to a substantial rework.
The principle problem was that the traction motor was mounted too low relative the the railhead and that the traction motor mounting bracket could in some cases strike a rail when passing through turnouts and diamonds. After careful consideration of what I was dealing with, I concluded that reworking the bolsters would consume at least as much time as making new ones, so I scrapped the old bolsters and fired up the CAD program to make some drawings for a new bolster assembly. As long as I was doing that, I decided to figure out how to add brakes. Furthermore, I decided that the truck side frames should be made into a one piece affair like the prototype, so I designed cross beams like those of the real Blomberg truck, which are what supports the outboard brake shoe hangers.
Brakes were an interesting design problem. The real Blomberg truck utilized a clasp brake arrangement that would press the brake shoes to the wheel treads. That arrangement can be scaled down, of course, but doesn't produce a whole lot of braking force. Also, duplicating the Blomberg brake rigging would mean making a lot of fiddly little parts that would be prone to bending and breaking. So I decided to take a different approach and instead adapted small pneumatically-actuated industrial disc brake calipers operating against cut-down go-cart rotors, one brake assembly per truck. Each caliper is supported on a floating bracket that allows automatic alignment with the rotor as the pads wear, and compensates for truck frame roll relative to the wheelset. Below is a picture of a wheelset with brake.
The caliper is a Tol-O-Matic product and its mounting bracket was machined from a piece of 3/4 inch thick extruded 6061-T6 aluminum. The bracket rides on a plain bearing on the axle (note the grease fitting) and engages a pin on the bolster to act as a reaction point. Below is the bracket drawing.
After rework, I finally had what I thought was a pretty decent truck, although initial operating experience would soon highlight more issues related to the wheelsets. So I bolted the mess together and what you see below is the result.
At the time these photos were taken, my camera was slowly dying, which accounted some of the poor picture clarity (I've since broken down and obtained a new camera). Also, I was undergoing heavy chemotherapy at the time, which had the annoying tendency to give me the shakes.
The roller chain is ANSI 40 and runs on stock Browning sprockets with case-hardened teeth. Gearing between the traction motor and axles is 1:1. ANSI 40 is the smallest roller chain in terms of pitch that can safely handle the expected loads. It would have been nice to use ANSI 35, which is somewhat more tolerant of centerline variations between sprockets, plus less prone to chordal vibration, a phenomenon that can in some cases cause a lightly-loaded chain to jump teeth. However, calculations proved that continuous loading on ANSI 35 chain during a hard drag would be approximately one-half its rated tensile strength, which was not acceptable. With ANSI 40, the loading was reduced to about 20 percent of tensile strength, which although higher than recommended, was still well within acceptable limits.
The spacing between the side frames is such that the wheelsets are able to laterally float in the bearing boxes about 1/4 inch. This produces a relationship between the truck and track that is analogous to that produced by the lateral springing and damping in the full size truck. Without lateral float, track conditions would have to be pristine to avoid derailment at any speed above a crawl.
In the second photo (above), you can see how the brake caliper bracket engages the bolster—I used a stripper bolt for a removable pin. The two large braided hoses extending to the right are how the hydraulic plumbing is connected to the traction motor—they engage bulkhead fittings on the underside of the frame's "tub." The hose and fittings are -10 size to mitigate the pumping losses that hoses insert into the system. This is the only place in the propulsion system's high pressure circuit in which hose is used.
The above picture is of the bearings used to support the locomotive's weight on the truck. The truck is physically attached to the frame via the flange bearing that is centrally attached to the bolster. This bearing is a 1/2 inch, self-aligning, industrial unit that is bolted to the underside of the frame. I'm taking advantage of the fact that being a self-aligning unit, the bearing cartridge can swivel in any direction inside the housing without binding or otherwise impeding movement. Hence it acts like a heavy duty spherical rod end, minus the threaded shank.
During assembly, the flange bearing for each truck is bolted to the underside of the frame. After the frame has been lowered onto the truck, the bearing's hub is secured to the truck bolster beam with a 1/2 inch socket head capscrew that is inserted through a hole in the frame and into a tapping plug in the bolster. The capscrew is torqued to around 120 lb/ft—if it were to come loose and work its way out, the truck and frame would part company and there'd be one heck of a wreck.
The two small ball bearings—lateral roll bearings—on the truck bolster make contact with wear pads on the underside of the frame. The center bearing is shimmed relative to the bolster so that a very small amount of contact between the lateral roll bearings and the frame is maintained at all times. Hence the truck is free to swivel and longitudinally roll relative to the frame, but cannot laterally roll. Any lateral roll that occurs with the unit is due to movement of the wheelsets in the truck frame pedestals, which is resisted by the springs. The result is that the truck can easily follow abrupt changes in track height, such as might be encountered when moving the unit from a transfer table to a lead track, yet maintain good lateral stability.
Each truck assembly weighs approximately 90 pounds and is handled with a cherry picker and a 1/2 inch lifting eye that threads into the bolster's center bearing hole. Additional pictures will be in the next post.