EMD F7 in SCALE
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In one of my earlier posts, I noted that the real F7 didn't have a separate frame, relying instead on a truss inside the carbody to support the machinery and absorb the buff and draft loads. Such a design isn't very practical in a model, so my F-unit has a separate frame and the body primarily plays a cosmetic role. Also, as I earlier noted, the frame has a dropped center to position the prime mover (Briggs & Stratton V-twin) relatively low in order to maintain clearance with the body.
A lot of riding scale Diesels are built on ladder frames fabricated from rolled shapes like angle and C-channel. This is style of construction is practical because the prime mover is small enough to not cause clearance issues with the roof of the body. For example, the well-known Backyard Rails F7 has a ladder frame built from sections of angle welded to form a tee shape, with no change in height from end to end. This was feasible because the single cylinder Kohler engine that powers locomotive is small enough to clear the roof. This is not the case with the V-twin.
As a beginning exercise in designing a frame, I had to decide the structural material I would use. Building a dropped center frame from angle and/or C-channel would entail a lot of cutting and welding. The cutting part isn't bad, as I have a pretty substantial horizontal band saw that can make accurate cuts. The problem would come in jigging and welding the mess.
Maintaining alignment in a 6 foot long weldment built up from angle—the weakest of the rolled structural shapes—would be difficult. Also, the resulting frame would not have good structural and torsional rigidity and thus would be prone to sagging and twisting due to the concentration of weight in the center (recall that the power assembly weighs about 200 pounds).
An alternative would be C-channel, which is a better structural shape for this application. However, I would still be faced with jigging and welding a lot of parts. Maintaining planar and longitudinal alignment in a complicated weldment would be quite a challenge. That's just a fancy way of saying that the frame would likely pull and distort from all the welding and end up with a twist or bow, or both. Frames like that tend to provoke derailments, since the trucks are operating in different planes, causing equalization problems. Needless to say, I wasn't at all keen on struggling with
that problem.
I gave this a lot of thought over a period of several weeks, going through a small mountain of paper in the process as I made and discarded sketch after sketch. In the end, I devised a frame design with only three structural members, requiring a minimal amount of welding. During the design process, I had decided that rather than try to fabricate the basic structural components myself I would get a properly equipped job shop to cut and form the required pieces. I could then fit them up in my shop and take care of the welding.
As the amount of welding would be relatively small, the welds' effect on planar and longitudinal alignment would be small as well. Plus with only three members to join, jigging would not be complicated. I should mention that among my various items in my shop is a large piece of 3/4 inch thick aluminum tool plate. This piece is rigid enough to act as an accurate base on which to jig parts for welding, and hence gets a lot of use. In the process of designing the frame, I also concocted a method of jigging the parts to produce and keep the necessary alignment while welding.
Two of the frame's members are formed into the shape of a shallow U-channel, which is a simple structural shape to fabricate. The center section of the frame resembles a tub (in fact, the filename for the drawing says "tub") and is where the power assembly is mounted. I devised a six-point mounting scheme for the power assembly, four bolts securing the base of the V-twin to the bottom of the tub and the two lower bolts that hold the coupling support to the PTO adapter tie rods passing through the tub's rear bulkhead.
As some pictures are worth many words, here are illustrations of the frame.
- Frame Assembly
- Frame Forechannel, 1 of 3 Structural Members
- Frame "Tub", 2 of 3 Structural Members
- Frame Aftchannel, 3 of 3 Structural Members
The first picture is the frame assembly, which is the basic frame weldment with other parts attached (the long narrow bracket with elongated holes going across the frame is the support for the air compressor and one end of the main air reservoir). If you study this illustration you will see that the weldment consists of the forechannel (second picture), midchannel or tub (third picture) and the aftchannel (fourth picture). The cut out area at the left (forward) end of the aftchannel clears the power assembly's pump adapter, as well as the pump's mounting flange.
Some of the holes you see in the structural members are tooling holes used for alignment on the jig plate. I made an adapter that allows me to use a Black & Decker laser level as a sighting device to align the three sections on a common centerline. I'll put up a picture of that arrangement in a future post.
Once I was satisfied with the basic premise of my frame design, I constructed a mockup in my basement shop to verify my calculations. With everything verified and corrections made as necessary, I submitted the drawings to a local job shop. The parts were laser-cut from 1/4 inch hot rolled P&O steel and then formed in a large brake press. The combined weight of the three members after welding was approximately 135 pounds. As I earlier noted, weight is your friend in a locomotive, and a strong frame is a good place to have it.
Laser cutting is generally quite accurate, even when forming holes, as long as you understand that there is always a small amount of taper in the cuts. In the case of holes that have to be a specific size, I specify them undersize and then finish them as necessary to get the proper size. In thick sections, I like to use piloted counterbores for sizing larger holes.
Of the three structural members, the tub is the most complex, as it is formed on all four sizes and the forward bulkhead has two bends for clearance with other parts. The three holes cut in the front bulkhead are there to provide unrestricted airflow to the V-twin's cooling blower. The tub's rear bulkhead is an exact 90 degree angle to the tub's bottom, since that bulkhead is an attachment point for the rear of the power assembly. Below is a picture I took when I attached the aftchannel to the tub.
- Rear View of Partially Welded Frame
In this view you can clearly see the aforementioned cooling holes in the tub's forward bulkhead, as well as internal gussets in the tub and crossmembers in the aftchannel. There are also tapping plugs welded into the vertical walls of the aftchannel, they being used for attaching various items to the frame.
I'll put up some more pictures in my next post.