EMD F7 in SCALE
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A number of posts ago, I had described how the components for the F7's trucks had been made by
Bob Snippe but due to a number of engineering issues, were less than satisfactory. There was more to it. I had gotten the unit out to the railroad in 2008 following a period of serious illness (I was a year behind schedule at that point in time) for a series of shakedown runs. Generally speaking, it ran well and things mostly worked as designed.
- First-Ever Run of the F7
However and almost immediately after putting the unit on the track, I encountered some new problems with the trucks.
Rather than press-fitting the wheels to the axles, as is customary practice, Mr. Snippe had made them a slip fit and secured them with setscrews. It didn't take long for the wheels to start working loose and going out of gauge. I discovered the problem when I was passing through a turnout and the rearmost wheelset picked the frog and derailed. One of the wheels had loosened and drifted outward enough to catch the point of the frog. My remedy at the time was to jack up the locomotive, get the wheel back into position and tighten the setscrew as much as possible. Later on, I returned the unit to my shop and replaced all of the original wheel setscrews with new ones with knurled cup points (made by SPS Technologies). These tend to lock into the surface against which they bear—the axle key in this case—reducing the likelihood of loosening. It was a temporary fix at best.
Over time and despite the better setscrews, loose wheels continue to be an issue and as I put more operating time on the unit, the wheels started to wear in odd ways, suggesting a faulty profile, metallurgy issues, or both.
- Old Wheel Profile
I noted that the tread-to-flange fillet was turning into a sharp corner, which would tend to increase the possibility of the flange climbing the edge of the railhead. I had already concluded that the constant loosening of the wheels was an artifact of the material with which they and the axles had been made, which was too soft, possibly some sort of free-machining steel. It was clear that these wheelsets were not going to work out and would have to be replaced with something better, with the wheels pressed onto the axles to keep them tight and in gauge. As it turned out, construction of a control (riding) car to go with my F-unit gave me the direction I needed to make proper wheelsets for the locomotive.
- F7 Control Car
The control car's trucks were built up from some old
Tom Bee Blomberg style side frames I had, with the wheelsets being made from 1.6 inch scale Diesel wheels also made by Tom—these wheels incorporate the same machining characteristics that Tom has long used in his freight car wheels.
- Tom Bee Diesel Wheel Profile
Note in the above photo of the Tom Bee wheel profile that the tread-to-flange fillet is more pronounced than on the Snippe-machined wheel. This enlarged fillet causes better tracking and a reduced tendency to pick turnout points and frogs.
Axles were machined from C1144 TGP, with the wheel mounting surfaces sized to produce a four to six ton press fit. Running under the control car during the 2011 and 2013 seasons (I again fell ill in late 2011 and didn't resume railroad activities until some eight months later), the wheels showed no tread or flange wear, and were staying in gauge.
Seeing how well the wheelsets were performing under the control car, I made up drawings for new locomotive wheelsets, and got the pieces made and assembled.
- New Wheelsets for One Truck
As with the control car's wheelsets, the wheels are a press fit to the axles, with a little bit of anti-seize compound being used to prevent galling as the wheels were pressed into place. The sprockets and disc brake rotor are a precision fit on the axle and are secured with knurled cup point socket screws that bear down on both the axle proper and on the key. Keys are made from C1144. The axles run on full sealed ball bearings.
Since the above photo was take, the wheelsets were assembled into the trucks and will soon (I hope) be rolling down the rails. However, I'm not yet done with improving the trucks.
I'll save that for another time.