Grand Forks turntable saga – Part 1: Problems with the Walthers kit
A turntable at Grand Forks was always part of the layout plan, but it took a while for me to get “around to it”. I decided on the simple Walthers 90′ kit as I was not in favour of scratchbuilding one. I figured that as it was widely available and had been on the market for a number of years, it should at least prove to be a decent basic unit, even it not powered. Newer versions are now available that arrive completely assembled, and include an electronic indexing system, but those were not yet out when I purchased my kit.
I decided to just assemble the kit as carefully as possible, according to the instructions, even though there were some aspects to the parts that gave me pause. A turntable is a very basic animal, but it has to be quite precise in a couple of aspects. It must rotate smoothly and stay where it stops, as there is not much room for error side-to-side if the locomotive is to stay on the rails. The height of the ends of the table must match very closely the height of the fixed track rails as well. This is all regardless of whether or not the unit is powered or rotated by hand.
Unfortunately, the assembled kit failed in all of these aspects, due to three major areas. First, the small wheels at the ends of the table that ride on the circular ring-rail were very badly formed during the plastic casting process. This caused the table to ride up and down as it rotated, due to the eccentric shape of the wheels. This caused the table to “cog” as it rotated, tending to a favour positions where the wheels were at a low spot. Accurate positioning was all but impossible. Second, the main bearings for the table shaft were very sloppy and would allow the table to move about side-to-side rather than maintaining an accurate centre position. This translated to significant movement at the ends of the table, again, making accurate positioning impossible. And finally, the gear drive for the motorization kit would not stay in mesh, rendering the power drive useless. The first two of these were corrected, and the motorisation was abandoned.
It was obvious that the table wheels and the centre bearings were flawed by simple visual inspection, but the drive gear required some more investigation to determine why it would not stay in mesh. There were two possible issues, only one of which could be corrected. The centre hole might be off-centre, which would be simple to fix, but if the outer edge of the gear was not circular, then it would never stay in mesh regardless of how well centred the hole was. Unfortunately it suffered from both of these problems, so I decided to abandon that part.
The solution to the poor centre bearings seemed to be to use ball bearings, but the issue was how best to mount and align them. The table wheels just needed new ones carefully turned on the lathe to have minimal play.
One other issue was that way that power was conducted up to the table rails. It used a pair of brass slip rings that were very difficult to get aligned correctly, even after they were rebuild a couple of times. The main problem was that the bits were mounted on the turntable pit base, the centre shaft of the table, and the plastic cover that held the lower bearing. It was impossible to see how well everything lined up because it was all inside the cover, so it took lots of trial and mostly error attempts to get it just right, sort of.
The dual ring design did not provide any means to reverse the track polarity of the table, which would have required something like an automatic reverser or even a manual switch (error prone!). This all had to go and a much better means devised for powering the table.
When I was assembling the kit, following the instructions, I had a bad feeling that it just might not work out so well that I decided to fasten together some of the major sub-assemblies with screws rather than gluing them, just in case!. Very fortuitous decision, as it did have to come apart many times during the investigations and rebuilding.
By this point, I was so fed up with messing with the turntable that it would be almost 4 years until I finally got back to tackling its problems. All of that process is detailed in Part 2 of this saga.
