fall inside a hole

Motor Tomica automatic turntable repairs

 In the mid 90s as part of the Battery Operated (B/O) and Motor Tomica ranges Tomy developed a small automatic turntable piece which would turn cars around automatically using rollers in the road surface that drive a gear along a curved rack inside. In the 2000s this was adapted to have a different entrance and exit in the Thomas and Terence Deluxe Timberyard Set with the help of some springs and later the same concept was adapted into an automatic rail turntable for a single power car as part of the Chuggington range.

With age the gears inside contract and split around the metal shafts they are mounted to. This locks up the mechanism and makes it impossible for cars to turn it. The turntable should be fairly easily actuated around by hand which will cause some of the rollers to spin. If a turntable is not working properly it is almost certainly because of split gears.

A single screw on the bottom of the turntable holds the rotating platform into the static base. Five more screws, two shorter countersunk screws and three longer ones with rounded heads, hold a plastic carrier plate onto the bottom of the road platform and keep the shafts in place. All shafts in the turntable are 2mm in diameter like many other Plarail shafts and all the gears are 10 tooth gears like those used in some other geartrains.

Two long ~58mm shafts run from one side of the turntable platform to the other and couple the rollers to free-spinning contrate gears with integrated gears that run along the curved rack in the static base. Four of the six roller shafts (the four closest to the middle of the turntable) are ~48.5mm and are geared against the rack while the outer two ~42mm shafts are free-spinning. Two much shorter ~9mm shafts contain only a ten tooth gear and couple the two active rollers on each side together. In operation, only one pair of these rollers is activated by the Tomica's rear drive wheels while the unpowered wheels sit on the other set of rollers. When the car turns the turntable around and exits the other pair of rollers is now in position to drive the turntable the other way around back to where it began when the next car arrives. I have noticed that some of the rollers also contract and split. If the rollers are free-spinning on the shaft they can probably be glued back in place without too much of an issue because the rollers already do not mesh "perfectly" like gears and also have help in being driven by the rubber of the vehicle's traction tires.

When trying to put one of the short shafts into a new gear I ended up dropping it on the floor and losing it. I ended up cutting a section that was slightly longer than the original out of some spare 2mm shaft from a junk gearbox but the length does not matter that much as long as it is not so long as to hit the inside of the outer lip of the platform as the gear holds it in the correct place and the outside of the molding to hold the shaft does not restrict the length of the shaft installed. I would recommend lining up the shaft with the new gear while still installed in the old gear and hammering the shaft down into the new gear to prevent the shaft from being lost. The old gear can then be pried or popped off and the shaft hammered the rest of the way down into the new gear. It occurs to me that a better long-term solution may be a plastic gear with integrated plastic shaft molded as a single piece, and I suspect that might be what Tomy would do if they were making these turntables today, as it would probably cost less (after initial startup cost - my guess is that as Tomy was already making these gears and metal axles it was less work and initial expense to make more of the same gear and cut some new axles than it would have been to design and manufacture new toolings for a plastic gear with axle and integrate that new machine into the production line. Tomy has used plastic gears with integrated axles for other things, even around the same time, but I do not think they already had one which would have worked in this situation.)

The failure rate on these gears seems fairly high. I had to replace eight out of ten of the gears in the green turntable shown and several people have reported that the Thomas series version of the turntables are also failing due to broken gears on the Blue Plastic Tracks forum. More details on the Thomas series turntables to come.

The Chuggington version of the turntable for rail vehicles operates similarly but only has two driven rollers instead of four and 12 tooth gears instead of 10. A similar rack and gear system is used but the rollers engage directly with the contrate gear molded into the gear that runs along the rack as multiple rollers do not need to be coupled together. As these turntables are more recent, I do not think the gears inside have had enough time to warp and contract with age. Perhaps a few decades from now these turntable will have similar gear problems.