fall inside a hole

Big Loader Chassis Repair

First written May 19, 2024

The Tomy Big Loader (ビッグローダー) is classic automaton-type toy in the vein of shuttle trains in which a powered chassis car traverses a series of tracks switching between different vehicle bodies and interacting with scoops, hoppers, lifts, and other components usually with the goal of cycling several small balls around between the different vehicles and accessories.

The Big Loader power car is rather ingenious but with time is prone to failure. The most common issues are an electrical problem, usually when batteries have been left inside the chassis and they have corroded, or a physical issue with one or more of the gears in the geartrain or the traction tires being worn out. Tires in my 1970s and 1980s sets have lost much of the flex to their rubber and on heavily-used examples can have the teeth worn flat. Newer sets from the 2000s often have intact tires and the 1998 Japanese release of the Thomas the Tank Engine Big Loader includes four spare tires that seem to also still be holding up. I suspect in Japan you may have been able to call Tomy and get more, and individual power cars were also sold in Japan in the 1990s.

Three screws hold the bottom of Big Loader's chassis on. Later on, the three cross-head screws were changed for triangular-headed security screws, seemingly in part because parents continued to unscrew the bottom of the chassis and drop all the gears on the floor when they just mean to put in the batteries.

The geartrain inside includes an interesting mechanical reverser that also uncouples the output of the wheels from the input, leaving the chassis free-wheeling when unpowered. The motor pinion drives a contrate gear which has an integrated smaller pinion.

The key to the Big Loader reversing mechanism is next - another gear that rotates inside a hole in the side of in a drum sitting freely on the shaft sits so that the drum-gear rotates against the smaller gear on the contrate gear. A small spring sits on the stem of the gear, keeping it pressed against the other cog integrated into the contrate gear - if this spring is missing, the chassis will have trouble reversing. This drum spins around freely until the protrusions on its surface catch with the reversing lever - this stops the "planet" gear riding against the gear integrated into the contrate gear and either engages with the clutch gear on the drive shaft directly or through a reversing idler that spins freely on the shaft the powered wheels are on, reversing the direction of the output shaft.

The drive shaft has brass bushings that I have seen in a few different orientations as the tooling and mechanism were adjusted over time.

The output shaft has a clutch mechanism to let the drive wheels slip if held in place while powered as well as the gear that drives the gear integrated into one of the rear wheels. The entire rear wheel assemble is pinned through a holder plate that holds two small gears that are positioned to engage with the racks and cogs sticking up out of the Big Loader tracks. The reversing idler is also free-spinning on the axle. On the Busy Robot Factory version of the mechanism, the hanging cogs are left out as the track has no racks to engage with.

The most common gears to split are the smaller ones, namely the 8 tooth gear on the 2mm motor shaft and the 12 tooth gear on the 2mm shaft that drives the drive wheel cluster. Another problem on the same shaft as the 12 tooth gear that engages with the drive wheel is the clutch mechanism holding the other gear on the shaft in place - a curvy interface between a collar and the gear is under spring compression and if the collar ring splits the spring will push the gear up against the plastic casing and the chassis will not operate properly. Unlike the gears, which usually do not mesh properly if reglued, these collars can usually be glued in place to fix this issue.

In the later 2010s the chassis was redesigned to have a screwed and angled battery cover which make its original two-AA origins a little less obvious.

The mechanism is mostly the same inside, although it appears to be an entirely new chassis with the rivets in the metal contact strips in different positions and the metal bar swinging up to the left instead of the right. The screws in the unpowered wheels now have flat caps, but the screws holding it together that had previously changed from cross-headed to triangular security screws were changed to five (instead of the older three) cross-headed screws.

A version of this reversing mechanism was also used in Turn the Terrible Tank, a 1979 "game" by Tomy with a spinning drill tank that reverses direction to face towards a player until they can hit the hanging reversing target underneath that makes them face towards their opponent.

Internally, it uses a souped-up version of the Big Loader-style reverser with a noisemaker and additional drive outputs to spin the tank's drill as well as make the "head" and the reversing target lift up and down.