One issue that I became aware of some time after placing the order for the GP40 was the difference in wheel standards for 7¼" gauge in the US compared to Australia. The common US profile is defined by the International Brotherhood of Live Steamers (IBLS). The corresponding organisation in Australia is the Australian Association of Live Steamers (AALS). The area of concern was the wheel check gauge, which affects how the wheels interact with the frog and check rails when negotiating points (turnouts). Despite the common 7¼" track gauge, the IBLS wheel check gauge was 3.5 mm wider.
Prior to installing the electronics and batteries in the GP40, I did a couple of test runs around the railway being pushed by the Planet 2 locomotive. As I had feared, the loco did not run smoothly through the curved, diverging, leg of points as the wheel flange hit the tip of the point frog, rather than being guided clear by the opposing wheel and check rail. I only tested at very low speed, but there was a very noticeable bump at the point frog.
A friend in Perth, Richard Stuart, who had a fully-equipped workshop for building miniature trains had previously offered to help if any modifications to the wheels were required, I removed the bogies from the GP40 and brought them to Perth on the next visit.
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| Underneath view of Accucraft GP40 bogie |
The bogies were designed to be convertible between 7 1/4” and 7 1/2” gauge, the wheels being held in place by circlips in axle grooves, with round ended feather keys to prevent the wheels from rotating on the axles. As supplied, each end of each axle has 2 sets of grooves, offset by 1/8” (3.2 mm), to allow for the gauge change.
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| An axle in the lathe, ready to machine a new groove for a circlip |
As there was no spare space on the side with the drive chain, the solution was to remove each axle and machine a new circlip groove, displaced by 3.5 mm on the non-drive side. Each axle was then offset by 1.75 mm from its original position, to re-centre the wheels. The axles are retained in their position by grub screws on the bearing hubs.
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| Showing the 3.5 gap between the wheel hub and the innermost circlip groove |
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| Washers inserted between the frame (black) and motor mounts moved the motors towards the bogie centreline, to maintain chain alignment |
Also, the motors were moved inwards by adding washers to their mounting bolts, to maintain clearance between the chain and the back of the wheel.
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| New 3.5 mm thick spacer secures the wheel in its new position. The notch in the spacer is necessary to allow for the end of the feather key. |
Finally, Richard machined 3.5 mm thick spacers to go between the outside of the adjusted wheels and the outer circlips. It wasn’t feasible to machine a new groove for the outer circlip as the end of the key protruded beyond the face of the wheel hub, which would have interfered with the circlip. The new spacers required a notch to accommodate the end of the key.
