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Bruce Hansen's Erector Clamshell Derrick

Bruce writes: "This model is called the 'Derrick with Clamshell Bucket' built from a 1960 12 1/2 set. The action in this model is a manually raised/lowered crane boom and an A49 powered clamshell bucket for scooping up a load. The derrick cab rotates about a base via a typical BN turret plate setup; the whole derrick is driven manually with NW/NX treads and pulleys.

I modified the model as shown in the manual by adding another 7 1/2 inches to the boom with additional C girders; the height seemed better proportioned with the large clamshell bucket. I also used an NK ratchet and P13 gear to keep the bucket from free-wheeling to the floor when the motor was shifted to neutral. Also, I put a CJ 36 tooth gear on the P24 crank used to raise/lower the boom and screwed an O pawl to the cab to stop the boom from dropping to the floor when not holding onto the crank.

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The clamshell bucket was unique to 12 1/2 sets from 1956 - 62. Here's how it works: The raising/lower string is attached to what I'll call a 'lifting link with hook' at the top. A second piece of string is tied to the bottom of the lifting link near the hook. When the clamshell is lowered in the open position, the 'lifting link' continues to drop after the bucket has touched the floor; it slides freely through the top links of the clamshell.

     

When the hook end of the 'lifting link' gets below the axle pivot for the two halves of the bucket, the motor can be shifted to neutral or reversed. The piece of string tied to the hook is maneuvered so the hook catches the pivot axle as the 'lifting link' is raised. As the 'lifting link' raises, the hook end pulls up on the pivot axle causing the clamshell to close. It's a clever mechanism that works well.

     

To dump a load, reverse the pickup process. The clamshell is lowered to the floor. After contact, the lifting link hook is maneuvered off the pivot axle with the string. Then the clamshell is raised which opens the clamshell back up. The finished model stands about 24" tall and went together with no problems."