The GSM 20 - The Giant Block-setting Crane 1971 by G. E. Rudings of South Africa.
Even though the leaflet text is quite clear, the illustrations of the gear box were puzzling. Thanks to Mr. David Williams, who sent me nice pictures via e-mail, I could finally figure out its mechanism.
For easier adjustments, and to increase the reliability of the mechanism, many parts of the original were replaced. For this article, I have decided to limit myself to those improvements that I thought were the most significant. Experienced builders and those who, in the past, have built this crane, will all easily identify the substitutions using the pictures.
1 - The Roller Bearing: I replaced the eight 4" Stepped Curved Strips and the 7½" Circular Strip with a 7½" Circular Plate, and fixed at its centre a Metallus long boss Bushwheel no. 4018-39.
2 - The Boom: To have perfect vertical girders, and to eliminate excessive stress, I used in some places end slotted strips and extra 1½" Strips under some of the 1½" Angle Girders. I replaced the 1½" Strips that joined transversally the sides of the boom by 1½" x 1" and 1½" x 1½" Flat Plates to ensure parallelism. To limit the number of washers needed, I replaced all the 1½" x ½" Double Angle Strips by Angle Brackets and 1½" Strips. I replaced the six 5½" x 3½" Flat Plates bolted together to form a compound plate by a single 9½" x 9½" Flat Plate (one that our Product Manager, Mr. David Duncan, ordered for me).
3 - The Traveler: To serve as guides for the rope under the traveler, I fixed 4 Handrail Couplings (part No. 136a), and to haul the traveler I replaced the ordinary cord by two 2 meters elastic cord that I bought at a very cheap price in a needlework shop. Only a few turns of this elastic rope around the wood roller were necessary for satisfactory hauling, and there is no need for springs to provide proper tension in the cord.
4 - The Pulley Block: I replaced the 2" Threaded Rods by Threaded Spacers, and the Corner Brackets by Metallus Bearing Mounts no. 4650-07.
5 - The Mechanism: In the original plan, there is one selector rod for each of the 4 movements of the crane. In this way it is theoretically possible to operate all the movements simultaneously.
I thought that I did not need such flexibility. Two movements at the same time, and changing the direction of the movement by only reversing the polarity of the motor power using an electrical switch was fine with me. In reality I found that I always operated my crane one movement at a time. So I replaced the four ¾" Pinions on the Rod with Keyway with only two, and I replaced the four 1" x ½" Double Brackets with 4 Cranks. By sliding one of the two selector rods from one end to the other, the ¾" Pinion enclosed in the Crank engaged with one of the two Contrate Gears under it. The neutral position was therefore just at the centre of the throw of the selector rod.
Under the gear box, I also replaced the Threaded Rod by Threaded Spacers, and I replaced the Coupling which journalled the rods for the slew and hoist with a 2½" x 1" Double Angle Strip.
For those who plan to build this magnificent model I suggest that you first find a proper place where it can be shown to good effect. Mine is now in the guestroom!
This article appeared in the December 2014 issue of "Canadian MeccaNotes". From this web page you can download the complete set of photos using the link below.
Copyright 2014 by CMAMAS.