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Helical Gearbox

The interconnect system requires right angle gearboxes be placable at any intersection between a longitudinal shaft and each of the shafts connecting to the functional units. Because of the length of the shafts involved it is not possible to provide bearings in the main framework close to each possible interconnection point. Consequently, the gearboxes hang from the interconnected shafts (just as in the prototype). The interconnect shafts are arranged at 1" pitch, so it must be possible for an adjacent rod to pass by the gearbox at this pitch. Further, the rods connecting to the functional units are at 2" pitch. Because we require to be able to connect at any intersection point, the overall length ot the gear box must be less than 2" so that two gearboxes can be located on adjacent interconnect shafts and simultaneously on adjacent function unit shafts.

Matters are complicated by the fact that Meccano small helical gears do not mesh in pairs at standard half inch spacing, but require a slightly larger separation.  The picture below shows my current favorite design. The bearings are made from triple stacked 1½" strips. In each corner a threaded boss is screwed onto a rod socket just to the point were a bolt can still pass through the transverse bore of the threaded boss. This gives perfect spacing for meshing the helical gears. Four 1" screwed rods are used to set the spacing between the pairs of threaded bosses and between the pairs of rod sockets. This design is remarkably strong, easy to construct and usually works smoothly right away with very little adjustment. There should be no end float in the rods. This may require carefully selecting collars for thickness so as to eliminate all slop. By using three collars on each shaft, the space between the bearings is completely filled, and there is no possibility ot the gear slipping longitudinally on the rod, since the end thrust is transferred directly to the framework.
helical gearbox type 1

I have tried many other designs for this simple element before finally settling on this one. Some of these are pictured below though several others have gone by the wayside without being recorded for posterity.. As with most things in Meccano, there are many possible solutions. In all these designs, good lubrication is essential.

Below is a very minor variant. This on uses 2" screwed rods.  While it looks less cluttered, it has the slight disadvantage that if the screwed rods are not exactly centered, then when two of these gearboxes are at minimum spacing the ends of the screwed rods foul each other. Non purists of course may just prefer to shorten the screwed rods slightly!
 helical gearbox type 3
Another minor variant to overcome the 2" length limitation moves one set of the 1½" strips to the other side of the threaded bosses and rod sockets. This allows use of 1 1/8" bolts from one side. The closer spacing of the bearings creates a little more play, though it does reduce the amount of collars required.
helical gearbox type 2

Here is a variant using bush wheels for the bearings. It looks great in 100% shiny brass, but in practice getting good enough alignment of the bosses, both in pairs, and the exact right angle between the pairs, makes this design very hard to construct. It also suffers from the 2" screwed rods potentially fouling the adjacent unit, though that could easily be solved by changing to 1" screwed rods and bolts for the assembly as in the first design above.
helical gearbox type 4

Helical gears used for a right angle drive produce end thrust. Being concerned about frictional losses in the interconnect, I tried a variant incorporating built up thrust bearings. These each consist of a 4 hole collar carrying 4 bolts, each with a washer acting as a roller, sandwiched between a pair of ¾" washers. I found it necessary to use carefully selected short round head bolts to get adequate clearance from the screwed rods. A different approach to spacing the rods is used here, relying on the slots of 1½" flat girders to provide adjustment. A problem with this design is that once installed in the machine, access to the grub screw in the lower helical is very difficult.

In practice I have found that there is very little benefit from these built up thrust bearings compared to just using a regular domed washer as the thrust element. In fact it is inherent in the helical design that there is greater loss in the sliding action ot the teeth than in the the loaded thrust washer, because of the much greater radius at which the teeth slide on each other. Still, the action of the thrust bearings is fascinating to watch, especially when a large number of these units are operating together in the machine.
helical gearbox type 8

Corner angle brackets provide another possible element to establish the non standard spacing requirement. Here is an example using double arm cranks as the bearings. It uses a combination of couplings, collars, threaded bosses and thin washers to get the correct spacing between the bearings. It is very strong, but as with the bush wheel example above, getting correct alignment of the bearings and an accurate right angle is very time consuming. In addition, access to the lower helical grub screw is difficult once installed in the machine.
helical gearbox type 6

By using stacked 1½" strips as the bearings and making some adjustment in the spacers, the alignment issues can be alleviated, though there is still the practical issue of poor access to the lower grub screw. Part to part variation in corner angle brackets makes getting precise spacing of the shafts difficult.
helical gearbox type 5

A very compact design is possible by turning the corner angle brackets inside out. Again there are the issues of bearing alignment and access to the lower grub screw.
helical gearbox type 7

Finally, here is a very early and very simple design using 1½" flat girders and 1½"x½" double angle strips. It is not very sturdy and so can deform under heavy load. In addition, since the spaces between the gears and the bearings are not filled with spacers, the gears can tend to slip longitudinally on the shafts under overload conditions. This of course could be addressed by adding washers or collars as appropriate.
helical gearbox type 9

Last modified:  11 December 2004

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