Experimental Detent Mechanism
I’ve found the most difficult part of building a gearbox is a reliable detent mechanism to hold the gears in or out of mesh. This attempt wasn’t entirely successful, but it does use only Meccano parts.
Essentially, the domed ends of axle rods are pushed part-way into the holes of a perforated strip by compression springs bearing on collars. The collars have just enough of the axle rods protruding to allow them to slide from one hole to the next. Using a 2” strip with the three close holes in the middle gives an appropriately short travel between ‘clicks’.