Universal joints (UJs) are essential for connecting motor and gearbox shafts in many 2mm scale steam locomotive builds, typically where the motor is in the tender and the gearbox in the locomotive drives the wheels.
Commercial options include N Brass Locomotive’s engineering plastic units for 1.5 mm shafts (now discontinued), Branchlines’ metal joints, and the 2mm Scale Association’s 3D-printed ball and socket type (shop ref. 3-264) designed by Julia Adams.
The Association’s joint is supplied as two pairs of male and female parts, enough for a complete motor-to-worm drive connection. The female part is intended to take a fine steel wire driveshaft. A small pilot hole is moulded into the female part to guide drilling for the driveshaft, with 0.3 mm wire recommended, although 0.4 mm or 0.5 mm can be used with care. Drilling through the part and gluing the wire in place strengthens the joint. The male part has a 0.9 mm hole that can be opened out for shaft sizes up to 2 mm, provided care is taken not to drill too deep and damage the socket. The joint is intended to be a light press-fit rather than forced together, and a light application of plastic-friendly grease in the socket can reduce wear.
Many modellers make their own UJs. A common home-made approach, described in the old Association handbook, uses telescopic brass tube for the sockets with a thin steel wire shaft between them. This produces a discreet and almost invisible connection compared with bulkier proprietary items. Variations include:
- Slotted Shaft Method: Both shafts are finished flush with the bearing face and slotted about 1.5 mm deep. A short length of 0.008 in guitar string (Top E) is used as the drive shaft, with each end bent into a loop of roughly 1 mm diameter. The overall length is made about 1 mm shorter than the distance between the bottoms of the two slots. This gives a simple, reliable, and slightly flexible coupling.
- Captive End Method: Similar to the slotted shaft method but with one end arranged to prevent the shaft dropping out during handling. This can be done by cross-drilling the shaft and threading the wire through, or by using a small retaining sleeve soldered over the slot.
- Etched Fork Type: Etched ends mounted in slotted 1.5 mm inside-diameter brass tube, connected by 0.25 mm or 0.3 mm sprung steel wire. Variants include turning the ends from bar stock, filing a “tooth” with a cross-drilled hole for the shaft, or using telescopic tube. These approaches don’t demand a lathe, though a lathe makes for neater results.
Success depends on accuracy and smoothness in assembly. While the principles are straightforward, consistent, free-running joints require careful measurement, drilling and alignment.