Saturday, October 20, 2012

An Unorthodox Coupling Nut Application

I've written before about coupling nuts -- about what a useful, versatile standard hardware item they are. Here's another example of an unorthodox application that got me what I needed with a minimum of machining.

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The Problem

I have an old 230V space heater in my workshop that works fine, and looks likely to last forever. It has one little annoying feature, though -- its thermostat is mounted at the lower rear of the unit, like so.

(That's actually a sensible place for the thermostat to be, where it can determine when to switch off based on the temperature of the incoming air.)

The way I have the heater situated, it's awkward to have to reach around to the lower rear of it to switch it on. I could see a way to install a control knob in front while leaving the thermostat in place at the rear, but I needed to fabricate a 3/16" diameter rod-coupling with a little bit of axial 'give' to it to compensate for imperfect alignment of components. A 1/4"-20 coupling nut came to the rescue. Here's a view of the rod-coupling I came up with.

I love that sort of work. I won't bore you with all the details of it as I'm usually prone to. The key to success is extremely careful layout, centre-punching and setup in a good drill press vise.  The key to the coupling's axial 'give' is the cotter-pinned end. That part of the coupling is 'sloppy' enough to compensate for an imperfect rod axis from front-to-back of the heater. The setscrew fastening at the coupling's other (left) side end gives angular adjustability, so the knob can be positioned correctly angularly.

The brass bushing at the left was glued onto its shaft-end with CA adhesive. The brass bushing at the right was left a free slip fit. Here's a view of the coupling installed in the heater.

And here's the front of the heater with its new control knob.

The knob's 'directionality' is the reverse of what it should be, but that's not difficult to get accustomed to. The whole arrangement works perfectly. (Concealed by the knob is a Heyco SB 312-3 snap bushing that's serving as a 'bearing' for the control shaft.)

Anyway, that's a good example of the versatility of coupling nuts. I could have fabricated much the same thing from scratch from a similar length of plain steel rod, but the hexagonal coupling nut was easier to work with for my purpose here.

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