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What are Coach Bolts and Nuts?

December 1, 2017

couch bolt

Just as a quick check, are we to assume every fastener has some kind of geometry on its head? You know, either a slot or a star shape? Strangely enough, no, that's not always the case. Coach bolts don't adhere to this rule, yet they still mate their threaded rods to an accompanying nut. How is that possible? Won't the bolt turn if it's not properly anchored?

Anchoring Coach Bolts

As we said, there isn't a slot or tool-mating edge on the head of a coach bolt. Viewed while fastening two thick beams of wood in place, the dome head is featureless. Nevertheless, it somehow functions as an efficient fastening rod, one that anchors itself while a heavy nut is threaded onto that rod. What gives here? What's the secret anchoring strength hiding behind the blank dome bolt head? The answer lies just below the fastener crown. Take a closer look. The neck of the bolt is shaped as a square. Inserted into an opening, that head engages against an iron plate. One on either side of the timber surface, the plates lock against the square section so that the nut can be easily tightened with a single tool.

A Real World Application

The smooth dome of a coach bolt eliminates ragged fastener edges, so imagine this product being used in the wooden panels of a boat or some other wooden surface that relies on a streamlined profile. Alternatively, there are countersunk versions of this fastener type, versions that completely eliminate the need for an ugly screw or bolt head. Like the rivets that fasten metal plates together, this wood-themed variant eliminates head geometry while providing an application approach that favours a single tool. Again, and this is most definitely a real-world application, many carpenters simply pull out their favourite hammer when they're putting in carriage bolts and nuts. A few taps of that heavy-impact tool are enough to drive the square segment into soft wood, at which point the workpiece is turned over, the nut is tightened, and there's no risk of a freewheeling bolt head.

Coach bolts (carriage bolts) and nuts have been around for a long time. Originally used to build old horsedrawn conveyances, they're still in use today, especially in the timber industry. Threaded at one end and bereft of threads at the shank end, a square self-locking neck sits below a featureless fastener head. It locks against iron plates, or, as is the usual practice today, against and through the bare lumber, at which point it's locked in place. Unable to rotate, the final action here involves the tightening of an accompanying nut.

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Address:
TCI Fasteners - Topcope
13 Slater Parade, Keilor East VIC 3033 Australia

Telephone: (03) 9336 0155

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