Quote:
Originally Posted by Rowglide
Acme thread and trapezoidal threads both have flat tops and bottoms, and very steep angles (29/30 degrees). If you look at the photo they appear to be triangular / pointed at the top and have a large angle... no need to make a simple problem complicated.
I would certainly try a standard thread first.
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Yes, I think you're right Rowglide. On careful
inspection of the photo, the OP's dog bolts do look as if they have a standard triangular machine screw thread.
And perhaps that's the source of the problem. Portlight dog bolts traditionally (and still from good manufacturers, such as from Dover Marine Manufacturing) were cut with Acme threads. The Acme Manufacturing Company invented the Acme thread as an improved trapezoidal thread for worm drives and the like. The idea being that the friction between the external thread and its nut was lower than in the case of a standard square-topped trapezoidal thread.
The triangular machine screw thread is designed for lots of friction: that's what keeps nuts fastened to bolts.
So my guess is excess friction between the dog bolt and dog nut, due perhaps to some surface
corrosion on the faces of those triangular threads, prompted a user to apply additional force to the dog nut, causing the dog bolt to fracture. Acme threads might not have led to the problem.
So perhaps time for an upgrade all round to Acme threaded dogs from Dover, so the
portlights last the life of the boat?