Prop coupling bolt failure

[Ian Grace]

Roger Burnett has reported a prop coupling bolt failure on his M Type Coupe at MGCC Silverstone. His bolts are high tensile steel.

Has anyone else ever had a prop coupling bolt break? I have not, and have always used mild steel bolts - maybe mild steel is more forgiving than HTS bolts when it comes to shear loads?

All I know is that when a bolt has to take shear loads, it should be tightened sufficiently so the friction between the bolted components takes the shear load, not the bolt, but with rubber couplings, tightening is not an option.

Who can say more?

[prharris25]

Is this a flexible coupling ? If so, yes. It did a lot of damage to the metal rear floor of my 10/4.....wouldn't like to think what it would do to the wooden back end on a Minor. One bolt went, followed immediately by another and the prop shaft then described a large arc smashing into the pressed steel floor pan. As to whether they were HT bolts, I don't know... but it took a lot of putting right ! As always of course it happened deep in the Cotswolds on a Sunday morning. Always glad I carry half a car load of spares behind the back seat.
Paul.

[Ian Grace]

Yes, it was a fabric copupling.

[Simon]

Some thoughts. I think it is important that bolts with sufficient unthreaded length to pass through the spider and at least 50% of the fabric coupling are used and NOT set screws (threaded for the full length). This ensures that the full area of the bolt is available to resist the shear and not just the core area of the threaded portion, there is quite a reduction in area for a 5/16" bolt. The bolts should also be inserted with the head of the bolt behind the spider. The spider should also be checked to ensure that the bolt holes are not worn oval and that all three bolts are parallel and do not flop about. I have taken to using Nylock nuts.