Shaft/coupling problem

[Guest]

I have a problem that I hope someone can solve!
I have a Seamaster 925 with a mitsubishi herald diesel engine which is maintained well, but I have had a problem with the drive shaft coupling on two occasions.

On both occasions I have been entering a berth or rafting along side, giving blasts of power in forward and reverse to manoever myself, and suddenly I lose all thrust, due to the shaft coming away from the coupling.

Luckily I have been in a situation where I got help from other boaters to secure myself.

The shaft slots into the coupling with a key inserted into the groove between the coupling and the shaft.
An M8 SS bolt slips through a hole in both to secure them.

On examination of the bolt, it had sheared cleanly at the centre.
What I don't understand is how any load is being applied to the bolt, surely the key should take this, and how it is shearing at the centre?

Luckily I could change the bolt while the boat was still, but with the engine in the usual awkward position on most yachts It is very fiddly, I wouldn't like to try it at sea.

Can anyone suggest either another type of bolt, or method of securing the shaft into the coupling, or any other advice?

Regards

Sean Foster

 

[tr7v8]

If the M8 bolt breaks then I'd assume it's taking engine torque. Yes you are correct it should be purely taken on the keyway, but normally this would be done with a taper and a keyway on the shaft. I suspect the keyway has been fretting which has made it loose in either the shaft and/or the coupling.
So bad news is that it needs to come apart and be investigated thoroughly.

Jim
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[yoda]

The key way will only transmit torsional load. When going from ahead to astern when manoevering there can be quite a considerable axial force. The bolt needs to be a good fit through both shaft and coupling to ensure maximum possible effectiveness by minimising the relative movement of the 3 components. If 2 bolts have already gone then it is quite likely that there is slack somewhere in the holes and they will need a careful check befor you go further. Matching holes and bolt size needs to be done carefully and may require a bolt making to suit (a 'fitted bolt')

Yoda

 

[30boat]

You can get shaft couplings from A.S.A.P that rely on friction to hold the shaft and are very reliable.