Coupling types and removal

This page looks first at fixed couplings, with flexible couplings below.

The propeller shaft transmits drive from the gearbox to the propeller. It is connected to the gearbox by a coupling comprising two similar halves, one bolted to the gearbox output shaft, the other connected to the shaft by some sort of clamp arrangement. This may be a simple clamp, usually with one or more grub screws for location, but a wide range of other locating devices may be used. These may be axial keys and keyways or pins/bolts through the shaft. Shaft ends are normally parallel but tapered ends are sometimes seen.

Flexible couplings can allow greater angular misalignment and movement between gearbox and shaft.

Coupling flange removal

  Never hammer the propeller, or any other component, to remove the shaft from its coupling. Hammer impacts will be resisted by the output shaft bearing in the gearbox, which may well be damaged by this treatment.This method is usually effective. Undo and remove all clamping bolts between the flange and shaft. If there are any roll-pins or through bolts connecting the two, remove them also. Note the Bukh design below in which the clamp bolts also prevent axial movement of the shaft. Now find some temporary flange bolts that are around 25 mm longer than the original ones, threaded throughout their length. Two will usually be sufficient but more may be needed for persistent cases. Re-assemble the gearbox and shaft flanges with these bolts, but insert a large nut or similar piece of metal between the gearbox flange and the shaft end. Tightening the temporary bolts will press the shaft out of the bore of its coupling flange.
  An alternative that can also be used is to make up a plate with holes that match those in the flange, as has been done in this photograph. A longer packing was needed here to drive the shaft completely free of the coupling.In this case the shaft coupling is not a clamped type but is a press fit on the shaft, retained by a pin.

Photo:  Peter Lyle

 Fixed couplings

  Yanmar coupling. This type relies only on the clamping load of the two pairs of bolts to retain the shaft.Photo: Andy Gray
A coupling on a Volvo 2002. Two pairs of three cap screws are attached to the opposite coupling half in tapped holes. The bolt with a Nyloc nut visible at the centre passes through the shaft.Photo: Andy Gray
A Bukh shaft coupling. Axial retention of the shaft is achieved by grooves in way of the coupling. The shaft cannot be withdrawn until all bolts have been removed.Photo: David Wray
  A simple and compact coupling with only single clamp bolts. The shaft in this photograph is 3/4 inch. Yanmar 1GM10 engine in a Corribee.Photo: Steve Pavey
An elderly set up, on a Petter Mini Twin.   The shaft is held by two set screws and a taper pin through the coupling and shaft.Photo: Brian Bird
  A Volvo type coupling with three pairs of cap screws, located in the opposite coupling half. In addition to the clamping load of the six bolts the shaft is retained by a grub screw that locates in a dimple in the shaft. The grub screw is driven home after tightening the clamp boltsPhoto: Roger Smith

Case history – A confusing coupling. Words and photos ex by Wil Bailey.

I’m struggling to disconnect my 1″ propshaft from its ‘shaft clamp’ and that from the flexible coupling. There are four clamp-bolts which have been eased OK using an Allen key. The problem is the locking screw or stud….This is about 8mm diameter, and there is a 6mm approx. hole down the approximate centre. That hole continues right through the body of the shaft-clamp and well down into the propshaft, but not all the way through. There is a corresponding but empty hole on the other side of the shaft-clamp body but no dimple or penetration on that side of the propshaft.The ‘screw’ top surface was recessed into its hole perhaps 1mm, and I suspect a previous owner had sheared off the top-slot intended for a screwdriver, then drilled down the centre in a failed attempt to remove it. A colleague considers that it is a roll pin, but there’s no sign of concentric/spiral layers.
 I’d hoped to drill out most of the remainder of this ‘screw’, then use a stud extractor on the thin remains, so freeing the shaft. I could then unbolt the shaft-clamp from the flexible coupling, but want/need to remove the ‘locking screw’ first.I’ve tried 2 new left-handed HSS drill bits, one of 6.2mm and one of 7.8mm. These have failed to cut, despite using CT-90 cutting paste, and have merely indented or ‘countersunk’ a little the top of the screw/stud – as can be seen.
  I started this morning re-measuring and confirming all the previously-recorded dimensions, for I suspected I’d need to order a set of cobalt drill bits. While measuring the depth of the clamp’s ‘rear’ hole, I noticed that it was not centred over the prop shaft as was the other one – it was ‘out’ by about 4-5mm – and on cleaning out the crud with an handy engineer’s scriber, I thought I could just make out ‘two shades of grey’ metal, one of which could possibly be part of the curved wall of a roll pin. Further, there was a pinprick of light coming through from the other side – inferring that the prop shaft was drilled right through.Cleaning up in there a bit more, I thought that I’d need to drill away the wall of the rear hole in the clamp so I could get a small drift-pin in there to try tapping the thing out. I anticipated that would not be easy, as several had suggested, but I thought to try a fine centre-punch that was to hand, just to see……
   Locating the point was done by feel, and a few light taps were tried – all I could manage in the tight space. Feeling round the back for any sign of movement, I was heartened to find the roll pin – for that is what it was – had raised its chewed-up head up by a couple of mm. Halleluia! A few more cautious taps and the pin head was proud by about 4mm – enough to grip! A few moments with a set of nippers, some twisting, and the darned thing was free. All 40mm of it……The way was now open to remove the flange-clamp entirely, which proved straightforward, then the two components of the damaged stern gland assembly and its old packing, which was the reason for tackling this in the first place. One can see the sheared-off bronze stud which has been drilled off-centre. That can now be taken to a workbench and ‘fettled’ properly.
   The two holes in my flange-clamp are, confusingly, not in line. Had they been, the makers’ intention would have been clear, and I would have contentedly punched the pin out form one side or the other.That was not the situation I found. The two holes were misaligned by about 20-25°, so that using a pin-punch would not have driven the roll pin out through the ‘bottom’ hole.

Flexible couplings

Steel shaft coupling as supplied by Beta Marine (around 2006). A similar design to the Volvo one shown above, in that cap screws are located in tapped holes in the opposite coupling half, although they alternate in direction.An R&D Marine flexible coupling is installed between the gearbox and shaft couplings.

Photo: Bjorn Forsman

  A Centaflex CF-M-127 coupling installed on a 3 cylinder 20 HP Beta engine model BD 722.Photo: Brian Bird

The method of removal of these couplings is not obvious. Following a query on PBO forum the following was posted:

Remove the plastic caps and you will find tapped holes into which you screw 10 mm bolts. Tightening these jacks out the conical sleeve that grips the shaft. Remove the bolts around the rim of the sleeve first.

Screw in a bolt into the tapped holes to force the coupling’s tapered collet (male) out of the housing (female) A screwdriver can then be inserted into a slot in the collet to open it, upon which the propshaft can be pulled out. Although there are three it seems that just one is adequate unless the joint is corroded.

Text: Parsifal and Minchsailor.

The manual for this coupling is available here

Thanks to Philip Leith.

 An elderly Layrub coupling using metalastic type bushes.They are still available:

Thanks to Andy Gray for the information

Note also the retention method of the shaft by an offset bolt through a groove in the shaft and possibly an additional pin.