Saturday, 6 May 2017

Lowdown on the gearbox

Lowdown on the gearbox
May 2017

Thanks to all of you who expressed such concern about our gearbox.

I am glad to say that our problems have now been resolved with a broken 120 PRM removed and replaced with a shiny silver 125, fresh from Beta.

The plastic bits shouldn't stick out.....

To be honest my biggest fear was that having removed and replaced the gearbox the problem would be diagnosed as a twopenny ha'penny washer, or something like it. As it turned out the contractors had real trouble getting the old one out. Firstly they tried to withdraw the propshaft only to see the driveshaft pulling out of the gearbox, which was unexpected as it is supposed to be held on by a locked nut. This problem resulted in a 24 hour delay whilst a puller was obtained.

On day two we had two engineers turn up at 8.30am and this time progress stalled when they attempted to remove the old drive plate from the flywheel. It appears that when it was first assembled the allen studs were imperial and, of course, the only keys they had were metric!

This is how a drive plate shouldn't look!

With this problem resolved the old driveplate was extracted and whaa sorry state it was in.
For those of you unfamiliar with the inner workings of the bell housing the driveshaft from the engine leads into a flywheel which is then attached to a driveplate which is two steel plates riveted to each other via a PVC buffer made by R&D. The old one had almost completely collapsed and was held together with just one rivet! Bits of the plastic buffer were splayed out all over the place and its a miracle it had held together at all.

Then the driveshaft passes into a gearbox which is a simple forward / backward / neutral mechanical affair bathed in automatic transmission fluid. This then attached to the propshaft which includes a Centaflex coupling which absorbs more of the shocks before it goues out to the propeller via the stern gland, which is lubricated by grease and packed with tarry rope.

With no retaining nut on inside the gearbox I am surprised the propeller hadn't pulled the shaft out when in reverse! From the hacksaw marks and silicon on the retaining nut there appears to have been a bodged repair in the boat's first three years of life, before we bought it 10 years ago.

All in all the system was completely shot, so whilst its a pain to have to spend over £1000 on a new gearbox, its a relief to get is all solidly fixed up. Whats more it is a bit weird to put it into forward gear and find is slipping in so smoothly. For years it has engaged with a clunk as all the slack was taken up.

Anyway, problem fixed and on we go. 

3 comments:

Nick said...

That drive plate looks just like mine! We had a similar sage (gearbox died - in our case it was the forward clutch plates - drive plate found to be likewise knackered when I removed the gearbox) on the first day of our Easter trip. In our case it was a HBW 125 and Norbury Wharf (half a mile walk back to the car, 15 mile drive) had one in. They also had what looked to be a suitable drive plate but wasn't, and were excellent at tracking a suitable one down at Midland Chandlers.

I don't think the same problem could occur - our gearbox ends with a plate which bolts onto the flexible coupling, which in turn bolts onto a coupling with a keyway into which the prop shaft goes. I just removed the flexible coupling from the gearbox, loosened the gland and slid the entire assembly back until the prop hit the rudder.

All done in under a day, but a month later and my last barked knuckle hasn't quite healed completely.

Andrew Tidy said...

Nick - having never seen the insides of a gearbox or bell housing before i don't think I would attempt this one myself! I take my hat off to you..... as Clint once said "a man has got to know his limitations"

Nick said...

There were times when I was lying on my stomach on a deck board, with my head two foot lower into the engine well, groping around underneath myself to get a key into the hex bolts that I did start to wonder just what I was playing at.