Sunday, 5 December 2010

Centerflex fix

Centerflex fix
5 December 2010

I feel a bit sorry for Wand'ring Bark right now, lying out there in the ice and cold with her prop well and truly disconnected from her engine. Hobbled and immobile.

Well I have been doing something about that. With the Centreflex Coupling extracted I set about trying to fix it back in my workshop at home.That was easier said than done.

Coupling on the bench - with broken stud in nearest hole

The sensible move would be to admit failure and buy a new coupling for £200, chucking the old one away. The problem was that I grew up on Scrapheap Challenge and the possibility of finding creative engineering solutions, and determined to have a go. After all, I had nothing to lose.

The problem lay in about a centimetre of stud embedded in the body of the casting. I invested in a set of screw extractors (£2.60 from Screwfix), drilled in a pilot hole and had a go at cranking it out. But it wouldn't budge, not even a tiny bit. The problem lay in the nature of the fracture - the stud had fractured but the thread remained leaving the stud flopping around in the hole for years and years, gradually flattening out the surrounding thread as you can see from the picture.

Remains of stud out - note the complete absence of thread in the hole

This process left the stud with no way out so I had to resort to careful drilling. Now we all know how that story ends, no matter how carefully one drills into the stud you always stray, damaging the thread which surrounds it. This was no exception.

Locking in a stud fashioned from an M10 bolt

I had called in at Midland Chandlers to buy a set of studs, but unfortunately they were out of stock. I was advised that they were made specially by an engineering firm but none had been available for ages - great! Then a moment of inspiration - the studs are really only M10 thread in High Tensile Steel and could be replicated by cutting the head off a commercially available bolt (£5.50 for 50 from Screwfix).

Three new studs in place - and one original

I prized the half crescent remains of the stud out and tentatively inserted a an M10 bolt to see what grip remained. The result was uncertain, the bolt screwed in but didn't really bite with any certainty as it bottomed out. With plenty of bolts to spare I sawed off the end of one and inserted it into the hole, liberally coated in thread glue to maximise the grip available. The problem is that the first half of the hole was burred out and now half the the bottom half was lost to the drill - that's a lot of thread missing! The coated bolt held ok, but for how long and how much tension will it stand? Not really satisfactory enough to give me peace of mind.

New stud retained by a cross bolt

A couple of days later I woke early with a start, and another bright idea. If the thread is very poor and the efficacy of the glue doubtful why not  find another way of locking the stud into the casting - something like how a split pin holds the retaining nut tight to the propeller. Of course, a split pin is inappropriate in this setting but how about drilling right through the casting and the stud and inserting a long thin bolt held in place by a nut in the outside threaded hole? It could work providing the thin bolt doesn't cause the stud to weaken and fracture, or to protrude and snag anything. 

Refurbished unit complete

The end result looks fit for purpose and there is no reason why the dodgy stud can't be tightened up to the gearbox flange and provide a full measure of grip, but only time will tell.

I will bolt the refurbished coupling back into the boat and see how it goes. It can't be any worse than the 2.5 studs which have been holding it all together for the last few years.

Another super bodge by a wannabe scrapheap challenger. If you see me in the engine bay wielding a spanner you can assume that my optimism was misplaced.

Update: After writing this post up I took the repaired coupling out to Wand'ring Bark. It slipped on like glove and within 45 mins the  engine was bellowing away on  full revs, the newly re attached prop churning away and the studs holding beautifully. I left it on full throttle for about ten mins and alll was well, no vibration or lose nuts, so the repair appears to be a success.



Halfie said...

Excellent! It's always good when you can find a way to cheat the throwaway culture. As I read through your post I was thinking "but what about vibration?" - but you say there isn't any. Good news, and well done.

Captain Ahab said...

You will be concerned about an unequal weight distribusion causing vibration like a car wheel out of balance?
I wondered about that but figured that everything would balance out. I had drilled out the hole to get the small bolt in and I added an extra washer on the opposite side so gram for gram its not far out. Whats more, it turns relatively slowly and the distance from the centre to the outside of the coupling is small so it should be well.

It ran smoothly with a whole stud missing so a slight imbalance in the other direction should be ok too.
See, I'm trying to convince myself now!
Hey, your snow melted much faster than Birmingham today!I was in Norwich three hours ago.....mayhem.

Halfie said...

You should have called in to The Forum - I was at work.

After my experiences on the Thames you can understand why I'm concerned about the possibility of vibration. Yes, of course, any slight imbalance in the coupling will be insignificant compared with the importance of getting the propshaft correctly aligned. And I should think that the prop itself is more likely to be slightly out of balance - a small dink on one of the blades would probably have more effect even than a completely missing coupling bolt!

Captain Ahab said...

Update 3 years on - have run the Trent flat out twice, stalled the engine several times on junk in the BCN and it has never failed.
Proper job!