Centaflex coupling fault / design flaw

[ianj99]

Good result.
Do let us know if they offer their explanation for the failure.

I will post an update when the replacement arrives on friday.

Even if this is identical to the original, I still don't understand why the holes need to be so close to the edge when it is quite possible to move them outwards by 1 or 2mm.

The other alternative would be to increase the depth of the holes by 5mm and lengthen the bolts by about 8mm, and also drill out the thread for the first 5mm.
(The supplied bolts stop 5mm short of the bottom of the hole).
This would avoid the stress at the top of the bolt hole where the edge distance is least.

The bolt holes are 25mm deep, the bolts supplied have 18mm of thread below the flange. The coupling bore is 40mm deep. Therefore I can see no reason why the holes can't be 30mm deep, the bolt 25mm long, and the top 5mm of the hole counterbored to remove the thread and the holes moved outwards by 2mm.

I am tempted to drill 6 new holes tapped at 6mm in the broken coupling between the existing 8mm ones and keep it as a spare.

 

[ianj99]

The engineer ought to have appreciated that this coupling relies for its effectiveness on the tapered collet gripping the shaft more tightly as it is drawn into its socket and thereby compressed, and that it should not have been tightened until something broke but that there would be a torque setting for the bolts which he could, and should, have ascertained. I don't have the torque wrench settings to hand at the moment but they are not very great.

I hope he also knows that the clamping bolts should be torqued up in stages, i.e. following a sequence so that the collet is drawn in evenly. Also that the shaft and inside face of the collet should be thoroughly degreased and the special grease supplied by the manufacturer should be applied to the outside (tapered) face of it.

Another thing to make sure of is that the collet is the right size for the shaft, ie don't use a metric size collet for an imperial size shaft

Don't place undue reliance on 'experience'. More damage is caused by people who think they have nothing more to learn than by those who know they don't know much and take the trouble to learn.

Finally, I think it's a good idea to paint the torque settings on the inside of the hull near the coupling so that the next man to install it has no excuse for overtightening the bolts.

I am sure the engineer is aware of all the points you mention but with so little edge distance at the top of the holes, the margin for error is very small. Yet so far as I can see, this need not be so had a bit more thought gone into the design.

"...next man to install it has no excuse for overtightening the bolts"

Your implication that the bolts were overtightened is unfair unless you know for a fact that this is what happens if they are overtightened.

 

[Poignard]

I am sure the engineer is aware of all the points you mention but with so little edge distance at the top of the holes, the margin for error is very small. Yet so far as I can see, this need not be so had a bit more thought gone into the design.

"...next man to install it has no excuse for overtightening the bolts"

Your implication that the bolts were overtightened is unfair unless you know for a fact that this is what happens if they are overtightened.

I have the same coupling. As you can see from the photo, it is not that accessible and requires working upside down with a torque wrench deep down in the bilges. Since I bought it I have had the shaft out twice, so the coupling has been connected up 3 times (by me). It takes quite a long time to do it because you have to tighten each bolt a little, then rotate the shaft to do one on the opposite side and so on.

With all due respect, if I can do it without damaging it I don't see why anyone else can't.

It might be worth asking the yard engineer if he used a torque wrench and what settings he used since, as you said in your original post,

No documentation was supplied with the coupling as regards tightening torque.

Presumably Centaflex will want to examine the coupling. I really would like to know what they have to say.

Anyway, I'm sorry if you don't like what I have said but I did think you were seeking opinions.

 

[doug748]

I will post an update when the replacement arrives on friday.

Even if this is identical to the original, I still don't understand why the holes need to be so close to the edge when it is quite possible to move them outwards by 1 or 2mm....

Thanks

Yes, I agree difficult to understand. Centaflex show a substantial wall thickness in most of their published publicity materials. Funny though that Neil_Y's Hydrodrive photo shows a similar narrow margin.

Looking again at your first photo, I am minded to think that the thread on the left was almost breaking out when manufactured It could simply be two components at or beyond their limit of tolerance, aided by over torque. However I find it difficult to believe that any savvy company would put a fragile design into such a robust arena (knowing the number of paid and unpaid apes out here :-D ). None of the published stuff I have seen mentions a danger of damage, simply:
"Tightening by "feel" will not give a satisfactory result"

 

[Poignard]

Ianj99

Looking at the left hand tapped hole in your first picture, there seems to be bits of metal in the threads. Any ideas why it is like that?

Do you have a photo of the clamping bolts, by any chance?

 

[ianj99]

update

The coupling was replaced but three other points arose.

1) I found that the bolt holes were not threaded fully so the bolts run out of thread before the clamping sleeve is tightly clamped. (about .5 to 1mm has to be taken up. Hole depth 25mm thread dept 17-18mm)

The manufacturers stated via the suppliers that this is okay when the specified tightening torque is used. I assume therefore that the bolt attempting to cut more thread is used as a locking device?

2) Due to the lack of edge distance, the hex head bolts supplied are too close to the shaft to permit a socket to be used so its impossible to use a torque wrench. (even a thin wall 3/8" drive socket cannot be used)
So the engineer had to obtain 6 cap head screws (socket screws) to be able to use a torqque wrench.

3) The instructions refer to 'special grease'
" Leave the special grease only in the tapered bore of the coupling hub, and on the outside of the clamping hub." What is special grease and why wasn't there any on the coupling I received?

Its now been fitted and time will tell if it proves reliable.

However I remain totally bemused by what appear to be design 'oversights'...

a) Lack of edge distance between clamping bolt holes and coupling sleeve bore.
b) Insufficient thread depth
c) Impossibility of using socket and torque wrench with the supplied hex head bolts.
d) If a 'special grease' is required during fitting, why not specifiy which type or supply a sachet?

No doubt these points are not an issue with the couplings for small diameter shafts, although I am dubious about the reason given for lack of thread depth. Why not provide a locking washer or recommned thread locking compound.

I hope I don't live to regret this purchase.
Ian

 

[doug748]

The coupling was replaced but three other points arose.
I hope I don't live to regret this purchase.
Ian

I think you are right to be concerned. I know the sound of engineer babble when I hear it. In this application, any product that you can bust with an 8in spanner is not up to the job

Looking on the bright side you should be ok now; I have found mine to be reliable and a useful piece of kit. Never have put a torque wrench near it though.
Thanks for the feedback.

 

[eagleswing]

get yourself the jackbolts...

while you are finishing up, be sure to get the proper length and thread for 3 jackbolts. on my M127 the jackbolts were a size up from the coupler bolts , and were medium thread. had to make 2 trips to the hardware store on a sunny day to get the right bolts ... as i recall, main 6 bolts were M10 11 or 13 mm and jackbolts were bigger M13 or maybe 17 mm ?? but dont quote me on the dimensions, the boat is on the hard the project was last june...

 

[ianj99]

while you are finishing up, be sure to get the proper length and thread for 3 jackbolts. on my M127 the jackbolts were a size up from the coupler bolts , and were medium thread. had to make 2 trips to the hardware store on a sunny day to get the right bolts ... as i recall, main 6 bolts were M10 11 or 13 mm and jackbolts were bigger M13 or maybe 17 mm ?? but dont quote me on the dimensions, the boat is on the hard the project was last june...

Thanks, that's something I have overlooked.
Access to under the cockpit is tight so I better sort them out before the Christmas festivities result in an expanded girth!
Ian

 

[thaisail]

Centaflex coupling M Type 12 stuck!

Whilst being fitted by the yard's engineer, the Centaflex-M coupling (sold under the Quietlife DB127 brand) split along one of the clamping bolt holes as you can see from the photo. There are hairline fractures of varying lengths visible adjacent to the other bolt hole locations. Even if the crack and fractures had not occurred during the initial fitting, I'm sure the coupling's reliability in service would be marginal and potentially life threatening if it was to fail at the 'wrong moment'.

I would appreciate comments as to the design because it seems to me and to the engineer and other experienced yard staff, that the holes are far too close to the edge. Either this is a design flaw or a manufacturing defect however Centa are dragging their heels over deciding whether to replace it despite a promise to have done so first thing on monday morning.

Photo 1 shows the cracks - the one on the left is full length and through into the bolt hole.
Photo 2 shows the bolts holes are not central in the clamp flange.
The coupling is the clamp fixing type and the clamp flange tapered and fits in the tapered bore and tightened by means of the 6 bolts.

Ian

I know that this is an old thread but i was hoping that someone can help me out with a similar issue. I bought my Centaflex M Type 12 coupling and fitted it on a 1-1/4 inch shaft three years ago. I did the torque according to the recommended specs. Now after three years I tried to remove the coupling and had no problem to get the insert free with the use of the three jacking bolts. However, the insert was stuck on the shaft. I used my prop puller tool to pull the shaft away from the gear box after fabricating a strong back to hold the insert against the boat frame and put about 3 tons of force on the shaft and the insert would not budge. I think that seawater from the packing gland had worked it's way up into the gap between the insert and the flange and the insert just got stuck from the corrosion. In the end after using all my marine engineering knowledge, I was forced to cut the insert with an angle grinder and a cutting disk and spread apart the grove that I cut to be able to get the insert free from the shaft so that I could remove the shaft from the boat.

I now have to order a new insert for the coupling from Centa but I feel that this should be considered a defect in the design or materials used in the insert for the coupling. I would like to know who you contacted at Centa to get them to provide a new coupling for free. I am now in Thailand and I bought my coupling from Beta Marine in the UK when I fitted my new Beta Engine. Please can you contact me direct by email at thaisail (at) gmail.com if you have any information about your claim with Centa that will be helpful.

 

[ianj99]

I now have to order a new insert for the coupling from Centa but I feel that this should be considered a defect in the design or materials used in the insert for the coupling. I would like to know who you contacted at Centa to get them to provide a new coupling for free. I am now in Thailand and I bought my coupling from Beta Marine in the UK when I fitted my new Beta Engine. Please can you contact me direct by email at thaisail (at) gmail.com if you have any information about your claim with Centa that will be helpful.

Heating it with a blow torch may have loosened it.

My replacement has been fine but I have not tried to remove it.
The excellent ASAP Supplies sorted out the replacement so I never had any contact with Centa directly.

Any Centa stockist should be able to order a replacement insert but I don't think you can expect a free one necessarily, since any water ingress & resulting corrosion is hardly down to them. Presumably the insert isn't made from stainless steel due to unsuitable mechanical characteristics.
Ian