Centaflex PTO coupling

I've noticed a fair bit of vibration on the port main engine and am now looking at the front of that engine where it drives a hydraulic pump (which powers the thrusters + the windlass).

On the front of the crankshaft is a Centiflex coupling which drives a Spicer flexible driveshaft (about 10" long) that in turn drives a clutch in front of the hydraulic pump. This Centiflex coupling has one bolt missing, and I believe it's an M20 bolt, so not a small one. In addition to this missing bolt, there's a fair bit of slop in the coupling, even near one of the bolts that's still there.

I took a short video of the setup and movement; hopefully this works:
https://vimeo.com/495100482

This type of coupling is completely new to me, and I'm trying to determine what's best to do with it. Should I just find a replacement for the missing bolt and tighten everything securely, or do these Centiflex couplings have a limited lifetime and need to be replaced periodically? It's just under 20 years old.

What about the Spicer driveshaft -- maybe get it rebuilt with new U-joints?

Thank you - very helpful. I'll need to get a 17mm hex socket and then follow your suggestions. The ABT Trac manual lists the torque at 300 ft-lbs, vs the 480 Nm specs you shared, equivalent to about 350 ft-lbs. In any case, they need to be tight!

The next step is to figure out why there is slop under one of the still-present bolts. I'm also seeing if I can get a new rubber element, just for peace of mind.

I like the idea of taking the Spicer unit to a driveshaft shop and get their recommendation.

Thanks again!

Update: I got a 17mm hex socket with a 1/2" breaker bar and tried to get the bolts out. I managed to get all four of the radial bolts out but only one of the axial bolts. The problem is that the engine spins... anyone know a good way to stop a Cat 3412 from rotating?

I did go get a Milwaukee 120V impact wrench, but it's only rated at 300 ft-lbs and these bolts were torqued to at least that, plus the 19 years of being stuck in there.

Any ideas?

Also, it appears that the missing bolt didn't just spin out, but rather broke off just below the surface of the flange on the crankshaft. Once I get the rubber element off, I'll need to go to work on extracting the broken part. Good fun.

A couple pictures, here's the center hub loose:


And these M20 x 65mm bolts are pretty big. I don't have small hands.

I actually looked at that DeWalt 20V but the one I saw was only 150 ft-lbs.

However, your idea of a pipe or 1/2" extension in one of the empty bolt holes is a great idea! Since I'll be replacing this rubber element anyway, I don't care if the hole gets bungled up. That and a cheater pipe on the breaker bar should do it. Thanks!!!

This is very good to know. Once I get to the point of extracting the broken bolt, this is probably going to be key... thanks!

Success. Dockmaster's suggestion of a pipe worked like a charm; most the work was just filing down a 1/2" steel pipe until it fit snugly into one of the radial holes in the rubber element. That, plus a 30" long, 3/4" extension pipe on my breaker bar, had the remaining two bolts out in about 10 minutes.


There is obviously a significant amount of wear on this whole assembly. This 1/4" thick brass disk sits behind inner coupling, plus one of the rubber element bolt holes is similarly ovaled-out:


The broken bolt inside the flange at the 9 o'clock position:


The great news is that the flange, to which the rubber element bolts, is removable so I can just take it down to a machine shop, have it cleaned up, get them to remove that bolt and chase the other threads, and then it will be time to reinstall everything.

Everything looks solid on the crank itself:


Thanks for all the thoughts and suggestions!

I now have a new Centaflex rubber element (with 8 new bolts), have rebuilt the Spicer driveline, and have cleaned up the other parts. The last missing piece is this bronze (?) disk/spacer that was sandwiched between the engine flange and the inner PTO coupling:


As posted previously, it's significantly worn / ovaled and can't be reused, so I'm trying to source a replacement. Centa (now Rexnord) doesn't make this; ABT Trac (who made the overall thruster system) doesn't, either. I'm checking with Horizon to see if it's something they manufactured and added, but haven't heard back yet.

I don't understand what the purpose of that disk is. The inner coupling will be very firmly attached to the rubber element and sees little axial force (really zero). I'm tempted to reassemble this without that disk in place, but would welcome any thoughts or insights into what it is and why it was there.

Thanks for the thoughts. The 8 bolts that attach the rubber element to both couplings are supplied by Centa, so I'm fairly confident that they are the correct length. There is nearly an inch of threads in both the axial holes of the crankshaft flange and the radial holes into the inner coupling, so I believe that there is adequate space for the bolts to not bottom out.

To clarify my earlier picture, this bronze spacer fit entirely inside the rubber element, so it's not intended to compensate for bolts being too long nor space the rubber element away from the crankshaft flange:



I think the slop shown in the video resulted from the bolts once being tight but the rubber element wearing out after 19 years of use.

Centa specifies (in their technical drawings) a spacing of 8mm between the pink and white couplings above. One theory might be that Horizon felt that this was so important that they machined an 8mm thick spacer. But with four M20 bolts connecting the rubber element to this center coupling, there just isn't any play.

One thing that surprised me initially is that the inner diameter of the new rubber element is actually about 30mm larger than the diameter of the inner coupling; after you snug all those four radial bolts down, the rubber element is significantly pre-loaded: