Propshaft Flexible Coupling Disks

[lockie]

Here is today's assignment:

Q 1. Have you heard of elastomer coupling disks hardening over time?

Q 2. Have you reduced drive vibration by replacing a disk with a lower torque-rated (ie more flexible) one?

No theorising please I can do that myself after a good smoke, only direct experience.

Engine mounts, alignment, shaft straightness, prop and cutless have all been replaced/adjusted/checked comprehensively

Cheers, Graeme

[skipperpete]

My experience is that yes, the coupling discs eventually lose resilience over time, more so if the alignment is poor and causing the disc to flex. The Polyflex brand were the ones that I had trouble with so started using the R&D skeletal flexible couplings with good results but even these failed if the lineup was out too much.
I replaced a hugely expensive set of Vulkan coupling disks behind an MTU for no reason other than than that the elastomer had hardened unacceptably. The driveline vibration was noticeably less with the new segments.

[skipperpete]

The Centaflex keyless coupling from Vetus are pretty damned good ,will easily handle 1° of permanent angular mis alignment and really smooth out driveline vibrations.
Pete.

[Dave852]

I use sigma drives, very happy with this product https://www.ab-marine.com/bruntons-sigmadrive/

[skipperpete]

Originally Posted by Dave852

I use sigma drives, very happy with this product https://www.ab-marine.com/bruntons-sigmadrive/
Attachment 208954

I like em. I never thought CV’s could handle thrust but it seems that Sigma have solved that problem. [emoji106]

[roland stockham]

Not had an issue with R&D after more than 10 years. Eventually had to replace it due to corrosion but the disc was still fine. I like the look of that sigma drive, very neet

[Davidhoy]

Slightly off topic, but I’m curious about electrical bonding or isolation between the two sides of a flex coupling. My boat was recently out of the water and I noticed a few small pink dots on the prop, so I checked if there was any ground connection between the boat’s negative bus bar and the prop, and it showed an open circuit. I have good zincs on the prop, shaft, and strut. So should I install some kind of ground strap over the flex coupler, or leave the prop and shaft isolated? I’ve read contradictory things online...

Regards,
David

[Jesse]

(1) no (2) no

[sailormed]

Originally Posted by Dave852

I use sigma drives, very happy with this product https://www.ab-marine.com/bruntons-sigmadrive/
Attachment 208954

I installed a Sigma drive as well. If I remember correctly they allow 3° of misalignment max. Very happy user I am. No maintenance, no vibration, no noise. Install correctly and forget.

[FPNC]

You don't give much in the way of details.

If your talking about an engine to drive a Compass 28 you should already be using the softest, lowest rated flex coupling they offer.

Everything has a life span and any plastic will eventually harden and fail. This would normally be after many years of operation.

If the plastic coupling is installed with improper alignment then it may fail quite quickly. Very soft engine mounts will also cause premature failure.

[slug]

Originally Posted by Davidhoy

Slightly off topic, but I’m curious about electrical bonding or isolation between the two sides of a flex coupling. My boat was recently out of the water and I noticed a few small pink dots on the prop, so I checked if there was any ground connection between the boat’s negative bus bar and the prop, and it showed an open circuit. I have good zincs on the prop, shaft, and strut. So should I install some kind of ground strap over the flex coupler, or leave the prop and shaft isolated? I’ve read contradictory things online...

Regards,
David

Regardless of couplings , Prop shafts are always a poor earth

The shaft enters the gearbox and floats on a film of oii. A very poor electrical connection

If you feel that you must earth the shaft , you need to use shaft brushes

Very few yachts fit theee

[GILow]

Originally Posted by Davidhoy

Slightly off topic, but I’m curious about electrical bonding or isolation between the two sides of a flex coupling. My boat was recently out of the water and I noticed a few small pink dots on the prop, so I checked if there was any ground connection between the boat’s negative bus bar and the prop, and it showed an open circuit. I have good zincs on the prop, shaft, and strut. So should I install some kind of ground strap over the flex coupler, or leave the prop and shaft isolated? I’ve read contradictory things online...

Regards,
David

Just add a simple shaft earth using a length of plaited earth strap loosely looped over the shaft and weighted with a few small lead sinkers. Bond this to your main anodes.

[a64pilot]

Originally Posted by Davidhoy

Slightly off topic, but I’m curious about electrical bonding or isolation between the two sides of a flex coupling. My boat was recently out of the water and I noticed a few small pink dots on the prop, so I checked if there was any ground connection between the boat’s negative bus bar and the prop, and it showed an open circuit. I have good zincs on the prop, shaft, and strut. So should I install some kind of ground strap over the flex coupler, or leave the prop and shaft isolated? I’ve read contradictory things online...

Regards,
David

Two camps, and both have disciples. Float and bond.
I’m in the bond it camp and the best way to do it is as you describe. I would use two flat braided tinned straps opposite of each other, that kind of RPM balance isn’t likely to much matter, but why not?

[a64pilot]

Originally Posted by skipperpete

I like em. I never thought CV’s could handle thrust but it seems that Sigma have solved that problem. [emoji106]

It’s nothing more or less than a Rzeppa joint, which has been around forever. The front axles of Military vehicles all had Rzeppa joints in those big round knuckles.

https://en.wikipedia.org/wiki/Alfred_H._Rzeppa

I have one trying to fix a drive line vibe, it helped but what I am now sure the source of my vibe isn’t easily fixable, but in truth I like it as engine alignment isn’t ever going to be an issue anymore, and I could go to more flexible mounts and let them absorb vibrations better now that the engine can be allowed to move around a bit.

[a64pilot]

Inside of a sigma drive
https://images.app.goo.gl/vYUdYJu5hiB89n667