Prop rattle after fitting flexible coupling.

[NYSail]

Originally Posted by Draughty

Hi,
The reason IS buried back there!
I hoped it would stop the recurring oil leak through the output shaft spline.
Oil in the bilge is a PIA.

Rgds

I did read that and I did read that the engine manufacturers recommended. Band aides will not fix another problem. The leak is there because something is broken..... I would pull it and send it to a qualified mechanic unless of course you can tackle the task. that is just my opinion of course.

Greg

[Capt Ben]

Originally Posted by DougR

If I understand the situation, you have:

- a standard strut/P bracket

- a Volvo Penta rubber stuffing box

- an R&D flex coupling

- a flexible mounted engine (?)



This means you have one fixed point in the system, the strut, and then three flexible unsupported points in a row. In most engine manufacturers installation manuals, (Volvo Penta for sure), this configuration is not recommended. When the engine is working hard the prop thrust is trying to cause the drive line to buckle and here there is little in the way of support to resist it. The rubber stuffing box is too flexible to exert any control of the shaft, the R&D coupling is made to flex, and the engine flex mounts allow engine movement as well. Once this flexing begins, higher RPM ( more thrust) makes the situation worse. Also, down angle gear boxes aggravate the situation, as the shaft thrust is providing lift against the engine flex mounts instead of just horizontal thrust. When these situations get really bad the drive line starts to look like a jump rope, and frequently the noise one hears is the shaft hitting the inside of the shaft log.



The removal of the flex coupling will probably restore the system to its prior smoothness. Another option could be to retain the flex coupling and to install a steady bearing toward the front end of the shaft to control its movement.



DougR

I’m curios if the engine mounts low, mid or high on the mounting studs? Having it aligned such that it’s low on the stud will minimize that point of flexibility.

[NYSail]

I have a water feed for my vp seal. It’s on my beneteau 423. No picture however will take one tomorrow and upload if you need.....
Greg

[Draughty]

Hi Capt Ben
The engine feet are only about 1/3 up the mount studs.

And Hi to NYsail; yes I think I've seen a pic of your installation on another thread.

Thanks

[PeterS]

If you only have 4" of shaft from the stern tube fitting to the back of the half coupling then in no way should you install a cutlass bearing there.

The forces from engine movement will be detrimental and at best will destroy the cutlass bearing prematurely and certainly will introduce severe vibration.

That is probably the reason why there is no bearing installed presently as the shaft has to flex with the movement of the engine. If a bearing were present the minimum shaft length from bearing to coupling should be around 15 times shaft diameter.

The way everything was set up was the correct way to do it. Your transmission leak from the rear spline is a transmission problem, nothing to do with the shaft system (thinking Alfa Romeo)!!!

The flex coupling with the unsupported shaft is definitely what is generating the rattle.

[Draughty]

Thanks PeterS,
Your reply very succinctly confirms reservations I had about fitting a bearing so close to the engine.
I have removed the coupling and all is back as before.
I think I will replace the cutlass brg (at least 15 years old) when I haul out in a few months time, just because it is so old and hope my last fix of the spline leak holds up better.
A major USA Beta dealer (I can't recall the name; they run a useful user forum on their web-site) kindly sent me a workshop manual for the gearbox which confirms that it is just sealant which retains the ATF; there is not an internal seal missing from new as was suggested by one mechanic.
I'm not sure how common this method of sealing is but it doesn't seem very robust to me.

Rgds

[PeterS]

Hi Draughty:

I am glad I could help relieve some of your concerns.

My profession leads me to work with most of the larger transmission manufacturers, in 45 years I have never heard of using a sealant to seal off a splined output, I have only ever seen positive mechanical seals (such as O-rings) in that application.

I note that Twin Disc use a sealant but as far as I am aware it is to ensure sealing around the OD of a lip seal. Quite a few TD output flanges are hydraulically installed and the rest are bolted, the hydraulic version is on a tapered shaft so self seals, the spline shafts have a shoulder and/or ring type seal among others. I have no experience of your particular transmission so really cant comment further but agree that it is a little feeble in approach.
Is there a specific torque setting specified on the retaining bolt, it may be possible they are relying on an angled shoulder and the sealant to seal the shaft ahead of the spline, if that is the case the coupling will be very clamp force dependent and a torque will be specified.

I think your 15 old cutlass bearing should be retired, I am sure a lot of gents on this forum would love it if theirs did that kind of distance, having said that I am sure I am in for a severe lambasting for suggesting it.

Regards and Happy New Year

[Draughty]

Thanks again PeterS
This TMC60A gearbox has a male parallel spline on the output shaft.
The coupling slides on to this, it's OD being the sealing surface to the output shaft oil seal. The coupling is held on by a nut with an integral "washer" which should be tightened to 155nm. There is a collar around the nut which is punched into a recess in the shaft to lock it.

I have tightened it as much as I could muster using a 36mm socket and 300mm ratchet handle; which I have been told should approximate this torque.
I have a torque wrench but unfortunately when I've had to do this job its been hundreds of miles away.

Hopefully my last fix will last.
I appreciate your comments.
Rgds

[PeterS]

Hi Draughty:

No problem 155 Nm is 115 Ft/Lbs.
Not huge, the easy way to do that is take a wrench 1 Ft long and apply (guestimate is OK) 115 Lbs to the end of it = 115 Lbs over 1 Ft, or any combination of weight over distance that equals 115 Lbs.
Sorry if you already know this, most people do not, even very experienced mechanics.

Regards Peter