To grease autoprop nut or not?

[Star-Lord]

On my fixed prop I was always advised to grease the main nut that keeps it on the shaft. In my current boatyard they advise against doing this. I think it needs a bit of grease... very stiff as it is. Thoughts?

 

[awol]

Grease is fine with caveats. It is easier to overtorque and there must be a physical lock - split-pin, bent tab washer, whatever.

 

[ean_p]

On my fixed prop I was always advised to grease the main nut that keeps it on the shaft. In my current boatyard they advise against doing this. I think it needs a bit of grease... very stiff as it is. Thoughts?

Maybe you should ask them as to why they advise against lubricating the threads? Are not Bruntons nuts 'locked' with locking screws ?

 

[Cantata]

Maybe you should ask them as to why they advise against lubricating the threads? Are not Bruntons nuts 'locked' with locking screws ?

I always grease the nut on mine. I also grease the large locking screw, then Loctite the smaller screw that locks the big one. So long as that one doesn't loosen, there's no way everything else can budge.

 

[Daydream believer]

I did have the smaller locking screw drop out once when not fitted with locktite. I found it laying in the void behind the anode. When I laid up the boat I removed the auto prop for cleaning & the main nut & locking bolt just fell off after a couple of turns. The whole assembly was loose.
I now put the main nut on lightly oiled & as tight as I can get it. I then locktite the locking screws. Actually it is an equivalent cheaper make of locking liquid & a pig to undo. However, I feel happier with that.

 

[ean_p]

I always grease the nut on mine. I also grease the large locking screw, then Loctite the smaller screw that locks the big one. So long as that one doesn't loosen, there's no way everything else can budge.

to be fair why wouldn't you when it needs to be loaded to a specific torque and as its locked then it presents no problems as far as I can see though the OP's yard may have a valid reason.....would be good to hear it!

 

[vyv_cox]

It's the torquing that may cause problems. A lubricated thread will tension the bolt up to 50% more than a dry one for the same torque.

 

[Daydream believer]

Brunton's MD first fitted my autoprop & did not use a torque wrench. Just did it up tight with a small amount of lubricant on the splines & main shaft thread. Nor did he mention any special torque. He applied locktight to the central locking bolt & the smaller locking bolt that locks that. Is there detail of specific torques in the instructions? Not that I have ever seen any installation instructions

 

[ean_p]

All very true and the spread of a dry torqued joint can be as much as 40% either way of the target tension due to the variability of the components. When you think you've applied the correct torque but its been dissipated in friction on the threads/ face contact etc etc . Not many people specify a dry torque in a critical application as even a lubricated fastening will still have a spread of 10-15% of target tension. All of course compounded by the common 25-30% inaccuracy of even quality torque wrenches. Drilling deeper into this Bruntons, though they specify torque values for various thread sizes of shaft nut, don't actually recommend (by implication) the setting of a torqued tension. Indeed in their installation documents and in their demonstration videos they simply show the nut tightened by the use of an appropriate socket on a long arm socket bar. Having seen the video it would seem to far exceed their recommended torque value by the way the installer 'swings' on the strong arm. But going forward I think that even if the Brunton recommendations are for dry settings then even if lubricant were to be used and the multiplying factor applied, a torque as indicated would still be well within the normal (and nominal) 75-90% yield of the shafting.

 

[RunAgroundHard]

... Not many people specify a dry torque in a critical application as even a lubricated fastening will still have a spread of 10-15% of target tension.

That is not my experience in critical bolting applications. Torque is specified with the bolt / nut / stud un lubricated. Friction factors are then applied based on the lubrication being used. Many lubricants actually have a friction factor of 1.0, meaning no adjustment required. My applications, API flanges with 20k psi, structure members on bolted drilling derricks. All my employers torque wrenches are calibrated, but not my personal devices.