Transmission to prop shaft coupler bolts loose

[pbyrne]

Good to know! I was having nightmares about pulling the shaft out to get enough room...   

It does seem in the photo that some have some thread peeking out.  I'll have to look in person.

Are these nuts reusable, or should I be replacing them for new ones to get the best adhesion from the nylon nuts? 

Also, I'm assuming I tighten to something like tight plus a 1/3 turn?

Guys : I agree with Kevin, that flexible coupling was not installed properly!!!

It appears that the nuts are nylon lock nuts and NO torque is required!!  With that type of nut the normal rule is to have at least 1 (best 2) thread showing!!  I believe that the bolts with the nuts on the transmission side just need to be tightened so there is 1-2 threads outside the nut!!

If you can't get a thread on the outside of the nuts, then get a thinner lock nut!!  Simple as that !!

A few thoughts

[KWKloeber]

If you can't find a manual or instructions for torque you can always go generic. There's tables online for torque values by bolt/thread size/type. i.e., (just a made up example) grade 5, steel 3/8" UNC (coarse pitch) or UNF (fine pitch) thread, max torque = nn ft-lbs.

In that application I'd think I'd trust lock tight, or lock washer, or spring lock, or deformed locking nut - over a nylock.  No data, just my gut.

[pbyrne]

Hmm.  How do you determine those values?

I emailed R&D marine, but they are GMT, so closed at the moment.  Hopefully they can help.

The link indicates torque numbers, but I'm not sure which is correct. http://www.randdmarine.com/flexiblescinst.asp

Can you loctite those nuts?  I have Loctite 222 low strength.  Maybe something higher?  This is something that should never be backing off unless deliberately removed.

You know, I didn't check if they are nylon to be honest.  It's just what the kind people here have been mentioning...maybe they aren't nylon?

If you can't find a manual or instructions for torque you can always go generic. There's tables online for torque values by bolt/thread size/type. i.e., (just a made up example) grade 5, steel 3/8" UNC (coarse pitch) or UNF (fine pitch) thread, max torque = nn ft-lbs.

In that application I'd think I'd trust lock tight, or lock washer, or spring lock, or deformed locking nut - over a nylock.  No data, just my gut.

[Stu Jackson]

Hmm.  How do you determine those values?

I emailed R&D marine, but they are GMT, so closed at the moment.  Hopefully they can help.

The link indicates torque numbers, but I'm not sure which is correct. http://www.randdmarine.com/flexiblescinst.asp

Can you loctite those nuts?  I have Loctite 222 low strength.  Maybe something higher?  This is something that should never be backing off unless deliberately removed.

You know, I didn't check if they are nylon to be honest.  It's just what the kind people here have been mentioning...maybe they aren't nylon?

pb, the reason they are suggesting they are nylocks nuts is because of the shape.  Looking at your photo in your OP, there is the body of the nut with a slight dimple and bump above.  Compare that to the nut at the bottom of that same bolt (or machine screw).  Often folks put nylocks in that kind of situation because there isn't room for a regular nut and lock washer, like yours.

I have had a coupling that for many years had a propensity for throwing nuts off the coupling bolts.  Mine is a standard box and coupling, but same concept, same issue.  Because of space or lack thereof in front of the coupling, I resorted to fine thread bolts and loctite blue, because I couldn't get a lock washer (neither split or star) in there.  Once I did that they stayed in place.  The fine threads helped.

You really don't need any torque tables for this, tighten them, use locktite.  If they ever do pop off, my experience is that they end up right below where they fell off.  Ask me how I know... 

Good luck.

[Noah]

Not sure you "don't need torque tables" Once you determine if it is indeed the R&D coupler, or whatever brand, I would use the torque values they recommend.

[KWKloeber]

Hmm.  How do you determine those values?

The link indicates torque numbers, but I'm not sure which is correct. http://www.randdmarine.com/flexiblescinst.asp

Ok you answered your own question. You have the torque values, you simply need to determine what size bolts they are - I simpluly guessed (they looked larger than 1/4," smaller than 1/2".) 
Measure one, take one off and match it up, etc etc.

[pbyrne]

Ah! Thanks for the explanation. 

So loctite can be used with the nylon nuts?  Do you think it best to get new ones?

Also, did you just go with a regular nut with the blue loctite for the fine thread bolt?

Hmm.  How do you determine those values?

I emailed R&D marine, but they are GMT, so closed at the moment.  Hopefully they can help.

The link indicates torque numbers, but I'm not sure which is correct. http://www.randdmarine.com/flexiblescinst.asp

Can you loctite those nuts?  I have Loctite 222 low strength.  Maybe something higher?  This is something that should never be backing off unless deliberately removed.

You know, I didn't check if they are nylon to be honest.  It's just what the kind people here have been mentioning...maybe they aren't nylon?

pb, the reason they are suggesting they are nylocks nuts is because of the shape.  Looking at your photo in your OP, there is the body of the nut with a slight dimple and bump above.  Compare that to the nut at the bottom of that same bolt (or machine screw).  Often folks put nylocks in that kind of situation because there isn't room for a regular nut and lock washer, like yours.

I have had a coupling that for many years had a propensity for throwing nuts off the coupling bolts.  Mine is a standard box and coupling, but same concept, same issue.  Because of space or lack thereof in front of the coupling, I resorted to fine thread bolts and loctite blue, because I couldn't get a lock washer (neither split or star) in there.  Once I did that they stayed in place.  The fine threads helped.

You really don't need any torque tables for this, tighten them, use locktite.  If they ever do pop off, my experience is that they end up right below where they fell off.  Ask me how I know... 

Good luck.

[KWKloeber]

So loctite can be used with the nylon nuts?  Do you think it best to get new ones?

Loctite is used on metal to metal threads, so technically yes.  But there is no reason to use nylocks.

Let me say, with all due respect to those who disagree, a nylock nut is for convenience, not to really to save space.  At least not to save space and do a critical fastening correctly.

I'll use 3/8" as an example:

A standard nut plus a split lock washer is 0.45" high, a nylock is 0.42" high.  Things have to be VERY tight for 0.03" to make a difference AND have enough bolt thread catching the nylock anyway.  Way too iffy.  PLUS a nylock has less (metal) thread height than a standard nut.  That can be ok for a non-critical situation, but not on a critical location.

If space is limited on a critical fastening there are better options -- locktite or a standard height distorted thread locknut (grips tighter than a nylock.)  If there's room there's also tall thread nuts (i.e., having more metal thread so better holding) but not in s/s.  A deformed locknut has a divot in the side or top that deforms the female thread and presses against/holds tight on the bolt half of the thread) AND it doesn't compromise the height of the nut (amount of thread.) 

Using a lock washer and thin nut (called a jam nut) is ok on a non-critical location but not here because it compromises the amount of metal thread.  The more thread there is = the greater the holding power of the connection.

IIWMB, I would use new standard nuts (and bolts if those are not long enough) and lock washers (you could add locktite for belt/suspenders.) What appears to be a heavy washer under the bolt head looks suspect to me.  I'd like to see the disassembled fasteners or a picture from the mfgr as to what goes together how.

[Craig Illman]

Check with PYI, they're in Lynnwood, Washington on PDT. They sell these couplings.

https://www.pyiinc.com/flexible-shaft-couplings.html

Craig

[Ron Hill]

pby : Don't try to make this complicated!!  The main pressure/stress is on the bolts the locknuts just keep the bolts in place!! 

I'd just tighten all the nuts and on the short bolts clean all the grease off the nuts and white coupler.  Take some of you 1st Mates red fingernail polish and make a strip on the nut that goes on to the white coupler.  That's called a slippage mark.  Then run the engine as you normally would and periodically look at those stripes.  If the nut starts to back off the painted strip line mark will break and you'll be able to see it !!!   

I have slippage marks on all of my engine mount top nuts - to let me know that the bottom nut is turning and the engine will be out of alignment if I don't get the bottom nut tightened (mount raised) so the top nut slippage mark is back in line!!  Clear as mud??? 

I would NOT use Locktight (red) you might consider using Locktight (blue) as a last resort if the nuts are infact backing off!!   

A few thoughts

[pbyrne]

Thanks Craig.

I spoke with PYI and they recommended replacing the nuts.

Check with PYI, they're in Lynnwood, Washington on PDT. They sell these couplings.

https://www.pyiinc.com/flexible-shaft-couplings.html

Craig

[pbyrne]

Thanks Ron, that's is a good tip.  Will do. 

I inspected the nuts and out of 8, 5 are loose.  I removed one to take to Fastenal for replacements.  It was not very hard to remove the nut.  I was able to use my fingers once a spun it with the wrench a 1/2 turn.  That was a bit alarming.  Probably past the nylon at that point.

My conversation with PYI provided the information that the nuts are not recommended to be reused (i.e. if they were removed) as the nylon will be worn down a bit and not hold as well as the first time.

They also suggested blue loctite for a belt and suspenders approach, but mentioned even loctite purple (low strength) is okay too.

On a standard installation it is just the nylock no threadlock.

So there you go!

pby : Don't try to make this complicated!!  The main pressure/stress is on the bolts the locknuts just keep the bolts in place!! 

I'd just tighten all the nuts and on the short bolts clean all the grease off the nuts and white coupler.  Take some of you 1st Mates red fingernail polish and make a strip on the nut that goes on to the white coupler.  That's called a slippage mark.  Then run the engine as you normally would and periodically look at those stripes.  If the nut starts to back off the painted strip line mark will break and you'll be able to see it !!!   

I have slippage marks on all of my engine mount top nuts - to let me know that the bottom nut is turning and the engine will be out of alignment if I don't get the bottom nut tightened (mount raised) so the top nut slippage mark is back in line!!  Clear as mud??? 

I would NOT use Locktight (red) you might consider using Locktight (blue) as a last resort if the nuts are infact backing off!!   

A few thoughts

[Ron Hill]

Guys : Nylon locknuts are really made for a one time use.  If you decide to use them a second time you need to inspect them periodically and I'd use a painted slippage mark for ease of inspection!!

A thought