Solution for shaft coupler bolts loosening?

Checking my shaft couplers was not a routine maintenance item, but a month ago I discovered that my starboard coupler bolts were so loose as to be ratthling around in their holes. Having never checked them in ~120 hours of operation, I couldn't say for sure they weren't always this way, but I didn't think they were. I tightened them as tight as I could, no idea the torque. I checked the port side, and all bolts were tight. I resolved to check the couplers before each outing, just as I'd check oil and coolant levels.

The couplers are held together with 4 large nut/bolt pairs with split lock washers between the nuts and the rim of the coupler. The couplers are solid metal with an alignment guide on one that slides into the center of the other that look like two hats bolted together along the brims when tightened down.

Subsequent checks of the couplers has shown that the starboard side requires retightening with as little as 10 hours running time. The port side remains tight. I can't see significant vibration at the couplers on either side when running, certainly no more on the starboard than on the port.

What should I do? Determine the required torque and ensure the bolts are tightened to spec? Any guidance on the appropriate torque? Continue to check and retighten as needed? Add some thread locker? If so, is blue good enough? Reconfigure the hardware to improve the locking mechanism? If so, what hardware would you recommend?

Thanks for your help.

Replace the all lock washers. They are supposed to have a raised edge that digs in a little to both surfaces.

Ted

Check the alignment and engine mounts and prop and shaft. Is there excessive vibration on that side at idle? really inspect stuff closely because I suspect something is wrong other than bolts getting loose.

If all is well consider new bolts (grade5?) nylon lock nuts in addition to washers.

Fine thread bolts and nylock nuts.

jleonard said:

Fine thread bolts and nylock nuts.

Click to expand...

What he said.

I replaced mine with fine thread Grade 8 bolts and nylock nuts.

Ken

In addition to minimum grade 5 or grade 8 (or metric 8.8 and 10.9) fasteners, you might also look into Nord-Lock wedge-locking washers. These are readily available at Amazon and Grainger.

For a typical shaft coupling, through holes require two pairs of Nord-Lock washers – one pair for securing the bolt and a second pair for securing the nut. Turn both fasteners in order to close the cams on both washer pairs before tightening to minimize settlements. Keep the nut secured while tightening the bolt. These washers are re-usable.

Over-kill, maybe, but for the small cost and minimal effort, the additional peace of mind is worth it to me. Especially where, in my case, the coupling is a single point of failure.

Thanks, All, for sharing your years of experience.

I'll likely start with the fine thread bolts held with nylock nuts while I wait for the Nord-Lock washers to come in. I'm not sure the bolt heads will have adequate clearance with a pair of washers underneath them, but I'll find out.

Greg.

greg; Unless this is a new boat something has changed to cause these old bolts to loosen. What is it that changed?

Check the alignment. If all is right with the joint, the fasteners will remain tight even without a lockwasher. Also make sure the mating flange faces are clean clean clean to grey metal. If rust is between flanges, as it works out the tension in the bolts will be lost.

Greg, as others have said-check alignment. also take a close look at your engine mounts. something going on to cause this other than old loc washers

A small tube of Loctite Thread Sealer, medium strength, should be in every boat owners repair arsenal. Don't use the high strength or you may have to use a jack hammer to get it apart some day. Keeps fasteners from vibrating loose. I was first introduced to this in the Air Force working on jet engines. Can get in most any auto parts store.

jleonard said:

Fine thread bolts and nylock nuts.

Click to expand...

Exactly & check vibration

Another possibility is that the coupler is worn out. If the engine has been out of alignment for any length of time the two parts of the coupler can wear. I had to replace my shaft and coupler at 5200 hours on my Tiawan trawler with a Lehman 120 in it. The new coulper and shaft had to be mated and lathe turned to square the coupler to the shaft. A dial indicator can show the runout on both the radial direction and the face of the coupler. The transmission half of the coupler is also suspect and should be checked.

@bayview & to Larry H's point: The boat isn't new, but it's 160 running hours new to me. ALL of the drivetrain components were either replaced (couplers and attaching hardware, 1 shaft, motor mounts, cutlass bearings) or machined/adjusted (the other shaft, both props), shaft logs repacked, etc. The shop that worked the shafts fabricated or matched the couplers to them. Both sides were aligned in the water and then test driven while the mechanic that did the job checked his work. The couplers shouldn't be worn out and aren't rusty, but either could have been defective to begin with or installed improperly. I'll keep those possibilities in mind as avenues to explore. I don't know 100% that the hardware securing the couplers was replaced, but moving to fine threads and nylock nuts will take care of that.

@ others who suspect the alignment: I know that the work done 160 hours ago doesn't mean the alignment was good to begin with or that it hasn't gone south since. (Why is 'gone south' a bad thing?) I don't know what a bad alignment looks like, but I can't see or feel significant vibration. Because of my ignorance, it's something I'll have checked.

@ Russel: I just added two small tubes of blue (medium) to my toolbox but hadn't decided to use any yet. They'll be there when I need them.

Thanks again for your thoughts.

If all else fails look in an aircraft parts catalog and purchase bolts of the proper length that have holes drilled in the end.

Get the special nuts for the bolt size and simply safety wire them

Works on aircraft for the past century.

Passed on to me by my secret santa...

"Has he referred to a bolt torque spec chart?....or used a torque wrench during his seemingly frequent re-tightening sessions?...... Or is continuing to use old, stretched bolts that are no longer elastic?"

"Basics first....."

My addition.....basic torque values can be found on the web for standard bolt and nut sizes...may want to start there or just go with a new set of fine bolts and choice of nuts and wadhers.