Shaft Coupling Bolt Torque

[joesig]

After recently installing new cutlass bearings and having my shafts and couplings checked I am performing the coupling alignment on both Allison MH 20 gears. The new cutlass bearings changed my shaft location slightly. I just finished aligning the port coupling after fighting rusty bolts and spending two days on my knees. I managed to align the faces within 0.001". This was only accomplished after using a portable Hyd. jack to move the rear mount in the lateral direction slightly. I purchased a new set of Buck Algonquin (Boltset626) coupling bolts, lock washers & nuts for both couplings. They are 5/8" -18 x 2" Grade 5 (3-lines on the bolt head). It looks like the are zinc plated, I calculated the torque dry at 150 ft. lbs. taking the 15% reduction for the zinc plating. I have enough room to get my 1/2" drive Snap-On torque wrench with a 2" extension on the bolt head to the rear of the coupling and a combination wrench on the nut and torque all 6- bolts (it's tight, my back hurts). I have read postings on this sight indicating there is no space for a torque wrench on the Allison gear, I agree if you try and get it on the nut, but what's wrong with placing it on the bolt head? I figure its better then guessing at a torque value.
Thanks, Joe

[krush]

In theoryland, it is best to torque from the nut. But life doesn't allow that all the time. In other theoryland, if the bolt is free to move, torquing from the bolt won't make much of a difference.

In my world, do what you can, and give a little extra if torquing from the bolt if you are worried or lube it up.

Small fasterners are usually overtoruqed, big ones usually under torqued. I wouldn't worry about overtoquing the coupling by hand...i'd concern myself with consistency among the bolts and ensure they are indeed tight enough. The bolts are not supposed to tranfer the torque via shear--they are meant to squeeze the coupling together (bolts remain in tension) and let friction do the work, just like the studs on your car/wheels.

[joesig]

Thanks for the input, I agree.
Joe