Torque vs cotter pin

Russell Reay

Russell Reay

Well-known member
This drive shaft nut is torqued to 100 lbs. The red dot at 1 o'clock is the location of the hole for the cotter pin. There is no way that the nut can be turned another 1/12 of a turn to align the next notch on the castle nut with the hole. This means I must back off the nut 1/12 of a turn to permit the insertion of the cotter pin. I have encountered this issue with other torqued nuts, but never with one that requires 100 lbs. Do I back it off and declare victory, or is there an alternative procedure?
 

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Russell, to begin with, fastener torque ratings back then were not used. Common sense was. The Mechanic used his judgment to determine whether the fastener was to be tightened, -or loosened to allow the holes to align.

With regard to your present condition, if you feel you cannot safely tighten another 1/12th of a turn, then remove the nut and sand the bottom-side of the nut to remove thickness. By removing a slight amount, the fastener will likely align with the cotter pin hole.
 
BRENT in 10-uh-C said:
(...) remove the nut and sand the bottom-side of the nut to remove thickness. By removing a slight amount, the fastener will likely align with the cotter pin hole.
Click to expand...


Since there is a risk of accidentally grinding the nut's contact surface slightly crooked, I prefer to place a thin washer in between.
 
I don't know what the threads per inch are, but say it is 16 tpi. Then one turn is 1/16 inch or 0.0625 inch. It looks like the hole is 30 degrees off or 1/12 of a turn. So 1/12 X 1/16 would be 0.005 inch. So you would have to grid off 0.005 inch or add a 0.005 shim. If not 16 tpi them plug in the correct number.
 
Castle nuts were not designed to be torqued. You snug them & advance to align the cotter pin hole. A castle nut has a lower structural strength than a conventional nut because it has fewer complete threads, so do not arbitrarily apply standard torque values to them. Moreover, none of the original fasteners in a Model "A" & "AA" were designed for SAE torque values.
A proper use of a torque wrench in a Model "A" would be to measure the torque applied by hand tightening as was done in on the original assembly line, and apply the same torque to all of the other fasteners in that particular joint. Even tightening is the goal of using a torque wrench.
In general, SAE torque values are designed to stress a bolt to about 75% of its tensile strength.
 
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In aviation, there are several washer thicknesses available for each diameter of bolt. They are most commonly 0.03125" and 0.0625" or 1/32" and 1/16". A lot also depends on whether it is a shear type fastener or a tension type fastener. Most stuff in the modern era use self locking nuts but on control systems the FAA requirement is two separate lock features. There re castellated nuts with fiber lock inserts for cotter pin types and pinched steel lock nuts with pal nuts. The big shear bolts such as used on main rotor blade retention are stretch torqued so get maximum shear strength of the bolts. I used a special micrometer to measure untorqued length and then torque it to recommended stretch. I had to use two long break over bars to torque most blade bolts.

The early era procedure was tighten the nut snug and then go to the next castellation for cotter pin use but stuff like the tapered axle shafts complicate things. Grinding a nut would be best done by use of a surface grinder or a piece of abrasive paper on a surface plate if done by hand. It's wise to consult thread torque recommendations for diameter of fastener and thread type on that fastener to see what recommended max torque might be. Tapered shafts should always have a specific torque as recommended by the manufacturer, The taper and the key have to share the rotational torque applied by the vehicle's drivetrain. This required a certain amount of stretch but the engineers in 1927 didn't follow that practice.
 
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Are you using the torque wrench that's in the picture? That one looks like it's in inch-lbs.
 
Don’t grind— use 80 grit wet/dry sandpaper with oil, rotate the nut in your fingers about every 10 strokes or so to help keep it even—-
with a grinder doing it freehand is hard to keep it even.
sometimes just loosening and tightening is enough.
never back off even just a little unless backing off all the way and tightening again.
 
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