ETLB Squawk Forums — Archive

Hi
At the point of putting a new Ed Sterba 54 x 24 wood prop on my Max with the Mosler CB-40, and I sense a dilemma coming up.  It is a six bolt pattern, with crush plate, with nuts forward, nuts safetied in pairs of two, safety wire forward of the prop.  Though I haven't done it yet, it seems to me that using cast nuts that require a fairly precise positioning with the bolt in order for the hole to work as a hole for the safety wire, is going to make getting the torque to coincide with that position a problem.  Using unsafetied nylocks for instance would not present this problem, you would merely apply pressure on the torque wrench until it clicks and then stop and go on to the next one.  Using cast nuts and safety wire requires the bolt hole and a slot in the cast nut to line up.  Seems as if it is going to be a pain in the butt.  Has anyone done through this and found it actually workable, hence I am merely creating a concern where there isn't one?  Thanks.
Kim Brown
New Hampshire

Nylock nuts can't be torqued to precise value since the nock friction works against the measured torque.

You really do want safety wired nuts.  Don't over think this. It won't be as hard to torque the nuts to the right value as you think.

Tom

The torque spec should indicate a small range so one can torque a castellated nut to the lower end of the range and go slightly higher to get the hole in the bolt to line up.  I'd use cotter pins instead of safety wire.  If you want to use the self-locking nuts you might want to know the prevailing torque.  AC 43.13-1B lists minimum prevailing torques but unfortunately not for 1/4-28 thread, not too useful here but maybe for a guestimate.  My torque wrench goes as low as 25 inch-pounds so I guess the prevailing torque is likely somewhere between 10 (8 is the minimum) and 20 inch-pounds, say 15 for a never-used AN365-428 nut.  If the torque spec for AN4 bolts is 120-140 inch-pounds, then for a castellated nut, torque to 120 plus a bit to line up a hole if needed. For the AN365 nut, torque to the high end of the range, 140 inch-pounds.

I'm just going to lob this one up for the more experienced ones here to weigh in on, but in rebuild old (1920's-1940's car and tractor engines) that had safety wired castellated nuts on the connecting rods I was taught to slide  the bottom of the nut down a flat file a stroke or two if it didn't line up at torque spec with the hole, that would let turn just a bit more to line up

My last 4 aircraft have had wood props, from 72 inch to 34 inch diameter, go with what Tom says, "don't over think it". Good torque technique is probably the most important element. You also might be interested in this from the FAA:

https://www.faasafety.gov/gslac/ALC/course_content_popup.aspx?cID=529&sID=907&preview=true

One thing that I don't believe they mention is to never torque twice once you have achieved the correct torque value, especially with a click type. Each time you tighten to the click you will add a small amount to the torque.

Also, never modify a critical fastener !!!

Hi KFB
Never torque one bolt at a time till it "clicks". Tighten a little at a time like torqueing a cylinder head. AN washers come in 2 thicknesses just for this purpose. Use cotter pins. The washers should get you in the torque range.
Bob