Torque values

06 Feb 2011 Torque values

When you see a chart like is in Chapter 7 of AC 43.13-1B it is for
standard Aircraft Hardware. The torque indicated in these charts is
calculated to pre-stress the standard aircraft bolt to 40,000 psi for a bolt
used in tension and 24,000 psi for a bolt in a shear application. “Clamping
Force” is arrived at by the size and/or quantity fasteners used in the
application.

Other requirements of the standard are, AN310 nuts used in tension, AN320
nuts used in shear, and all standard Aircraft bolts are Cadmium plated. One
thing that is not made as clear as it could be is what constitutes the face
bearing area. The head of the bolt and the nut should both bear on a
standard AN960 Washer which is also cadmium plated. This Cadmium plating
provides the proper and uniform lubricant required to arrive at the
pre-stress requirement consistently.

On close examination of most washers you can see that one side is almost
perfectly flat and the other side is some what rounded over at the inside
and outside diameters from the manufacturing process. The flat side goes
toward the head of the bolt or the nut to provide the proper bearing area,
and the rounded side goes toward whatever is being bolted together so as not
to make a grove or nick that would be a stress riser.

Special applications torque values are set so as to arrive at the
pre-stress values required for the application and special fastener being
used. As in the case of a rod bolt, which is considerably stronger than a
standard bolt of the same size, it is not plated, the threads are most times
oiled, the nut is longer including more threads and the threads are somewhat
differently shaped. All this leads to a different torque value. The measured
torque values of lock nuts is also considered in torque values.

These torque values and how they are applied are critical. Someone one on
the list recently stated that “~ it might be time to come up with a new AC
43.13-1B for BOAM use”. That publication now applies to Heavy Aircraft only
as FAA states on their web site that puts the responsibility for Standards
for Light Aircraft in the lap of the manufacturer.

From: RV-List message posted by: “John D. Heath”