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For applications requiring
high installation torque values and using standard NAS bolts:
achievable torque is dependent on plating, length of thread
engagement, class of mating threads and lubricant (if
any). |
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For maximum fatigue
resistance and clamping force in the bolted assembly, we want
to produce a level of installed tensile stress that is just
below the yield strength of the bolt material (typically
around 80% of the ultimate tensile strength). Achieving this
ensures that the stresses encountered in the
assembly/tightening process will be the most the bolt ever
sees. |
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At the yield point, the bolt
will undergo "plastic" deformation (permanent
stretch). Bolts for critical applications should be test-torqued
to determine the point of measurable permanent stretch. As a
rule, approximately 90% of this value will give the best
performance in a bolted assembly. |
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Example: '93 INDY-LOLA ring
gear bolts NAS 1307-4H- with light lubrication, these bolts
stretch permanently on a consistent basis between 105 and 110
ft/lbs of installed torque. 90% of this value give an assembly
torque of 95 ft/lbs. |
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The mating of aerospace spec
(MS.NAS.AN) bolts and nuts to fasteners of other
specifications and thread classes can cause problems. Nut
splitting failures are primarily caused by overtorquing and/or
thread class mismatches. Generally, NAS wrenching torque
values should be reduced when aerospace nuts are matched with
non-aerospace bolts or studs. |
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Wrench torque values in the
NAS nut tables below are achieved under laboratory conditions
using aerospace spec bolts (Class 3 threads) of higher
strength that the nuts they are mated to. THE TABLES DO NOT
STATE, NOR SHOULD ONE INFER THAT THIS PERFORMANCE CAN BE
DUPLICATED WITH OTHER TYPES OF FASTENERS AND/OR CLASSES OF
THREADS. |
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For applications requiring
high clamping forces, 12 point nuts should be used whenever
possible. Their greater thread contact area and higher rated
strength make higher wrenching torque values possible |
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As a general rule, the
wrenching torques values for aerospace spec nuts should be
reduced approximately 25% as a starting point when these nuts
are mated to non-aerospace bolts or studs. |
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