Capscrews can be used in tapped holes or with a nut (like a bolt). Most
designs with capscrews where a clamp load is needed, require the
capscrew to be tightened to 75% of the proof load for a safe working
range. Shear strength of a capscrew loaded in single shear through the
threads is about 60% of the ultimate tensile strength, so working loads
should be lower than this breaking strength. The fatique strength of
capscrews as measured by the endurance limit will vary widely
depending upon the amplitude, frequency, and type of applied load.
Endurance limits of 10-20% of the tensile strength have been reported
for high strength capscrews.
SUGGESTED STARTING TORQUE VALUES
All torque/tension relationships should be viewed with a cautious eye
since no one table can indicate the range of conditions expected to be
experienced by a fastener. Torque is only an indirect indication of
tension. The torque value to use in an application is best obtained by
using a calibrated torque wrench (or transducer) and a
Skidmore-Wilhelm type load indicating device to equate actual torgue to
desired tension. Nearly all of the torque/tension tables which have been
developed, including the one shown in Table 3, are based on the
following formula:
T = (k*d*P)/12 where T=Torque (ft-lbs), d=nominal diameter
(inches), P=Tension (Lbs), and k is the "torque coefficient" or "nut
factor" (dimensionless)
The value of k is a dimensionless "fudge factor" which includes
variables such as friction, thread conditions, etc. The value of k can
range from 0.10 for a well lubricated connection to greater than 0.30
for a rusted assembly. Normally, k is approximately 0.20 for plain steel
(increase by about 10% for Zinc plated and decrease by about 25% for
parts which have been well lubricated). The torques shown below
represent starting values for plain capscrews (k=0.20) at 60%-90% of the
proof load tension:
tds009d.txt
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