Consolidated Tech Man - Holo-Krome

FASTENER JOINT DESIGN
In applications which experience high cyclic loading analysis of
the fastener loading is critical. Based on the relative stiffness of the
assembly and the screw, and the external load applied, the fastener size
and proper preload can be determined.
Preload is normally achieved by tightening the screw. The
torque required to achieve the specific preload depends on the surface
lubricity at the threads and at the head, the surface finish, thread pitch,
head bearing configuration, and torque wrench accuracy.
PRELOAD AND EXTERNAL LOAD CONSIDERATIONS
When a fastener is tightened, the load on the assembly and
fastener increase. In the ideal case, the screw stretches, and the joint
compresses, within the elastic limit, according to Hooke's Law.
Suppose a joint has been tightened to a preload P i and additional
load, P e , tending to separate the members is applied. In general in rigid
assemblies, as long as the external load is less then P t it primarily
decompresses the joint and has little effect on the tension in the screw.
Thus even if such a load is repeatedly applied, the fastener will not fail in
fatigue. However, if a repeated external load greater than P i must be
applied, it should be kept to a minimum, since it produces cyclic tensioning
in the screw and may lead to fatigue failure.
This principle has important practical applications. Often when
fasteners fail, the solution used is to switch to the next larger size. This
involves changing the tooling for hole preparation and tightening, and
possibly changing assembly fixtures. Most likely, merely tightening the
fastener above the external load would solve the problem. For instance,
a 180,000 psi tensile strength socket screw has an average endurance limit
of only 15,000 psi. This means that the fastener is capable of withstanding
a maximum one-time applied stress of 180,000 psi, but that a stress
change felt by the fastener of more than 15,000 psi could result in a fatigue
failure within a given number of cycles.
A conservative formula giving the tension on a fastener which
has an external load, P e is:
[ ] Ks P = P + K + K P t i s c e
Nomenclature
Pt = Total Bolt load, lb.
Pi = Preload, lb.
Pe = External load, lb.
Ks = Screw spring constant, lb./inch.
Kc = Assembly spring constant, lb./inch.
The spring constant, K, for a member is given by:
52
(area, sq. in.) (modulus of elasticity, psi)
(length, in.)
Since these calculations ignore factors such as bending, heat,
and impact loading, they are useful as a guide only.
Torque to achieve preload
When screws are torqued to achieve preload, the torsional
stresses on them reduces the total tensile force they can withstand
before yielding. The following formula can be used to determine the
effect of this torsional component:
P ( S = ±
t
A
√ 2 J A
P)
+( ) t x r 2
2
The desired tightening torque can be estimated using the
empirical formula:
T = KDP
Nomenclature
S t = Total tensile stress felt by bolt, psi
P = Preload exerted by bolt, lb.
A = Thread stress area, sq. inches
T = Tightening torque, in.-lb.
t = Torsion felt by screw (approximately 40% of applied
torque, depending on lubricity), in.-lb.
r = √A/π
J = πr4 (polar moment of inertia), in. 4
2
K = Constant from 0.05 to 0.35, dimensionless
D = Nominal screw diameter, inches
The constant, K, is normally from 0.19 to 0.25 for a black screw.
For a lubricated fastener or one with cadmium plating, K is from 0.13 to
0.17. Zinc plated screws, not lubricated, may have a K value as high as
0.30 to 0.35.
For rigid parts of steel, the conservative practice is to tighten the
fasteners to 75% of yield. Lower torques should be considered for
flexible joints, joints with gaskets, or assemblies subject to high
temperatures.
Joint Design Steps
1. Calculate the assembly service load.
2. Determine relative loading on each fastener.
3. Determine the style fastener desired (hex, socket, etc.).
4. Using the highest loaded fastener in assembly, apply an appropriate
safety factory based on the estimated reliability of the service load
value, the quality level of the type fastener used, estimate of
assembly techniques consistency, and danger of an assembly
failure. Normally a safety factory could range from 1.5 to 6.
5. Determine fastener size, tightening torque, and cycle life, where
applicable.
6. Specify tightening torques on drawing.
Fastener Assembly Tips:
1. Preload properly.
2. Do not use split type lock washers on socket head screws or Grade
8 hex screws.
3. Use long screws when possible.
4. Avoid transverse or shear loading against the threads.
5. Keep the members clean during assembly.
6. Use wrenches that fit properly and are correctly heat treated.