Every bolted joint is unique and the optimum
tightening torque should be determined for each application by
careful examination. A properly tightened bolt is one that is
stretched such that it acts like a very ridged spring pulling
mating surfaces together. The rotation of a bolt (torque) at some
point causes it to stretch (tension).
Several factors affect how much tension occurs when a given
amount of tightening torque is applied.
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The first factor is the bolt's diameter.
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The second factor is the bolts grade. It takes
more force to stretch an SAE grade 8 bolt (or a 12.9 quality
bolt) than it does to stretch an SAE grade 5 bolt (or a 8.8
quality bolt) because of the greater material strenght.
-
The third factor indicates that harder, smoother
and/or slicker bolting surfaces, such as threads and bearing
surfaces, require less rotational force (torque) to stretch
(tension) a bolt than do softer, rougher, and stickier surfaces.
The basic formula T = K x D x P takes these factors into account
and provided users with a starting point for establishing an
initial target tightening torque:
T = target tighten torque (result in this formula is inch pounds,
dividing by 12 yields foot pounds)
K = coefficient of friction (nut factor) always an estimation
in this formula *
D = Bolts nominal diameter in inches
P = Bolts desired tensible load in pounds (generally 75% of
yield strength)
The reason all applications should be evaluated to determine
the optimum tightening torque is that the K factor in this formula
is always an estimate. The most commonly used bolting factors
K are 0.20 for plain finished bolts, 0.22 for zinc plated bolts,
and 0.10 for waxed or highly lubricated bolts.
Bolt Conditions can range from:
Lubricated: Moly Based 0.10 - 0.18, Nickel Based 0.10 - 0.12,
Copper Based 0.08 - 0.23,
Plated: Cadmium 0.11 - 0.33, Zinc 0.08 - 0.53, Black Oxide 0.11
- 0.28,
As Received: Mild Alloy Steel on Steel 0.16 - 0.27
Example: 3/4"-10 Grade 5 zinc plated bolt
K = 0.22 (zinc plated)
D = 0.75 (3/4"-10 nominal diameter)
P = 23'046 pounds (Yield strength for SAE Grade 5 bolt = 69000
PSI * Tensile stress area 0.344 for 3/4"x10bolt)
T = 0.22 * 0.75 * 23046 Pounds / 12 = 317 ft.lb (317 ft.lb
* 1.3558 = 430Nm)
In order to work out your specific required torque use the
above formula using the values P = tensile load in pound which
usually is 75% of yield strength and the tensile stress area
for the specific bolt used).
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