Page 462 - Shigley's Mechanical Engineering Design

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Screws, Fasteners, and the Design of Nonpermanent Joints 437
Table 8–12
Stiffnesses, M lbf/in
Computation of Bolt Bolt Grip, in k b k m C 1 C
and Member Stiffnesses. 2 2.57 12.69 0.168 0.832
Steel members clamped 3 1.79 11.33 0.136 0.864
1
using a in-13 NC
2 4 1.37 10.63 0.114 0.886
k b
steel bolt. C =
k b + k m
washers. The ratios C and 1 − C are the coefficients of P in Eqs. (8–24) and (8–25),
respectively. They describe the proportion of the external load taken by the bolt and
by the members, respectively. In all cases, the members take over 80 percent of the
external load. Think how important this is when fatigue loading is present. Note also
that making the grip longer causes the members to take an even greater percentage
of the external load.

8–8 Relating Bolt Torque to Bolt Tension

Having learned that a high preload is very desirable in important bolted connections,
we must next consider means of ensuring that the preload is actually developed when
the parts are assembled.
If the overall length of the bolt can actually be measured with a micrometer when
it is assembled, the bolt elongation due to the preload F i can be computed using the
formula δ = F i l/(AE). Then the nut is simply tightened until the bolt elongates
through the distance δ. This ensures that the desired preload has been attained.
The elongation of a screw cannot usually be measured, because the threaded end is
often in a blind hole. It is also impractical in many cases to measure bolt elongation. In
such cases the wrench torque required to develop the specified preload must be estimated.
Then torque wrenching, pneumatic-impact wrenching, or the turn-of-the-nut method may
be used.
The torque wrench has a built-in dial that indicates the proper torque.
With impact wrenching, the air pressure is adjusted so that the wrench stalls when
the proper torque is obtained, or in some wrenches, the air automatically shuts off at
the desired torque.
The turn-of-the-nut method requires that we first define the meaning of snug-tight.
The snug-tight condition is the tightness attained by a few impacts of an impact
wrench, or the full effort of a person using an ordinary wrench. When the snug-tight
condition is attained, all additional turning develops useful tension in the bolt. The
turn-of-the-nut method requires that you compute the fractional number of turns nec-
essary to develop the required preload from the snug-tight condition. For example, for
heavy hexagonal structural bolts, the turn-of-the-nut specification states that the nut
should be turned a minimum of 180 from the snug-tight condition under optimum
◦
conditions. Note that this is also about the correct rotation for the wheel nuts of a pas-
senger car. Problems 8–15 to 8–17 illustrate the method further.
Although the coefficients of friction may vary widely, we can obtain a good estimate
of the torque required to produce a given preload by combining Eqs. (8–5) and (8–6):

F i d m l + π fd m sec α F i f c d c
T = + (a)
2 πd m − fl sec α 2