Page 88 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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(a) (b) (c) (d)
Figure 3.5 Typical points of setscrews: (a) cone point, (b) cup point, (c) flat point and (d) full-dog point.

Washers are used under a bolt head or under both a bolt head and nut. The basic
type of washer is a plain flat washer, which distributes clamping forces over a wide area.
When loaded, a split helical lock washer will deform axially, generating axial forces on
the fastener to prevent loosing, as used in Figure 3.3a, d.
This book only presents typical types of threaded fasteners. Detailed information for
almost endless threaded fasteners can be found in various standards, design handbooks
[6, 7] and on the Internet.

3.3.3 Tightening Torque and Preloading
The purpose of a bolt fastening is to clamp two or more elements together. For most
applications, a bolted joint is tightened during assembly before it starts to carry
operating loads. While tightening, the bolt head is usually held stationary and the nut
is twisted. The nut moves along the screw and, when resistance is encountered, axial
force will generate in the thread [5]. This axial force is called preload, initial tension or
pretension. It is this force that clamps two or more members together. The consequence
of tightening process is that the bolt is preloaded in tension, while the clamped elements
in compression.
The value of preload must be properly controlled to enhance resistance to potential
fastener loosening and fatigue failure. Too small a preload may cause leakage, while too
high a preload may twist off bolts or screws. The maximum preload is taken to be 75% of
the proof load [2, 4], which is the product of the proof strength times the tensile stress
area of the bolt or screw.
The required preload can be controlled by tightening torque T,which canbeestimated
by [8]
T = T + T ≈ 0.2Q d (3.1)
p
2
1
where T and T are frictional torque in the thread and at the bearing surface, respec-
1 2
tively. Q is the preload and d is the major diameter of the thread.
p
In practice, tightening torques are controlled or monitored approximately by a built-in
dial that indicates the torque in a torque wrench. A more accurate approach to determine
the value of preload is by bolt elongation measurement, especially for high reliability
design.
3.3.4 Fastener Loosening and Locking

Twisting a nut stretches the bolt and produces clamping force within the connected
members. However, the initial tension or preload may be lost gradually due to wear,
creep, impact, vibration or corrosion during operation. When the initial preload is
lost, the in-service threaded fasteners will loosen, causing separation of the connected
members and resulting in malfunctioning of machines. Periodically retightening is a