Chapter 8. Screws, Fasteners, and the Design of Nonpermanent Joints. Chapter Outline. 8-1 Thread Standards and Definitions 8-2 The Mechanics of Power Screws 8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness
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Screws, Fasteners, and the Design of Nonpermanent Joints
8-1 Thread Standards and Definitions8-2 The Mechanics of Power Screws8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness
8-5 Joints-Member Stiffness 8-6 Bolt Strength 8-7 Tension Joints-The External Load8-8 Relating Bolt Torque to Bolt Tension
8-9 Statically Loaded Tension Joint with Preload
8-10 Gasketed Joints
8-11 Fatigue Loading of Tension Joints
8-12 Shear Joints
8-14 Keys and Pins
8-15 Stochastic Considerations
8-1 Thread Standards and Definitions
The fundamental operation in manufacture is the creation of shape - this includes assembly, where a number of components are fastened or joined together either permanently by welding (Ch9) for example or detachably(nonpermanent) by screws,nuts and bolts and so on. Since there is such a variety of shapes in engineering to be assembled, it is hardly surprising that there is more variety in demountable fasteners than in any other machine element. Fasteners based upon screw threads are the most common, so it is important that their performance is understood, and the limitations of the fastened assemblies appreciated.
a) Square (b) ACME; (c) UN, ISO
Terminology of screw threads. Sharp Vee Threads shown for clarity; the crests and roots are actually flattened or rounded during the forming operation
(a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws.
Text Reference: Figure 15.2, page 667
A thread 'system' is a set of basic thread proportions which is scaled to different screw sizes to define the thread geometry. Whitworth, Sellers, British Standard Pipe (BSP) are just three of the many systems which proliferated before the adoption of the
ISO Metric thread system.
The American National (Unified) Thread standards is used mainly in the US.
Square and ACME
The basic profile of ISO Metric threadsis built up from contiguous equiangular triangles of height H disposed symmetrically about a pitch line which becomes the pitch cylinder of diameter d2 when the profile is rotated about the axis to form the thread. The distance between adjacent triangles - the pitch - is p = 2 H /√3. The tips of the triangles are truncated by h/8 to form the major diameter ( size ) d of the thread, and the bases are truncated by h/4 to form the minor diameter d1. It follows that d1 = d - 5 h/4 = d - 1.08 p. This leads to the rule of thumb for suitable tapping size
H= 0.5(3)1/2 p
US N= # threads/in
a ISO 68= a American National (Unified) thread standard= 60°
Power screw thread forms. [Note: All threads shown are external (i.e., on the screw, not on the nut); dm is the mean diameter of the thread contact and is approximately equal to (d + dr)/2.]
Figure 15.5 Details of ACME thread profile. (All dimensions are in inches.)
Text Reference: Figure 15.5, page 670
Crest diameters, threads per inch, and stresses for Acme thread.
The joyce warm-gear screw jack
Weight supported by three screw jacks. In each screw jack, only the shaded member rotates.
Example of catalogue:
Different types of collars