Screws, Fasteners, and the Design of Nonpermanent Joints Chapter 8 Screws, Fasteners, and the Design of Nonpermanent Joints Dr. A. Aziz Bazoune 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 8-5 Joints-Member Stiffness 8-6 Bolt Strength 8-7 Tension Joints-The External Load 8-8 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 8-13 Setscrews 8-14 Keys and Pins 8-15 Stochastic Considerations Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

LECTURE 39 8-12 Shear Joints 8-13 Setscrews 8-14 Keys and Pins Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-12 Shear Joints Joints should be loaded in shear so that the fasteners see no additional stress beyond the tightening. Shear loading is resisted in two ways: The shear load is carried by friction between the members and ensured by the clamping action of the bolts or cap screws. The shear load is carried by dowel pins in reamed holes, placed in both parts while clamped together to ensure alignment. The dowels will carry the shear load. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Bolt loaded in double shear. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Failure modes due to shear loading of riveted fasteners Failure modes due to shear loading of riveted fasteners. (a) Bending of member; (b) shear of rivet; (c) tensile failure of member; (d) bearing of rivet on member or bearing of member on rivet. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Centroid of pins, rivets or bolts Integral to the analysis of a shear joint is locating the center of relative motion between two members. In Fig. 8-23 let A1 to A5 be the respective cross-sectional areas of a group of 5 points, or hot driven members. Under this assumption the rotational pivot point lies in the centroid of the cross-sectional area pattern of the pins, rivets or bolts. The centroid G is located by and , where and are the distances to the ith area center Figure 8-23 Centroid of pins, rivets, or bolts. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Centroid of pins, rivets or bolts (8-49) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

An example an eccentric loading of fasteners is shown in Figure 8-24. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

If bolts are not same size only bolts with maximum load should be considered Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

See Example 8-6 and 8-7(Textbook) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Example 8-6 (Textbook) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Example 8-7 (Textbook) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-13 Setscrews Used to prevent relative motion between the two parts. Unlike bolts and screws, which depend on tension to develop a clamping force, the setscrew depends on compression to develop the clamping force. A set screw is screwed through a threaded hole in one part so that its point presses against the other part. A set screw is screwed through a threaded hole in one part so that its point presses against the other part. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-13 Setscrews Setscrews depend on compression to develop a clamping force The resistance to axial motion of the collar or hub relative to the shaft is called holding power. This holding power, is a force resistance due to frictional resistance of the contacting portions of the collar and shaft as well as any slight penetration of the setscrew into the shaft. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Table 8-18 Table 8-18 lists values of the seating torque and the corresponding holding power for inch-series setscrews. The values listed apply to both axial holding power and resisting torsion. Typical factors of safety are 1.5 and 2.0 for static loads and 4 to 8 for various dynamic loads. Setscrews should have a length of about half of the shaft diameter. This practice also provides a rough rule for the radial thickness of a hub or collar. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-13 Keys and Pins Keys and pins are used on shafts to secure rotating elements, such as gears, pulleys, or other wheels. Keys are used to enable the transmission of torque from the shaft to the shaft-supported elements. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-13 Keys and Pins Pins are used for axial positioning and for the transfer of torque or thrust or both. Pins are useful when the principal loading is shear and when both torsion and thrust are present. Taper pins are sized according to the diameter at the large end. Some of the most useful sizes of these are listed in Table 8-19. The diameter of the small end is where d = diameter at small end, in D = diameter at large end, in L = length, in (8-51) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Square Keys The standard sizes of square and rectangular keys, together with the range of applicable shaft diameters are listed in Table 8-20. The length of key is based on the hub length and the torsional load to be transferred . Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Gib-Head Key It is tapered. When firmly driven, it acts to prevent relative axial motion. This also gives the advantage that the hub position can be adjusted for the axial location. Head makes removal possible without access to the other end, but the projection may be hazardous. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Woodruff Key It is useful when a wheel is to be positioned against a shaft shoulder. It also yields better concentricity after assembly of the wheel and shaft. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dimensions for some standard Woodruff key sizes can be found in Table 8-21, and Table 8-22 gives shaft diameters for which the different keyseat widths are suitable. Stress-concentration factors for keyways depend for their values upon the fillet radius at the bottom and ends of the keyway. For fillet cut by standard milling-machine cutters, Peterson charts give Kt = 2.4 for bending and Kts = 2.62 for torsion. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Retaining Ring A retaining ring is frequently used instead of a shaft shoulder or a sleeve to axially position a component on a shaft or in a housing bore. A groove is cut in the shaft or bore to receive the spring retainer. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Example 8-8 (Textbook) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints