2-lectures lec 39

ME 307 Machine Design I

CH-8 LEC 39 Slide 1Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints


8-1 8-1 Thread Standards and DefinitionsThread Standards and Definitions8-28-2 The Mechanics of Power ScrewsThe Mechanics of Power Screws8-38-3 Strength Constraints Strength Constraints 8-48-4 Joints-Fasteners StiffnessJoints-Fasteners Stiffness8-58-5 Joints-Member Stiffness Joints-Member Stiffness 8-68-6 Bolt Strength Bolt Strength 8-78-7 Tension Joints-The External LoadTension Joints-The External Load8-88-8 Relating Bolt Torque to Bolt TensionRelating Bolt Torque to Bolt Tension8-98-9 Statically Loaded Tension Joint with PreloadStatically Loaded Tension Joint with Preload8-108-10 Gasketed JointsGasketed Joints8-118-11 Fatigue Loading of Tension JointsFatigue Loading of Tension Joints8-128-12 Shear JointsShear Joints8-138-13 SetscrewsSetscrews8-148-14 Keys and PinsKeys and Pins8-158-15 Stochastic ConsiderationsStochastic Considerations



8-128-12 Shear JointsShear Joints8-138-13 SetscrewsSetscrews8-148-14 Keys and PinsKeys and Pins


ME 307 Machine Design I 8-128-12 Shear Joints 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.




Bolt loaded in double shear.



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.



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

x yix iy

Figure 8-23 Centroid of pins, rivets, or bolts.

Centroid of pins, rivets or boltsCentroid of pins, rivets or bolts



1 1 2 2 3 3 4 4 5 5 1

1 2 3 4 5

1

1 1 2 2 3 3 4 4 5 5 1

1 2 3 4 5

1

n

i i

n

i

n

i i

n

i

A xA x A x A x A x A x

xA A A A A A

A yA y A y A y A y A y

yA A A A A A

Centroid of pins, rivets or boltsCentroid of pins, rivets or bolts

(8-49)



An example an eccentric loading of fasteners is shown in Figure 8-24.



If bolts are not same size only bolts with maximum load should be considered



See Example 8-6 and 8-See Example 8-6 and 8-7(Textbook)7(Textbook)



Example 8-6 (Textbook)Example 8-6 (Textbook)


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.


8-13 Setscrews8-13 Setscrews

A set screw is screwed through a threaded hole in one part so that its point presses against the other part.


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.


8-13 Setscrews8-13 Setscrews



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.

Table 8-18



8-13 8-13 Keys and PinsKeys 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.


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


8-13 8-13 Keys and PinsKeys and Pins

0.0208d D L (8-51)



Square KeysSquare 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 .



Gib-Head KeyGib-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.



Woodruff KeyWoodruff 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.


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.




Retaining RingRetaining 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.

2-lectures lec 39

Documents

clamping force

nonpermanent

retaining

aziz bazoune

point presses

set screw

threaded hole

nonpermanent