Fatseners

BOLTED JOINTS

Different types of stresses developed in screw fasteners due to initial tightening and external load. Combined effect of initial tightening and external load on a bolted joint. Leak proof joints and condition for joint separation

When a connection is desired that can be disassembled without destructive methods and that is strong enough to resist external tensile loads, moment loads, and shear loads, or a combination of these, then the simple bolted joint using hardened steel washers is a good solution. Twisting the nut stretches the bolt to produce the clamping force. This clamping force is called the pretention or bolt preload.This force exists in the connection after the nut has been properly tightened.

Figure 8-13A bolted connection loaded in tension by the forces P. Note the use of two washers. Note how the threads extend into the body of the connection. This is usual and is desired. LG is the grip of the connection.

Figure 8-14Section of cylindrical pressure vessel. Hexagon-head caps crews are used to fasten the cylinder head to the body. Note the use of an O-ring seal. LG is the effective length of the connection (See Table 8-7) .Figure 8-14 shows another tension-loaded connection. This joint uses cap screws threaded into one of the members.

An alternative approach to this problem (of not using a nut) would be to use studs.

A stud is a rod threaded on both ends. The stud is screwed into the lower member first; then the top member is positioned and fastened down with hardened washers and nuts

Studs are regarded as permanent, and so the joint can be disassembled merely by removing the nut and washer.

Spring Rate : The ratio between the force applied to the member and the deflection produced by that force.

The grip LG of a connection is the total thickness of the clamped material.

Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc.

The grip LG here is the sum of the thicknesses of both members and both washers.

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent JointsTable 8-7Suggested Procedure for Finding fastener Stiffness

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

In joint under tension the members are under compression and the bolt under tension: kb = equivalent spring constant of bolt composed of threaded kt and unthreaded kd parts acting as springs in series.

There may be more than two members included in the grip of the fastener. All together these act like compressive springs in series.Equivalent spring constant km

If one of the members is a soft gasket, its stiffness relative to the other members is usually so small that for all practical purposes the others can be neglected and only gasket stiffness used.With no gasket, the stiffness of the members is difficult to obtain, except by experimentation.Compression spreads out between the bolt head and the nut and area is not uniform.

Ito used ultrasonic techniques to determine pressure distribution at the member interface. Results show that pressure stays high out to about 1.5 bolt radii.Ito suggested the use of Rotschers pressure cone method for stiffness calculations with a variable cone angle. This method is quite complicated.

We choose a simpler approach using a fixed cone angle.The contraction of an element of the cone of thickness dx is subjected to a compressive force P is, from Eq. (5-3),

The area of the element is

Substituting this into the previous equation and integrating the resulting equation from 0 to t gives.

For Members made of Aluminum, hardened steel and cast iron 25o

Substituting this into the previous equation and integrating the resulting equation from 0 to t gives.

With =30o, this becomes

8-5Joints Member Stiffness (8-19)(8-20)

If members of the joint have the same E with symmetrical frusta (l=2t), then they act as two identical springs in series km = k/2. For = 30 and D = dw = 1.5 d, this can be written as,

Finite element analysis agree with = 30o recommendation coinciding exactly at the aspect ratio d/l = 0.4.Additionally, FEM offered an exponential curve-fit of the form

where A and B are given in Table 8-8. (8-22)(8-23)8-5Joints Member Stiffness

Joints Member Stiffness Figure 8-16The dimensionless plot of stiffness versus aspect ratio of the members of a bolted joint, showing the relative accuracy of method of Rotsher, Michke and Motosh, compared to Finite Element Analysis (FEA) conducted by Wileman, Choudury, and Green.

Joints Member Stiffness

Bolt strength is specified by:

minimum proof strength Sp or minimum proof load Fp, and minimum tensile strength, Sut

The proof load is the maximum load (force) that a bolt can withstand without acquiring a permanent set.The proof strength is the quotient of the proof load and the tensile-stress area.The proof strength is about 90% of the 0.2% offset yield strength.

Bolt Strength

The SAE specifications are given in

Table 8-9 bolt grades are numbered according to minimum tensile strength.The ASTM Specs for steel bolts (structural) are in Table 8-10.Metric Specs are in Table 8-11.

If Sp not available use: Sp =0.85 Sy

Fp = At Sp Bolt Strength

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints8-6Bolt Strength

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

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

END