UNIT-4

BOLTED JOINTS

Objectives :

At the end of this lesson, the students should have the knowledge of

• 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.

POWER SCREW:

It is mechanical device used for converting rotary motion into linear motion and

transmitting power. It is also called a translation screw.

Applications of power screws:

1. To raise the load e.g.: screw jack

2. To obtained accurate motion in machining operations e.g.: lead screw of lathe

3. To clamp a work piece e.g.: vice

4. To load a specimen e.g.: universal testing machine

Advantages of power screw:

1. Large load carrying capacity.

2. The Overall dimensions of the power screw are small, resulting in compact

construction.

3. It provides large mechanical advantages. A load of 15kN can be raised by applying an

effort as small as 400N.

4. It can give highly accurate liner motion.

5. A power screw is simple to design.

6. The manufacturing of a power screw is easy without requiring specialised machinery.

Square threads are turned on the lathe. Trapezoidal threads are manufactured on a

thread milling machines.

7. A power screw gives smooth and noiseless service without any maintenance.

8. A power screw can be designed with self locking property.

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Disadvantages of power screw:

1. A power screw has very poor efficiency, as low as 40%. Therefore, it is not used in

continuous power transmission is machine tools, with the exception of the lead screw.

2. High friction in threads causes rapid were of the screw or the nut.

FORMS OF THREADS:

The threads used for fastening purposes, such as V-threads are not suitable for power

screws.

The purpose of fastening threads is to provide high frictional forces, which lessens the

possibility of loosening the parts assembled by threaded joint. On the other hand, the purpose

of power transmission threads is to reduce friction between the screw and nut.

There are two popular types of threads used for power screws

1. Square threads

2. ISO metric trapezoidal threads

Square threads:

Efficiency of the square thread is more.

No radial pressure or side thrust on the nut. Life of the nut is more.

Difficult to manufacture, usually turned on lathe with a single point cutting tool.

Square thread has less thickness at the core diameter thus reduces load carrying

capacity.

Wear is compensated by making nut of soft material.

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ISO metric trapezoidal thread:

Less efficiency

Side thrust is present

Easy to manufacture. Thread milling machine is used which has multi point cutting tool

so result in less cost.

More thread thickness at core diameter more strength.

The axial wear on the surface can be compensated by means of split type of nut.

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Special types of threads:

ACME thread: (power transmission screw, lead screw) It is identical thread with

respect to trapezoidal thread in all respect but thread angle is 29 degree instead of 30

degrees. The relative advantages and disadvantages of ACME threads are same as those

of trapezoidal thread.

Buttress thread: It combines the advantages of square and trapezoidal threads.

Buttress threads are used where a heavy axial force acts along the screw axis in one

direction only.

Advantages:

1. It has higher efficiency compared with trapezoidal thread

2. Thread milling machine is used to manufacture it.

3. Stronger then square and trapezoidal thread both.

Disadvantages:

1. It can transmit power and motion only in one direction.

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Note: buttress threads are ideally suited for connecting tubular components (barrel housing in

anti-aircraft gun).

Multi threaded screws:

It is used where higher travelling speed is required. The travelling speed is twice that of

single start screw.

Efficiency of multi start screw is higher than efficiency of single-start screw. This is

due to increase in helix angle (α).

Mechanical advantage of multi start screw is less as compare to mechanical advantage

of single-start screw. Thus effort required to raise a particular load is more in multiple

threaded screw.

It is likely that the self-locking property may be lost in a multi-threaded screw.

Multiple threaded screws are used in high speed actuators and sluice valve.

Terminology of power screw:

1. Pitch (P): The pitch is defined as the distance measured parallel to the axis of the screw

from a point on one thread to the corresponding point on the adjacent thread.

2. Lead (l): It is defined as the distance measured parallel to the axis of the screw which

the nut will advance in one revolution of the screw.

3. Nominal diameter (d): It is the largest diameter of the screw thread. It is also called

major diameter.

4. Core diameter (dc): It is the smallest diameter of the screw thread. It is also called

minor diameter.

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5. Helix angle (α): The helix angle is defined as the angle made by the helix of the thread

with a plane perpendicular to the axis of the screw. The helix angle is related to the lead

and the mean diameter of the screw. It is also called lead angle.

Different types of screw threads:

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Locking devices:

Ordinary thread fastenings, generally, remain tight under static loads, but many of these

fastenings become loose under the action of variable loads or when machine is subjected to

vibrations. The loosening of fastening is very dangerous and must be prevented. In order to

prevent this, a large number of locking devices are available.

1. Jam nut or lock nut:

A most common locking device is a jam, lock or check nut. It has about one-half to

two-third thickness of the standard nut.

2. Castle nut: It consists of a hexagonal nut but with an addition of upper cylindrical part.

This cylindrical portion has six slots located at the centre of each face of the hexagonal

part. Due to these slots, castle nut is sometimes called a slotted nut. A split pin is

passed through diametrically opposite slots in the castle nut and a hole in the bold.

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The split pin consists of a wire of semicircular cross-section. It is bent over in such a way that

its flat sides are in contact. The split pin has a looped head on one side. The castle nut is

extensively used for joints that are subjected to vibrations.

3. Split nut: It is similar to ordinary hexagonal nut but with a saw cut. A cap screw is

provided to tighten or loosen the two parts of the nut separated by the saw cut. Initially,

the nut is tightened on the screw and then the slot is opened by means of the cap screw.

Opening of the slot results in deformation of nut and introduces additional friction at

the threads.

4. Locking with setscrew: A positive locking arrangement with the help of setscrew in

shown in fig. The elastic piece is made of soft material like copper or lead. It prevents

damage to the threads when the setscrew is tightened in the nut. This arrangement

results in additional friction between the threads of the bolt and the elastic pice and

prevents loosening.

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Terminology of screw threads :

1. Pitch (P): The pitch is defined as the distance measured parallel to the axis of the screw

from a point on one thread to the corresponding point on the adjacent thread.

Pitch= 1No.of threads per unit lenght of screw

2. Lead (l): It is defined as the distance measured parallel to the axis of the screw which

the nut will advance in one revolution of the screw.

3. Nominal diameter (d): It is the largest diameter of the screw thread. It is also called

major diameter.

4. Core diameter (dc): It is the smallest diameter of the screw thread. It is also called

minor diameter.

5. Pitch diameter: It is the diameter of an imaginary cylinder, on a cylindrical screw

thread, the surface of which would pass through the thread at such points as to make

equal the width of the spaces between the threads. It is also called as effective diameter.

6. Crest: It is the top surface of the thread.

7. Root: It is the bottom surface created by the two adjacent flanks of the thread.

8. Flank: It is the surface joining the crest and root.

9. Angle of thread: It is the angle included by the flanks of the thread.

10. Slope: It is half the pitch of the thread.

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ISO (International Organisation for Standards) metric threads:

It consists of an equilateral triangle with a thread angle of 60 degrees. The tooth profile that the

crests and the roots of the threads are either flattened or rounded to a circular arc. This is

essential in fastening threads. It has the following advantages.

It reduces the stress concentration in threads.

It increases the tool life of the thread cutting tool life of the thread cutting tool.

It reduces the possibility of damaging the threads by denting.

In fluid tight threads, it prevents leakage because there is engagement in the threads

on crests and roots.

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