Topics• Introduction• Terminology• Thread Specifications• Bolts and Screws• Nuts and Locking Devices• Exercise Questions• Note: Refer to Chapter 6 in the text.

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Introduction

• There are two thread standards:– The Unified Screw Thread Standard: Used in the U.S. It

is based on a 60o thread angle. • Unified National Coarse (UNC) series: this is the most common

series. Bolts, screws, and nuts of this series are used for general applications in which a quick assembly is required.

• Unified National Fine (UNF) series: threads of this series are used for handling high tensile stresses, or when the length of engagement is limited such as screws in thin walls.

• Unified National Special (UNS) series

– The International Screw Thread Standard: most common standard, based also on the 60o thread angle.

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Terminology– Thread angle: the angle included between the sides of the

thread, The ISO thread angle is 60 degrees.– Pitch: the distance between one point on a thread to the

corresponding point on the next thread.– Number of threads per inch– Minor (root) diameter– Major (Outside) diameter– Lead: the distance a screw

thread advances in one revolution.

- Thread Fit: How tight is the thread

engagement. Three classes of fit are used in the

unified system, loose, medium, and tight fits.

A for external and B for internal fits. (see p. 110)

Source: Siemens Technik Akademie Berlin

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Thread Specifications• Unified system threads:– Example: ¼ - 20 UNC-BX1 indicates a thread with an outer

diameter is ¼ inch, with 20 threads per inch, the type of thread is UNC, and a B (internal) fit and a thread length of 1 inch.

• Metric system threads:– Example: M10 1.5-6g indicates a Metric thread with an outside

diameter of 10 mm and a pitch of 1.5 mm, the fit of the thread is 6g.

Bolts and Screws: The two words are used interchangeably, but generally, bolt is used when a nut is required to be used.

The shank of a bolt has a diameter that is equal to the major diameter.

Source: Siemens Technik Akademie Berlin

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Carriage Bolt used to fasten metal to wood. It has a square body right under the head to keep the wood from turning.

Machine Screw used for vehicle body and metal sheets

Sheet Metal Screw used for vehicle body and metal sheets

Set Screw usually has no head, used to lock pulleys or gears to shafts

Socket Head Screw has the strongest head style

Common Bolts and Screws MECH 1200

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Stud (Standing) Bolt has threads on both ends, screwed permanently in one end and a nut on the other end.

Hexagon Through BoltHexagon Fit Bolt

If the bolt goes through all parts and is fastened with a nut, then it is called a Through Bolt. When used to fix a part in position against movement it is called a Fit Bolt.

Anti-Fatigue Bolt Has a reduced shank, used for applications in which shock loads occur.

Source of images in this slide: Siemens Technik Akademie Berlin

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Nuts

Cap (Acorn) NutWing nuts

Hex nuts: Have a hexagonal shape, and has different grades

Castle Nut: Named so because it looks like a castle, used in applications where heavy vibrations occur, such as on wheels. Can handle light torques.only.

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Fastener Locking Devices

Spring lock washer

Split pins

Safety wiresExternal tooth lock washer

- Screw locking devices are used to keep the nut from getting loose. - They can be friction based or non-friction based:

Lock washer Internal tooth lock washer

Tab washerNut lock plate

Friction based Locking devices

Non-Friction based Locking devices

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Exercise Questions

1. What are the two thread standards?

The Unified Screw Thread System and the ISO metric thread standard.

2. Thoroughly describe the bolt given by: ¼ - 28 UNF 2A

The bolt has an outer diameter of ¼ inch and a fine thread of 28 threads per inch (Unified National Fine thread series) and has a medium fit with external thread.

3. Thoroughly describe the thread bolt given by: M10 x 1.25

The bolt is metric and its outer diameter is 10 mm and has a pitch of 1.25 mm

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3. If a thread has 40 threads per inch what is its pitch?

Pitch = 1 inch / 40 = 0.025

4. Is each of the screw locking devices below friction based or not?

Yes No No Yes No

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3. What is the component shown in the figure below?

Castle Nut: used in applications that require low amount of torque, and where vibrations are common.

MECH 1200