Bolts and Fasteners

• Fasteners are manufactured in a wide range of materials:– common steel

– titanium

– plastic

– and other exotic materials.

Bolt IdentificationBolts are identified by 3 measurements:

• Diameter– Measured across the threaded area with a

caliper or micrometer.

• Length– Measured from the bottom of the head to the

end of the shank

• Thread Pitch- (Discussed on the next slide)

Thread Pitch

• Thread pitch (English system)– The number of threads per inch

• Thread pitch (Metric system)– The distance in millimeters between two

adjacent threads

Measuring Thread Pitch –

Using a Thread Pitch Guage.

Bolt Grade MarkingsA bolt’s grade is a measurement of its Tensile Strength

• U.S.C. (inch) bolts– Radial lines on the head of the bolt correspond to

tensile strength.– More lines represent increasing strength.– Count the number of lines and add 2 to determine

the grade of the bolt (ex. 3 lines indicates a Grade 5 bolt).

Bolt Grade MarkingsA bolt’s grade is a measurement of its Tensile Strength

• Metric bolts– Property class numbers on the head of the bolt

correspond to tensile strength and yield strength.– The Property Class is expressed with 2 numbers

(ex. 10.9).The 1st number represents the Tensile strength of the

bolt (ex. 10 = 1,000 MPa).The 2nd number represents the percentage rating of

the bolt’s Yield strength (ex. 9 = 90% of 1,000 MPa).

– Increasing numbers represent increasing strength.

USC Bolt Identification Example

• An Imperial size bolt that is identified as 3/8" x 2" x 24:– Has a shank diameter of 3/8 inch.– Is 2 inches long from the bottom of the head to

the end of the shank.– Has 24 threads per inch.

Metric Bolt Identification Example

• A metric size bolt that is identified as 6mm x 30mm x 1.00mm:– Has a shank diameter of 6 millimeters.– Is 30 millimeters long from the bottom of the

head to the end of the shank.– Has a distance of 1.00 millimeters between its

threads.

Key TermsThread Pitch •In the U.S.C. system, thread pitch is the number of threads per

inch (TPI) on a bolt or nut.

•In the Metric system, thread pitch is the distance between each thread and is measured in millimeters (mm).

Tensile Strength The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.

Yield Strength The maximum load at which a material exhibits a specific permanent deformation.

Torque-To-Yield A bolt that must be tightened to a specific torque and then a specified number of degrees.

Torque-To-Yield Bolts

• A torque-to-yield bolt must be tightened to its Yield Strength. This means tightening the bolt to a specific torque and then a specified number of degrees (Using a Torque Angle Gauge).

• Some Head Bolts are Torque-To-Yield Bolts

Unified Thread Standard

The Unified Thread Standard defines a standard thread form and series used commonly in the United States and Canada.

Example:• UNC – Unified National Course Thread• UNF – Unified National Fine Thread

– A 3/8” UNC bolt has 16 Threads Per Inch

– A 3/8” UNF bolt has 24 Threads Per Inch

Unified Thread Standard

Other UTS designations include:• UNEF – Unified National Extra Fine• UNPT – Unified National Pipe Thread (used

commonly in plumbing and other areas where the threads are used to seal off a liquid)– Oil Drain Plug

– Pinion Nut

– Engine Coolant Temperature Sensor

– Other pipe like things

Thread Sealers and Lockers

Thread Sealer

• Thread sealers prevent leaks around bolt threads.

Thread Locker

• Thread lockers such as Locktite prevent bolts from coming loose due to vibrations.

Anti-seize Compound

• Anti-seize prevents bolts from getting stuck due to rust and corrosion.

Thread Repair

• Threads can get damaged on bolts and nuts from excessive use and from misuse.

• A Tap & Die set is designed to create or repair threads.

Tap & Die

• A Tap cuts internal threads.

• A Die cuts external threads.

Tap & Die Sets

There are 3 common types of Taps used by machinists today:

• Bottoming Tap - The tap illustrated at the left of the image has a continuous cutting edge with almost no taper. A bottoming tap is used on holes that already have threads.

Tap & Die Sets

• Intermediate Tap - The tap illustrated in the middle of the image has tapered cutting edges, which assist in aligning and starting the tap into an untapped hole

Tap & Die Sets

• Taper Tap - The small tap illustrated at the bottom of the image is similar to a plug tap but has a more pronounced taper to the cutting edges. This feature gives the taper tap a very gradual cutting action that is less aggressive than that of the bottoming tap.

Bolt Extraction

• Broken bolts have to be removed before the threads can be repaired.

• Bolt Extractors are tools designed to remove broken bolts.