Major Diameter: The largest diameter of the threadMinor Diameter: The smallest diameter of the threadPitch: Distance between two threads (mm) or no of threads per inch.

• Pitch Diameter

The diameter of an imaginary cylinder passing through the threads to make equal the widths of the threads and the widths of the spaces cut by the cylinder.

• Lead

The distance a screw thread advances axially in one turn.

• Angle of thread

The angle included between the sides of the thread measured in a plane through the axis of the screw

Crest: The top surface joining the two sides of the threads.Root: The bottom surface joining the sides of two adjacent threads. Side: The surface of the thread that connects the crest with the root.

• Axis of screw: The longitudinal center line through the screw.

• Depth of thread: The distance between the crest and the root of the thread measured normal to the axis.

• Form of thread: The cross section of thread cut by a plane containing the axis.

13.3 SCREW THREADS FORM• Sharp V threads called United States Standard

threads. Used on brass pipe works.

• American National Threads replaces the Sharp V for general use.

• Unified thread is the standard agreed upon by the USA, Canada, Britain in 1948 and has replaced American National form.

• Square Thread are to transmission of power.

13.3 SCREW THREADS FORM (Continue)

• ACME Thread replaces the Square threads since it is difficult to manufacture.

• Whitworth thread (British standard) has been replaced by the Unified thread.

• Knuckle thread is used in electrical bulb and other electrical applications.

• Buttress Thread is designed to transmit power in one direction.

13.4 THREAD PITCH

Thread Symbol

• Three method of representing thread on drawing.

• Schematic, use for smaller diameter, about 25mm (1 inch)

• Detailed, for diameter more than 25 mm (1 inch)

• Simplified, commonly used.

13.20 Metric and Unified Threads fit

• Metric threads fit

• Two class of fit, one for general purpose, 6H for internal threads and 6g for external threads, therefore default class of fit is 6H/6g if not specified.

• Second class for closer fit, 6H for internal threads and 5g6g for external threads.

Unified threads fit

• A represents for external threads and B represents for internal threads.

• Number 1, 2 and 3 represents the class of fit.

• 2A/2B is general class and equivalent to 6H/6g

• 3A/3B for close fit.

• 1A/1B for Rapid assembly.

Bolt: Through bolt, head on one end and passes through clearance holes and nut on other side.

Cap Screw: Similar to a bolt, has greater length, use without a nut. Other member has thread and acts as a nut.

Stud: A steel rod threaded on both ends. Screwed in one member and nut on other member.

• MACHINE SCREW:

is similar to the slotted head cap screws but in general is smaller. Can be used with or without nut.

• SET SCREW:

is a screw with or without a head that is screwed through one member and whose special point is forced against another member to prevent relative motion between the two parts.

• Clearance hole: A simple hole without thread used to insert a bolt.

• Tapped hole: A hole with internal thread. Read 13.24 page # 379 for more detail.

• There is clearance between clearance hole and the bolt or screw. For standard, see page # 379.

• .75-10 UNC-2A x 2.50 HEXAGON CAP SCREW• M8 X 1.25-40 HEX CAP SCR• 5/8-11 UNC-2B SQURE NUT• M8 X 1.25 HEX NUT

Key

A key seat in a shaft and a key way in the hub or surrounding part.

• ¼ x 1 ½ SQ KEY

• No. 204 WOODRUFF KEY

• ¼ x 1/16 x 1 ½ FLAT KEY

• No. 10 P & W KEY

Rivets