To hold parts together

To adjust parts with reference to each other

To transmit power

PITCH: The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis. The pitch P is equal to 1 divided by the number of threads per inch.

LEAD: The distance a screw thread advances in one turn.

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

Pitch

The pitch of a thread is the distance from one point on the thread form to the corresponding point of the next thread.

Pitch

Notice the pitch can be measured

from crest-to-crest OR

Root-to-Root

The lead (pronounced "leed") is the distance that a threaded part (such as a nut) would move or travel in the horizontal direction if the screw thread was rotated 360 degrees.

In triple threads, the lead is three times the pitch.

In double threads, the lead is equal to twice the pitch.

In single threads, the lead is equal to the pitch.

What term refers to the distance a threaded part would move parallel to the axis in one rotation? Lead

British Standard Whitworth Thread (B.S.W) British Standard Fine Thread (B.S.F) British Standard Pipe Thread (B.S.P) British Association Thread (B.A) Square Thread Buttress Thread Acme Thread Knuckle Thread

Examples of where an ACME thread would be used

Example of where a Buttress Thread would be used

Example of where a Square Thread would be used

British Standard Whitworth Thread (B.S.W) A general standard

British Standard Fine Thread (B.S.F) A standard developed for accurate threads

British Standard Pipe Thread (B.S.P) A standard developed for pipe work

Not used as commonly as the Metric standard

Plug Gauges For checking internal

threads

Ring Gauges For checking external

threads

Threads are always assumed to be right handed threads unless marked with the initials LH (Left Hand) at the end of the thread note.

Right-hand threads

x θ

Left-hand threads

xx θ

T

Right Hand ThreadLeft Hand Thread

T

W

θy

W moves up when T is rotating

clockwise

http://www.youtube.com/watch?v=QeB2YxMqogI

Most screws have single threads. Assume single thread unless otherwise noted. Multiple threads are used where parts must screw together or apart quickly. For example, the compass adjustment bar and toothpaste tube caps have multiple threads. Notice in the picture above that:

› Single Thread - The lead is equal to the pitch. › Double Thread - The lead is twice the pitch › Triple Thread - The lead is three times the pitch.

Thread series refers to standard number of threads per inch set for different diameters.

They include:› UNC: A course-thread series› UNF: A fine-thread series› UNEF: An extra-fine thread series

and constant pitch threads, with a constant number of threads per inch:› 8-thread series› 12-thread series› 16-thread series

Coarse-thread Series (UNC or NC) In this series the pitch for each diameter is relatively large. This series is used for screws, bolts, and nuts produced in quantity, and also for fastening in general.

Fine-thread Series (UNF or NF) In this series, the pitch for each diameter is smaller (there are more threads per inch) than in the coarse- thread series. This series is used where a finer thread is needed, as in making automobiles and airplanes.

Extra-fine-thread Series (UNEF or NEF) In this series, the pitch is even smaller than in the fine-thread series. This series is used where the thread depth must be very small, as on aircraft gear or thin-walled tubes.

Thread Class refers to sets of standard tolerances and allowances

Classes 1A and 1B› Loose fit for ease of assembly

Classes 2A and 2B› Ordinary good grade of commercial products, like

machine screws and fasteners Classes 3A and 3B

› Snug fit, used for high-grade commercial products

Grade numbers reflect the size of tolerance in that grade

Grade 4 tolerances are smaller than grade 6 tolerance

› Grade 6: Medium-quality length of engagement› Grades higher than 6: Course quality and/or long

length of engagement› Grades lower than 6: Fine quality and/or short

length of engagement

Does the term UNF refer to a grade, class or series?

Series

1/2” - 13 UNC - 2Aexternal thread(B means internal)

Class of fit(1 is loosest tolerance, 3 is tightest)

Thread SeriesUNC (Unified Coarse)UNF (Unified Fine)

Pitch (threads/inch)

Nominal Diameter(also shown as decimal or screw #)

The screw is enormously useful in when pressure is needed to seal the lid onto a jar, a top on a bottle, hold a light bulb in its socket or hold two pieces of metal together.

*(Note: Ball screws are used in some machines to greatly reduce friction.)

There are three methods of representing screw threads on drawings:

1. Detailed2. Simplified3. Schematic

exterior

interior

Step 1 1. Establish the pitch and half-pitch at the

major diameter.

2. Measure intervals the size of the pitch along the major diameter from the first two marks.

3. Add crest lines.

Step 2 1. Add the V profile for one thread, top

and bottom, locating the root diameter.

2. Add construction lines for the root diameter.

Step 3 1. Add one side of the remaining Vs

(thread profile). 2. Add the other side of the Vs, completing

the thread profile.

Step 4 Add the root lines to complete the detailed

representation of the threads.

Staggered lines perpendicular to the axis of the thread represent the thread root and crests.

Spacing between root and crest lines is any convenient size.

Root lines are any convenient length.

Machine screw› Used in tapped

hole or with a nut

Cap screw Passes through a

clearance hole in one part and screws into a tapped hole in a second part

Bolt › Passes through a

clearance hole and threads into a nut

Stud A shaft threaded at both

ends used in assemblies

What is the term for a shaft that is threaded on both ends?

Stud

Screw Fasteners 1 、 bolts

2 、 studs

3 、 screws

(a) = slotted nut

(b) = castle nut

Nuts

standard nut

thin nut thick nut

round nut

castellated nut

Ball Screw & Ball Screw housing

What is the purpose of a ball screw thread?

Motion and torque transmission

•Right angle crossing shafts•Self locking•High friction•High speed reduction

•Right angle torque•Smooth tooth interaction•Low noise

Rotary-Linear power transmission

Flexible transmission Chain-sprocket Belt-pulley

Summary There are many types of threads for

different applications These come in various sizes with a set

standard tolerance Metric sizes are more commonly used

today Mechanical bolts, screws and rivets

come in various forms and sizes for different purposes