Chapter 11 Fasteners and Springs

Screw Threads Fasteners Transmitting power Helix or conical spiral formed on the external surface of a shaft or the internal surface of a hole Threads are used for: Fasteners Transmitting power Adjusting locations (leveling)

Unified Thread Series Developed to standardize threads so that there would be interchangeability U.S. used American National System before 1949

Screw Thread Terminology Axis-centerline of the cylindrical thread shape Body-portion of the screw left unthreaded Chamfer-angular relief at the last thread to help engagement Lead-the distance the thread travels in one revolution. Right hand thread-thread that engages when rotated clockwise (standard) Left hand thread-thread that engages when rotated counter-clockwise.

Die-tool used for cutting external threads Tap-tool used for cutting internal threads. Tap drill-size of the drill used to make a hole before the tapping operation (each thread size requires a specific tap drill size) Fit-range of thread tightness or looseness. Included Angle-angle between the sides of the threads.

Crest-top of external threads, bottom of internal threads Root-bottom of external threads, top of internal Depth of thread-distance from the crest to the root Major Diameter-size of the largest diameter on the thread Minor Diameter-size of the smallest diameter on the thread Pitch-the distance from a point on a thread form to the corresponding point on the next thread form Pitch diameter-diameter halfway between the major dia. and the minor dia.

Thread form-profile of a thread (shape) Thread series-groups of major diameters and pitches. Threads per inch-number of threads included in one inch of length

Thread Forms Unified Threads Most common Aka. Unified National Several thread series: UNC UNF UNEF

American National Old standard-pre 1949 Flat root compared to Unified National Harder to make

Sharp V Forms a tight seal Difficult to make without damaging the crests and roots Was original U.S. standard Not common

Metric Thread ISO standard for metric sizes Similar shape to Unified National

Whitworth Threads Original British standard Developed in 1841 Only used for replacement parts

Square Threads Used to transmit power (not fasten) Difficult to manufacture

Acme Threads Transmit power Replaced square threads Used for screw jacks and vice threads

Buttress Threads Used to transmit power in one direction

Dardelet Threads Used where self locking threads are required Resistant to loosening under vibration

Rolled Threads A formed thread (not cut) Used for electrical parts (light bulbs, sockets)

American National Standard Taper Pipe Used for pipe fittings Designed to be pressure tight

Thread Representaion Different methods of showing threads on engineering drawings. 3 Methods

Detailed Thread Representation Shows the true shape of the threads Not commonly used because of the time needed to draw and computing time to model

Schematic Thread Representation A simple way to graphically represent threads Not commonly used any more

Simplified Thread Representation Similar to schematic but shows more accurate information Preferred method

How to draw simplified