THREADS, FASTENERS, AND SPRINGS C H A P T E R T W E LV E

OBJECTIVES 1. Define and label the parts of a screw thread. 2. Identify various screw thread forms. 3. Draw detailed, schematic, and simplified threads. 4. Define typical thread specifications. 5. Identify various fasteners and describe their use. 6. Draw various screw head types. 7. Draw springs.

UNDERSTANDING THREADS AND FASTENERS The three basic applications of Screw threads are as follows: 1. To hold parts together… 2. To provide for adjustment between parts… 3. To transmit power… Thread Used for Attachment. (Courtesy of Arthur S. Aubry/Getty Images, Inc.-Photodisc.)

Screw Thread Terms Screw Thread Nomenclature Study Page 455 of your textbook for all thread and fastener terms Screw Thread Nomenclature

Screw Thread Forms

Thread Pitch The pitch of any thread form is the distance parallel to the axis between corresponding points on adjacent threads. The pitch or the number of threads per inch can be measured with a scale or with a thread pitch gage.

Thread Series The thread series is the detail of the shape and number of threads per inch composing different groups of fasteners. Five series of threads were used in the old ANSI standards: Coarse thread Fine thread 8-pitch thread 12-pitch thread 16-pitch thread

Right-Hand and Left-Hand Threads Aright-hand thread is one that advances into a nut when turned clockwise, and a left-hand thread is one that advances into a nut when turned counterclockwise.

Single and Multiple Threads A single thread, as the name implies, is composed of one ridge, and the lead is therefore equal to the pitch. Multiple threads are used wherever quick motion, but not great power, is desired, as on ballpoint pens, toothpaste caps, valve stems, and so on.

American National Thread Fits and Metric and Unified Thread Fits For general use, three classes of fits between mating threads (as between bolt and nut) have been established by ANSI. These fits are produced by the application of tolerances listed in the standard and are as follows: Class 1 fit Class 2 fit Class 3 fit There are two general classes of metric thread fits: 6H for internal threads 6g for external threads

Three Methods for Drawing Thread Detailed Schematic Simplified

THREAD NOTES ASME/ANSI Y14.6, Screw Thread Representation, is a standard for representing, specifying, and dimensioning screw threads on drawings

EXTERNAL THREAD SYMBOLS With Simplified representations, threaded portions are indicated by hidden lines parallel to the axis at the approximate depth of the thread, whether the cylinder appears rectangular or circular. Schematic threads are indicated by alternating long and short lines.

INTERNAL THREAD SYMBOLS The only differences between the schematic and simplified internal thread symbols occur in the sectional views. In the case of blind tapped holes, the drill depth normally is drawn at least three schematic pitches beyond the thread length

DETAILED REPRESENTATION: METRIC, UNIFIED, AND AMERICAN NATIONAL THREADS The detailed representation for metric, unified, and American national threads is the same, since the flats are disregarded. Notice that for right-hand threads the lines slope upward to the right Notice that for left-hand threads the lines slope upward to the left

DETAILED REPRESENTATION (External and Internal Square Thread): Sometimes in assemblies the root and crest lines may be omitted from the nut only portion of the drawing so that it is easier to identify the inserted screw.

Threads in PHANTOM and Assemblies Use phantom lines to save time when representing identical Features Threaded shafts and springs may be shortened without using conventional breaks but must be correctly dimensioned. When external and internal threads are sectioned in assembly, the V’s are required to show the threaded connection.

AMERICAN NATIONAL STANDARD PIPE THREADS The tapered profile of the pipe thread… American National Standard Taper Pipe Thread. (Reprinted from ASME B1.20.1-1983 (R1992), by permission of The American Society of Mechanical Engineers. All rights reserved.)

BOLTS, STUDS, AND SCREWS The term bolt is generally used to denote a “through bolt” that has a head on one end, is passed through clearance holes in two or more aligned parts, and is threaded on the other end to receive a nut to tighten and hold the parts together.

Screw Heads A machine screw is similar to a slotted head cap screw but usually smaller. A 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 motion between the two parts. Do not section bolts, nuts, screws, and similar parts when drawn in assembly because they do not have interior detail that needs to be shown.

TAPPED HOLES When an ordinary drill is used to make holes that will be tapped, it is referred to as a tap drill. When drawing the drill point, use an angle of 30° to approximate the actual 31° slope of the drill bit. The thread length is the length of full or perfect threads. The tap drill depth does not include the cone point of the drill.

DRAWING STANDARD BOLTS Bolt Proportions (Regular)

Locknuts and Locking Devices Many types of special nuts and devices to prevent nuts from unscrewing are available, and some of the most common are Shown below.

STANDARD CAP SCREWS Cap screws are normally finished and are used on machine tools and other machines when accuracy and appearance are important.

STANDARD MACHINE SCREWS Machine screws are similar to cap screws but are usually smaller (.060" to .750" diameter) and the threads generally go all the way to the head. Clearance holes and counterbores should be made slightly larger than the screws. Typical machine screw notes are…

STANDARD SET SCREWS A set screw is screwed into one part so that its point bears firmly against another part. If the point of the set screw is cupped, or if a flat is milled on the shaft, the screw will hold much more firmly. Set Screws. (Courtesy of Penninsula Components Inc.) Set screws are specified as follows…

AMERICAN NATIONAL STANDARD WOOD SCREWS Wood screws with three types of heads— flat, round, and oval—have been standardized. (Courtesy of Michael Newman/ PhotoEdit Inc.)

Miscellaneous Bolts and Screws

KEYS Keys are used to prevent movement between shafts and wheels, couplings, cranks, and similar machine parts attached to or supported by shafts. Typical specifications for keys are…

MACHINE PINS and RIVETS Machine pins include taper pins, straight pins, dowel pins, clevis pins, and cotter pins. For light work, taper pins can be used to fasten hubs or collars to shafts. Taper Pin Note that the rectangular view of each rivet shows the shank of the rivet with both heads made with circular arcs, and the circular view of each rivet is represented by only the outside circle of the head.

Rivet Symbols Because many engineering structures are too large to be built in the shop, they are built in the largest units possible and then are transported to the desired location. Trusses are common examples. The rivets driven in the shop are called shop rivets, and those driven on the job are called field rivets.

SPRINGS A spring is a mechanical device designed to store energy when deflected and to return the equivalent amount o f energy when released, ANSI Y14.13M. Springs are commonly made of spring steel, which may be musicwire, hard-drawn wire, or oil-tempered wire. Other materials used for compression springs include stainless steel, beryllium copper, and phosphor bronze. Urethane plastic is used in applications where conventional springs would be affected by corrosion, vibration, or acoustic or magnetic forces. Steps in Detailed Representation of Spring Springs. Norton, Robert L., Machine Design: An Integrated Approach, 3rd, © 2006. Printed and electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.)