Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 19 Fastening Devices and Methods
Fastening- Method of connecting or joining two or more parts together, using either devices or processes 3 Methods Mechanical- uses manufactured device to hold parts together Bonding- uses material to hold parts together (permanent) Forming- relies on shape of components to hold them together (least expensive, may be permanent) What are some Examples of Each?
Threaded Fasteners Formed w/ TAP for internal threads and DIE for external threads What are some Examples of threaded objects that are used for purposes other than fastening?
Helix Basic geometric form used for threaded fasteners (curve is formed by a point moving uniformly both an angular and a linear rate around a cylinder) Lead- Distance point moves parallel to axis in 1 revolution
3 Application of Threads Joining- connecting two or more parts temporarily or permanently Adjusting- Locating or modifying position of a part Power Transmission- transmit power force in one direction to another Examples of each? Bolts Wrench Ball Screw
Thread Terminology Chamfer-the angular relief at the last thread; allows easier engagement with the mating part. Crest-the peak or top surface of a screw thread. Depth-the distance between the crest and the root of a thread, measured normal to the axis. Root-the bottom of a screw thread cut into a cylinder. Lead-the distance a screw will travel when turned 360 degrees, or one revolution. Major diameter-the largest diameter of an internal or external thread. Minor diameter-the smallest diameter of an internal or external thread. Pitch-the distance between corresponding points on adjacent thread forms.
Thread Specifications Information Needed ¼-20UNC-2A-LH Major Diameter Threads per inch Thread Form Thread Series Class of fit Internal vs. External If left hand noted
Thread Forms Unified Standard for fasteners Acme, Buttress, & Square transmit power Knuckle- Light-bulbs, bottle-caps, glass jars
Thread Series Standard # of threads per inch (C) Course – quick assembly (F) Fine- lots of force necessary (EF) Extra Fine- Length of engagement is short
Appendix 17 Thread Series Standard # of threads per inch How many threads per inch can a ½” thread have in each of the series?
Class of Fit 1.Loose fit for quick assembly 2.General purpose 3.Close fit, for precision, high stress, and vibration Internal vs. External A=External B=Internal
Thread Notes English Units VS. Metric
Single and Multiple Threads Single- advances distance of one pitch for every 360 degree revolution (Pitch is equal to lead) Multiple- advances # of pitch faster with one revolution (used where quick assembly is required) Right and Left Hand Threads Righty Tighty Rule LH must be noted if left hand thread or counter clock wise tightening takes place
Tap Drill Notes Blind Hole without Bottom Tap
3 Method of Thread Drawings Detailed Schematic Simplified
Pipe Threads Carry liquids and Gases 2 Types Taper and Straight
Design for Assembly (DFA) Design for Manufacturability (DFM) More than 50% of production time is spent in assembly or fastening Only 5% of cost is from fasteners Ways to Design Smarter: Select standard fastener sizes Select same size of fasteners as often as possible Less tooling Less Stations Leave plenty of room for standard tooling (wrenches) Use database of standard fasteners Use Bolt Design Principles John Deere Example
Standard Bolt, Nuts, & Screws Main difference? Bolt- intended to be tightened/ loosened using a nut Screw- intended to be mated with an internal thread in a part Finished vs. Unfinished
Standard Nuts
Set Screws & other Threaded Fasteners
Shoulder Screws Hold machine parts together, have toleranced/ unthreaded shafts used as pivots and linkages Self tapping and wood screws
Locking Devices/ Retaining Rings Prevent threaded fasteners from coming loose Act as fastener due to profile 3 Series of washer N- Narrow R- Regular W- Wide Used to improve assembly surface and increase the strength
Keys Used to attach two components such as a shaft and hub to transmit power to gears, pulleys and other devices Key sits in Keyseat 4 Major types of Keys
Rivets & Springs Attach parts permanently Expand or contract due to pressure or force