ENGR 1304: Chapter 8: Threaded Fasteners

"Seventy-five percent of the assembly labor cost of an automobile is spent on fasteners.

Eighty to eighty-five percent of all automobile recalls are fastener related."

- Joseph R. Dudley, V.P. Automotive Marketing, Nylok Fastener Corp. May 1995

http://www.space.com/11745-loose-bolts-space-station-spacewalkers-trouble.html

Fastening:
Two or more components that are joined. Do not require orthographic views, just the info to purchase the part.

Permanent Fasteners:
adhesives, welds, braces, solders, rivets, nails.

Non-Permanent Fasteners:
Screws, bolts, nuts, pins

Notes: http://www.gizmology.net/nutsbolts.htm

https://www.fastenal.com/web/en/78/screw-thread-design

Thread Characteristics:

Direction
Most are right handed (righty tighty, lefty loosy)

Left handed threads - used where right handed threads would become loose in the application (lawn mower blades)

Lead
Distance traveled during one revolution of the fastener.
Lead = Pitch for single threaded screws.

Double and triple threaded bolts - one revoltuon = 2 or 3 times the pitch.

Double and triple threading give rapid motion + large thread surface.

Form
Cross-sectional shape, profile.

N - Rounded V profile (rounded for ease of manufacturing)
ACME - Heavy loads, high accuracy
BUTTRESS - High axial loads
SQ - hard to manufacture

Series
Number of threads per inch for a specific diameter.
coarse - general purpose used for holding
fine - to support added force, used in aircraft and automotive industries
extra fine - for short and small diameter fasteners.

Thread spec's:
3 standard representations:

Class of Fit - How tight fasteners fit together.

Loose fit (1) for harsh corrosive environments.
Regular and Average fits (2) General purpose
Tight (3) For high pressures, stresses, and vibrating systems.

Thread Notes:
Notes on drawing that provide specs about thread.

Use a "leader" to connect note to thread

English Thread:

←0.375-32UNEF-3A-LH
0.375 = diameter
32 = threads per inch
UN = Form
EF = Extra fine (could be C for coarse, or F for fine)
3 = class of fit (1 = loose, 2 = regular, 3 = tight)
A = external thread (B = internal)
LH = left handed

Above: Diameter = .25", 28 threads per inch, United National, Fine series, average fit, external, right handed. (LH would be left handed, nothing written assumes right handed)

Thread Table:
1st column - diameter
(a coarse threaded 1.25" diameter bolt has 7 threads per inch)

Metric:
only listed as coarse or fine.
M - metric
90 - thread diameter
X
6 - pitch (in mm)

Metric threads are usually defined by their pitch, that is, how much distance per thread, inch-based standards usually use the reverse logic, that is, how many threads occur per a given distance. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw (360°). Pitch is the distance from the crest of one thread to the next. Because the vast majority of screw thread forms are single-start thread forms, their lead and pitch are the same.