Page 52 - banneredThermosealCatalog2010

PAGE 2 of 2 Bolts and Screws061604-002
Screw Thread and Material Strength Systems
Basically, screws and bolts are described according to their diameters, thread pattern, and
material strength. There are English and metric systems for both sets of properties.
English unit designations for bolt sizes and screw threads follow the unified standard thread
system in the United States. Unified standard threaded fasteners are available in three series
of diameter-and-pitch combinations: unified coarse (UNC), unified fine (UNF), and unified extra
fine (UNEF). The UNC coarse series finds the most common use with gaskets and is
recommended for general assemblies where vibration is not a problem and where disassembly
may be needed.
A shorthand notation that includes the nominal diameter, the number of threads per inch, and
the thread-pitch series is used to identi fy unified thread bolts and screws on drawings or in parts
lists. For example, 1/4" 20 UNC indicates a bolt or screw with a nominal diameter of 1/4", with
twenty threads per inch and with unified coarse threads. Fastener sizes frequently encountered
in gasketing applications include 1/4"-20 UNC, 5/16"-18 UNC, 3/8"-16 UNC, and 5/8"-11 UNC.
Currently proposed U.S. standards for metric-size fasteners call for only one series of diameter-
pitch combinations. Metric threads are designated by a capital letter M, followed by the major
diameter in millimeters, followed by the symbol "X," and then followed by the pitch distance in
millimeters. For example, M6 x 1 indicates a metric thread with a major diameter of 6 mm and a
pitch of 1 mm. The diameter is approximately 1/4 in., and the pitch is approximately 25
threads/inch similar to, but not the same as, 1/4"-28 UNF. Unified series and metric fasteners
cannot be interchanged.
In addition to size and thread type, fastener specifications also include material tensile strength
properties. As a screw or bolt is tightened, it first stretches elastically, then yields, and
ultimately fails and breaks. Four load or stress levels describe this process. As the fastener is
tightened, increasing tensile (stretch) load is created until a recommended preload is reached.
This is the load that sufficiently stretches the bolt/screw to give a tight joint and overcome any
forces tending to pull the joint apart. If the bolt were tightened further, the proof load would be
reached, the maximum load that still allows a sufficient margin of safety before yield. Further
tightening would stress the bolt/screw until the yield load was reached, the load at which the
fastener begins to permanently stretch and not behave elastically. Finally, further tightening
would take things to the ultimate load which is the greatest load the fastener can withstand. Still
further tightening will cause significant plastic flow and failure. Typically, the preload is 75-85%
of the proof load; the proof load is typically 90-95% of the yield strength and approximately 65%
of the ultimate load.
These bolt/screw strength properties are quantified in terms of tensile stress in SAE grade
designations and metric property classes. The SAE grades (2, 5,7, 8) specify proof load stress
(psi), minimum tensile strength (psi), alloy composition, and treatment requirements for fastener
steel. Within a grade, the requirements may depend on the fastener diameter. SAE grade 8
requirements are the most stringent. Metric requirements include proof stress measured in
megapascals (MPa), minimum tensile strength (MPa), minimum yield strength (MPa), and
maximum and minimum Rockwell hardness.