Page 126 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

Each grade of fastener has specific limits based on the
strength of the material it is manufactured from. In general, indus-
trial fasteners used as hold-down bolts, will normally be either
grade 5 or grade 8. These fasteners are identified by the number
of radial lines present on their heads. Grades 2 and below have no
markings. Grade 5 has three lines and grade 8 has six lines.

GRADE 2 GRADE 5 GRADE 8

Figure 8-2. Determining Bolt Grade

Due to their poor characteristics, grade 2 bolts are seldom
used in industrial applications. It is very important that the fas-
tener is properly identified and that it is properly torqued to the
specifications for that particular bolt.
As an example, a 1-1/4 inch bolt with 7 threads per inch has
a tensile stress area of .9646 square inches. Table 8-1 reflects the
various properties for grade 2, 5 and 8 bolts.

Table 8-1. Properties of Various Grade Fasteners
————————————————————————————————
GRADE CLAMP FORCE TORQUE
(lbs.) (Ft.-lbs.)
————————————————————————————————
2 19,176 – 28,719 300 - 450
5 43,023 – 64,535 672 – 1,008
8 69,768 – 104,652 1,090 – 1,635
————————————————————————————————

Table 8-1 reflects the danger of selecting the wrong grade
fastener for an application. If a grade 2 fastener were used, and
torqued as if it were a grade 5 fastener, it would fail prior to reach-