TM-9-2350-311-20-2
HOWITZER, MEDIUM, SELF-PROPELLED, 155MM; M109A2, M109A3, M109A4 AND M109A5
TECHNICAL MANUAL; UNIT MAINTENANCE MANUAL FOR CAB, ARMAMENT, SIGHTING AND FIRE CONTROL, ELEVATING AND TRAVERSING SYSTEMS AND ASSOCIATED COMPONENTS
DECEMBER 1993
TM-9-2350-311-20-2 - Page 784 of 828
TM 9–2350–311–20–2
F–4
Change 2
F–3
TIGHTENING METAL FASTENERS
When torquing a fastener, select a torque wrench whose range (Table F–2) fits the required torque value. A torque wrench
is most accurate from 25% to 75% of its stated range. A torque wrench with a stated range of 0 to 100 lb-ft will be most
accurate from 25 to 75 lb-ft. Accuracy of readings will decrease as you approach 0 lb-ft or 100 lb-ft. The following ranges are
based on this principle.
Table F–2 TORQUE RANGES
STATED RANGE
MOST EFFECTIVE RANGE
0–18 lb-ft (0–24 N
.
m)
4–13 lb-ft (5–18 N
.
m)
0–600 lb-ft (0–813 N
.
m)
50–450 lb-ft (68–610 N
.
m)
0–170 lb-ft (0–230 N
.
m)
44–131 lb-ft (60–178 N
.
m)
15–75 lb-ft (20–102 N
.
m)
30–60 lb-ft (41–81 N
.
m)
F–4
FASTENER SIZE AND THREAD PATTERN
Threaded fasteners are categorized according to diameter of the fastener shank. Thread styles are divided into broad
groups, the two most common being coarse (Unified Coarse–UNC) and fine (Unified Fine–UNF). These groups are de-
fined by the number of threads per inch on the bolt shanks. In addition, threads are categorized by thread class (Table F–3),
which is a measure of the degree of fit between the threads of the bolt or screw (external threads) and the threads of the
attaching nut or tapped hole (internal threads). The most common thread class for bolts and screws is class 2.
Table F–3 THREAD CLASSES AND DESCRIPTION
EXTERNAL
INTERNAL
FIT
1A
1B
Loose fit
2A
2B
Medium fit
3A
3B
Close fit
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