National Crane Published 8-01-2017 Control # 287-11 9-13
NBT40 SERVICE MANUAL CRANE INSTALLATION
TRUCK FRAME STRENGTH
For a truck frame to be suitable for a Series NBT40 crane,
the truck frame must:
• be rigid enough to allow excessive boom movement due
to truck frame deflection when lifting over the front of the
unit.
• be strong enough to resist the loading induced by the
crane.
• not permanently bend or deform.
The Section Modulus (S.M.), which determines the rigidity of
the frame, is a measurement of the cross-sectional area of
the truck frame. Resistance to bending moment (RBM) is a
measurement of strength and is determined by multiplying
the section modulus of each frame rail by the yield strength
of the rail material.
The NBT40 Series require a minimum of 3,300,000 in-lb
(372,850 N·m) RBM and 27.5 in
3
(451 cm
3
) S.M. from the
rear of the truck frame to the front of the front outrigger
boxes.
The truck frame strength required from the front of the
outrigger boxes to the front stabilizer attachment point is
variable and is listed in the table below.
Most truck frames have reduced section properties through
the front suspension due to truck frame cut-outs or because
outer channel reinforcement stops short of the front
suspension. In these cases it is imperative that the truck
frame is measured and the section modulus is calculated
and compared to the table below to ensure adequate
strength exists for front stabilizer loading.
The distances listed in the chart below are shown in
Figure 9-10.
Engine and PTO rotate
in opposite directions.
PTO rotates CW
Engine and PTO rotate
in the same direction.
PTO rotates CCW
FIGURE 9-9
Distance From SFO
Attachment
Inches (centimeters)
Section
Modulus Per
Rail in.
3
(cm
3
)
0 - 12 (0 - 30) 2.7 (44)
12 - 24 (30 - 61) 5.5 (90)
24 - 36 (61 - 91) 8.2 (134)
36 - 48 (91 - 121) 11.0 (180)
48 - 60 (121 - 152) 13.7 (224)
60 - 72 (152 - 182) 16.5 (270)
72 - 84 (182 - 213) 19.2 (315)
84 - 96 (213 - 243) 21.9 (359)
96 - 108 (243 - 274) 24.7 (405)
108 - 120 (274 - 304) 27.4 (449)
120+ (304 +) 30.0 (492)
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