NATIONAL CRANE Published 11-01-2020 Control # 710-00 2-37
NBT60XL OPERATOR MANUAL SAFETY PRECAUTIONS
would retract approximately 196 mm (7 3/4 in) [see Table 2-
8]. A cylinder extended 1.5 m (5 ft) in which the oil cools
15.5°C (60°F) would only retract approximately 38 mm (1 1/
2 in). The rate at which the oil cools depends on many
factors and will be more noticeable with a larger difference in
oil temperature verses the ambient temperature.
Thermal contraction coupled with improper lubrication or
improper wear pad adjustments may, under certain
conditions, cause a “stick-slip” condition in the boom. This
“stick-slip” condition could result in the load not moving
smoothly. Proper boom lubrication and wear pad adjustment
is important to permit the boom sections to slide freely. Slow
movement of the boom may be undetected by the operator
unless a load is suspended for a long period of time. To
minimize the effects of thermal contraction or “Stick-slip” it is
recommended that the telescope control lever is activated
periodically in the extend position to mitigate the effects of
cooling oil.
If a load and the boom is allowed to remain stationary for a
period of time and the ambient temperature is cooler than the
trapped oil temperature, the trapped oil in the cylinders will
cool. The load will lower as the telescope cylinder(s) retracts
allowing the boom to come in. Also, the boom angle will
decrease as the lift cylinder(s) retracts causing an increase
in radius and a decrease in load height.
This situation will also occur in reverse. If the equipment is
set up in the morning with cool oil and the daytime ambient
temperature heats the oil, the cylinders will extend in similar
proportions.
Table 2-8 and Table 2-9 have been prepared to assist you in
determining the approximate amount of retraction/extension
that may be expected from a hydraulic cylinder as a result of
change in the temperature of the hydraulic oil inside the
cylinder. The chart is for dry rod cylinders. If the cylinder rod
is filled with hydraulic oil, the contraction rate is somewhat
greater.
NOTE: Operators and service personnel must be aware
that load movement, as a result of this phenomena,
can be easily mistaken as leaking cylinder seals or
faulty holding valves.
Table 2-8: Boom Drift Chart (Cylinder length change in inches)
Coeff. = 0.00043
(in
3
/in
3
/ °F)
STROKE Temperature Change (°F)
(FT.)102030405060708090100
5 0.260.520.771.031.291.551.812.062.322.58
10 0.52 1.03 1.55 2.06 2.58 3.10 3.61 4.13 4.64 5.16
15 0.77 1.55 2.32 3.10 3.87 4.64 5.42 6.19 6.97 7.74
20 1.03 2.06 3.10 4.13 5.16 6.19 7.22 8.26 9.29 10.32
25 1.29 2.58 3.87 5.16 6.45 7.74 9.03 10.32 11.61 12.90
30 1.55 3.10 4.64 6.19 7.74 9.29 10.84 12.38 13.93 15.48
35 1.81 3.61 5.42 7.22 9.03 10.84 12.64 14.45 16.25 18.06
40 2.06 4.13 6.19 8.26 10.32 12.38 14.45 16.51 18.58 20.64
45 2.32 4.64 6.97 9.29 11.61 13.93 16.25 18.58 20.90 23.22
50 2.58 5.16 7.74 10.32 12.90 15.48 18.06 20.64 23.22 25.80
55 2.84 5.68 8.51 11.35 14.19 17.03 19.87 22.70 25.54 28.38
60 3.10 6.19 9.29 12.38 15.48 18.58 21.67 24.77 27.86 30.96
Length change in inches = Stroke (Ft.) X Temperature Change ( °F) X Coeff. (in
3
/in
3
/ °F) X 12 in/ft
Fo
r
Reference
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