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 MIL-HDBK-1038
The number of lower roller path segments should be limited to between 12
and 18, and the upper segments should be of different length to avoid simultaneous
alignment of all upper and lower joints during rotation. Cast segments should be
alloy steel hardened to a minimum of 225 BHN and their joint faces should be
angled from the radial direction. Bent rail segments should be of standard
commercial rail sections connected with standard railroad type of joint splice
hardware.
5.2.6.1
Maximum Roller Load. In determining the maximum roller load, it is
assumed that the resultant of all loads from the upperworks is applied to the
centroids of the front (boom) or rear (counterweight) quadrants of rollers. The
loads in each quadrant are divided equally among the rollers in that quadrant.
Both the front and rear quadrant roller loads must be calculated. The load cases
for straight line rated cranes are:
a)
Rated load on the main hoist load hook at maximum radius;
b)
No load on any hoist load hook and the boom at minimum radius.
For variably rated cranes, other load cases must also be analyzed to determine the
highest roller load of either quadrant within the full operating range of the
crane.
The minimum roller diameter is determined using the following
equation:
EQUATION:
MRL = K*W*D
(4)
Where
MRL = maximum roller load (in pounds) that is imposed on the roller
K
= dimensionless sizing factor (1200 for bent rail; 1440 for cast
alloy steel)
W
= effective width of the running surface; which is the width of
the rail head top minus the corner radii, or the flat machined
surface between the corner chamfers (in inches)
D = roller tread diameter (in inches).
5.2.6.2
Bull Gears. The bull gear, whether separate from or integral with the
roller path is comprised of fitted spur gear segments. The gear teeth on older
cranes are often relatively crude either as-cast or cast and semi-finished by
hand. However, because of the slow rotational speeds of 1/3 to 1/2 revolutions
per minute, the operation is satisfactory even if noticeably rough. The teeth may
be external or internal, but they are always exposed to the weather. On newer
cranes, NCC policy is to require the segments to be cast alloy steel with gear
teeth machined to ANSI/AGMA 2000, Gear Classification and Inspection Handbook
Tolerances and Measuring Methods for Unassembled Spur and Helical Gears (Including
Metric Equivalents), Quality Number Q6 accuracy and with provisions for accurate
alignment between the segments and with the center steadiment. The maximum stress
anywhere on the segments is limited to 35 percent of the material yield strength
at maximum design load condition. NCC, in order to simplify roller path and gear
segment alignments and obtain the best possible mounting accuracy, recommends that
the bull gear be integral with the roller path.
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