Bolts and other fasteners must be tensioned to the stress level specified
by the bearing manufacturer. Usually, this value is 70 percent of the fastener
material yield strength. Furthermore, the tension in the bolts and other
fasteners must be checked periodically to ensure that no relaxation has taken
place due to contact surface seating. The initial check is usually after six
months of normal crane operations following the initial tensioning. Subsequent
checks are less frequent usually once a year for several years and then once
every three to five years. During such tension checks the amount of additional
applied tension, as gauged by nut rotation, should be recorded and the frequency
of ensuing checks adjusted accordingly until no further seating is evident.
Fastener Sizing. The maximum tensile load imposed on any mounting
fastener is calculated by the following method, derived from American Petroleum
Institute Specification 2C:
P = (4M/ND) (H/N)
P = fastener tensile load (in pounds)
M = maximum overturning moment (in pound-feet)
N = number of fasteners
D = bolt circle diameter of the fasteners (in feet)
H = total axial load (in pounds)
Mounting fasteners should be equally spaced over the entire bolt circle diameter.
The fasteners must be sized to provide a higher installation pre-load
(installation tensioning) than the value of "P".
Maximum Wheel Load (Portal Cranes). The determination of maximum wheel
loads of portal cranes involves a statically indeterminate problem. There are
several valid methods to obtaining the solution, depending on which simplifying
assumptions (such as, condition of the rail system and the portal base structure)
are invoked, and the differences between them are not great. The traditional Navy
procedure followed for older and newer portal cranes is commonly referred to as
the "beaming method." The essential element of all these methods is the
calculation of the maximum corner load imposed on the portal base by the most
adverse loading condition. The maximum corner load occurs when either the boom
(with rated load at maximum radius) or the counterweight (with no load on the boom
at minimum radius) is approximately at right angles to the diagonal between the
adjacent corners. The system of equalizers serves to distribute the corner load
equally among all the wheels in each corner of the portal base. The maximum wheel
loads calculated by the beaming method are taken as the loads that may not exceed
the allowable loads at the assumed spacing for which the rail system was designed.
Secondary effects, such as wind or impact are not included in this value.
The inherent rail system design safety margin is understood to be sufficient for
tolerating extraneous secondary loads without any degradation of its strength.
The accepted practice in rail system design is to assume 4-foot spacing
for the wheels. For this assumption, the maximum wheel loads of the crane must be
reduced proportionally for closer wheel spacing, but they may not be increased for