MIL-HDBK-1038
complex process which must be performed or approved by NCC for all new crane
designs. As a practical matter, the amount of float on existing cranes on a
particular crane rail system is a logical point of reference in establishing float
requirements.
5.2.13
Float Pin/Bushing Assemblies. These assemblies are exceptionally
sensitive to design details for proper operation. They are subjected to severe
bending moments with edge loading on the bushings and exposure to the weather. The
lateral (spreading/squeezing) loads and moments that are imposed on the entire
travel truck system and the portal base structure. One means of minimizing such
loads is to locate the float pins at the lowest level in the travel truck
frames. It is also critical that the friction coefficient be maintained at the
design value. The design details of float pin installations described in this
handbook have proven to be successful and must be followed for new crane designs
unless permission for deviation is given by NCC.
5.2.13.1 Float Pins. Float pins have longer unsupported spans than rocker pins
and are expected to have noticeable deflection (bowing) under load. There is no
particular limit on deflection, and in practice the pin diameter is defined by the
float bushing design considerations. They should be machined from solid steel
with a hardness of at least 300 BHN and with a 63 micro-inch (or smoother) surface
finish on the diameter. Particular care must be taken to avoid any machine lead
on the sliding surface.
5.2.13.2 Float Bushings. Float bushings must be sized to limit the bearing
pressure in the bore (base on the net projected area) to a maximum of 1500 pounds
per square inch. Each bushing must have two independent grease groove patterns
one for the loaded surface, and one for the unloaded surface. (The area of the
grease grooves, excluding edge chamfers or radii, in the center quadrant of the
loaded surface is subtracted from the total projected area to obtain the net
projected area). Grooves on the loaded surface must have sufficient area to lift
the float pin and lubricate the adjacent surface with grease under pressure of
8000 pounds per square inch. Grooves on the unloaded surface are intended only to
fill the clearance space with grease to preclude infiltration of water.
The eccentric loading due to float and travel wheel flange-to-rail
clearance and the lateral spreading/squeezing forces, cause moment loads that
either increase or decrease the nominal bearing pressure on the float bushing
bores. The bearing stress distribution is further influenced by edge loading that
develops at the end of the bushings. The combining effects of these stresses and
influences on a typical float pin/bushing assembly is represented graphically in
figure 28. Float bushing material is required to be a high-leaded tin bronze,
copper alloy UNS Number C93700, with a bore surface finish of 63 micro-inches or
smoother. Thrust washers should be of the same material as the bushings.
The expected deflection of the float pin, although not usually
calculated, is taken into account by providing a generous bore-to pin clearance of
2.0 to 3.0 times that of the medium running fits (RC5 or RC6 of ANSI B4.1) for the
same diameter.
5.2.14
Wire Rope Pendants. Wire rope pendants, used as stationary structural
components, are assembled from 6x37 or 6x19 classification wire rope, and end
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