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 MIL-HDBK-1038
castings) which are rigidly built into the full depth of the machinery deck and
the gantry cap. The two facing ends which overlap and turn on each other, are
separated by a bronze bushing. The bushing and each center steadiment section are
sized for radial loads due to rotate drive gear forces, wind, list/trim, and
acceleration. The bushing fit maintains the concentric alignment between the
roller path rails. The bushing requires grease or oil lubrication. Figure 15
shows a cross section of a typical arrangement.
The relative tilting between the crane upperworks and the portal base cap
or barge tub may impose severe bending moments on the center steadiment sections.
The overall design of the assembly should minimize this condition. An effective
means of avoiding such bending moments is to mount one of the steadiment sections
on a trunnion with its axis parallel with the boom hinges. (Trunnion ends in the
base plates do not require bushings or lubrication.) There is no restriction of
the axial movement at the bushing; during normal operation the separation of the
two steadiment sections is prevented only by the inherent balance of the
upperworks on the roller path (with no contact with the locking nut),and in case
of unanticipated excessive overturning movement, by the king pin and its locking
nut.
The steadiment sections, trunnion, and trunnion base plates are designed
according to the structural criteria; the bushings according to the mechanical
criteria.
4.2.5.1
King Pins. King pins are installed through the open center of the
steadiment sections as an integral part of the assembly. The king pins are heavy-
walled tubes, with a nut or a shoulder at the top and a locking nut at the bottom.
The shoulder and nut clamping faces have thrust washers installed between them and
the ends of the steadiment sections. The hollow king pin is used to route the
electrical conductors between the machinery deck and the portal base or barge. In
normal operation, the clamping faces are not subjected to any axial (separating)
forces from the steadiment sections, and some clearance is maintained between
them. However, the clamping capacity of the king pin is designed to withstand a
theoretical separating (lift-off) force due to 150 percent of rated capacity.
Mechanical design criteria apply only to the bushings and thrust washers.
4.2.6
Roller Paths. Roller path assemblies include the upper and lower rail
circles, bull gear, rollers, and the roller cage. The rail circles may be in the
form of standard rails bent to the desired curvature or a number of cast segments
with a machined running surface for the rollers. The lower rail circle is always
a full 360-degree circle; the upper circle may be full or just two circle
quadrants one under the front (boom end) and the other under the back
(counterweight end) of the machinery deck. The rail circles are separated by a
full complement of rollers, mounted on fixed axles in a circular roller cage,
which carry all vertical compressive loads.
All rollers rotate on grease lubricated bushings. The roller cage serves
to keep the rollers in proper alignment, but is free to follow their movement
around the rail circle. The standard diameter of a portal or floating crane
roller circle is 26 to 30 feet.
The bull gear may be inside or outside the lower rail circle and the gear
teeth may be external or internal. The space between the bull gear and the rail
circle is drained at many locations to avoid accumulation of water. Roller path
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