MIL-HDBK-1038
manufacturer's recommendations must be followed. The final installation,
including tensioning of the mounting bolts, is required to provide an accurate
assembly, permanently secured against all vertical, radial, and tangential design
loads.
4.2.7
Rotate Bearings. Rotate bearings are precision assemblies of rings
(races), rolling elements (balls or rollers), and separators. There are several
standard designs available - one-row or two-row ball, crossed roller, three-row
roller, and a variety of raceway profiles.
The mounting provisions include
through bolts or studs, tapped holes for threaded fasteners, or weld bands. (The
weld bands are welded to the bearing races by the bearing manufacturer, and to the
crane structure by the crane manufacturer.) The three-row roller rotate bearing
design is the most robust, and NCC policy is to specify it for all new cranes.
The three-row roller bearing uses three sets of independent cylindrical rollers,
each sized for the particular load that is to be imposed on it downward, upward
(due to the overturning moment), and radial. In the three-row bearing design,
shown in Figure 16, one of the rings is split into two circular sections so that
it can be assembled over the rollers and the other, one-piece ring. The two
sections are held in place with assembly screws, whose contribution to the load
carrying capacity is insignificant, and may be removed or left in place after the
bearing is mounted. The split ring must be mounted using through bolts or studs;
the one-piece ring may use tapped holes for mounting, but through bolts or studs
are preferred. In crane applications, a ring gear is required for rotating the
upperworks. As depicted in Figure 16, the one-piece inner ring, with gear teeth
machined on its inside diameter, functions as the gear ring. The lower section of
the two-piece ring incorporates a back-up restraining feature against bearing
separation in the form of an extended flange that overlaps the shoulder of the
gear ring below its lower raceway.
4.2.7.1
Rotate Bearing Mounting. Being precision assemblies, the mounting
details of rotate bearings are of critical importance. The rings must be mounted
on accurately machined surfaces supported by substantial circular webs aligned as
closely as practical with the upper and lower rollers. The complete bearing
support structure, both upper and lower, must be rigid and of uniform stiffness.
The lower support structure is normally a circular column rising from the portal
base or barge. The machining tolerances of the mounting surfaces
circumferential waviness and radial tilt must be within the limits prescribed by
the bearing manufacturer. (Shimming or grouting of the mounting surfaces is
prohibited.) The surface finish roughness height ratings should be between 125
and 250 micro-inches.
Rotate bearings have a slender cross section compared to their diameter.
Consequently, the rings invariably are slightly oval, or out-of-round. The
bearing manufacturers customarily mark the location of the long axis on each ring.
In mounting and permanently securing the rings to their support structures, it is
imperative to take steps to minimize this inherent ovality.
Otherwise, the
mismatch between the long axis of one ring with respect to the short axis of the
other ring may cause excessive tightness at some locations during rotation. Each
ring is also marked with the location of the hardness gap (soft spot) of its
raceway surface. The mounting procedure requires that the rings be oriented with
respect to each other such that the long axis of one is aligned with the short
axis of the other ring. Then, the lower ring is positioned with its hardness gap
(soft spot) in the location that is expected to be subjected to the lowest loads,
and the upper ring with the soft spot 90 degrees from the boom-counterweight axis.
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