MIL-HDBK-1038
allow slow oscillating rotation, or "dithering motion", of steel shafts or pins.
The copper alloy contains substantial amounts of lead and tin to minimize the
friction coefficient with the steel surfaces; however, lubrication is still
required. The bushing material must always be substantially softer (at least 100
BHN points) than the steel shaft or pin and all grease grooves should be cut in
the bushing. The grease groove pattern should be carefully laid out to ensure
delivery of grease to locations of high contact stress.
Equalizer sheaves, fleeting sheaves, equalizer bars, travel truck
gudgeons and equalizers, and various pivoting and sliding mechanical elements
require bushings. Lubrication grooves should be of adequate size to permit easy
passage of grease or oil, and their edges must be chamfered or rounded to ensure
proper distribution of the lubricant without scraping it off the contacting
surface. Gudgeon bushings have specific design requirements, which are described
in detail in paragraph 5.2.11.
Bushings should have integral flanges or separate thrust washers of the
same material. The outside diameter of the flange or thrust washer should be
sized to provide a low bearing stress with the adjacent structure.
4.3.6.12 Keys. Keys should be of the parallel type, made from
cold-finished low carbon steel commercial keystock or heat-treated alloy steel.
The keys must have the corners chamfered to clear the fillet radii in the keyseats
or keyways. The keys should be installed so that it is impossible for them to
shift out of position. Normally the keys are square in cross section, but flat
keys, sometimes two per hub, may be used where required by the loading conditions.
It is important that the keys be properly fitted in their keyseats or
keyways. ANSI B17.1, Keys and Keyseats, prescribes the fit-up criteria for
various sizes of shaft diameters and key sizes, and classes of fit.
4.3.6.13 Sheaves. Sheaves are commercially available in standard sizes as steel
castings or roll forgings. Cast iron (gray or ductile) sheaves may be permitted
only on commercial packaged hoists in light duty applications. Otherwise, when
the packaged hoist design makes it practical, cast iron sheaves should be replaced
with steel sheaves.
Sheave groove depth, side angle, and bottom radius follow commercial
practice and are often intended for two wire rope sizes. For heavy duty
applications, the sheave grooves should be hardened by a controlled process. Hard
grooves extend the service life of the sheave and the wire rope.
Running sheaves turn on roller bearings which may be permanently
lubricated (sealed) or relubricatable through grease fittings in the hub.
Fleeting and equalizer sheaves turn on bronze bushings, as described above.
On
packaged hoists, equalizer sheaves may turn on sintered bronze bushings
impregnated with oil.
4.3.6.14 Travel Wheels. All wheels that run on crane rails are double flanged.
The tread width is governed by the size of the rail head, with total clearance of
1/4 to 1-1/2 inches, depending on wheel size and crane component. Roll forged and
cast steel wheels are available in tread diameters 8 to 36 inches and of various
tread
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