stub shafts mounted in
bearings. The mating ring gear and pinion must be
accurately aligned and
positively secured to ensure proper gear tooth contact.
required, the outboard
end of the output shaft may be supported by a third
bearing, in which case
precise alignment of the three bearings is critical.
For the most severe duty, the preferred arrangement is to install an
intervening pinion on a shaft, independently supported by two bearings, between
the output shaft and the drum ring gear. The pinion shaft is connected to the
output shaft of the gear reducer by a full-flexible coupling and the pinion is
aligned with the ring gear as in (d) above.
Arrangements (d) and (e) above, are sometimes modified by replacing the
stub shafts with hubs and bushings or bearings which are supported by and rotate
on a stationary through shaft. The pinion and ring gears should be spur type to
avoid axial forces on the bearings and the support structure (pedestals).
Luffing (boom) hoists, being under the constant load of the boom, require
a ratchet-and-pawl mechanism to positively lock the drum against any possible
drift. The ratchet is located either in the center of the drum or at one end
(opposite the ring gear, if present). The pawl is controlled from the operator's
cab by an electric or a pneumatic actuator. The operating system should be
designed so that application of power is not required to maintain the pawl in the
position in which it is placed by the actuator. Over-the-center counterweighing
is a common method of providing that feature. When engaged, the pawl is moved
into the space between the ratchet teeth, but it need not be in contact with the
face of the ratchet tooth.
All hoist assemblies are required to be equipped with two limit switches
to block the hoist motion beyond the intended operating range. One of the limit
switches should be block actuated and the other geared type. Normally geared
limit switches are coupled directly to the exposed end of the drum shaft. On
hoists with drums mounted on stationary shafts, the limit switches are connected
to the drums by means of roller chains and sprockets.
Commercial Base/Deck Mounted Electric Hoists. Electric hoists with wire
rope reeving and suitable for installation on twin girder bridge, traveling wall,
gantry, and semi-gantry cranes are designed and constructed according to the
commercial standards of ASME HST-4M. These hoists are mass produced by
established manufacturers of the crane industry. They are compact designs
intended for base mounting on trolley frames, and have many optional features.
They range in capacities up to 35 tons and should be selected for all general
Commercial Hydraulic Hoists. Hydraulic hoists utilize either a high-
torque/low-speed (HT/LS) hydraulic motor flange mounted directly to the end plate
of a wire rope drum or a low-torque/high-speed hydraulic motor coupled to a
planetary gear reducer inside the drum. Both designs are standard commercial
units and are exceptionally compact. Normally the wire rope drum is the only
component that is custom designed for the specific application. The HT/LS hoists
include one or two band brakes acting on the outer circumference of the motor
housing. The HT/LS design is the more robust of the two and is well suited for
the most severe service and, with completely sealed and oil-submerged working
parts, for environments which cause corrosion problems for relatively exposed
electric and mechanical components associated with built-up electric hoists.