A crane is a lifting or lowering device that is motivated by a source of power remote from the load hook (as distinct from a hoist) and which is carried at the end of a boom or jib. The boom pivots on a mast, which is supported by fixed legs or which is guyed by ropes or cables. The unique capability of cranes to lift objects below their support level, combined with their lift and swing characteristics, makes them indispensable for loading and unloading ships, barges, railway trucks, etc.
An overhead traveling bridge crane (or more correctly, hoist) can service the width and length of the area encompassed by its runways. It carries its load suspended from a trolley carried on two transverse beams. The load may be lifted and lowered, caused to transverse the support beam and/or move along the transverse beams giving accurately controlled three-dimensional movement.
A pillar crane is a fixed-elevation jib crane. These find many applications ranging from retort charging to the handling and positioning of heavy packages and equipment.
5.4.2 Hoists
A hoist differs from a crane in that the load is attached directly to the motivator, which is suspended from a fixed point or a horizontal beam. Fixed-point mounting is provided where only vertical lifting is required for, say, loading and unloading wheeled transport. Beam mounting is used where the hoist is required to service a wider area. The beam may be fixed, pivoted at one end so it can be moved through an angle, or mounted on rails so that it can track in a direction perpendicular to its length.
Hoists may be manually, electrically, or pneumatically powered, the loads being carried, raised, or lowered on chains or wire ropes. Some form of gearing is normally employed to increase the mechanical advantage.
5.4.2.1 Pneumatic Hoists
Pneumatic hoists are sparkproof, safe in dusty conditions, cheap to maintain, and resistant to corrosive, hot, or humid conditions. They are of two types, cylinder and air-motor. Both types operate by compressed air, are economical to install and use, and provide smooth, accurate control of the load. Cylinder hoists may be either single- or double-acting. The latter provides more accurate control. Air-motor hoists require a greater initial investment than cylinder hoists but are as economi-cal to operate and maintain. The motor is totally enclosed and an automatic brake holds the load at any position, even when the air supply is shut off.
5.4.2.2 Electric Hoists
These range in capacity from 1/8 to 30 t and have a far wider application than chain or air hoists when high speeds are required for economic operation. They are designed to meet varying needs in operation and are furnished with one or two motors, single- or multiple-speed control, and push-button, pendant-rope, or remote-operating facilities. Other desirable features include travel limit switches, radio-controlled units, and creep-speed controls for accurate positioning of loads. Two brakes, one mechanical and the other electrical, should be provided on electric hoists.
5.5 Elevators
It is possible to divide elevators into two major classes: (a) package elevators for individual objects and goods in containers and (b) bulk elevators for loose material.
5.5.1 Package Elevators
Package elevators are of three main types, namely, (a) rigid-arm elevators, for short lifts;
(b) swing-tray elevators, for either long or short lifts; and (c) finger-tray elevators, mainly for relatively long and heavy lifts.
5.5.1.1 Rigid-Arm Elevators
This is the simplest type of elevator for raising packaged materials vertically or at large angles. It is a simple, inexpensive, compact, and efficient labor-saving device. It consists basically of two endless chains or belts running over top and bottom sprocketed wheels, to which rigid arms that carry the load are fastened at specific intervals. The rigid arm picks up the load at the loading station and discharges it over the top of the terminal pulley. The running gear is perfectly balanced and the speed slow; the power absorbed in lifting a series of loads (barrels, bags, boxes, etc.) is often so small as to be almost negligible.
5.5.1.2 Swing-Tray Elevators
The rigid-arm elevator has its limitations, as it is unsuitable for serving several floors and for lifting breakable goods. Of more widespread use is the swing-tray elevator, which is well suited for lifting and lowering goods in bottles, cans, baskets, crates, or other containers through any required number of floors.
The machine consists of two endless chains or belts running over chain wheels at the top and bottom. At intervals, swinging trays are suspended between the chains. These act as pivoted carriers that always maintain a horizontal position, as their center of gravity is well below the point of suspension. It is usual to locate the drive motor at the top of the elevator and the tensioning mechanism at the bottom.
It should be observed that no harm is done if loads are left on the swing trays, as they merely go round and round the circuit. Carefully designed and foolproof guarding of the charge and discharge points is obligatory. Although the load-carrying capacity of the carriers may be quite high, the effective capacity of these elevators is controlled by the “lift-on–lift-off” capacity and by the intermittent nature of the charge and discharge arrangements.
The swing-tray elevator caters for many materials-handling applications. However, there are instances where even the swing experienced with these trays may be sufficient to give rise to serious production problems. These can be eliminated with the aid of a stabilized tray.
5.5.1.3 Finger-Tray Elevators
These are similar in construction to swing-tray elevators, but they incorporate automatic charge and discharge gear, which allows safe, substantially continuous operation in multistory buildings.
Effectively, the swinging trays consist of “fingers,” which pass through matching gaps in hinged roller platforms located at the conveyorized charge and discharge points. Packages charged to a loading platform are picked up by the ascending fingers on the up leg and are discharged by the descending fingers on to discharge platforms on the down leg. The location of the charge and discharge points can be altered by raising or lowering these platforms.
As with other mechanical elevators, stringent safety precautions are necessary with these devices.
5.5.2 Bulk Elevators
Bulk elevators, for loose materials, include bucket elevators (described below) and elevators which function on the screw, flight, and pneumatic conveying principles. These are discussed elsewhere in this chapter.
Bucket elevators are high-capacity machines used primarily to elevate relatively free-flowing materials that discharge cleanly. They comprise metal or plastic buckets, which are carried on a pair of endless chains, running in a casing. The material to be conveyed is fed into the bottom (or boot) of the casing where it is dredged up by the buckets and ultimately discharged through a chute at the top of the casing.
Four main types of bucket elevator are recognized. These are characterized by the type of discharge method employed, i.e., gravity, positive, centrifugal, and continuous discharge. Gravity-discharge bucket elevators use high-capacity buckets carried at low speeds (0.25 m/s), which are tipped mechanically. This system is often used in conveyor–elevator applications. Positive-discharge bucket elevators use closely spaced small-volume buckets, which are tipped by means of change-of-direction sprockets located on the downside of the conveyor chain. Centrifugal-discharge bucket elevators are high-capacity machines, which discharge the conveyed material centrifugally into the outlet chute. In this system, the drive speed is critical (typically 1.5 m/s) if the material is to be thrown cleanly into the outlet chute. At these high speeds, wear and tear is considerable and precautions need to be taken with foods that exhibit dust explosion risks. Continuous-discharge bucket elevators run at low speed (0.5 m/s) with closely spaced buckets. A clean discharge results from using the backs of preceding buckets as chutes.