Material Transport Equipment- Part 3:
Monorails, Hoists and Cranes
Monorails and cranes are generally used to transfer material from one point to another in the same general area; hoists are used to facilitate the positioning, lifting, and transferring of material within a small area. Monorails, cranes and hoists generally provide more flexibility in the movement path than do conveyors; however, they do not have the degree of flexibility provided by variable path equipment, such as industrial trucks.
Typically, the loads handled by monorails, hoists and cranes are much more varied than those handled by conveyors. Also, the movement of materials is generally much more intermittent when using monorails, cranes and hoists than when using conveyors.
Monorail:
A monorail consists of an overhead track on which a carrying device rides. The carrier can be either top running or underhung. The carrying device can be either powered or unpowered. If powered, then the carrying device itself is generally powered electrically or pneumatically. Additionally, intelligent carriers have been developed through the application of microprocessors. The monorail functions like a trolley, except the carrying devices operate independently and the track need not be a closed loop.
Figure 1. Monorail used in assembly line
Figure 2. Monorail used to move the ladle in a steel foundry.
Figure 3. Monorail drive and tractor
Hoist:
A hoist is a lifting device that is frequently attached to a monorail or crane. The hoist may be manually, electrically, or pneumatically powered.
Figure 4. Hoist
Figure 5. Hoist
Cranes:
Jib Crane
Bridge Crane
Gantry Crane
Tower Crane
Stacker Crane
Jib Crane:
A jib crane has the appearance of an arm that extends over a work area. A hoist is attached to the arm to provide lifting capability. The arm may be mounted on a wall or attached to a floor-mounted support. The arm can rotate and the hoist can move along the arm to achieve a wide range of coverage.
Figure 6a. Jib crane
Figure 6b. Jib crane work envelope
Figure 7. Jib crane
Bridge Crane:
A bridge crane resembles a bridge that spans a work area. The bridge is mounted on tracks so that a wide area can be covered. The bridge crane and hoist combination can provide three-dimensional coverage of a department. The bridge can either be top riding or underhung. The top-riding crane can accommodate heavier loads. However, the underhung crane is considered to be more versatile than the top-riding crane because of its ability to transfer loads and interface with monorail systems.
Figure 8. Bridge crane
Figure 9. Bridge crane used in computer manufacturing industry.
Figure 10. Bridge cranes
Gantry Crane:
The gantry crane spans a work area in a manner similar to the bridge crane; however, it is generally floor supported rather than overhead supported on one or both ends of the spanning section. The support can either be fixed in position or travel on runways.
Figure 11. Gantry crane
Figure 12. A Double Leg Gantry at a Midwestern steel facility.
The gantry is used where overhead runways are not practical. The length of the runway easily can be changed. Because they have a much greater bridge weight than an equivalent overhead crane, the gantry's bridge motor must be more powerful or the speed must be slower. The speed also must be reduced if there is a possibility of ground personnel being struck by the crane. Because of these design limitations, gantries are generally used for moving heavy loads, often outdoors.
Figure 13. This Gantry at Boeing plant has a span of 133 ft and a height of 40 ft.
Figure 14. Single leg, double girder gantry crane.
Tower Crane:
A tower crane is most often seen on construction sites, but is also suitable for ongoing material handling operations. The tower carne consists of a single upright which may be fixed or on a track having a cantilever boom. A hoist operates on the boom which may be rotated 360 0 about the upright.
|
|
|
Figure 15. Tower cranes
Stacker Crane:
The stacker crane is similar to a bridge crane. Instead of using a hosit. A mast is supported by the bridge; the mast is equipped with forks or a platform, which are used to lift the loads. The stacking crane is often used to sort or retrieve the unit loads in storage racks. The stacker crane can operate in multiple aisles. They are often used in high-rise applications, with storage racks more than 50 ft high.
|
|
|
Figure 16 (a), (b) Stacker cranes
What type of crane is most appropriate?
Bridge cranes cover rectangular areas, while jib cranes cover circular or semicircular areas.
Bridge cranes can be hung from the ceiling or be floor-supported. Jib cranes can be floor-supported or wall-mounted and may require a special foundation.
An enclosed track work station bridge crane provides consistent ease of operation over the full range of movement.
Jib cranes move more easily at the very end of the boom and are more difficult to move as the load approaches the pivot point.
Should the crane be manual or motorized?
Ease of movement and light weight are key features of enclosed track work station crane systems. In fact, manual work station cranes do the job faster than motorized cranes. If the operator cannot control the load throughout the operation (for instance, over a vat, pit, or other inaccessible area), then the crane should be motorized.
What type of suspension: ceiling mounted or free standing?
With ceiling-mounted systems, support steel doesn't interfere with the handling operation. Ceiling-mounted systems require a building with an adequate overhead structure to support the crane. Free-standing (floor-supported) systems don't put stress on the building's overhead structure. Installation is usually more straightforward, and these cranes are also easier to relocate in the future. These systems require a reinforced concrete floor of at least 6 inches.
What capacity, bridge length, and height?
The general rule is "less is more."
Keep capacities to a minimum. If you "over-buy capacity," the operator will need to move extra bridge dead weight, which isn't a good ergonomic or economic solution.
Keep bridge lengths to a minimum. The less dead weight an operator has to move, the better.
Short bridge lengths are better for higher-cycle production areas.
Longer bridges are acceptable for lower-production cycle or maintenance areas.
Keep bridge heights to a minimum. Keeping the overall height less than 16 feet is desirable because it makes it easier to control and position the load.
|
Last Update: July 22, 1999 |
|
Prepared by : Serdar Z. Elgun |
(a)
(b)