Hoists: Weight lifters in all load classes
From raising multi-ton loads to hoisting 15 pound items repeatedly, here's help on matching the muscle power of hoists to your applications.
By Tom Feare, Senior Editor -- Modern Materials Handling, 12/1/2000
Hoists are the weight lifters of materials handling. Their mechanical muscle power makes light work out of the tasks of lifting or lowering loads. They lift loads from hundreds of tons down to tens of pounds. A custom hoist safely handles 20 tons of molten metal, for example. Or a hoist simply raises, then lowers 15 lb. machinery components repeatedly in a work cell so an operator readily performs assembly with little manual effort.
It's an oversimplification, however, to characterize modern hoists simply as all mechanical brawn and no brains. With today's capabilities of very fine electronic control over hoists, the equipment has added features of smartness and precision. Hoists carry out many assignments where very exact positioning of loads or tools is the key requirement.
It's not all up, then down, with hoists, either. Combined with an overhead crane or mounted on a trolley running on an overhead beam, hoists increase their operational versatility: These hoist/crane or hoist/trolley systems add horizontal movement to the hoist's basic skills of vertical movement and positioning.
Hoists can be general purpose, "packaged" hoists, or they can be "engineered" hoists. The latter usually are intended for overhead crane operations. Among manual hoists, ratchet lever and hand chain models are the two basic categories. Within the general class of electric-powered hoists there are chain and wire rope models. Air hoists, powered by plant air, make up another major category.
What will your hoists do?
Selecting a hoist for a particular task involves determining and weighing a number of factors. Here's an overview of what's involved:
Mounting options: Hoists can be suspended from an overhead fixed point if just a simple lift/lower motion is all that's desired. Or the hoists can be trolley mounted, giving the added dimension of motion along the horizontal axis of the I-beam or monorail upon which the trolley runs. Or, if mounted on an overhead crane, the hoist can be moved throughout most or all of the points in a horizontal plane.
Power sources. Manual hoists have their uses. Hand ratchet or lever action hoists, for example, find uses in rigging operations and in installing machinery. Hand chain hoists, meantime, lift the lighter loads. One pulls upon a so-called "endless" chain; the hoist's internal gearing provides the mechanical advantage to reduce the force necessary to move the load.
Even multi-ton loads can be handled with these manual hoists, so long as the user heeds the duty service class limits.
Electric- or air-powered hoists are increasingly preferred, however, as the duty the equipment performs becomes more frequent and load weights increase.
Pneumatically-powered hoists should be considered when the plant or warehouse facility already is supplied with a pressurized air system. In certain process industry applications and other situations involving explosive or hazardous environments, a pneumatic hoist (and perhaps an air-driven trolley/crane operation as well) becomes a necessity, moreover. Here, use of air-driven hoists helps to avoid risks of injury or plant damage caused by an electrical spark from a motor or by static electricity building up and leading to an explosion or fire.
You need not sacrifice positioning accuracy in load handling with pneumatic hoists. These hoists can perform precise spotting motions by varying the air pressure driving them.
Lifting medium. For both electric- and for air-driven hoists, the lifting medium can be either metal chain or wire rope. (Manual hoists are chain hoists, generally.) Typically, powered chain models will be used for the lighter loads while wire rope models serve in the heavier-duty applications.
Lifting distance and speed. Be sure to get out your tape measure and take some critical measurements for your application. They include the headroom required for a specific hoist model as mounted as well as the lifting distance.
The rate of speed (typically in feet/minute) that a hoist will lift your load is important, particularly when many lift/lower moves will be made over a work period.
Motor speeds, controls. Determining the load lifting speed are the hoist's motor speed(s) and the motor's drive controls. Here the choices often include single-speed and two-speed electric motors/controls as well as variable-speed motors/controls. With a variety of speeds to select from, there's wide latitude in precise load positioning capability and generally a smoother operation. Less flexibility in moving the load goes with single- and two-speed controls; but in some applications simplicity of operation is what's really needed.
Hoists also need to be controlled by an attached pendant or by a remote system. With a pendant hard-wired to the hoist, an operator pushes control buttons to direct hoist movements overhead.
Alternatively, controls can be sent to the hoist remotely through radio or infrared signals to an onboard sensor. Remote control in some applications enables the hoist operator to be less a part of the direct, work "envelope." Thus, he or she is working at a safer distance should some part of the lifting system fail.
Load weights, hoist capacity. Determine your typical load weight as well as the weight of the largest load expected to be lifted. Be sure these calculations include the weights of any associated lifting devices, slings, or other attachments. Match your loads to the tonnage capacities of the hoists you're considering.
Duty classes. But you're not quite done yet. You should also consider hoist duty class rating for the service you expect from this equipment.
Hoists are categorized into the duty classes H1 to H5, depending upon factors such as frequency of service and stops/starts per hour.
Basically, the higher the number after the "H," the more frequently the hoist runs and the heavier the service it performs. Thus, the classes proceed from H1 (infrequent, standby duty use) through H2 (light duty) to H3 (medium) to H4 (heavy) and finally to the H5 (severe) duty class.
An H1 hoist might make only one or two lifts a month. H3 hoists run between 15% and 25% of the work period and have a maximum of 150 to 250 starts/stops per hour. For H2 hoists the average is about 75 starts/stops per hour, in contrast.
Lifting a lid on productivity
Ritz-Craft Corporation is well established in the modular-home manufacturing business. So when the company, based in Mifflinburg, Penn., began building its fourth plant in Jonesville, owners Eric and Paul John had three goals:
Make it a state-of-the-art facility,
Increase productivity, and
Improve safety and ergonomics.
They achieved these goals with the help of an overhead handling system comprised of 63 electric chain hoists and 19 cranes ( Harrington Hoists, 717-665-2000, www.harringtonhoists.com ) in a new 175,000 sq. ft. facility which opened earlier in 2000.
"When we conducted a review of our three other factories as we prepared to build a new plant, we observed a number of places where we could improve our production lines," Eric John explains, "and increase our employees' access to materials."
Most of the 63 hoists and 19 cranes provide either one-two- or three-ton load capacities. They are used for basic activities such as lifting, moving, or placing the floors, exterior walls, interior partition walls, ceilings, and roofs that are assembled into a modular home.
Ritz-Craft dedicates 24 two-ton hoists to a very high-tech scaffolding system. This system features a moveable scaffold, 3 ft. wide by 80 ft. long. It moves to each of 12 stations. One two-ton hoist is placed on each end of the scaffold, allowing the scaffold to be raised and lowered anywhere from 4 ft. to 22 ft. high.
This scaffolding helps increase productivity when putting up siding and working on a roof. Single-fall two-ton hoists meet the duty cycle demands of the Ritz-Craft operation and provide the travel and lifting speeds needed as well as operation under a 15 deg side pull. Multiple hoist lifts, common in the modular-home industry, frequently result in side pulling and can ruin chain and chain guides when the hoist is not properly designed.
In other Ritz-Craft plants, Eric John says, "we spend a lot more hours on the forklift. For instance, when doing the roofing work there, our employees use a forklift to bring in a skid of shingles, then hoist the skid up to the top of the roof. Then they go back and get another skid until the roof has been spread with shingles," he explains.
In Jonesville, however, the crane/hoist system moves and picks skids off the mezzanine where employees have set up enough shingles to do all the houses for the day. A forklift isn't lifting a load of shingles every 1-1/2 hours.
"In this business, productivity is very important and the lift speed of the hoist has a direct effect on productivity," says Eric John. "Obviously, we are really pleased with our hoists because they have given us a big productivity boost.
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