hoist

Builder's hoist, with small petrol engine

A hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which rope or chain wraps. It may be manually operated, electrically or pneumatically driven and may use chain, fiber or wire rope as its lifting medium. The load is attached to the hoist by means of a lifting hook.

Types of Hoist

The basic hoist has two important characteristics to define it: Lifting medium and power type. The lifting medium is either wire rope, wrapped around a drum, or load-chain, raised by a pulley with a special profile to engage the chain. The power type can be either electric motor or air motor. Both the wire rope hoist and chain hoist have been in common use since the 1800s. A hoist can be built as one integral-package unit, designed for cost-effective purchasing and moderate use, or it can be built as a built-up custom unit, designed for durability and performance. The built-up hoist will be much more expensive, but will also be easier to repair and more durable. Package units are designed for light to moderate usage, while built-up units are designed for heavy to severe service. A machine shop or fabricating shop will use an integral-package hoist, while a Steel Mill or NASA would use a built-up unit to meet durabilty, performance, and repairability requirements.

Construction hoists

Also known as a Man-Lift, or construction elevator, this type of hoist is commonly used on large scale construction projects, such as high-rise buildings or major hospitals. The purpose being to carry personnel and materials quickly between the ground and higher floors, or between upper floors.

The construction hoist is made up of either one or two metallic cages which travel vertically along stacked mast tower sections. For controlled travel along the mast sections, most modern construction hoists utilize a motorized rack-and-pinion system mounted onto the mast sections, while some older hoists still utilize a system of counterweights.

Chain or rope

For a given rated load wire rope lighter in weight per unit length but is limited to drum diameters far larger than the liftwheel over which chain may function. Therefore, a high-performance chain hoist may be of significantly smaller physical size than a wire rope hoist rated at the same working load. High speed lifting, greater than about 60 feet per minute (20 m/min), requires wire rope wound on a drum, because chain over a pocket wheel generates fatigue-inducing resonance for long lifts.

Both systems fail over time through fatigue fractures if operated repeatedly at loads more than a small percentage of their tensile breaking strength. Hoists are often designed with internal clutches to limit operating loads below this threshold. Within such limits wire rope rusts from the inside outward while chain links are markedly reduced in cross section through wear on the inner surfaces. Regular lubrication of both tensile systems is recommended to reduce frequency of replacement.

The unloaded wire rope of small hand powered hoists often exhibits a snarled "set", making the use of a chain hoist in this application less frustrating, but heavier. In addition, if the wire in a wire hoist fails, it can whip and cause injury, while a chain will simply break.

A hand powered hoist with a ratchet wheel is known as a "ratchet lever hoist" or, colloquially, a "Come-A-Long". The original hoist of this type was developed by Abraham Maasdam of Deep Creek, Colorado about 1919, and later commercialized by his son, Felber Maasdam, about 1946. It has been copied by many manufacturers in recent decades.

A "hoist" normally is used for lifting (near vertical) and a winch is normally used for pulling (near horizontal).

Construction sites, especially for buildings of more than five floors in height, often employ hoists to lift workers and building materials.

Chainfall

A chainfall is another example of a hoisting device consisting of a chain suspended from or laid over a fixed structure such as a beam, or on well built saw horses resting on well supported joists used to lift heavy objects, such as steel beams, microlaminated structural beams, vehicle engines etc. Depending on the situation there are many other temporary rigs that can be improvised for lifting heavy objects with the tool. This principle is very similar to a block and tackle.

See also

• Rigging

External links

• Using a two-post hoist (automotive) Free video content from CDX eTextbook
• Hoist Configuration Basics Sketches of Hoists mounted to Cranes

Donate to Wikimedia