Hoist
From Free net encyclopedia
Image:Builders hoist.jpg Hoist or hoist can mean:-
- A verb meaning "to lift".
- In flag terminology, the half of a flag nearest to the flagpole.
- A 1994 Phish album: see Hoist (album)
- A Transformers toy and fictional character: see Hoist (Transformers)
- A lifting device: see below.
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. "Overhead" lifting operates under special legal restrictions.
For a given Rated Load wire rope is of lighter weight per running foot but is limited to drum diameters far large 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 (60 ft./min +) requires wire rope over a drum because chain over a pocket wheel generates fatigue inducing resonance for long lifts.
Both systems over time fail 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 markedly are reduced in cross section through innerlink wear. Sigificant repeat lubrication of both tensile systems is recommended to limit replacement cycles.
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.
A "hoist" normally is used for lifting (near vertical) and a "winch" is normally used for pulling (near horizontal).