Text 3. Hoists

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 hoists

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 can be provided by different means. Common means are hydraulics, electrical and air driven motors. Both the wire rope hoist and chain hoist have been in common use since the 1800s. Mass production of an electric hoist did not start until the early 1900s and was first adapted by Germany. 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 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 were once regarded as being designed for light to moderate usage, but since the 60s this has changed. Built-up units are designed for heavy to severe service, but over the years that market has decreased in size since the invention of the more durable packaged hoist. A machine shop will use an integral-package hoist, while a Steel Mill or NASA would use a built-up unit to meet durability, performance, and repairability requirements.

Wire Rope Hoist or Chain Hoist

More commonly used hoist in today's market is an electrically powered hoist. These are either the chain type or the wire rope type.

Nowadays many hoists are package hoists, built as one unit in a single housing, generally designed for ten-year life.

Common small portable hoists are of two main types, the chain hoist or chain block and the wire rope or cable type. Chain hoists may have a lever to actuate the hoist or have a loop of operating chain that one pulls through the block (known traditionally as a chain fall) which then activates the block to take up the main lifting chain.

For a given rated load wire rope is lighter in weight per unit length but overall length is limited by the drum diameter that the cable must be wound onto. The lift chain of a chain hoist is far larger than the lift-wheel 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.

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. High speed lifting, greater than about 60 feet per minute (18.3 m/min), requires wire rope wound on a drum, because chain over a pocket wheel generates fatigue- inducing resonance for long lifts.

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.

" Chain hoist" also describes a hoist using a differential pulley system, in which a compound pulley with two different radii and teeth engage an endless chain, allowing the exerted force to be multiplied according to the ratio of the radii.

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Определите, верны ли утверждения. Если не верны, дайте верный вариант.

1. A package hoist as usually cheaper than a build-up hoist.

2. The overall length of the operating chain is limited by the drum diameter.

3. Package hoists are regarded as being designed for light usage.

4. Functioning over the tensile breaking threshold may cause the hoist failure.

5. Chain as a lifting medium is always a better choice over wire rope.