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, Buckhoist, temporary elevator 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 cars (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 hoists have been predominantly produced the United States and Europe, China is emerging as a leading manufacturer of hoists.
In the US, General Contractors rent or lease hoists for a specific project. Rental companies provide erection, dismantling, and repair services to their hoists to provide General Contractors with turnkey services.
Mine hoists
These are, effectively, elevators built in mine shafts. Human, animal and water power were used to power the mine hoists documented in Agricola's De Re Metallica, published in 1556.
Stationary steam engines were commonly used to power mine hoists through the 19th century and into the 20th, as at the Quincy Mine, where a 4-cylinder cross-compound corliss engine was used.[1] In Brittain, the term winding engine is used for steam engines used to power mine hoists. Modern mine hoists are usually electric powered.
A common feature of mine hoists is a system of bell or whistle signals by which those below could signal the hoist operator to raise or lower the hoist. Where no working signalling device was available, rapping on the hoist cable even the walls of the hoist shaft was an acceptable substitute. Here is the bell code from Alaska's 1915 mining law:[2]
- One bell -- hoist.
- One bell -- stop, if in motion.
- Two bells -- lower.
- Three bells -- hoist men, run slow.
- Two slow bells -- lower very slow.
- Three slow bells -- hoist very slow.
- Four bells -- blasting signal. This is a caution signal, and if the engineer is prepared to accept it he must acknowledge by raising the bucket or cage a few feet, then lowering it again. After accepting this signal an engineer must be prepared to hoist the men away from the blast as soon as the signal (one bell) is given, and must accept no other signal in the meantime.
- Six bells -- skip or cage call. To be followed by the station signal, when the skip or cage is desired.
- Nine bells -- danger signal. Followed by the station signal, calls cage to that station. This signal takes precedence over all others, except an accepted blast signal.
These bell codes are typically written into mining law and apply to all mines within a particular jurisdicion. There were significant variations from one mining district to another. For example, Pennsylvania defined different codes used for whistles and bells, and included "rattle" modifiers on some codes.[3] Occasionally, the meaning of codes depended on the time of day, as in the U.S. Indian Territory bell code of 1901.[4]
Chain or wire rope
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 which then activates the block to take up the main lifting chain.
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 ratchet lever hoist (Come-A-Long).
Ratchet lever hoists have the advantage that they can usually be operated in any orientation, for pulling, lifting or binding. Chain block type hoists are usually suitable only for vertical lifting.
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 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.
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 (20 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.
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
References
- ^ Quincy No. 2 Mine Hoist (1920) National Historic Engineering Landmark brochure, The American Society of Mechanical Engineers, 1984.
- ^ The Mining Industry in the Territory of Alaska During the Calendar Year 1915, Bulletin 142, U.S. Department of the Interior, 1917, page 60
- ^ http://www.pacode.com/secure/data/034/chapter43/s43.85 Chapter 43.85, Signal Codes, The Pennsylvania Code, 2008
- ^ Department of the Interior Annual Report for the Fiscal Year Ended June 30, 1901, Government Printing Office, 1901, page 599
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