Safe working load (SWL)
is the load that a lifting device such as a crane, a cherry picker, or a
lifting arrangement can safely lift, suspend or lower. Other synonyms
include working load limit (WLL), which is the maximum working load
designed by the manufacturer. The load represents a mass or force that is
much less than that required to make the lifting equipment fail or yield.
The SWL is calculated using a given safety factor (SF) which for lifting
slings could be given for example 5:1. The failing load is also known as
minimum breaking load (MBL).
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| Factor of safety
(FoS), also known as safety factor (SF), is a term
describing the structural capacity of a system beyond the expected loads or
actual loads. Essentially, how much stronger the system is than it usually
needs to be for an intended load. Safety factors are often calculated using
detailed analysis because comprehensive testing is impractical on many
projects, such as bridges and buildings, but the structure's ability to
carry load must be determined to a reasonable accuracy.
Many systems are purposefully built much stronger than needed
for normal usage to allow for emergency situations, unexpected loads,
misuse, or degradation.
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Ultimate load, strength requirements are
specified in terms of limit loads (the maximum loads to be
expected in service) and ultimate loads (limit loads multiplied
by prescribed factors of safety). With respect to aircraft
structure and design, ultimate load is the amount of load
applied to a component beyond which the component will fail.
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A chain is a series of connected links
which are typically made of metal. A chain may consist of two or
more links.Chains are usually made in
one of two styles, according to their intended use:
- Those designed for lifting, such as when used with a
hoist; for pulling; or for securing, such as with a bicycle
lock, have links that are torus shaped, which makes the
chain flexible in two dimensions (The fixed third dimension
being a chain's length.)
- Those designed for transferring power in machines have
links designed to mesh with the teeth of the sprockets of
the machine, and are flexible in only one dimension. They
are known as Roller chains, though there are also non-roller
chains such as block chain.
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Wire rope is a type
of rope which consists of several strands of metal wire laid (or
'twisted') into a helix. Initially wrought iron wires were used,
but today steel is the main material used for wire ropes.
Historically wire rope evolved from steel chains
which had a record of mechanical failure. While flaws in chain
links or solid steel bars can lead to catastrophic failure,
flaws in the wires making up a steel cable are less critical as
the other wires easily take up the load. Friction between the
individual wires and strands, as a consequence of their twist,
further compensates for any flaws.
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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. however; Mass production of an electric hoist did not
start until the early 1900's 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 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
where once regarded as being designed for light to moderate
usage, but since the 60's 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 advent of the more
durable packaged hoist. 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 durability, performance, and repairability requirements.
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| A hoist is a suspended machinery unit
that is used for lifting or lowering a freely suspended (unguided) load. It
may be manually operated, electrically operated or pneumatically driven and
may use chain or wire rope as its lifting medium. |
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| An electric hoist is a suspended
(overhead) hoist that is powered by electrically driven motors and is used
to lift or lower a freely suspended (unguided) load. It generally uses chain
or wire rope as its lifting medium |
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A manual hoist is a suspended
machinery unit that, by use of manual operation, is used for
the lifting and lowering of a freely suspended (unguided)
load. A manual hoist generally uses chain (roller or link)
as its lifting medium.
Manual hoists have a wide range of lifting
capacities and can be used either for vertical lifting and
lowering or for horizontal pulling.
A manual hoist is operated by hand. An operator will pull
down on one of the chain loops on one side of the chain.
This will turn a pulley mechanism inside the chain hoist
housing. When this pulley turns, it will lift up the end of
the other chain which usually has a hook on the end. By
pulling down on one chain, the manual hoist is actually able
to increase the mechanical work that is being done. This is
caused by the gear ratio inside the manual chain hoist.
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Generally, a chain hoist is any hoist
which utilizes link or roller chain as its lifting medium. Chain hoists can
be manually operated (hand or lever), pneumatically driven or electrically
driven
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A lever hoist is a manual device used
to lift, lower, or pull a load and to apply or release
tension. It utilizes a ratchet and pawl mechanical
configuration to incrementally raise or lower a load or to
apply or release tension.
Lever hoists and pullers have the
advantage of being easy to transport, require minimum
set-up, and are a practical option in confined spaces. They
are suitable for different applications ranging from
heavy-duty construction work to day-to-day maintenance and
repair jobs.
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A ratchet lever hoist or "ratchet
hoist" is a lever operated manual device used to lift, lower, or pull a load
and to apply or release tension. It utilizes a ratchet and pawl mechanical
configuration to incrementally raise or lower a load or to apply or release
tension.
The ratchet lever hoist is a versatile tool used in a wide variety of
applications, both in industry and in the private sector. Its versatility
allows it to be used in confined spaces, and also allows for heavy loads to
be maneuvered and positioned safely and efficiently。 |
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A trolley hoist is a
hoist suspended from a trolley. A hoist can be connected to
a trolley by hook or clevis, or a hoist can be integral with
the trolley.
Trolley Hoists are available
with varying spans and capacities. Trolley Hoists feature
adjustable I-beams and heights for optimal use. Trolley
Hoists are weather and moisture proof. Trolley Hoists need
only one operator and are capable of heavy-duty lifting.
A trolley is a wheeled carriage, cage
or basket that is suspended from and travels on an overhead track. |
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| A monorail is a single run of
overhead track on which carriers (trolleys) travel |
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| A single girder crane is an overhead
travelling crane that utilizes a single bridge beam attached to the two
runway/end trucks. This bridge beam or single girder supports a lifting
mechanism or hoist that "runs" on the bottom flange of the bridge beam. |
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| A double girder crane is an overhead
traveling bridge crane that utilizes two bridge beams set atop the runway
(end) trucks. Generally this type of crane utilizes a top running trolley
hoist, which moves along the top of the two bridge beams on its own set of
trucks/trolley wheels. The hook from the hoist "falls" between the two
bridge beams. Headroom under the crane is increased by utilizing this
hoist/crane configuration |
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An overhead crane is a crane with a
single or multiple girder (bridge girder) bridge carrying a
movable or fixed hoisting mechanism and traveling on an
overhead fixed runway structure.
An overhead crane, also known as a bridge crane, is a type
of crane where the hook-and-line mechanism runs along a
horizontal beam that itself runs along two widely separated
rails. Often it is in a long factory building and runs along
rails along the building's two long walls. It is similar to
a gantry crane. Overhead cranes typically consist of a hoist
to lift the items, the bridge, which spans the area covered
by the crane, and a trolley to move along the bridge.
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A crane (also known as a bridge crane
or overhead crane) is a type of machine used for lifting,
generally equipped with a hoist) (also called a wire rope drum),
wire ropes or chains and sheaves, that can be used both to lift
and lower materials and to move them horizontally. It uses one
or more simple machines like a hoist to create mechanical
advantage and thus move loads beyond the normal capability of a
human. Cranes are commonly employed in the transport industry
for the loading and unloading of freight, in the construction
industry for the movement of materials and in the manufacturing
industry for the assembling of heavy equipment.
The first construction cranes were invented by
the Ancient Greeks and were powered by men or beasts of burden,
such as donkeys. These cranes were used for the construction of
tall buildings. Larger cranes were later developed, employing
the use of human
treadwheels, permitting the lifting of heavier weights. In
the High Middle Ages, harbour cranes were introduced to load and
unload ships and assist with their construction – some were
built into stone towers for extra strength and stability. The
earliest cranes were constructed from wood, but cast iron and
steel took over with the coming of the Industrial Revolution.
For many centuries, power was supplied by the
physical exertion of men or animals, although hoists in
watermills and windmills could be driven by the harnessed
natural power. The first 'mechanical' power was provided by
steam engines, the earliest steam crane being introduced in the
18th or 19th century, with many remaining in use well into the
late 20th century. Modern cranes usually use internal combustion
engines or electric motors and hydraulic systems to provide a
much greater lifting capability than was previously possible,
although manual cranes are still utilised where the provision of
power would be uneconomic.
Cranes exist in an enormous variety of forms –
each tailored to a specific use. Sometimes sizes range from the
smallest jib cranes, used inside workshops, to the tallest tower
cranes, used for constructing high buildings. For a while, mini
- cranes are also used for constructing high buildings, in order
to facilitate constructions by reaching tight spaces. Finally,
we can find larger floating cranes, generally used to build oil
rigs and salvage sunken ships.
This article also covers lifting machines that
do not strictly fit the above definition of a crane, but are
generally known as cranes, such as stacker cranes and loader
cranes.
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| A top running crane is an electric
overhead traveling crane having the end trucks supported on rails attached
to the top of the crane runways |
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| An overhead traveling crane is a
crane with a single or multiple girder (bridge girder) bridge carrying a
movable or fixed hoisting mechanism and traveling on an overhead fixed
runway structure |
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| A runway is an assembly of rails,
beams, girders, brackets and framework on which a crane or trolley travels |
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| An I-beam crane is an overhead
traveling bridge crane that utilizes standard I-beams (S beams) as the
bridge girder and at times, also utilizes I-beams as the runway beams.
Cranes that utilize H-beams (structural beams) as girders and runways are
sometimes referred to as I-beam cranes as opposed to patented track cranes |
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| A box girder crane is an overhead
travelling crane that utilizes a "box" configuration in fabricating the
bridge girder. This box girder design incorporates a four-sided box with a
running surface plate for the hoist trolley attached to the bottom of the
box. The advantage of the box girder is that it possesses greater loading
capabilities and is able to span greater bridge distances. The crane is
generally utilized in pairs with the hoisting mechanism operating on rails
attached to the top of each box girder |
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A jib crane is a stationary or fixed
crane that utilizes a cantilevered bridge (girder) supported
from a stationary vertical support. A jib crane consists of
a hoist attached to a rotating arm that functions as a
manipulator. The arm can rotate 360 degrees and is mounted
on a wall or attached to a floor-mounted support.
Jib cranes can have varying boom styles,
including some that articulate or pivot. Jib cranes may also
mount to walls, ceilings or floors, or may be portable.
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| The bottom block is the device at the
bottom of a hoist's lifting medium (chain/wire rope) through which the
medium is reeved and supports the hook and/or an attachment. It may be
stationary or rotating depending upon the requirements for the piece of
lifting equipment |
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| Material handling
is the movement, storage, control and protection of materials, goods and
products throughout the process of manufacturing, distribution, consumption
and disposal. The focus is on the methods, mechanical equipment, systems and
related controls used to achieve these functions. See also, supply chain,
supply chain management, logistics and third party logistics. Note that all
of referenced terms are highly interrelated and their definitions are
frequently intermingled. |
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