As their name suggests, overhead hoists are the
machines used to lift/lower loads. These devices are powered by electricity,
manual power, or air pressure, and primarily consist of a rope or chain
arrangement wound around a lift wheel that acts as the fulcrum.
If you’re planning on using one of these at your
construction site or for renovation/remodelling purposes, it is extremely
helpful to understand how these popular machines work.
When classifying overhead hoists, the
primary factors include the lifting medium, the source of lifting power, and
the means of suspension of the load.
Lifting Mediums
This refers to the material that is used to
lift/lower loads. Depending on the application, the common lifting mediums include
welded link chains, roller chains, and wire ropes.
Welded Link Chains
These are made of a series of interwoven links
that are formed and welded. Chains of this type are made according to
product-specific dimensions and material strength. This means that they are not
interchangeable between hoists made by various manufacturers.
Rolled Chains
Rolled chains are a series of
alternating-arranged roller links and pin links, where the pins articulate
within the bushings while rollers are free to turn on the bushings. Roller
chains made for hoists cannot be exchanged with those made for power
transmissions. Also, only those roller chains whose specifications match the
manufacturer’s recommendations can be used on a particular hoist.
Wire Ropes
These are made using wire strands wrapped around
a suitable core. The wire rope fits into grooves on the hoist drum, and the
latter transmits the lifting/lowering motion.
Lifting Power Sources
Manual Power
This consists of manually grasping and pulling a
continuous hand chain suspended from the hoist. The hand chain is usually a
welded link chain or a roller chain. For higher loads, multiple personnel pull
on multiple chains.
Electric Power
Hoists powered this way are the easiest to
operate; the operator simply raises or lowers the load by pressing the
respective button on an electric switch. Radio controls are used sometimes.
Pneumatic Power
Air power devices are operated using switches
that control, through a series of valves and other air devices, an air motor.
The motor transmits power through hoist gearing.
Suspension Types
Hook mounted hoists have a hook at
the top of the hoist frame or body; this hook suspends the load from the
suspension pin of the trolley, or a suspension device fixed onto the main
structure.
Lug mounted devices have a lug
mounting attached to the top of the hoist frame.
Trolleys enable
horizontal motion of the hoist unit as well as the load being handled.
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