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Cherry Picker on lorry

An Elevating work platfoem (Cherry picker) being used for an unusual application of tree prunning on O'Connel Street in Dublin. This looks an over large machine for the job but the tall boom allows the busses to pass below and not have to close the lane

An aerial work platform (AWP) or Mobile elevated work platform (MEWP) is a mechanical device used to provide temporary access for people or equipment to inaccessible areas, usually at height. An AWP may also be known as a lift table, lift platform or cherry picker (these terms actually describe specific sub-types).

They are generally used for temporary, flexible access purposes such as maintenance and construction work or by firefighters for emergency access, which distinguishes them from permanent access equipment such as elevators. They are designed to lift limited weights (usually less than a tonne, although some have a higher safe working load (SWL)[1]), distinguishing them from most types of cranes. They are usually capable of being fully operated (including setup) by a single person.

Regardless of the task they are used for, aerial work platforms may provide additional features beyond transport and access, including being equipped with electrical outlets or compressed air connectors for power tools. They may also be equipped with specialist equipment, such as carrying frames for window glass.[1]

Lifting mechanismsEdit

There are several distinct types of aerial work platform, which all have specific features which make them more or less desirable for different applications. The key difference is in the drive mechanism which propels the working platform to the desired location. Most are moved by either hydraulics or possibly pneumatics actuators, with the motive power from a small engine running on Diesel or petrol/LPG or by electric motors for indoor use mainly utilising a battery pack which is charged over night. The different techniques also reflect in the pricing and availability of each type.

Articulated Edit

Main article: Cherry picker
Compact 4-wd MEWP -IMG 4523

A compact 4-wd MEWP of about 10m lift used for setting up the tents at a trade show

Articulated lifts, also known as boom lifts or hydraulic platform are almost exclusively hydraulically powered, and are the closest in appearance to a crane. They consist of a number of jointed sections, which can be controlled to extend the lift in a number of different directions, which can often include 'up and over' applications.

This type of AWP is the most likely of the types to be known as a 'cherry picker', owing to its origins, where it was designed for use in orchards (though not just cherry orchards). It lets the picker standing in the transport basket pick fruit high in a tree with relative ease (with the jointed design ensuring minimum damage to the tree). The term 'cherry picker' has become generic, and is commonly used to describe articulated lifts (and more rarely all AWPs).

This type of AWP is now widely used for maintenance and construction of all types, including extensively in the power and telecommunications industries to service overhead lines, and in arboriculture to provide an independent work platform on difficult or dangerous trees. A specialist type of the articulated lift is the type of fire apparatus used by firefighters worldwide as a vehicle to provide high level or difficult access. These types of platforms often have additional features such as a piped water supply and water cannon to aid firefighters in their task.

Some articulated lifts are limited to only the distance accessible by the length of each boom arm, however, by the use of telescoping sections, the range can be vastly increased. Some large hydraulic platforms mounted on a lorry can reach heights of over 100 metres.[2]

The majority of articulated lifts require a wide supportive base to operate safely, and most models have extending legs/struts to help accomplish this. These legs can be manual or hydraulic (usually depending on size and price of the machine). Some AWPS are classified as 'spiders' due to the appearance of these legs. Spiders are also available in especially compact form, to fit through doorways for use inside buildings, usually mounted on an adjustable width tracked chassis.

Scissor Edit

A scissor lift is a type of platform which can usually only move in the vertical plane. The mechanism to achieve this is the use of linked, folding supports in a criss-cross 'X' pattern. The upward motion is achieved by the application of pressure to the outside of the lowest set of supports, elongating the crossing pattern, and propelling the work platform vertically. The Platform may also have an extending 'bridge' to allow closer access to the work area (because of the inherent limits of vertical only movement).

The contraction of the scissor action can be hydraulic, pneumatic or mechanical (via a leadscrew or rack and pinion system). Depending on the power system employed on the lift, it may require no power to enter 'descent' mode, but rather a simple release of hydraulic or pneumatic pressure. This is the main reason that these methods of powering the lifts are preferred, as it allows a 'fail safe' option of returning the platform to the ground by release of a manual valve.

Mechanical Edit

There are a number of smaller lifts which use mechanical devices such as rack and pinion or screw threads to extend. These often have juxtaposed sections which move past each other in order to create the movement, usually in a vertical direction only. This lifts usually have limited capability in terms of weight and extension, and are most often used for internal maintenance tasks such as changing lightbulbs.

Motive mechanismsEdit

AWPs, by their nature, are designed for temporary works and therefore frequently require transportation between sites, or simply around a single site (often as part of the same job). For this reason, they are almost all designed for easy movement.

UnpoweredEdit

These usually smaller units have no motive drive and require external force to move them. Dependent on size and whether they are wheeled or otherwise supported, this may be possible by hand, or may require a vehicle for towing or transport. Small non-powered AWPs can be light enough to be transported in a pickup truck bed, and can usually be moved through a standard doorway.

Self propelledEdit

These units are able to drive themselves (on wheels or tracks) around a site (they usually require to be transported to a site, for reasons of safety and economy). In some instances, these units will be able to move whilst the job is in progress, although this is not possible on units which require secure outriggers, and therefore most common on the scissor lift types. The power can be almost any form of standard mechanical drive system, including electric or gasoline powered, or in some cases, a hybrid (especially where it may be used both inside and outside).

Vehicle mountedEdit

Facelift aerial platforms for TV News crews - Sheffield Ulley Dam Disaster 07 - DSCF0302

A pair of Aerial platforms hired by TV News crew to get a better view of the Sheffield Floods Ulley Dam disaster from the top of the hill a mile away

Some units are mounted on a vehicle, usually a truck, though other vehicles are possible, such as railway cars. This vehicle provides mobility, and may also help stabilize the unit - though outriggers are still typical, especially as vehicle-mounted AWPs are amongst the largest of their kind. The vehicle may also increase functionality by serving as mobile workshop or store.

These units are used by TV News crews and Sports Outside Broadcast Units to get photos and Video of events over crowds and other obstructions. Another common use is at Horse racing events were the camera can pan round and follow the action from just a couple of locations.

The main application of these units is building maintenance such as window cleaning and external inspections of the façade close up without the time and expense of erecting scaffolding for short term operations or light work.

Control Edit

The power assisted drive (if fitted) and lift functions of an AWP are controlled by an operator, who can be situated either on the work platform itself, or at a control panel at the base of the unit. Some models are fitted with a panel at both locations or with a remote control, giving operator a choice of position. A control panel at the base can also function as a safety feature if for any reason the operator is at height and becomes unable to operate his controls. Even models not fitted with a control panel at the base are usually fitted with an emergency switch of some sort, which allows manual lowering of the lift (usually by the release of hydraulic or pneumatic pressure) in the event of an emergency or power failure.

Controls vary by model, but are frequently either buttons or a joystick. The type and complexity of these will depend on the functions the platform is able to perform. The controls can control features such as:

  • Vertical movement
  • Lateral movement
  • Rotational movement (cardinal direction)
  • Platform / basket movement – Normally, the system automatically levels the platform regardless of boom position, but some allow overrides, tilting up to 90 degrees for work in difficult locations.
  • Ground movement (in self propelled models)

Safety Edit

The majority of manufacturers and operators have strict safety criteria for the operation of AWPs. In some countries, a licence and/or insurance may be required to operate some or all types of AWP. Most protocols advocate the giving of training to every operator, whether mandated or not. Before usage, most operators also prescribe a range of pre-usage checks on the unit itself, and manufacturers recommend regular maintenance schedules.

Most AWPs are fitted with safety or guard rails around the platform itself, in order to help contain operators and passengers. This is supplemented in most models by a restraining point, which designed to secure a harness or fall arrestor. Some work platforms may also have a lip around the floor of the platform itself to avoid tools or supplies being accidentally kicked off the platform. Some protocols require all equipment to be attached to the structure by its own lanyard.

AWPs often come equipped with a variety of tilt sensors. The most commonly activated sensor (especially with two people on a lift), will cause the machine to refuse to raise the platform beyond a certain height. Sensors within the machine detect that weight on the platform is off balance to such a point as to risk a possible tip-over if the platform is raised further. Another sensor will refuse to extend the platform if the machine is on a significant incline. Some models of AWPs may additionally feature counterweights, which extend in order to offset the danger of tipping the machine inherent in extending items like booms or bridges. Some lifts are also fitted with sensors which will prevent operation if the weight on the platform exceeds the safe working load.

As with most potentially dangerous mechanical devices, almost all AWPs are fitted with an emergency stop button or buttons for use in the event of a malfunction or potentially dangerous occurrence. Best practice indicates fitting of emergency stop buttons on the platform and at the base as a minimum. Other safety features include automatic self checking of the AWPs working parts, including a voltmeter, which detects if the lift may not have sufficient power to complete its tasks (which will result in it refusing to operate for safety).

Business modelEdit

AWPs are often bought by equipment rental companies,[3] who then rent them out to construction companies or individuals needing these specialised machines. The market for these machines is known to be marked by especially strong boom and bust cycles, and after a great demand in the 1990s (with up to 70,000 AWPs sold per year in the US) the market crashed in the early 2000s, leading to a strong contraction amongst the manufacturers.[3][4]

The UK market has experienced a similar cycle with demand fuelled by recent Health and Safety rule changes and a move away from brickwork to other cladding systems and a subsequent reduction in the need to scaffold the exteriors of building. Firms instead working from mobile access platforms or mast climber systems. The growth in the creation of atriums has also increased the use of compact machines for indoor maintenance use in place of step ladders or aluminium towers for cleaning, decorating and maintaining lights etc.

ManufacturersEdit

See also Edit

References Edit

  1. 1.0 1.1 Specialized Aerial Work Platforms Stretch Makers' Market Share (from www.constructionequipment.com, Thursday 1 February 2007)
  2. Bronto Skylift (Industrial applications: HLA Range) (from the Bronto Skylift website, Retrieved 28 April 2007)
  3. 3.0 3.1 Sales of aerial work platforms have gone up! up! and away! over the last three years - Diesel Progress, North American Edition, August 1997
  4. Things looking up for aerial work platforms - Diesel Progress, North American Edition, January 2005


External links Edit

Smallwikipedialogo This page uses some content from Wikipedia. The original article was at Aerial work platform. The list of authors can be seen in the page history. As with Tractor & Construction Plant Wiki, the text of Wikipedia is available under the Creative Commons by Attribution License and/or GNU Free Documentation License. Please check page history for when the original article was copied to Wikia

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