Ladder
|
|
Ladder_and_telegraph_pole.jpg
A ladder is a vertical set of steps. There are two types: rigid ladders that can be leaned against a vertical surface such as a wall, and rope ladders that are hung from the top. The steps of a ladder are called rungs, and the vertical members of a rigid ladder are called stiles. Rigid ladders are usually portable, but some types are permanently fixed to buildings.
Rigid ladders are available in many forms, such as:
- fixed ladder (two stiles joined by several rungs; no moving parts)
- extension ladder (fixed ladder divided into two or more lengths for more convenient storage; the lengths can be slid together for storage or slid apart to maximise the length of the ladder; a pulley system may be fitted so that the ladder can be easily extended by an operator on the ground)
- step ladder (hinged in the middle to form an inverted V, with stays to keep the two halves at a fixed angle)
- platform steps (step ladder with small horizontal platform at the top)
- telescopic ladder (stiles consist of short lengths of concentric tubing that can be slid inside each other for storage)
- roof ladder (rigid ladder with large hook at the top to grip the ridge of a pitched roof)
- hook ladder (rigid ladder with a hook at the top to grip a windowsill; used by firefighters)
- turntable ladder (extension ladder fitted to rotating platform on top of a fire engine)
Rigid ladders were originally made of wood, but in the 20th century tubular aluminium became more common because of its lighter weight. Ladders with fibreglass stiles are used for working on or near overhead electrical wires, because fibreglass is an electrical insulator.
For safety, a rigid ladder should be leaned at an angle of about fifteen degrees to the vertical. In other words, the distance from the foot of the ladder to the wall should be about one quarter of the height of the top of the ladder. At steeper angles, the ladder is at risk of toppling backwards when the climber leans away from it. At shallower angles, the ladder may lose its grip on the ground. Ladder stabilisers are available that increase the ladder's grip on the ground.
A ladder standoff, or stay, is a device fitted to the top of a ladder to hold it away from the wall. This enables the ladder to clear overhanging obstacles such as the eaves of a roof, and increases the safe working height for a given length of ladder.
Rope ladders are used where storage space is extremely limited, or weight must be kept to a minimum. They may have rigid or flexible rungs. Climbing a rope ladder requires more skill than climbing a rigid ladder, because the ladder tends to swing like a pendulum.