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An escalator has a balustrade with a handrail and the handrail moves on an endless

chain in step with the stairs. There are separate chains for the handrail and the steps

but they are both driven through a gearbox from the same motor (though there can be

synchronization problem from time to time)

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Typical speed of an escalator is 0.5m/s, 0.75m/s, etc. Typical inclination ranges from

27 o to 35 o to the horizontal.

Safety devices of an escalator

1) Comb plate switches –The surface of each tread has grooves parallel to the

direction of motion. The stationary platforms at the top and bottom of the

escalator have fixed combs which mesh with the grooves in the treads in order to

ensure a smooth run in and run out of the treads under the fixed floor. The

stationary platform is actually a floor plate over the recess under the moving

staircase and covers the working mechanism at the top and bottom landings. It has

an extension known as a combplate and the combplate carries the projecting comb

teeth. There is a switch called combplate switch. The combplate switch is actuated

by any object caught between the step and the teeth of the comb plate – this will

stop the escalator.

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An escalator meshing into the combplate at a landing

2) Combplate lights to give confidence to nervous passengers by lighting up the

entry and exit points.

3) Overload relays that trip if the motor should take an excessive current due to an

overload, mechanical problem or any other cause. An escalator must have a brake

which has to fail-safe if there is an interruption to the electrical supply. The brake

is applied by a spring or a hydraulic force and is held off against the mechanical

force by an electrically energized solenoid. After removal of power supply and

application of brake, the escalator shall be brought to a smooth stop.

Escalator gear unit

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3) Interlock contacts which open if the step chain stretches unduly or breaks. There

are further safety devices to disconnect the power supply if one of the driving

chains breaks.

4) A non-reversing device to prevent an ‘up’ travelling escalator from reversing in

the event of failure of the driving gear.

5) Some escalators are fitted with a speed governor should it overspeed in the ‘down’

direction – the governor disconnects the power supply and activates the

mechanical brake.

6) Switches to stop the escalator if any object is carried by the handrail into the

newel openings.

7) It would be dangerous for the escalator to be started or stopped by someone who

could not see the people on it and the switch must thus be in a place from which

the escalator can be seen. Such a place is almost inevitably in reach of the public

therefore the on/off switch must always be key operated.

8) Emergency stops are provided in the machinery spaces under the escalator and

also in position accessible to the public at the top and bottom of the escalator.

Most escalators are reversible. The driving motor is a squirrel cage induction motor

and the drive is reversed by contactors which change the phase sequence of the supply

to the motor.

There are indoor and outdoor versions of escalators. Outdoor versions are fitted with

weatherproof (not necessarily waterproof though) accessories such as sealed bearings

so that the escalator can be run under adverse weather.

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Key parameters:-

Typical inclination – 30 o, 35 o etc.

Typical step width – 600mm, 800mm, 1000mm

Typical pit depth – 1150mm

Typical balustrade height – 900mm

(Adopted from BARNEY, G.C., Elevator & Escalator Micropedia)