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K.F. Chan (Mr.) F of G Page F1 of 5
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)
K.F. Chan (Mr.) F of G Page F2 of 5
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.
K.F. Chan (Mr.) F of G Page F3 of 5
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
K.F. Chan (Mr.) F of G Page F4 of 5
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.
K.F. Chan (Mr.) F of G Page F5 of 5
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)
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