Page 6 - Brochure

TECHNICAL
Elevator Safety – continued
The design engineer must consider the stroke requirements in the overall height of the bu er. If telescopic solutions are not to be used then the overall height must be at least double the minimum stroke with a further height requirement to restrict lateral movement when the bu er is fully extended.
Lateral movement should be restricted to +/-5mm per metre of stroke from the centre.
EMERGENCY TERMINAL SPEED LIMITING DEVICE
The function of an emergency terminal speed limiting device is to automatically reduce the speed of a car or counterweight by removing power from the driving machine. The device effectively slows the car or counterweight to the rated speed of the buffer before impact. This device would normally be independent of the normal terminal slowdown devices.
This is important when selecting a bu er for a particular application. If the emergency terminal speed limiting device is part of the installation then the ‘reduced stroke’ rules can apply. This e ectively reduces the size of the bu er required for a particular application.
REDUCED STROKE
The calculation for reduced stroke is based on the stroke of the bu er and not the speed of the elevator. The reduced stroke calculation di ers in some countries but the basic rules are as follows:
The stroke must not be less than:
a) One half (50%) of the stroke for elevators that do not exceed 4.00m/s.
b) One third (33.3%) or 0.45m, whichever is greater, of the stroke for elevators where the speed exceeds 4.00m/s.
Minimum strokes also apply under some code requirements including EN81-20. Under EN81-20 the minimum stroke should not be less than 0.42m for rated elevator speeds above 2.50m/s.. This does not apply under all code requirements.
Using the reduced stroke calculation a bu er rated at 5.09m/s could be used on an installation of 8.8m/s if used with a terminal speed limiting device.
BUFFER PERFORMANCE
The minimum stroke for an elevator bu er is specified (within EN81-20 and ASME A17.1), as the necessary distance to bring an impacting mass, travelling at 115% of the bu er's rated speed, to rest with a uniform deceleration of 1g. However, this is only true if the bu er exerts a constant retardation force over its entire stroke.
A hydraulic buffer can be designed to closely match this idealised performance. This is achieved by precise control of hydraulic oil flow across an orifice throughout the buffer stroke. However, this can only be achieved for one specific impact mass. The same performance is not achievable for the range of elevator masses that are encountered in the real world where the elevator car mass varies with passenger load.
In the elevator application, where there is a need to protect passenger safety, it is important to try to minimise the deceleration experienced during stopping. This can be easily resolved when the elevator is fully loaded but at low loads the same retardation force will slow the elevator more quickly and therefore initially result in higher deceleration for the passenger.
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