Page 6 - Elevator-series-brochure-USA

ELEVATOR SAFETY
The design engineer must consider the stroke requirements in the overall height of the buffer. 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 buffer is fully extended.
Lateral movement should be restricted to +/-0.2 in per ft of stroke from the center.
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 buffer for a particular application. If the emergency terminal speed limiting device is part of the installation then the ‘reduced stroke’ rules can apply. This effectively reduces the size of the buffer required for a particular application.
REDUCED STROKE
The calculation for reduced stroke is based on the stroke of the buffer and not the speed of the elevator. The reduced stroke calculation differs 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 787 ft/min.
b) One third (33.3%) of the stroke for elevators where the speed exceeds 787 ft/min.
Minimum strokes also apply under some code requirements including EN81.1. Under EN81.1 the minimum stroke should be 16.5 in for 50% calculation and 21.25 in for the 33.3% calculation. This does not apply under all code requirements.
Using the reduced stroke calculation a buffer rated at 1002ft/min could be used on an installation of 1752 ft/min if used with a terminal speed limiting device.
BUFFER PERFORMANCE
The minimum stroke for an elevator buffer is specified (within EN81.1 and ASME A17.1), as the necessary distance to bring an impacting mass, travelling at 115% of the buffer's rated speed, to rest with a uniform deceleration of 1g. However, this is only true if the buffer exerts a constant retardation force over its entire stroke.
A hydraulic buffer can be designed to closely match this idealized 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 minimize 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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