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ELEVATOR SAFETY
Elevator buffers are safety devices which are required to be mounted at the base of an elevator shaft. As with any safety device, elevator buffers have to meet with a variety of specifications. One of the most important of these specifications is the manner in which the buffers must bring an impacting elevator car to rest. There are different technical specifications for elevator buffers in different regions worldwide however all employ the same basic performance criteria.
Since the very early days of elevators, a variety of safety systems have been employed to ensure that the elevator will not free fall. The purpose of elevator buffers is to provide protection against the malfunction of an elevator control system resulting in the elevator continuing to travel past the lowest stop to the base of the elevator shaft. The buffers are specified in accordance with the operating velocity and mass of the elevator.
Although freefall is not a realistic event for an elevator, the specification and code requirements are based on the assumption of freefall.
The requirement for elevator buffers fall into two categories depending on the type of buffer.
1. Energy accumulation buffers: These can take the form of simple mechanical springs or polymer buffers which store the absorbed energy of the impact in the form of strain energy. In some accumulation buffers this stored energy can be dissipated on the return movement of the buffer leading to two separate requirements:
a) Buffers with linear and non linear characteristics – these can be used if the elevator does not exceed 197 ft/min.
b) Buffers with buffered return movement – these can be used for elevators that do not exceed 315 ft/min.
2. Energy dissipation buffers: These are usually hydraulic buffers which dissipate the energy of the impact in the form of heat during the travel of the buffer. This type of buffer can be used for all rated speeds, but must be used for speeds of 315ft/min or over.
BUFFER PERFORMANCE CRITERIA – ENERGY DISSIPATION BUFFERS
Performance criteria in all specifications is governed by 2 underlying rules which state that the buffer must arrest a freefalling mass travelling at 115% of the rated speed of the elevator:
(i) With an average deceleration not exceeding 1g.
(ii) Without exceeding a deceleration of 2.5g for a time period greater than 0.04 seconds.
In addition a further, but separate, requirement states that the buffer stroke must be at least as great as free fall distance required to reach 115% of the rated elevator velocity. It is this requirement that dictates the stroke and consequently the installation height of elevator buffers. Due to customer demands, most elevator buffers do not deviate far from the minimum stroke requirement.
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