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Elevator and Escalator Consulting Engineers

Buffer Bumph

The buffer is a device that provides a "soft landing" or a "buffer" if the car travels past the terminal landing. The buffers are installed in the elevator pit under the cab and under the counterweight. Elevators with speeds under 1 meter per second (200 feet per minute) use simple spring buffers; elevators traveling at higher speeds use oil buffers.

The use of buffers on elevators quite likely evolved from their use on trains. In the early elevators with simple control systems and manual attendants operating the elevators it was not unusual to slide past the terminal landing occasionally just as with trains it was not unusual to have some bumps when shunting box cars.
 
There are a couple of problems with buffers.
 
Spring buffers do not improve safety much. If the elevator hits a spring buffer at 1 meter per second the spring compresses and then bounces back. A passenger in the car would get a bit of a jolt. The oil buffer is better since it provides a regulated rate of deceleration. But there is a significant difference between the deceleration with a full load in the car and with just one or two people in the cab. Since the buffer has to be designed for the full load condition, the near empty car condition can result in some fairly severe stops. Nor is the deceleration always uniform; there will be peaks that exceed the average. To alleviate this problem the buffer can have a longer stroke but there are penalties to be paid in increased pit depths and overheads.
 
One answer is to provide speed slowdown switches to restrict the speed of the elevator as it approaches the terminal floor. This is a typical "Catch 22" situation: the buffer is there to provide a reasonable emergency stop in the event that the speed control system fails but the electrical control system must function to bring the elevator to a safe speed prior to striking the buffer. If we are going to rely on the speed control system to do this, why not rely on it to bring the elevator to a safe stop without using a buffer?
 
The industry appears to be moving, as in aircraft, to a 'fly-by-wire' approach. More reliance will be placed on the speed control systems with fewer mechanical backups such as buffers. With the improvements in the technology of elevator speed control systems and their increasingly high level of reliability, the buffer may become an anachronism on the elevator of the future.