Paul Sleath asked:
I recall once watching a program about elevators which inferred that there was almost no restriction in the speed an elevator could ascend (for the human body) but there was a limiting speed for the descent. Or was it vice versa? Not sure but is there any truth in this statement
Graihagh Jackson put this question to Cambridge engineer Dr Philip Garsed...
Philip - A typical express lift can travel at speeds of up to 22 miles per hour, although this year weíll see the first high speed lifts capable of traveling at over 40 miler per hour.
Graihagh - Now, what confuses me is that trains can travel at up to 360 miles per hour, lifts go at a measly 22 miles per hour on average. I wanted to know why elevator engineers havenít quite cracked this nut. So come on Philip - explain yourself...
Philip - The main difference with a lift is it goes up and down. Because the Earthís atmosphere gets thinner as you go higher, a person in a lift experiences a change in air pressure as they travel. On a fast lift this changes rapid enough to cause notable pressure differences in the body.
One of the most sensitive parts of the body to changes in pressure is the ear. This is because the inner ear is quite well sealed and air has to travel along the thin tube, known as the eustachian tube, to leave or enter. As a result, it can take a while for a change in pressure to equalise across the eardrum, and a pressure difference across the eardrum causes it to bulge and thatís uncomfortable at best and can even be a painful experience. It just so happens that this pressure equalisation works better if you're ascending rather than descending and this is because the walls of the eustachian tube are a little bit floppy, a bit like the neck of a balloon. Air comes out easily when itís inflated but itís a lot more difficult to get air in and, if youíve been in a plane, youíll probably have noticed that landing is much more painful on the ears than takeoff even though the aircraft ascends much faster than it descends.
Graihagh - And itís the same with lifts. They can go fast on the way up but have to go slower on the way down.
Philip - The maximum speed of ascent and descent is set by how much the pain the passengers can reasonably bear. Lift manufacturers can get round the problem a bit by pressurising the lift but, even so, the new and fancy 40 miles per hour lifts can only go up the building at that speed, they come down at a much more sedate 22 miles per hour.
So, yes youíre right, a lift does have different limits on its speed depending on whether itís going up or down but itís our biology that prevents us from going faster, not our engineering!
The first thing to remember is that the speed of the elevator doesn't actually matter. What matters is the magnitude of the acceleration the elevator goes through to get to its top speed. For ascent the acceleration phase pushes you against the floor making you feel slightly heavier. This is usually not too uncomfortable for the passengers. For the descent the acceleration phase actually reduces your weight (you push against the floor less) and if the magnitude of the acceleration is too high you'll become weightless or even be pushed against the top of the elevator. Reductions in weight tend to make people feel sick to their stomach and clearly you can't make people feel weightless or hit the ceiling without causing significant discomfort.