1
$\begingroup$

I'm referring to the kind of lift that takes people up and down floors.

I notice lifts are very stable as people get on or off. Since the lift is presumably stationary because the tension in the cords above it balances the weight of the lift + passengers, this is surprising since the masses of the people involved can be several hundred kilograms. What mechanism prevents the lift from moving?

Only mechanism I can think of is electromagnetic in nature, where magnetic forces resist large changes in velocity. This is the kind of mechanism that slows doors down just before they shut. However, lifts appear not to move at all, which is in contrast to doors (doors move, just move slower).

I'm sure the answer to this is Googleable, but I'm apparently searching for the wrong terms (e.g. "lift stable people embark disembark" finds results about people getting on or off boats).

$\endgroup$
2
  • $\begingroup$ Not all elevators are supported by cables. Some are moved by pistons. Also some elevators do bounce a little when people get off an on. $\endgroup$
    – Forward Ed
    Commented May 8 at 10:42
  • $\begingroup$ There can certainly be a noticeable elasticity in some of them (try bouncing!). The supports are just massively over-sized for safety, compared to what would be necessary to hold the weight, so it is not much. I suppose damping of the spring-mass resonance would also be necessary. $\endgroup$
    – Pete W
    Commented May 8 at 13:40

2 Answers 2

3
$\begingroup$

First of there will be many cables going through pulleys which increases the effective young's modulus of the suspension mechanism. This will reduce the tendency of the cabin to move as weight is added or removed.

In addition they can apply a brake on the cabin. The elevator already needs to physically move something, might as well move a set of brake pads to lock it onto the guide rails.

This brake gets interlocked with the door mechanism to prevent the cabin from moving while the door is open.

There are more benefits to having the cabin stabilized this way when the cabin doesn't need to move. Like being able to add a load cell to the brake mechanism that sounds an alarm and locks out the elevator when the cabin is overloaded.

$\endgroup$
2
$\begingroup$

When an elevator car is stopped, brakes are automatically applied which grasp the vertical rails inside the elevator shaft so the car does not bounce as its load changes. These brakes automatically disengage when the drive motors are turned on, and automatically engage if one of the lift cables snaps.

$\endgroup$
2
  • $\begingroup$ Does this suggest that, if I consider a hovering helicopter instead, the helicopter will indeed bounce as people embark or disembark? $\endgroup$
    – Allure
    Commented May 9 at 2:18
  • $\begingroup$ Yes it does and the pilot has to compensate for it right away. $\endgroup$ Commented May 9 at 2:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.