Recoil of hydraulic lift/elevator

Question

I noticed that when a group of people enter in an hydraulic lift/elevator often when the last person steps in the lift/elevator has a small but noticeable recoil: the lift goes down a few mm then it compensates. This creates a recoil.

It seems that regardless of the number of persons on the lift/elevator this effect triggers when several people enter the cabin in succession and always happens when the last one steps in.

My hypothesis is that a sensor perceives the increased pressure but does nothing while the pressure is building, instead when the pressure is stabilised because no more person enters it gives a bit of extra oil to the piston and we experience the recoil.

Is my idea correct? Or is there a better explanation?

Answer 1

When an elevator stops it will set a brake to immobilize the elevator cabin. This way a sudden leak doesn't drop the cabin while the door is open.

When the controller prepares to move the elevator cabin, it will release the brake which allows the cabin to move again.

In an ideal system, there will be no recoil. However, air bubbles and elasticity of materials will allow the piston to compress a bit under the weight of passengers entering the cabin. This is then detected by a sensor and compensated for.

Answer 2

The sinking of the cabin in a hydraulic lift is a result of the oil compressing in the cylinder (or piston), due to the extra weight (there should be no air in the system). This will be more profound when more oil is in the cylinder, i.e. at high buildings at the top floors. Also keep in mind that the piston travel that you experience is dependent on the suspension. This is "dynamically" compensated by a system that detects if the cabin has moved from the floor level and is activated if the cabin moved beyond a limit.

Usually, the cabin is held in place by a mechanism if the load difference expected is quite significant (ex. loading by a forklift). If the customer pays, you may find it in smaller lifts, but it is extremely rare.