Tagged: motor

Why do escalator handrails move at different speed?

Did you notice that the handrails on escalators are moving at a different speed than the stairs? Well I did too and it bugged me enough to do the research on why is that happening.

There are many different theories around the interwebz to answer the question. One of the most popular one is saying that the slight speed difference is there on purpose to prevent the tired traveler from falling asleep / not paying attention and hurting themselves (basically to prevent lawsuits). I do refuse to believe that theory. My reasoning is simple: The same speed difference exists in Budapest’s metro system and I’m 100% sure they don’t give a rat’s ass about the safety of the travelers.

If you look at the picture above one thing is obvious. The distance that the handrail have to travel is longer than the distance the stairs need to do. Now imagine two circles (or ellipsis rather) that only have one touch point.

If you put a motor at that single point to spin both circles the speed of each circle will be very different. They will be approximately the same speed as you get closer to the touch point but will differ wildly when they are farther apart.

Now you can obviously modify this behavior by adding gears and essentially changing the speed each circle does on a per section basis, but surprisingly enough escalator manufacturers tend not to do that. Instead they have two motors running each thing.

And here is where the speed difference comes in: Turns out when an escalator is new both the handrail and the stairs are running at almost exactly the same speed. But since the handrail motor is doing more or less work it’s gears will degrade at a different rate and sooner or later the handrail will be slightly out of sync from the stairs.

Every day we learn something new (and useless)