The Band Should Slip Off But It Does The Opposite!

Crowned Pulleys do this bizarre counterintuitive thing. Instead of slipping off the curved surface, the band actually moves to the middle and stays there.

Not to be confused with snatch block type pulleys! Destin's video here:

Here's the video I made with Tom Lipton:

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[smartereveryday]

SNATCH BLOCK!.... er... uh, I mean. . NOT A SNATCH BLOCK

[xmtxx]

That explains it..
It don't remember what for, but a few years ago, I designed the concave thingy for a homemade belt system, and I couldn't understand why the belt was, always flying off.
I had to put on guards to prevent it... It was a mess.
Now I know. Thanks! :)

[raydunakin]

I explore ghost towns and mining camps, and have seen a lot of old mines and mills which used belt-driven machinery. I have often wondered why the pulleys were slightly crowned. Now I finally know! Thanks!

[somedood]

2:12
"Our intuition is wrong."
Learning physics summed up in one sentence.

[tyrind]

You've just turned everything around that i knew about conveyors. I work in the metal recycling industry and usually we have to track conveyors to keep them in line. Usually we tighten the side to make the conveyor move opposite of the side we tighten. Then we'd get some conveyors that just didn't seem to wanna do that. They'd move opposite the way they should and everyone was always confused about it and just chocked it up to basically being a odd quirky attribute of that particular conveyor.

[TusharGoyal]

That concave pulley clip really answered all lingering questions in one shot for me! Great work as always Steve!

[PlasmaChannel]

Hey now, this type of pulley system looks familiar ; P It's exactly what stabilizes the band on a Van De Graaff Generator.

[estivalbloom]

The instant a video starts talking about pulleys I'm already waiting for "SNATCH BLOCK"

[mikenolan]

I've worked in manufacturing for years and have observed this effect on lots of conveyors, belt drives, belt sanders, etc. And this is the first real explanation I've heard. Thanks for clearing up something I've been mystified about!

[AmeriBradeOfficial]

Hey Steve! I've followed you for years, so it was super exciting to see our belt grinder in the video Tom sent you. I definitely didn't expect that when I started watching. Obviously, belt tracking is a phenomenon we spend a lot of time thinking about in belt sander design, so it was nice seeing someone explain the crowning effect scientifically. Home builders often run into problems when they add a tracking adjustment because the interaction with the crown isn't necessarily intuitive. The belt seems to ride to the high point of a crown, but it also looks like it rides down the slope when you tilt a wheel to adjust the tracking (as it does in the video Tom sent you). We have our own working theories on how crowning interacts with adjustable belt tracking, but I'm sure I would learn something and see it in a new light if you ever looked into it.
Eric

[inspiringengineer]

Anthropomorphising a rubber band is streching it a bit! ;)

[FrozenFirestorm]

Hey thanks for this! I work on a plant where we have long conveyor belts. We crown the centre of our head pullies to help centre the belts and prevent them from running skew. Been using this method for years but now i understand why it works!!💪

[GW_Live]

There is a pulley on my car that always bothered me that it's crowned, always felt like it was gonna be a problem one day. Guess I was wrong lol

[DavidCaldwell]

When I did my degree (mechanical engineering, a long time ago) we spent half a lecture on this. The lecturer (a tenured professor) actually explained it in terms of velocity and angular acceleration of elements of the belt. I was more confused after than before. This deflection-based explanation is way better, and makes me wonder if the comparative angular acceleration explanation was just totally wrong. Also had no idea elastomer elasticity was from entropy... awesome

[kevinsmith]

Amazing!!! I have been working on a homemade conveyor and tracking was proving a problem. Based off this video, I wrapped layers of tape around one of the pullies (to make it convex) and it immediately started tracking perfectly! (And when I manually disrupt it, it returns to center.) Thank you!

[justdoingodswork]

You can never miss "Destin", when you say snatch block

[dynamicgecko]

"Heat is just molecular jiggle"


It's kind of unsettling how simple and accurate this is.

[nyx]

I love how all the science communicators on youtube reference each other in their videos

[LFTRnow]

Something else interesting (and entropy related) is when you stretch an elastic band, it gets hot. If you let it cool down and then retract it, the same area (middle mostly) that heated up will actually turn cool. Yes, it works like a refrigerant.

[rybec]

That was strange for me. When I first saw it, my intuition told me the band should ride up on the pulley, but at the same time that _felt_ unintuitive. Excellent explanation.


Also worth noting: Take two tires and connect them with an axle, but leave them free spinning. Now, accelerate the system. Next, slow down one tire but not the other. The system will turn in the direction of the breaking tire, because there is more friction on that side.

Now, you have a band across a curved pulley. When you stretch the band, the side that is further from the opposite end is pulled tighter and thus has more friction. This will turn the band in the direction of the tighter side.

In short, there are probably two effects causing this behavior. The stretch widening the band on one side is one effect, but the asymmetrical friction is probably also causing the band to turn toward the higher portion of the pulley as well.