A spanner in the wercs
June 17, 2019 12:50 PM Subscribe
Mr Olds' Remarkable Elevator - Tom Scott explores a (recently discovered) counterintuitive engineering solution that had been hiding in plain sight
Stupid question: what is the advantage of rotating the casing instead of rotating the screw? I wasn't sure if it had to do with the dust/air displacement issue, but I'm not sure why rotating the screw would be significantly different from rotating the casing in that respect.
The Olds Elevator site shows several advantages but I think they're comparing the elevator to non-screw elevation methods like buckets on a conveyor.
posted by chrominance at 1:30 PM on June 17, 2019 [4 favorites]
The Olds Elevator site shows several advantages but I think they're comparing the elevator to non-screw elevation methods like buckets on a conveyor.
posted by chrominance at 1:30 PM on June 17, 2019 [4 favorites]
"it's a very forgiving machine"
just what we need these days
posted by chavenet at 1:41 PM on June 17, 2019 [5 favorites]
just what we need these days
posted by chavenet at 1:41 PM on June 17, 2019 [5 favorites]
OK wait... so you're telling me that a screw is basically a RAMP that is twisted to save space?
And that the mechanical energy has nothing to do with the rotation, but rather the "scoop" at the end of the screw serves as a little bulldozer to shove more product up the ramp????
posted by rebent at 1:43 PM on June 17, 2019 [8 favorites]
And that the mechanical energy has nothing to do with the rotation, but rather the "scoop" at the end of the screw serves as a little bulldozer to shove more product up the ramp????
posted by rebent at 1:43 PM on June 17, 2019 [8 favorites]
Pretty sure it would still work without the scoop just due to the friction between the bulk product and the cylinder (but it might be slower).
posted by doomsey at 1:45 PM on June 17, 2019 [1 favorite]
posted by doomsey at 1:45 PM on June 17, 2019 [1 favorite]
what is the advantage of rotating the casing instead of rotating the screw?
I guess one advantage might be in isolating whatever-it-is that's driving the elevator from the stuff one is elevating. With a conventional auger, driving the screw part, you need extra bits to form a seal so that the shaft can exit the system and be driven while not letting all the stuff out. Driving the casing instead, from the outside of the system, eliminates all that. Maybe??
I, personally, find it really cool that nobody thought to do this before(!) The Archimedes screw is thousands of years old.
posted by selenized at 1:46 PM on June 17, 2019 [7 favorites]
I guess one advantage might be in isolating whatever-it-is that's driving the elevator from the stuff one is elevating. With a conventional auger, driving the screw part, you need extra bits to form a seal so that the shaft can exit the system and be driven while not letting all the stuff out. Driving the casing instead, from the outside of the system, eliminates all that. Maybe??
I, personally, find it really cool that nobody thought to do this before(!) The Archimedes screw is thousands of years old.
posted by selenized at 1:46 PM on June 17, 2019 [7 favorites]
With a regular auger, you have the rotating part being inside the rest of the machine, so there's parts, bearings, etc., in where the item being lifted is in contact with; also, the center pole of the screw has to be strong and not distort, because torque is being applied to it and if it gets resistance it will bend, causing physical damage. The advantage is that in this example the column can have all of its moving parts, bearings, gears, etc., externally, and the tube can be built as thick or as strong as it needs to be, as long as it has a smooth channel in the middle. It's very ingenious, really.
Also, I wonder if they've thought of this: the rotating part of the column really only needs to be the scoops at the bottom, the spinning column's height doesn't help other than to keep the objects moving along the screw/ramp.
My only thought is that the grain being forced in on the bottom is what's pushing everything else up the column, so the weight of the grain in the lift being pushed does offer some resistance, there must be either a maximum height, or a design for the 'scoops' at the bottom, involved. Because, looking at the screw as a ramp, if you put enough heavy stuff on a ramp it'll eventually want to slide downwards, which is fighting against the scoops.
posted by AzraelBrown at 1:50 PM on June 17, 2019 [5 favorites]
Also, I wonder if they've thought of this: the rotating part of the column really only needs to be the scoops at the bottom, the spinning column's height doesn't help other than to keep the objects moving along the screw/ramp.
My only thought is that the grain being forced in on the bottom is what's pushing everything else up the column, so the weight of the grain in the lift being pushed does offer some resistance, there must be either a maximum height, or a design for the 'scoops' at the bottom, involved. Because, looking at the screw as a ramp, if you put enough heavy stuff on a ramp it'll eventually want to slide downwards, which is fighting against the scoops.
posted by AzraelBrown at 1:50 PM on June 17, 2019 [5 favorites]
Stupid question: what is the advantage of rotating the casing instead of rotating the screw?
I think it might be the feeding system. You could put the Olds Elevator at the bottom of a bin and it will clear the bin out. Plus, I think it would be safer to work around.
posted by Bee'sWing at 1:53 PM on June 17, 2019
I think it might be the feeding system. You could put the Olds Elevator at the bottom of a bin and it will clear the bin out. Plus, I think it would be safer to work around.
posted by Bee'sWing at 1:53 PM on June 17, 2019
It seems like the intake is the really important part of this machine. The cylinder as a whole needn't rotate, and I doubt they get much advantage from doing so along most of its length, unless the cylinder's very rough. I bet an elevator with a fixed shaft but a freely rotating intake on the end of the shaft would work roughly as well. In fact, to some extent, just having the intake and the cylinder without the screw might even work, although eventually the weight of the material within the tube versus the force of the intake trying to shove more in would crush or damage the material.
One obvious disadvantage I see to this design is that it's not quite suitable to completely transfer a quantity of material. Once there's no more material in the intake, what's in the tube stops lifting. If the screw itself is rotated, then material keeps being lifted even when there's no more coming in the bottom (although for liquids and fine--grained solids, they'll fall right back down even as you attempt to raise them, unless you tilt the elevator).
posted by jackbishop at 1:55 PM on June 17, 2019 [2 favorites]
One obvious disadvantage I see to this design is that it's not quite suitable to completely transfer a quantity of material. Once there's no more material in the intake, what's in the tube stops lifting. If the screw itself is rotated, then material keeps being lifted even when there's no more coming in the bottom (although for liquids and fine--grained solids, they'll fall right back down even as you attempt to raise them, unless you tilt the elevator).
posted by jackbishop at 1:55 PM on June 17, 2019 [2 favorites]
@Azrael I suspect that you need to rotate the whole column to reduce friction between the wall and the solids inventory.
Also further to other comments: agreed that it's a huge advantage to not need to drive something captured inside the motive area; stuffing boxes etc are all a big maintenance headache and an additional source of friction besides. Also rotating a cylinder is the easiest thing going in terms of motion transfer.
Having spec'd a few "conventional" screw-in-sleeve conveyors it seems to me that the other big advantage is that there's minimal risk of startup torque snapping the thin section of the screw.
I wonder how it works with material that's wet or sticky? This is when "open helix" or "chain drag" style conveyors typically come into play, at least in my experience.
posted by hearthpig at 1:56 PM on June 17, 2019 [5 favorites]
Also further to other comments: agreed that it's a huge advantage to not need to drive something captured inside the motive area; stuffing boxes etc are all a big maintenance headache and an additional source of friction besides. Also rotating a cylinder is the easiest thing going in terms of motion transfer.
Having spec'd a few "conventional" screw-in-sleeve conveyors it seems to me that the other big advantage is that there's minimal risk of startup torque snapping the thin section of the screw.
I wonder how it works with material that's wet or sticky? This is when "open helix" or "chain drag" style conveyors typically come into play, at least in my experience.
posted by hearthpig at 1:56 PM on June 17, 2019 [5 favorites]
Wait, never mind, my addendum is wrong. A vertical screw wouldn't keep lifting material even when there's nothing in the intake (a tilted screw would, though).
posted by jackbishop at 1:59 PM on June 17, 2019 [2 favorites]
posted by jackbishop at 1:59 PM on June 17, 2019 [2 favorites]
I think the maximum height would probably be dictated by the torque/power output of the motor (and the casings ability to withstand it).
I don't understand the stated advantage of not mixing in air with the stuff you are moving. I wouldn't think the screw turning would necessarily introduce air at all?
The main reason to have the whole casing (or most of it) turning instead of just the bottom would be so that you can have a deep bin of your material and not have have a motor or gears or belts exposed to that material. (or what selenized said)
posted by ArgentCorvid at 2:08 PM on June 17, 2019
I don't understand the stated advantage of not mixing in air with the stuff you are moving. I wouldn't think the screw turning would necessarily introduce air at all?
The main reason to have the whole casing (or most of it) turning instead of just the bottom would be so that you can have a deep bin of your material and not have have a motor or gears or belts exposed to that material. (or what selenized said)
posted by ArgentCorvid at 2:08 PM on June 17, 2019
The main reason to have the whole casing (or most of it) turning instead of just the bottom would be so that you can have a deep bin of your material and not have have a motor or gears or belts exposed to that material. (or what selenized said)
I would think the main advantage would be that the moving wall would impart some force to the material via friction along the entire column and provide a traction force—otherwise, the only driving force is the jamming of new material in at the bottom of the column and that will eventually come into balance with the friction on the screw surface when the column gets sufficiently tall. By having a rotating outer cylinder, you also get a driving force that scales with the column height.
posted by BlueDuke at 2:13 PM on June 17, 2019 [10 favorites]
I would think the main advantage would be that the moving wall would impart some force to the material via friction along the entire column and provide a traction force—otherwise, the only driving force is the jamming of new material in at the bottom of the column and that will eventually come into balance with the friction on the screw surface when the column gets sufficiently tall. By having a rotating outer cylinder, you also get a driving force that scales with the column height.
posted by BlueDuke at 2:13 PM on June 17, 2019 [10 favorites]
I don't understand the stated advantage of not mixing in air with the stuff you are moving. I wouldn't think the screw turning would necessarily introduce air at all?
I think the advantage is versus non-screw techniques like conveyors. The advantage of not introducing air is that grain dust can make a nifty fuel-air explosive, for one.
posted by Homeboy Trouble at 2:26 PM on June 17, 2019 [1 favorite]
I think the advantage is versus non-screw techniques like conveyors. The advantage of not introducing air is that grain dust can make a nifty fuel-air explosive, for one.
posted by Homeboy Trouble at 2:26 PM on June 17, 2019 [1 favorite]
"I wonder how it works with material that's wet or sticky?"
My guess is that this would not augur well.
posted by hal9k at 2:44 PM on June 17, 2019 [53 favorites]
My guess is that this would not augur well.
posted by hal9k at 2:44 PM on June 17, 2019 [53 favorites]
There's a detailed CGI re-enactment of the sugar dust explosion he mentioned on one of the truly great YouTube channels, the one for the US Chemical Safety and Hazard Investigation Board.
posted by Copronymus at 3:00 PM on June 17, 2019 [15 favorites]
posted by Copronymus at 3:00 PM on June 17, 2019 [15 favorites]
The technical term is inclined plane.
posted by Splunge at 3:44 PM on June 17, 2019 [1 favorite]
posted by Splunge at 3:44 PM on June 17, 2019 [1 favorite]
AgentCorvid: I wouldn't think the screw turning would necessarily introduce air at all?
I'm not 100% sure what the guy was referring too, but in my mind, I was picturing something like this video. Granted, this is probably an extreme case (probably set up this way for educational purposes), but I can imagine this phenomenon may exist even in more realistic scenarios. For example, here's a computer simulation of corn going up a screw conveyor where you can see a good amount of air introduced by the action of the screw.
posted by mhum at 4:21 PM on June 17, 2019 [2 favorites]
I'm not 100% sure what the guy was referring too, but in my mind, I was picturing something like this video. Granted, this is probably an extreme case (probably set up this way for educational purposes), but I can imagine this phenomenon may exist even in more realistic scenarios. For example, here's a computer simulation of corn going up a screw conveyor where you can see a good amount of air introduced by the action of the screw.
posted by mhum at 4:21 PM on June 17, 2019 [2 favorites]
If the screw itself is rotated, then material keeps being lifted even when there's no more coming in the bottom
...and that highlights the one disadvantage of the Olds Elevator - the Achilles heel of this Archimedes screw, if you will* - which is height. With all the friction-fighting pressure coming from the bottom, a stationary screw cannot lift a material nearly as high as a rotating one for a given power input.
*Pun Dog smirk
posted by CynicalKnight at 4:38 PM on June 17, 2019 [1 favorite]
...and that highlights the one disadvantage of the Olds Elevator - the Achilles heel of this Archimedes screw, if you will* - which is height. With all the friction-fighting pressure coming from the bottom, a stationary screw cannot lift a material nearly as high as a rotating one for a given power input.
*Pun Dog smirk
posted by CynicalKnight at 4:38 PM on June 17, 2019 [1 favorite]
Bee'sWing, I had the exact same experience today. Saw the Smarter Every Day grain bin video and immediately the Things You Might Not Know video! Both are eyeopeners.
posted by Zedcaster at 5:12 PM on June 17, 2019
posted by Zedcaster at 5:12 PM on June 17, 2019
I think people here are missing out that friction between the outer casing and the material being lifted is continual applying pressure against the screw at all levels so lifting pressure is continually being applied all the way up. If the material gets too clogged at any one place that applies more friction from the casing which would tend to redistribute the material more evenly (and upward because thats where the pressure is being released
posted by mbo at 5:56 PM on June 17, 2019 [4 favorites]
posted by mbo at 5:56 PM on June 17, 2019 [4 favorites]
Yeah, my understanding of it is that the rotation of the casing is important. Doesn't it impart energy to the bulk material via friction? I don't think the intake is doing most of the pushing, and the height of the elevator can be large because the casing is imparting energy along the entire height. That is, there is a constant amount of force applied to lift the material per unit of height.
posted by whatnotever at 6:21 PM on June 17, 2019 [2 favorites]
posted by whatnotever at 6:21 PM on June 17, 2019 [2 favorites]
If you think about it, you realize that the friction interaction at the wall of the tube is the essential ingredient. If there was no friction, the rotating screw wouldn't work. The material would just spin around in place.
posted by StickyCarpet at 7:32 PM on June 17, 2019 [1 favorite]
posted by StickyCarpet at 7:32 PM on June 17, 2019 [1 favorite]
I'm curious how the screw shaft and the cylinder remain concentric enough that there is no metal to metal contact, which is one of the stated advantages, without some kind of bearing at the bottom end for the cylinder to ensure it remains concentric, which the list of advantages says is not there, as a "hygienic" feature?
posted by 2N2222 at 8:19 PM on June 17, 2019
posted by 2N2222 at 8:19 PM on June 17, 2019
I'm pretty sure the whole cylinder has to move, not just the scoops. But I'm basing this opinion on symmetry and not much else.
posted by M-x shell at 8:24 PM on June 17, 2019
posted by M-x shell at 8:24 PM on June 17, 2019
some kind of bearing at the bottom end for the cylinder to ensure it remains concentric
The video shows a bearing sleeve at the bottom end which is clear of the scoop section by a couple of feet.
posted by Johnny Wallflower at 8:38 PM on June 17, 2019
The video shows a bearing sleeve at the bottom end which is clear of the scoop section by a couple of feet.
posted by Johnny Wallflower at 8:38 PM on June 17, 2019
you're telling me that a screw is basically a RAMP
The technical term is inclined plane.
Yes, it's one of the fundamental machines. Any threaded screw is just a rolled-up inclined plane.
posted by Rash at 9:40 PM on June 17, 2019
The technical term is inclined plane.
Yes, it's one of the fundamental machines. Any threaded screw is just a rolled-up inclined plane.
posted by Rash at 9:40 PM on June 17, 2019
the rotating part of the column really only needs to be the scoops at the bottom, the spinning column's height doesn't help other than to keep the objects moving along the screw/ramp
The cylinder as a whole needn't rotate, and I doubt they get much advantage from doing so along most of its length, unless the cylinder's very rough. I bet an elevator with a fixed shaft but a freely rotating intake on the end of the shaft would work roughly as well.
With all the friction-fighting pressure coming from the bottom, a stationary screw cannot lift a material nearly as high as a rotating one for a given power input
Anybody who truly believes that these things are true needs to go back to Tom Scott's video and ask themselves why the birdseed inside the smooth transparent casing of his hand cranked demo elevator is so obviously rotating along with the casing.
The physics of this device's internals are almost identical to those of a standard vertical auger elevator; the only real difference is that because the feedstock is in fact spinning, it will experience a centrifugal force that tends to drive it to the outside wall and might therefore tend to reduce friction with the auger somewhat. However, given the fairly low speed at which the casing is turning, this effect is probably small enough to neglect.
In particular I can see no reason why the central shaft for the screw would need to be any less strong for this elevator than if it were driven rather than fixed; in either case it's going to need to resist the same distributed torque from friction with the feedstock.
The cunning part of the Olds design is the way that it deals with feeding the material onto the screw in the first place. A traditional auger removes material from the feed point and relies on other material falling into the resulting space - a space somewhat restricted by the casing - in order to be fed in its turn. This is why a traditional auger introduces more air into the lifting column. The Olds elevator positively shovels material onto the leading end of the screw.
Attempting to achieve the same positive scooping action with a traditional auger by attaching feed blades to the bottom of the auger would result in feedstock being pulled in against a ramp that isn't moving with respect to the feedstock, creating a jam point. With the Olds elevator, the feedstock's transition to being in motion with respect to the screw happens outside the elevator, at the tip of the scoops, where there's plenty of room and jamming is unlikely.
I would expect that a traditional auger could achieve similar performance by extending the screw some way below the bottom of the casing and having it flare out to a somewhat larger diameter. But unless the drive for the traditional auger's shaft came up through the floor, it wouldn't get out of the feedstock's way at the top of the column the way the Olds design does.
posted by flabdablet at 9:41 PM on June 17, 2019 [8 favorites]
The cylinder as a whole needn't rotate, and I doubt they get much advantage from doing so along most of its length, unless the cylinder's very rough. I bet an elevator with a fixed shaft but a freely rotating intake on the end of the shaft would work roughly as well.
With all the friction-fighting pressure coming from the bottom, a stationary screw cannot lift a material nearly as high as a rotating one for a given power input
Anybody who truly believes that these things are true needs to go back to Tom Scott's video and ask themselves why the birdseed inside the smooth transparent casing of his hand cranked demo elevator is so obviously rotating along with the casing.
The physics of this device's internals are almost identical to those of a standard vertical auger elevator; the only real difference is that because the feedstock is in fact spinning, it will experience a centrifugal force that tends to drive it to the outside wall and might therefore tend to reduce friction with the auger somewhat. However, given the fairly low speed at which the casing is turning, this effect is probably small enough to neglect.
In particular I can see no reason why the central shaft for the screw would need to be any less strong for this elevator than if it were driven rather than fixed; in either case it's going to need to resist the same distributed torque from friction with the feedstock.
The cunning part of the Olds design is the way that it deals with feeding the material onto the screw in the first place. A traditional auger removes material from the feed point and relies on other material falling into the resulting space - a space somewhat restricted by the casing - in order to be fed in its turn. This is why a traditional auger introduces more air into the lifting column. The Olds elevator positively shovels material onto the leading end of the screw.
Attempting to achieve the same positive scooping action with a traditional auger by attaching feed blades to the bottom of the auger would result in feedstock being pulled in against a ramp that isn't moving with respect to the feedstock, creating a jam point. With the Olds elevator, the feedstock's transition to being in motion with respect to the screw happens outside the elevator, at the tip of the scoops, where there's plenty of room and jamming is unlikely.
I would expect that a traditional auger could achieve similar performance by extending the screw some way below the bottom of the casing and having it flare out to a somewhat larger diameter. But unless the drive for the traditional auger's shaft came up through the floor, it wouldn't get out of the feedstock's way at the top of the column the way the Olds design does.
posted by flabdablet at 9:41 PM on June 17, 2019 [8 favorites]
I just love it when someone looks outside turns the box inside out! Great post CWAA
Bee'sWing - I design different uses for grain bins -shelters- a lot and I've never seem one put up; that was so useful to me (we do call 'em silos here in the antipodes).
The technical thing I design most are probably stairs and ramps and I often wonder if there's another way of ascending a height change. Crazy thought I know but Mr Olds looked differently at something we all take for granted and massively improved it.
posted by unearthed at 10:18 PM on June 17, 2019 [3 favorites]
Bee'sWing - I design different uses for grain bins -shelters- a lot and I've never seem one put up; that was so useful to me (we do call 'em silos here in the antipodes).
The technical thing I design most are probably stairs and ramps and I often wonder if there's another way of ascending a height change. Crazy thought I know but Mr Olds looked differently at something we all take for granted and massively improved it.
posted by unearthed at 10:18 PM on June 17, 2019 [3 favorites]
Unearthed: have you thought of having the person remain stationary while having the grain silo rotate around them?
posted by Joe in Australia at 4:56 AM on June 18, 2019 [4 favorites]
posted by Joe in Australia at 4:56 AM on June 18, 2019 [4 favorites]
I design different uses for grain bins -shelters- a lot
Do any of them make use of the counterintuitive cooling ventilation described by Buckminster Fuller?
posted by flabdablet at 4:59 AM on June 18, 2019
Do any of them make use of the counterintuitive cooling ventilation described by Buckminster Fuller?
posted by flabdablet at 4:59 AM on June 18, 2019
I'm curious how the screw shaft and the cylinder remain concentric enough that there is no metal to metal contact, which is one of the stated advantages, without some kind of bearing at the bottom end for the cylinder to ensure it remains concentric, which the list of advantages says is not there, as a "hygienic" feature?
I think they just have lots of clearance, and the overall hydrostatic effect of the material tends to keep it centered. Alternately, if you only have a bearing at the top, and you're supporting a large-diameter moving cylinder instead of a shaft, you can have a Really Big Bearing.
Some benefit might be gained by adding more clearance and throwing some baffles on the inside of the cylinder to further drive the material in the upper parts of the cylinder, but maybe not. Might just crunch things up more.
[^^^^^^^ PRIOR ART AND EVIDENCE OF OBVIOUSNESS IN THAT PARAGRAPH, NOT FOR PATENTING BY ANYONE ELSE]
posted by Huffy Puffy at 5:22 AM on June 18, 2019 [1 favorite]
I think they just have lots of clearance, and the overall hydrostatic effect of the material tends to keep it centered. Alternately, if you only have a bearing at the top, and you're supporting a large-diameter moving cylinder instead of a shaft, you can have a Really Big Bearing.
Some benefit might be gained by adding more clearance and throwing some baffles on the inside of the cylinder to further drive the material in the upper parts of the cylinder, but maybe not. Might just crunch things up more.
[^^^^^^^ PRIOR ART AND EVIDENCE OF OBVIOUSNESS IN THAT PARAGRAPH, NOT FOR PATENTING BY ANYONE ELSE]
posted by Huffy Puffy at 5:22 AM on June 18, 2019 [1 favorite]
throwing some baffles on the inside of the cylinder to further drive the material in the upper parts of the cylinder
It seems to me that as long as the slope of the screw thread is less steep than 45° at the outside edge, and the coefficients of friction for auger surface and inner wall are similar, then the way the tangential and axial components of the frictional forces on all nearby surfaces resolve is always going to result in the outer part of the feedstock sliding over the surface of the auger and up the wall rather than sticking to the auger and rotating around the wall; feedstock nearer the centre is always going to be dragged along by feedstock further out regardless. I can't see your proposed baffles achieving anything beyond increasing the inner surface area of the tube and therefore the frictional losses involved in shoving stuff along it.
posted by flabdablet at 6:11 AM on June 18, 2019
It seems to me that as long as the slope of the screw thread is less steep than 45° at the outside edge, and the coefficients of friction for auger surface and inner wall are similar, then the way the tangential and axial components of the frictional forces on all nearby surfaces resolve is always going to result in the outer part of the feedstock sliding over the surface of the auger and up the wall rather than sticking to the auger and rotating around the wall; feedstock nearer the centre is always going to be dragged along by feedstock further out regardless. I can't see your proposed baffles achieving anything beyond increasing the inner surface area of the tube and therefore the frictional losses involved in shoving stuff along it.
posted by flabdablet at 6:11 AM on June 18, 2019
I think they just have lots of clearance, and the overall hydrostatic effect of the material tends to keep it centered
I agree with this, and also note that because the central shaft isn't spinning it generates no centrifugal bending forces on its own. That means that the only radial forces that do act on it come from the feedstock surrounding it. Any tendency of the central shaft to drift toward the outside would cause local compression of the feedstock, in turn causing pressure to be applied to the auger in a restoring direction.
I'm sure that the same kind of restoring effect would also occur inside a conventional screw elevator, but depending on the rotation rate and the length and stiffness of the screw shaft it might not be enough to oppose positive feedbacks from centrifugal forces acting on the spinning shaft.
posted by flabdablet at 6:28 AM on June 18, 2019 [1 favorite]
I agree with this, and also note that because the central shaft isn't spinning it generates no centrifugal bending forces on its own. That means that the only radial forces that do act on it come from the feedstock surrounding it. Any tendency of the central shaft to drift toward the outside would cause local compression of the feedstock, in turn causing pressure to be applied to the auger in a restoring direction.
I'm sure that the same kind of restoring effect would also occur inside a conventional screw elevator, but depending on the rotation rate and the length and stiffness of the screw shaft it might not be enough to oppose positive feedbacks from centrifugal forces acting on the spinning shaft.
posted by flabdablet at 6:28 AM on June 18, 2019 [1 favorite]
Yes, it's one of the fundamental machines. Any threaded screw is just a rolled-up inclined plane.
More detail on the four fundamental machines
posted by wilberforce at 3:53 PM on June 18, 2019
More detail on the four fundamental machines
posted by wilberforce at 3:53 PM on June 18, 2019
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posted by Bee'sWing at 1:13 PM on June 17, 2019 [4 favorites]