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New version of this question

So, I've tried putting two bounties on this question in hopes that somebody understands what I am asking, in hopes for a satisfactory answer.

I'm also going to include some background information to specify some qualities of these spindles:

So historically, most cultures used bottom-whorl spindles because of their stability. Top-whorl spindles are require the whorl to be perfectly balanced, otherwise the spindle would wobble. Today, top-whorl spindles are far more common because technology has made making them easier. What I don't understand is that many people say top-whorls spin faster than bottom-whorls, making them better for short staple-length fibres like cotton. I'm trying to figure out why.

Here's an image of a top and bottom-whorl respectively, as well as two abstracted versions:

spindles

I want to make it clear, I'm asking specifically how the difference in placement of the whorl affects the speed. I'm interested specifically in how the elevation of the whorl along the spindle's shaft changes the speed it rotates. Assume other factors are the same (i.e. both spindles have hooks at the top, and both spindles have the yarn connecting the side of the whorl directly to the hook; the bottom-whorl spindle does not have its yarn wrapped around the shaft, as shown in the spindles above and their abstractions.


Old Version of this Question

I'm including this, in case anyone wants to read it, but it isn't necessary.

Apparently spindles spin faster with their whorl, a kind of weighted disc, at the top of them instead of the bottom. I'm wondering why?

For reference, here is an image depicting a bottom-whorl (left) and a top-whorl (right) spindle: spindles

Addendum: I've added a rough sketch to indicate that I am asking this as an 'all else being equal' thing. Assume both spindles have hooks and are wound the way (i.e. the bottom-whorl isn't wrapped by the yarn; like the top-whorl, the yarn goes straight from the side of the whorl to the hook at the top). How does changing the placement of the whorl change the speed of rotation? I'm guessing it has something to do with the centre of gravity being moved (while bottom-whorls are slower, they are also significantly more stable).

rough sketch

Please note that differences in the diameter/length/etc. are just limitations of drawing; those aspects are supposed to be interpreted as the same.

Here's another image to show a similar idea. new image

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  • $\begingroup$ To clarify, when you write "spin faster" you mean "twist the fiber into usable yarn faster" rather that "automatically have more rotation per minute", right? $\endgroup$ Nov 9, 2019 at 8:42
  • $\begingroup$ @dmckee I kind of mean both. The top-whorl can get to a higher rpm and as a result, it puts twist into the fibres faster. $\endgroup$
    – Nicholas
    Nov 10, 2019 at 5:27

2 Answers 2

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Resistance against the rotation greatly depends on the point of suspension. Hanging the spindle on a hook will always provide lesser resistance than hanging it straight on the rod.

Top-whorl enter image description here

When you hang the spindle on a hook, the line connecting the point of suspension with the centre of gravity coincides with the axis of rotation. When it is not so (no hook case), there would always be a component of the string tension that would cause the spindle to climb up the string (just like a yoyo). This tension component would act tangentially on the rotating spindle, exerting a retarding torque on the spindle. This would make the spindle slow down faster.

I think it would be more useful thinking about the effectiveness of the two arrangements rather than comparing their spinning speed.

Here, the arrangement where you hang the spindle on a hook would be more effective.


Even if you put hooks to both the spindle's ends, I think the top-whorl would be more efficient.

Top whorl

enter image description here


Note that angle of contact of yarn with the disk's edge in top-whorl is fairly low. This amounts for a higher normal reaction, and hence a higher friction between the two surfaces.

Bottom whorl

enter image description here


In bottom-whorl, angle of contact of yarn with the rod's edge would be higher as compared to top-whorl. So the friction between yarn and the rod's edge would be low.

When this happens, the coil of yarn can slip on the rod's surface and make the twisting process inefficient. This would cause the pre-spun yarn to wind further, instead of making fresh yarn.

In top-whorl, the disk locks the yarn in place and prevents any slipping. This allows for an efficient spinning.

The only retarding torque in the top whorl is due to torsional strain, whereas in bottom whorl there is an extra torque due to friction between slipping yarn and the rod. So the spindle slows down faster in the bottom whorl, even if you put hooks to it.

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  • $\begingroup$ But why does the point of suspension affect the resistance to rotation? What forces are responsible for causing the difference in speed? $\endgroup$
    – Nicholas
    Nov 19, 2019 at 3:11
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    $\begingroup$ @Nicholas When the point of suspension is not in line with axis of rotation (no hook case), there would always be a component of of the string tension that would cause the spindle to climb up the string (just like a yoyo). This tension component would act tangentially on the rotating spindle, exerting a retarding torque on the spindle. This makes the spindle slow down faster. $\endgroup$
    – AlphaLife
    Nov 19, 2019 at 3:40
  • $\begingroup$ @Nicholas I hope my explanation is clear and convincing for you. $\endgroup$
    – AlphaLife
    Nov 19, 2019 at 4:00
  • $\begingroup$ But what about if you have the whorl at the bottom of the spindle and the yarn goes up to a hook at the top without wrapping around the spindle (just like the top whorl, but the whorl is at the bottom). There is a hook in this case so the point of suspension is in line with the axis of rotation, but the whorl is on the bottom; why does it spin slower? $\endgroup$
    – Nicholas
    Nov 20, 2019 at 22:27
  • $\begingroup$ @Nicholas As I've pointed out in the last part of my answer, the disc can prevent the yarn from slipping and sliding in the top whorl. So in top whorl, only retarding torque is due to the torsional strain of the yarn which is quite low in the beginning. Whereas in the bottom whorl, there is an extra torque due to friction between yarn and the spindle rod which slows it down. $\endgroup$
    – AlphaLife
    Nov 22, 2019 at 13:54
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Well, their moment of inertia are the same if the weight distribution of whorl in both the cases is the same. So, any torque given, should in theory, produce same net change in angular momentum. So, if you are doing this with a machine, you would need to spend same amount of electricity(work done) to spin a particular amount of yarn. I think your question makes more sense if you are doing this by hand(as shown in your picture). If you are doing this by hand, the spindle is free to move in other directions also. In those axes of rotation, the top whorl has a less moment of inertia than the bottom whorl. So, it provides less resistance when you accidentally move it sideways, which may give the illusion that you are spending less effort to make it spin faster.

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