My friend wants to get into spindle making. That is, spindles used for spinning thread for weaving.
I asked her what qualities make a great spindle. She said they spin fast and they spin for a long time.
This got me wondering about the ideal placement of mass in a spindle that you wanted to have those qualities.
Does a spindle spin faster with the majority of mass near the center or vice versa? Does a spindle spin for a longer time with the mass near the outer rim or vice versa? What about raising or lowering the inner mass relative to the outer mass?
In what way would making the shaft diameter larger or smaller affect the speed and spin time? What about the diameter and vertical thickness of the whorl?
My friend says that these spindles are considered the best in the spindle world: http://lahottee.info/pictures/BosworthMidi.jpg
Does that shape do something special to make it spin fast and for a long time?
I guess all my questions boiled down to one is: what would be the mass distribution for an ideal spindle in terms of balancing spinning fast and for a long time?
A few thoughts.
To spin for a log time you want a large moment of inertia. Rotational inertia is mass times the radius squared. So, a given bit of mass contributes four times the inertia if it is located twice the distance from the centre. So, for a given mass spindle, the most inertia will be achieved by placing the mass in a ring around the spindle shaft. Which, funnily enough is what the cited picture depicts.
The total mass is going to be limited b the constraints of the yarn. The mass will determine the tightness of the spun yarn and the dynamics of the spinning. You won’t have much wiggle room here.
Fast is a more complex issue. Because the spindle is spun up by twisting the shaft between the fingers of the spinner, you need to balance a number of things in the design. The shaft needs to be within a couple of constraints in size. Too large a diameter and the spinner’s fingers won’t be able to make it spin very fast no matter how quickly they run their fingers past it. Too small a diameter and they won’t be able to get enough leverage to allow their fingers to put the available energy into the spindle. There will be a magic compromise where the inertia of the spindle, and the diameter of the spindle shaft allow the spinner’s finger to exert maximum effort in transferring energy into the spindle. This will probably vary from person to person. (This is an impedance matching problem.)
Spinning for a long time is going to be matter of balancing a few other things. One assumes the majority of the energy in the spindle is going into deforming the spun yarn. This will depend upon the nature of the yarn desired. There will also be other losses, air drag being one. You don’t want to make the spindle too large, so a very large ring (to get the inertia up) is not going to work well. To reduce air drag you want to reduce the area of fast moving parts of the spindle - which is at odds with the desire to get the mass to the outside. The best spindle may therefore be a very high density metal ring. Say Tungsten. (I’m sure Gold or depleted Uranium could be pressed into service as well.
) Then you need to match the diameter of the shaft to the spinner, picking a diameter that the user can spin up the spindle with easily to a satisfactory speed. A very fine scored finish on a metal shaft might work well to improve grip and allow a slightly smaller diameter shaft to be used - which will increase speed.
