Quality and usability
Quality of these exercises varies between manufacturers and between models. Design and quality of PB-188, PB-188C ("250 Hz", "250 Hz Pro" in RPM Sports' range) balls from NSD is such that using these balls at speeds in excess of 13,000 rpm can result in severe structural failures, e.g. breakage of rotor axle. Balls with LEDs are reportedly even more fragile.
Records on NSD balls registered by RPM Sports
Balls manufactured by NSD are available with electronic revolution counters, which allow performance data to be gathered during use.
The current records for PB-188C ball are as follows: 16,732 rpm peak, 7,561 revolutions in 30 seconds, 21,228 revolutions in 90 seconds, and a combined 31,816 rpm in the Dual category (one Powerball in each hand).
The peak speed record for PB-388C ball is 14,091 rpm.
The peak speed record for PB-388HC ball is 20,090 rpm.
Above-mentioned model names are used by NSD. These products are also known by names used by retailers selling NSD balls under their own brands, e.g. "250 Hz Pro", "350 Hz Pro" etc. in case of RPM Sports.
All of the above records are currently held by Akis Kritsinelis from Greece. For other results, see the scoreboard managed by RPM Sports.
There exist videos showing the process, however independent expertise pointed out inconsistencies and anomalies in the world-record video by Akis Kritsinelis and others accepted for the scoreboard. Other evidence, e.g. of people admitting use of modified NSD balls or of WR holder Akis Kritsinelis offering powerballs capable of speeds in excess of 15000 rpm for £100/$200 supports these findings.
Certain scoreboards registered even higher scores, e.g. scoreboard managed by RPM Sports' official partner Gross-Snab contains the score of 16973 rpm achieved on PB-188LC ball (PB-188C with LEDs).
Patents
The device is covered by US patents 3,726,146 (1973) and 5,353,655 (1994) by L.A. Mishler and US patent 5,800,311 (1998) by P.S.Chuang and 6,942,601 (2001) by P.S. Chuang.
How does it work
The device essentially consists of a spinning mass inside an outer shell. The shell almost completely covers the mass inside, with only a small round opening allowing the gyroscope to be manually started. The spinning mass is fixed to a thin metal axle, each end of which is trapped in a circular, equatorial groove in the outer shell. A lightweight ring with two notches in it for the ends of the axle rests in the groove. This ring can slip in the groove; it holds the spinning gyroscope centred in the shell, preventing the two from coming into contact (which would slow the gyro down), but still allowing the orientation of the axle to change.
Since the spinning mass is balanced, the only possibility to speed up the rotation is for the sides of the groove to exert forces on the ends of the axle. Furthermore, the normal and axial forces will have no effect, so tangential force must be provided by friction. If the axle is stationary, the friction will only act to slow down the rotation, but the situation is very different if the axle is turned by applying a torque.
This can be accomplished by tilting the shell in any direction except exactly in the plane of the groove, and results in a shift of the axle ends along the groove. The direction and speed of the shift can be found from the formula for the precession of a gyroscope: the applied torque is equal to the cross product of the angular velocity of precession and the angular momentum of the spinning mass. The most important observation here is that the direction is such that, if the torque is large enough, the friction between the axle and the surface of the groove will speed up the rotation.
This may seem odd. After all, if the axle were shifting in a horizontal groove, the friction on one end that acts to speed up the rotation would be cancelled by the friction at the other end, operating in the opposite direction. The difference is that a torque is being applied, so one end of the axle is pushing against one side of the groove, while the other end is pushing against the other side. Likewise, it doesn't matter in which direction the torque is applied. If the torque is reversed, each end of the axle will then be pressing against the opposite side of the groove, but the direction of precession is also reversed. The only restriction is that the relative speed of the surface of the axle and the side of the groove due to precession, ΩPRgroove, must exceed the relative speed due to the rotation of the spinning mass, ωraxle. The minimum torque required to meet this condition is , where I is the moment of inertia of the spinning mass, and ω is its angular velocity.
Since an acceleration of the rotation will occur regardless of the direction of the applied torque, as long as it is large enough, the device will function without any fine-tuning of the driving motion. The tilting of the shell does not have to have a particular phase relationship with the precession or even to have the same frequency. Since sliding (kinetic) friction is usually nearly as strong as static (sticking) friction, it is also not necessary to apply precisely the value of torque which will result in the axle rolling without slipping along the side of the groove. These factors allow beginners to learn to speed up the rotation after only a few minutes of practice.
By applying the proportionality of the force of friction to the normal force, Ff = μkFn, where μk is the kinetic coefficient of friction, it can be shown that the torque spinning up the mass is a factor of smaller than the torque applied to the shell. Since frictional force is essential for the device's operation, the groove must not be lubricated.
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