Actuator mechanism spinner

Actuator mechanism spinner is unlike anything else on the market so the design had to be developed from scratch. A toy car inertial mechanism served as the foundation for the initial iteration. When you release the spring after rolling the toy backwards, it moves forward at a faster rate. This design was rejected because it was too expensive and complicated to build in large quantities, with the budget limit of $5 per unit.

The second mechanism is based on a humming-top, for it is similarly constructed. The humming-top mechanism uses a helical axial rod for unwinding, which converts the translational motion into a rotational one. The return of the nut to the screw is due to gravity, so the design only works when the axis of rotation is vertical. Due to this design flaw, the spinner cannot be used when tilted or horizontally.

To select the right mechanism, we held an internal competition. As a result, our engineers submitted 11 device concepts. As part of the competition, we made prototypes of the concepts: humming-top, ball screw, air drive and magnetic drive. The result was that the ball screw was selected. Ball screws are a type of linear actuator that converts the rotational motion of a screw into the translational motion of a nut. The screw does not interact directly with the nut, but via balls, which results in very low friction and consequently high efficiency of the ball screw.

When prototyping the first concepts for ball screws, the problems with this solution became apparent. The drive mechanism allowed a stable conversion of a linear motion into a rotational one in only the "bearing up" position. When the guide (7) was pressed, the ball (5) was pressed into the helical groove of the bushing (3). When no force was applied, the ball was in the plunger cavity and did not interact with the screw groove. When pressed slowly, the ball did not have time to enter the screw groove, skipped it and did not go along it. When the spinner was turned with the lid (10) up, the ball rolled into the screw groove under the action of gravity and the action started immediately. However, when the plunger (7) touched the ball, the mechanism started to "slip". This disadvantage is eliminated by ridding the influence of gravity on the position of the balls in the device. The drive mechanism was sent in for further development.

In total, over 20 iterations of the ball screw design were performed. To solve the problem with ball slips, a second bearing was added to the design and a system for controlling the ball circulation was implemented during the changes. The system is know-how and allows the spinner to be used at different angles as the balls are always in the operational position.

The second problem was the noise emanating from the bearings as the balls moved in the screw grooves.

To eliminate the bearing noises, we purchased them from different manufacturers and tested them. During the tests, we found that all the bearings had different angular play. For the actuator mechanism, we chose the ones with the smallest angular play, which reduced the noise somewhat. Further experiments showed that installing two smaller bearings would eliminate the effects of their play on the mechanism operation.

Once the bearing noise was eliminated, we moved on to eliminating the noise in the screw grooves. The noise was caused by the movement of a metal ball through the grooves. The screw on which the balls moved was 3D printed. Since 3D printed parts have a layered structure, the balls made a loud noise when they moved through the grooves. This problem was removed by altering the position of the screw grooved part on the table of the 3D printer.

When printing parts on SLA printers, parts with ball running tracks should be positioned vertically with the axis of rotation (although this is not recommended). This way, printing defects like overlapping layers won't provide extra resistance to the balls' ability to roll.

After agreeing with the customer on the final version of the mechanism, we moved on to the industrial design stage and refined the design for injection moulding. We chose glass-filled polyamide (PA) as the material for the hub and plunger and polyoxymethylene (POM) for the separators and the ring part with the running track. In the manufacture of the mechanisms, all threaded joints, with the exception of the push button, were replaced by adhesive bonding. These changes were made in the final stage of refinement, although the problems with the previous solutions were known long before the final stage. The engineers first solved important problems that affected the use of the device and then brought the mechanism to the level necessary for assembly in production.

The final design and mechanism of the spinner can be seen in the illustration.