At the core of this machine are 2 independently controllable coaxial shafts - that is, one inside the other. The outer shaft (black hatched) carries the platter with the legs, the inner one (green hatched) carries the offset cable hub which causes the rise - fall action of the legs. An arrangement of ball bearings and collars holds these 2 shafts in place, yet leaves them free to rotate. The 1/4 thick aluminum "base plate" forms the anchor to the whole system, and a second smaller outline plate is held below by 4 standoffs. Between this plate pair, the shafts (and everything that rides on them) are mounted. A thin bottom plate and 4 smoked plexiglass walls form the home to the electronics and a cradle for the main physical system.
Motor Mounting and Power Transmission
The 2 motors are Pittman 9413-2 types (McMaster part # 6331K33). They work at plus/minus 24 VDC and have 19.7:1 gear ratio. It is a good idea to design around always-available motors like these, and several gear ratios are available with the same dimensions. They have plenty of speed and torque for this application and minimal low speed cogging. A standard plate was made to attach to the motor faces.
The legs require most of the labor for this project, involving many operations on the 16 aluminum plates and other parts. In retrospect, hiring-out the leg outline as a waterjet cutting job would have been worth it. The plates, which have a curved outline, are force-warped during assembly, resulting in a flowing final shape. This "force warping" concept is easy compared to preshaping the plates - just bolt plates together and they must assume nice curves dictated by the positions and orientations of the mate-too structures.
The legs start as 1/8" aluminum 6061 sheet. A paper template is used to mark the outline which is then bandsawn out. Pairs which will constitute a leg are bolted together, and then sanding cleans up the shape - belt sander for convex areas and drum sander for concave areas. The 1/2" polycarbonate end spacers are joined in and sander-finished to match thier respective leg plates. Once the outlines look good the pair can come apart for filing and hand sanding. Several drilling, milling and slotting operations are required for lamp wiring placement (more below on this). Finally, the leg's physical structures are finished, and ready for electrical installation.
CABLE TYPE: (properly called "wire rope") .024" diameter including nylon coat, 7x7 strand, 18-8 stainless steel. McMaster Carr part # 8930T22. It is nice thin stuff, rated at 40 pounds. It cuts cleanly and easily with a cable cutting tool (no fray-out due to nylon coat).
BASIC PRINCIPLE: The legs are free to pivot up and down, without the lifting cable they would all sit at thier lowest position. The 8 lifting cables are what make the legs rise and fall as the offset cable hub rotates with respect to the leg platter. Each cable has 2 anchorages - one at the top hub, and one on a leg. As the system moves, the angles at which the cables enter thier anchorages change, and so there must be smooth bell-curved entry ways so the cable never has to make a sharp kink.
Gearmotor on Motor Plate will Transmit motion via Pulley and V-Belt
Transmitting the motion from the motors' shafts to the actual system shafts is accomplished with small (2L type) V-belts and homemade V-belt pulleys. The pulleys are of the "one-piece clamp" variety, and have 3 different bores: 1/4" for the motors, 3/8" for the inner shaft, and 1" for the outer shaft. The shaft pulleys also serve as collars, which helps keep the design compact. The motors are sideways adjustable in slots in order to install and tension the belts.
The Legs pivot on an axel mounted to a strut via a pair of ball bearings. This forms a complete leg unit. The 8 leg units bolt to the platter, which is connected to the outer shaft via the commutator.
CABLE LONGEVITY: How many times can a cable bend before it breaks? This is a key issue for such a machine. There are 3 main factors that will influence cable longevity:
- Loading, as a percentage of the cables breaking strength. Since this design uses only a few percent of the cables strength we are in good shape here.
- Strand design. This 7x7 cable is composed of 49 hair-fine individual strands. The ability of cable to withstand repeated bending around tight curves is directly related to strand size: the smaller the better. And these are pretty small. Although even smaller strand size is available with 7x19 cable, it is so limp that it tends to kink and tangle.
- Bend Radius. By using "radius drills" and being carefull with the entry angles in the design, the cable never has to make a sharp bend. In this design it is as if the cable is going over a 1/2" pulley, which is easy for such thin cable.
CABLE END FIXING: The 2 ends of each of the 8 lifting cables must be fixed to their structures. Different methods are used at the 2 ends. At the hub end, a "ball squish" method is used. A 1/8" diameter ball gets forced against the cable which is sitting in a tunnel by a set screw. This is safe for the cable and holds well for moderate loads. It is easy to adjust the cable to get the legs even - just loosen the set screw and slide the cable to desired position then tighten. At the leg end (which is installed first), a knot method is used. A knot is made and a bit of epoxy put on it to make a little blob. This blob cannot go through the thin passage, thus fixing the leg end.
Octopus: Structure and Fabrication