Overall Turntable Design

Revised:  8/5/2012            Subject to Revision

The design of the overall turntable was done in order to size the key components and determine a reasonable structure that could be modelled and operate in a manner analogous to the storyline prototype.

The overall concept has previously been presented.  The turntable structure is built around two wheel assemblies, one at the bottom and another at the top held together by a central mast.  The radial arms of the wheels serve to support the sails and transmit the torque to the central hub and thence down into the waggon cabin for power takeoff.

The wheels consist of wood radial arms bolted to a central hub with spacers between the ends to form a wheel.  Just inside the tips of each arm is a bearing for a sail assembly.  The bearing allows the sail to pivot freely.

The sails are free to rotate up to a sail stop mechanism that allows the sails to freely rotate counter-clockwise but constrains the rear bottom spar to rotate only to the radial arm when rotating clockwise.  This is done to permit the sails to freely rotate into alignment with the wind on one side of the wheel while holding the sails on the other side aligned similar to the paddles on a water wheel.

The sail stop mechanisms are pivoted with a counter weight that keeps the stop tip up when the sail attempts to push past when traveling clockwise but tilts down to allow the sail bottom spar to depress the tip when traveling counter-clockwise.

The top portion of the hub on the bottom wheel holds a wood mast.  The mast is secured with bolts to prevent rotation in the hub and transmission of torque coming down from the mast top. 

At the top of the mast another wheel is mounted less the sail limit stops.  This wheel has bearings facing those on the bottom wheel.  The bottom and top wheels are identical except the top wheel does not have the sail limits mounted.

The picture above shows three of the eight sail assemblies installed between the top and bottom wheels.  The two at the left and bottom center are aligned with a wind coming at the wheel from the lower right side at a ten degree angle to the right-lower arm. 

Above all eight sail panels are installed showing how the four at the bottom and left and facing into the wind while those at the right and top are held rotated to the wind by the limit stops.  Those at the bottom and left apply no torque to rotate the wheels while those at the right and top catch the wind and apply torque to rotate the wheel counter-clockwise looking down from above the turntable.

The sail panel assembly consists of a center mast, a bottom spar with pivot, a top spar with pivot and a cloth sail panel with cord edges.  The spars are metal while the mast is wood.  The mast is held in each of the spars with bolts to transmit torque between the spars so that the limit stop at the bottom holds the entire sail panel.  The cloth sail panel is held at the four corners with bolts at the ends of the spars.  Because the leading edge of the sail panel is tapered more sail area lies behind the pivots than in front.  This causes the sail to to align the leading edge facing the into the wind unless the sail panel is constrained by the sail stop.  The pivots top and bottom fit into bearings near the tips of the wood radial arms of the wheels.  The wheels hold the sails in place with sufficient clearance to permit the sail panels to freely rotate.
The above parts define the turntable with the exception of the bottom hub and shaft that will transmit torque to the drive mechanisms below the deck.  The bottom hub will attach to the bolts holding the wood radial arms on the bottom wheel.  A threaded cap plate is used above the top wheel to provide a way to torque the upper wheel bolts.  The bottom wheel lower hub will use the bolts in a similar fashion.  The shaft and hub assembly below decks will provide support bearings and clamps to hold the turntable assembly in place while allowing it to rotate freely.

The upper deck portion of the turntable is sufficiently designed and the parts detailed to point where model parts can be fabricated.  The wood parts will be made of a hard wood such as oak while the metal parts will be made of 6061 aluminum.  The bolts will be stainless steel or other steel depending on availability.

The assembly has many, many parts to be made.  The next entries in the blog will likely discuss and illustrate results of parts fabrication.  Most of the work will use the Sherline CNC mill and CNC lathe.  Since most of the parts are replicated many times the CNC program feature will be a near necessity to limit drudgery and boredom.

The companion blog about the Wind Waggon Trek (see links on page header) can be embellished with more storyline about the expected design of the prototype and fabrication of prototype parts as model parts fabrication occurs.  Meanwhile more model design work can be done with the drive shaft, drive mechanisms and waggon box structure and later use the model design to drive the story.  An unrelated part of the storyline consists of character development and threading together various storyline actions and conflicts to make it interesting.