Thursday, May 31, 2012

Dinette Table Riser Mockup and Load Limits

So, in order to help determine feasibility of this idea, a lab mock-up has been constructed to simulate the real (future) dinette installation. The mockup represents one side of the dinette table mechanism. It includes the hardware planned for the idea, as it currently exists in my (whacko) mind at the moment. In summary, there are...

  • two assemblies each with two roller bearing wheels that ride on...
  • a semi-captive vertical aluminum rail (which will be thru-bolted to the boat's interior structure)
  • the two roller bearing assemblies are bolted to a custom cut support bracket (plywood for the moment, aluminum in the final) which will support...
  • the dinette table frame itself
Laptop for quick PC programming of the motor controller. Motor and lower bearing shown left.

The linear motion is achieved by a vertical lead screw, driven with a stepper motor, through an anti-backlash nut within a special side-mount bracket, thru-bolted to the table support bracket. The stepper motor is driven by an Arduino+MotorDriver electronics powered by a 24volt power source, running a custom C++ program.

Vertical lead screw and linear motion bracket.

The lead screw is a "single start", meaning it has only one continuous thread spiraling around one "shaft". Advantages over a multi-start screw are, the torque from the motor is used more for "lift" rather than "speed". In other words, lifting power is more at the expense of speed, compared to, say, a "five start" screw (five parallel threads around one "shaft"). As the table is supported on only one end (ideally), because of opposite torque due to gravity, we wish to emphasize lifting power over speed.
Closeup of the semi-captive roller bearings riding in a rail.

One goal of this mock-up is to determine the lifting power, given the components I have selected (so far). Initial tests show no problems with the motor lifting 25 pounds on a 36 inch lever arm. Attempting 35 pounds, at 36 inches, causes the motor to slip. It should be noted that I do not yet have the lead screw, and pillow block bearings perfectly aligned. They could stand adjustment by an eighth of an inch, but I am doubtful that will make much difference in this test. A large misalignment could cause binding in the anti-backlash nut and lead screw resulting in unnecessary friction.
25lbs of dive weights hung from the 36-inch lever arm to test motor lift power.

If this is not exactly clear, I might post a video later.

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