Friday, March 25, 2011

Replacing a tube motor

Along the ridges of the greenhouse are venting skylights that open and close by means of tube motors that activate (via remote computer command) to rotate a steel shaft affixed to a rack and pinion assembly that is attached to the wood framed skylights. In the picture below, notice that a tube motor is in place along the left bank of skylights, while none appears along the right side. That's because it burned out, and is being replaced.

A bronze bracket on which the motor mounts is shown on the leftmost tube motor below:

The one on the right side has been removed in the photo below because it was deemed partially responsible for the failure of the motor.

These tube motors have adjustable "stop limits" that tell the motor when to stop turning, either closing or opening. This is because the rack shaft is only so long, and the motor must stop cranking when the skylight is fully open or fully closed. The bronze bracket has a square hole where the square shaft of the motor fits, providing a fixed point against which the rotating motor bears to cause the steel shaft to turn, operating the skylights. In this case, it seems the square hole was slightly oversize, giving the motor shaft a 'sloppy' fit, and playing havoc with the adjustment of the stop limit setting. Along with the immense combined weight of 10 triple glazed skylight vents on a motor just barely rated to lift them, and 11 years of faithful service, we theorize that this is why the previous motor burned out.

The solution, we hope, is to modify the bronze plate on its backside, which will be out of view, with an additional plate cut to a much tighter fit, and a new motor.

The motor has a key way at the steel shaft end to mate and lock rotation with the steel shaft.

Below you can see the steel shaft and the detached rack and pinion setup; detaching it from a couple of skylights let's us pull it back to facillitate sliding the new motor onto the shaft.

The new motor slips in nicely with the modified bracket and awaits electrical reconnection and stop limit adjustments:

Looking back down the greenhouse ridge:

That was fun!


Jeannette said...

Better than the manual...oh wait, there is no manual! Great job trouble shooting, describing the mechanics, fixing it all and photographing it too.

John said...

Nice job with the modifications, how is it holding up?

Mark said...

I'm glad you asked! I have a whole 'nother blog post planned on what Paul Harvvey would have called 'the rest of the story'. Interestingly, the replaced motor is performing admirably and according to expectations. The next post details the results of us turning our attention to the other motor pictured above. Thanks for stopping by, John!