Drive mechanism for lift-off roof.

The drive mechanism consists of a 1750 RPM DC motor, combination clutch/brake unit and a right angle speed reducer.  The motor is at the bottom of the drive train.  Above that is an electric clutch/brake unit and above that is the speed reducer.  The motor is reversible and its speed, direction and torque are set with a controller mounted behind the motor.   The speed reducer is chain coupled to the shaft shown which spans the width of the south side of the observatory.  A pair of sprockets at either end of this shaft turn a pair of chains at either side of the observatory.  This pair of chains, in turn, drives a pair of large sprockets which turn shafts which pass through the wall and impart rotatory force to the pair of powered cantilever arms on both sides of the observatory.

There are two shafts in the upper part of the picture.  The uppermost shaft passes completely across the southern base of the observatory.  The far ends of the powered cantilever arms outside the observatory are attached to this shaft and kept in synch with keys and keyways.  There is an identical shaft passing through the observatory roof on the opposite side  which connects to the passive cantilever arm outside the north side of the observatory.

The gear reducer shown here has a 1:20 reduction.  The 1:20 ratio of the speed reducer was initially guessed at, and as it turns out, the roof moves too quickly even with the motor speed reduced.  After doing some more research, here is how we figured what gear reduction was needed:

• Maximum roof opening speed should be 15 seconds at full motor speed of 1750 RPM.  Slower speeds can be obtained by slowing the motor with the controller.

• At maximum, the roof opens with a 180 degree pivot of the powered arm in 15 seconds which corresponds to a 1/2 turn of the large output sprocket which is 1 full turn in 30 seconds, or 2 RPM.

• There are two chain reductions.  These are figured as ratios of the number of teeth on the sprockets.

• The speed reducer output shaft has a 10 tooth sprocket.  This connects by chain to a 24 tooth sprocket on the rear shaft.  The reduction is therefore 1:2.4

• The small sprockets on the ends of the rear shaft have 10 teeth and connect by chain to the final drive sprocket with 60 teeth.   This is a reduction of 1:6

• Reductions in series are the product of the single reductions so the final reduction across the chains is 1:14.4  (Note: single reductions should not exceed 1:7).

• At a final output speed of 2 RPM, the output shaft of the gear reducer needs to be turning at 28.8 RPM.

• At full motor speed of 1750 RPM, the reduction needed for the speed reducer is therefore 1750/28.8 = 60.8, or effectively 1:60

• Speed reducers come in various configurations including single, double and triple reduction units.  Single and triple reduction units are typically worm gears and double reduction units can be either worm/worm or helical/worm units.

• Single gear 1:60 reduction units are quite large and heavy and also have less torque for a given motor horsepower.  To maximize torque and  minimize size and weight a double reduction worm/worm unit was selected.

• The problem with double reduction units is that the lowest reduction available is 1:75

• To keep close to the max opening time of 15 seconds, the sprocket on the speed reducer output shaft was changed to a 12 tooth unit.  This sprocket had to be changed anyway as the output shaft diameter on the original 1:20 speed reducer was smaller than the shaft on the new 1:75 unit.

• The end result at maximum speed is reductions of 1:75, 1:2 and 1:6 for a final output shaft speed of 1.94 RPM.  Plenty close enough to our goal of 2 RPM.

The new speed reducer has been ordered.  It is custom built and will have stainless steel output shafts.  It will also have an unused shaft at the motor side of the reducer which will allow for a hand crank to be attached in case it is ever necessary to open or close the roof manually.