Albireo Observatory

The motor for both the Azimuth and Shutter drives is a 12VDC gear motor originally designed for RV camper slide-outs. The online site I purchased these from states the amperage, output RPM, and torque ranges from 12RPM @ 4-amps no load, to 6RPM @ 25 Amps with a max 850 in-lbs of torque. However, according to a manufacturer spec sheet I found on another website, it appears that in “normal” operation the motor is actually meant to provide around 400 in-lbs of torque at about 9.5-amps. It also has an integrated brake with a holding torque of 500 in-lbs, so around 400 in-lbs in normal operation makes sense. That’s plenty of torque for this dome.

The metal components for a simple motor mount were made for me by a local metal fabrication shop. They cut and bent the pieces, but I will do all the drilling and assembling of the plates and motor. This is more or less of the same type hinged motor mount that Ash shows in the dome construction manual I have. But this one is built specifically for this gear motor.

The hinge shown is a commercial-grade door hinge, which is higher quality than typical cheap door hinges. I learned from my ExploraDome drive not to use a really cheap hinge, because it rattled incessantly whenever the dome was in motion. Unfortunately, the only version of this hinge I could find is spring loaded to automatically close the hinge (and door) which is opposite of what I need. It’s adjustable so I released all the tension on the spring and it worked OK for the azimuth drive. But not so much for the shutter. On the shutter motor mount, the hinge had too much side-to-side flop (it is supporting a much heavier motor mount than the azimuth), so I disassembled the hinge and replaced the spring loaded shaft with a solid metal one. That took care of any flop.

In this photo, drilling and assembling the mount is done, and the motor is bolted in place. The two hinged plates are spring loaded to control how the sprocket engages with the drive track, and all that’s left to do is bolt the mount onto the dome ring, and install the drive sprocket on the front motor shaft. Once it’s manually rotating the dome I’ll make a gray coder to integrate it with the motor drive circuitry.

The azimuth motor drive is now installed. In testing, a 12-Volt 8-Amp gel cell battery ran the drive easily. Measurements show the motor is only drawing about 5-amps in operation. It’s performing well and completes one full rotation in 3-1/4 minutes (the circumference is 39-ft 3-in).

In the Ash manual I have, they use a right-angle gearbox on the azimuth drive, so that their big AC motor sits parallel with the dome. But because my DC motor already has a 163:1 gearbox attached, and is much smaller, it can sit perpendicular to the track without being a hazard. The drive is fairly quiet as well, mostly just the hum of the motor plus whatever noise is generated by the sprocket teeth on the gear track, but it’s at a level that I find completely acceptable.

While the Azimuth drive was relatively simple to construct, the shutter drive is a bit more of a challenge. I hadn’t really wrapped my head around what this drive was going to look like until I completed the dome to the point of sliding the shutter frame in place on top. Then I could see that the motor and sprocket were not going to engage into the track with the gear directly mounted on the motor shaft. And I couldn’t use a larger sprocket because it would require too large a hole in the top of the dome. So I ended up building a mount somewhat similar to what is shown in the Ash manual. This mount has a separate shaft near the top for the sprocket, and is driven by a chain drive to the motor mounted below. In the Ash manual it looks like they used some type of gearbox, but gearboxes can be quite expensive, so I went with the chain drive as a simpler, cheaper option. And one that reduces the total weight. But even with the weight reduction, it was still almost too heavy for me to get up there by myself.

The drive assembly is made from 3/8″ thick aluminum plate that was cut and bent at a fabrication shop that builds semi truck beds. This is in fact the same material they use for the beds. Just as with the azimuth mount, I measured, drilled and assembled the drive myself. The upper shaft is 3/4″ in diameter, is keyed, and rotates in bearing blocks. The chain drive between the sprocket and motor is made with #41 chain and 10-tooth sprockets. The hinged mounting plates are spring loaded with a fairly heavy spring, in order to keep the sprocket pushed up and engaged with the track at all times.

It took a while to work out how to mount the shutter drive motor up in the dome, but it’s finally safely up there. In front it is mounted with 1/2″ threaded rods through angle iron mounting points that are bolted to the cross-member at the back of the shutter opening. This allows full adjustment of motor height and engagement of the drive gear into the track.

In the rear a long right angle bracket was mounted across the dome to create attachment points for two 1/4″ thick angled metal brackets between the bottom motor mount plate and the dome.

A look from behind at the motor drive and rear brackets. In initial testing the motor easily opens and closes the shutter, which is a real relief. The spring loading to keep the sprocket and track engaged appears to be working well, too. Now I need to run a cable up to the motor so I can stop climbing a 10-foot ladder with a battery.