Albireo Observatory

The motors for both the Azimuth and Shutter drives are 12VDC gear motors originally designed for extending RV camper slide-outs. The website I purchased these from stated the output RPM, amperage and torque varied from 12RPM @ 4-amps no load, to 6RPM @ 25 Amps with a max 850 in-lbs of torque. However, according to a manufacturer’s spec sheet I found at another website, in normal operation the motor is actually meant to provide approximately 400 in-lbs of torque at about 9.5-amps. It also has an integrated brake with a holding torque of 500 in-lbs. this is plenty of torque for this dome.

The metal components for a simple motor mount were made for me by a local metal fabrication shop that builds semi and dump truck beds. This is the same material they use in the truck beds.The shop cut and bent the pieces, but I did all the drilling and assembling of the plates, hinge and motor. This is mount is more or less of the same type of design that Ash shows in their 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 as-is for the azimuth drive, but not for the shutter. On the shutter motor mount, the hinge had too much side-to-side flop (it is supporting a heavier and taller motor mount than for the azimuth), so I disassembled the hinge and replaced the spring loaded shaft with a solid metal one. That took care of the 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 much the sprocket engages with the drive track,\. 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 directly attached, and is much smaller, it can just sit perpendicular to the track and directly drive the dome, without protruding enough to be a hazard. The drive is fairly quiet and the noise generated by the sprocket teeth on the gear track is at a level that I find completely acceptable.

While the Azimuth drive was relatively simple to construct, the shutter drive was a bit more of a challenge. I hadn’t really wrapped my head around what this drive was going to look like until I got 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 drive 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 get expensive and heavy, so I went with the chain drive as a simpler, cheaper option. And it reduces the total weight that has to be suspended from the dome.

The drive assembly is made from 3/8″ thick aluminum plate that was cut and bent at the same fabrication shop as the azimuth mount. Again, 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 pushing up and engaged with the track at all times.

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

In the rear a 24″ long right angle bracket is mounted across the dome to create attachment points for two 1/4″ thick angled metal brackets that tie the bottom motor mount plate to 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.