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New drive mechanics for the 28 inch scope

I started thinking about new drive mechanics of my 28 inch scope in 2012, and taught myself enough about Google's free 3D program SketchUp to start designing it in 2013. With the help of Dan Gray and Mel Bartels I soon came up with a design that seemed like it would work and had the two major components cut out by laser within a few months. I was sure it would be completed in a few more months but life, as it often does, had other plans. 

My objective was to simplify the original mechanics, which were effective but bulky and somewhat temperamental. The main idea was to utilize the 300+ pound weight of the 28 inch telescope for a friction azimuth drive - no more big timing belts wrapped around the perimeter of the ground ring.  Also, the slip clutches for both axis had to be built-in so an external device wouldn't be needed so none of the drive components would stick out. 

Clean, simple and easy to make was also important. For comparison, here's a photo of the original drive mechanics - that's Dan Gray helping me out - and note the huge slip clutch discs on both the azimuth and altitude drive shafts. As you'll see, these big slip clutch discs are now replaced by a more integrated drive system.

After three years whirled by, I finally started again on the new drive in May 2016 and had the first iteration ready to try at the August 2016 Oregon Star Party. It tracked quite well (great actually, but I'm too modest to say so...) but I wanted to make a few refinements to try to eliminate backlash in the azimuth drive . I tested the revised system in October 2016 at Likely Place in northern California for two nights - still tracks great, but now without any backlash. I should mention that the the scope moves more smoothly and easily now because it takes less tension in the slip clutches to drive the scope - it moves more like a regular Dobsonian.

I also hope this project will be a prototype for a larger scope. That may happen in the next few years, time will tell, but for now I'll just enjoy my 28 inch even more than I had been the previous 12 years.

So what is my design? Basically, I've made one of the three azimuth wheels into a combination friction drive wheel/slip clutch, and came up with a way to combine the altitude slip clutch with the altitude friction drive roller. Turned out to be clean, simple and almost 100% easy to make. I'll get into the details below, and will point out the one part that wasn't easy.

Azimuth Drive Photos
This photo shows the general layout of the azimuth drive. The ground ring base is 0.25 inch thick aluminum and was cut by laser from the SketchUp file I created. The ground feet are placed directly below the azimuth idler and encoder wheels, and as close as possible to the drive wheel. The ground feet are glass filled urethane balls that have been cut in half.

Going from the drive wheel toward the center pivot bearing are the:
1. Drive wheel
2. Clutch plate (faced with Ebony Star laminate)
3. Drive shaft
4. Harmonic Drive 50:1 gear reducer (eliminates backlash)
5. Servo motor with built in planetary gear reducer

This photo was taken before the azimuth encoder cable had been installed (the encoder is attached directly the azimuth wheel on the left). That cable along with the servo motor cable are now routed through the hole in the central pivot.

Also note that the long nut protruding from the end of the azimuth drive wheel is on the threaded end of the drive shaft and is used to adjust the tension on the slip clutch. The long nut has since been cut so it doesn't protrude past the edge of the aluminum ground ring.

This is the original layout of the azimuth drive which connected the Harmonic Drive reducer/servo motor assembly to the drive shaft via a belt and pulley. It worked fine this way but even with tremendous tension on the belt there was an appreciable amount of backlash, plus the belt tension probably wouldn't be good for the Harmonic Drive reducer over time. A direct mechanical connection as shown in the photo above made more sense, and indeed eliminates backlash.

With the belt removed, it's really the Harmonic Drive gear reducer that eliminates backlash. Dan helped me find two used units on eBay three years ago, and I was surprised how small they were - the body is 2 inches long, 1 5/8 inches in diameter, and the shafts on each end are 0.75 inch long for a total length of 3.5 inches. This photo shows the info printed on the body for reference.

The next two photos show both sides of the 3.3 inch diameter, 0.5 inch thick stainless steel azimuth drive wheel - the photo on the left shows the outward facing side with it's insert ball bearing (which is part of the slip clutch system) and the right hand photo shows the back side with three Teflon pads mounted on multi-wave springs. The Teflon pads/spring units are now attached with RTV adhesive inside the three recesses machined into the back side of the azimuth drive wheel, and provide tension adjustment for the slip clutch system.

The Teflon pads ride on Ebony Star laminate that's adhered to the face of the clutch plate, which insures the slip clutch moves without binding.

Machining the three circular cutouts was the only tricky part of the whole project and required the help of a professional machinist (thanks to Dan and his Sitech crew). Although there are other ways to provide tension, I like this approach because the cutouts hold the Teflon pads in place when under tension and there's no worry about them slipping. By the way, the three small holes drilled all the way through the drive wheel were needed to hold it on the milling machine while the circular cutouts were machined.

Very important - the rolling surface of the drive wheel is not flat, but has a slightly crowned shape. This insures it won't bind as it drives the telescope in azimuth.

Below is the wheel with azimuth encoder. I machined this 2.25 inch diameter stainless steel wheel and axle as one piece to make sure the encoder ticks were accurately keeping track of the scopes azimuth motion. The spring holds the encoder in place without binding. This wheel also has a slight crown on its rolling surface, and is just slightly larger in diameter than the encoder.

The bracket holding the wheel is Delrin plastic. The two blocks are stabilized with the aluminum plates on either end. The hole on top of the Delrin block closest to the encoder is for access to the set screw that connects the encoder shaft to the axle.

This is the azimuth idler wheel, also made of stainless steel and with a crowned rolling surface. It has the smallest diameter of the three azimuth wheels because it only needs to be mounted so its rolling surface is at the same height as the other two.

By the way, all three azimuth wheels ride on a 3/16 inch thick stainless steel ring that's attached to the bottom of the rocker - which isn't shown in any of the photos but here's the 2D design file.

Altitude Drive Photos
The next photo is a closeup of the altitude drive system. Like the azimuth drive it uses the same servo motor/Harmonic Drive reducer assembly, but the clutch system is different because it rides entirely on the altitude drive rod. There are several components that make this work:

1. The drive pulleys connected to the Harmonic gear reducer are mounted in their own bracket so they - not the reducer - bear all the tension from the two belts. I used two belts because the tension needs to be quite high to eliminate backlash, and spreading the load over two belts makes this a stout system. Plus, I had an extra belt and pulley laying around after taking them off the first azimuth drive configuration, so what the heck.
2. By the way, the ends of the shaft that the two small pulleys ride on are supported by ball bearings (set inside the Delrin brackets) so the shaft adds minimal friction to the altitude drive system.
3. The pulley mounted on the horizontal altitude drive shaft spins freely and has Teflon/Ebony Star pads attached on each end of the pulley. The wide, black shaft collar on the left of the pulley is adjustable - and a key to making this work. The part closest to the pulley is threaded on the part to it's left, which is locked tight on the altitude drive shaft. This sets the tension to the slip clutch, and it's a ready made part from McMaster-Carr 
(part number 9666T5, Adjustable Width Clamp On Shaft Collars).
4. Riding on the horizontal altitude drive shaft, but hidden between the adjustable width shaft collar and the left-side Teflon pad is a multi-wave spring, which provides the adjustable tension of the slip clutch.
5. Also ready made are the shaft connectors between the  servo motor and Harmonic Drive gear reducer. They're also from McMaster-Carr and are called Servo Motor Flexible Shaft Couplings. They're available for lots of shaft diameters and torsion ratings.
6. The mounting blocks for the servo motor and Harmonic Drive reducer are made of Delrin, as is the bracket for the the two small pulleys.

This photo shows the altitude drive within the context of the rest of the scope. For more photos of the scope showing how everything fits together go to Mel Bartels site about the 2016 Oregon Star Party Telescope Walkabout here
The power switch to the left of the drive assembly is sitting on top of the 12 volt battery that runs the entire drive system. 

For reference, below is the layout of both the altitude and azimuth drives that was used successfully at the 2016 Oregon Star Party.