The Drive Assembly and Engagement Mechanism

The drive for my platform consists of a stepper motor coupled to a 100:1 planetary gearbox which turns a urethane roller in contact with the bearing surface of the circular segment. Pressure between the roller and the bearing surface is maintained by a spring under the base on which the motor is mounted.

The drive can be disengaged by depressing the base to compress the spring, allowing the platform to free-roll for reset. When the platform is not in use, a knob depresses the base to hold it in the disengaged position, preventing flat spots from developing on the roller.

Motor (rear), gearbox, and drive roller, mounted to spring-loaded base

The roller came from McMaster-Carr. It's described as a Tight-Tolerance Drive Roller. It comes in two hardnesses. I got the softer (blue) one but I think the harder one (orange) would also work. It was a bit pricey ($31) but it's so perfect for this application that I think it's worth it. The small roller (10mm radius) coupled with the large radius of the segment (mine is 446mm) provides a mechanical advantage of 44.6:1. 

The motor is a small NEMA-11 stepper motor, model 42HD4027-01. The gearbox is model EGS11-G100. I got both from StepperOnline. I chose this motor because I wanted to run the platform from a 5v USB phone charger and this motor only needs 4.6v to achieve its rated current. The downside is that small, low voltage steppers do not have much torque. This one has a rated torque of only 0.1nM, which is further reduced by the lower-than-optimal voltage and microstepping. But then we apply the mechanical advantage: 44.6 from the roller-platform interface and 100 from the gearbox, giving a whopping 4460:1 for the total system. So the theoretical maximum torque applied to the platform's axis is 446nM, or 330 ft-lbs. Even if the real-world torque is a fraction of that, it's more than enough.

OK, so we have enough torque from the motor, but will the spring provide enough pressure to keep the roller from slipping? In fact, it's turned out to be excellent. The roller is fairly soft and has very good grip on the bearing surface, especially with the polyurethane finish on the wood. With the weight of the telescope on the platform and the roller engaged by the spring, it's very difficult to slide the platform by hand.

So the grippy roller and the mechanical advantage combine to produce a very robust drive system that provide more than enough torque to rotate the platform, even if there is significant off-axis mass.

Links to the motor, gearbox, roller, and mount are on the 'Bill of Materials' tab of the design spreadsheet.

With the design discussion out of the way, let's turn to the construction. 

The first step is to assemble the motor and gearbox onto the motor mount. The mount is a NEMA-11 mount from Amazon. I had to shorten the motor shaft to fit into the gearbox; I did it with a Dremel and cut-off wheel. Mount the roller onto the gearbox shaft by tightening the set screws. I used a Sharpie to put a dot on the roller; otherwise it's hard to tell if it's moving!

The next step is to cut the motor base. I used a piece of 12mm Baltic birch (the same material as the platform). I used cheap 1/2" hinges from the hardware store. Make the length so that the base fits comfortably between the roller blocks. Mount the hinges on the end of the base, then screw them to the platform base. The base should now articulate vertically toward the bearing surface of the circular segment.

Place the motor assembly with its mount onto the motor base. The goal is to position the motor so that with the base in its lowered position, the roller is disengaged from the bearing surface, leaving a gap of maybe 1mm or so. The motor does not need to be centered under the circular segment. If it's too high, move it to one side or the other. If it's too low, center it and shim it up using washers under the mount. I was lucky: with the motor centered and no shims I had barely enough gap.

Base lowered all the way; motor in position; gap barely visible

With the position established, screw the motor mount to the motor base.

Now it's time to mount the engagement spring and disengagement knob. The spring I used came from a cheap spring assortment that I got from somewhere years ago. It's 3/8" in diameter, about an inch long, and fairly stout (I did not measure it but I'd estimate 5-10 lbs to compress it.) The knob is a M8 through-hole star knob that screws onto a M8 bolt mounted into the platform base. 

Place the spring and the knob on the motor base and decide where they go. Mark the corner of the motor base on the platform base. Measure the position of the spring and knob relative to the corner and mark those spots on the platform base underneath. Drill a hole partway through the platform base for the spring so that part of the spring extends above the surface. Insert the spring. With the spring inserted, the spring should lift the motor base so the roller makes firm contact with the bearing surface.

Place the knob and spring and mark their positions

Drill the hole and insert the spring. It's hard to tell from the photo but most of the spring is in the hole below the surface.

To mount the knob shaft, start by making a mortise for the T-nut. (If there is room for the T-nut under the motor base and still have the roller disengaged you can skip this step. On my platform, the motor base had to go all the way down in contact with the platform base to have any gap at the roller, so the T-nut needed to be recessed like this.) Then drill a 10 mm hole concentrically and mount the T-nut. Put a washer on the bolt and thread it up through the T-nut.

With the bolt inserted, make an oversized, oval-shaped hole in the motor base such that the base can be lowered over the shaft without interference. Mark the extent of the hole from below. I used a Forstner bit slightly larger than the bolt and made two overlapping holes to form the oval.

Re-assemble everything and screw the knob onto the bolt with a washer underneath it. You should be able to screw the knob down to disengage the roller, and unscrew to re-engage it. You can also disengage the roller momentarily by pushing the base down by hand to compress the spring.

Complete drive assembly, with engagement knob threaded onto shaft and spring underneath

I did a lot of head-scratching over the design of the drive and engagement mechanism. Overall I'm really happy with how it turned out.

That's it for the drive assembly. A subsequent post will cover the electronics that drive the motor.