I replaced the failed Pittman dome motor last night, and thought I’d post some pictures of the failed gearbox.
The pen tip is pointing to the failed gear in the center of the picture. The teeth are almost stripped flat.
And this is the new motor opened up and how the gearbox should look. Again the gear in question is in the center of the shot. Notice the teeth!
I’m not confident that it will not fail again, and I may need to look at an alternative. I’m hoping the issue was my failed attempt to use batting tape to line the dome bearing to help with traction, but the gear may have been slowly failing with the constant harsh stop/starts on such a small gear. I know of at least one other builder that had a Pittman dome motor fail in the exact same spot.
I stripped my first set of gears in a motor this weekend. I was goofing around on the driveway showing Artoo to some neighbors across the street when suddenly the dome stopped working. I heard this really bad crunching noise then a very quiet motor spinning.
I quickly pulled him into the workshop and the dome off to see what was up.
A couple of months ago I had lined the inside track of my dome bearing with some batting tape to help the dome wheel grip. It had been slipping and the resulting noise was very annoying. The tape seemed to be the perfect solution …… until the gearbox failed.
I think what had happened was I’d left R2 sitting in the van all day and when I finally unloaded him the glue on the tape had heated up and when I started to spin the dome it balled up – jamming the wheel and breaking the gearbox.
Gerard had a similar failure trying to fix the squeaky slipping wheel. He’d over tightened the tension spring which stopped the dome the moment the motor stopped – but again this put extra pressure on the gears in the Pittman motor causing them to fail.
Last week I made a new aluminum dome drive mech to replace the Daren Murrer’s HDPE version I’d been using. There wasn’t much wrong with the one I had to start with, but I gotten it in my head I wanted to try making one myself from aluminum to go with my new dome motor.
The one I created is a combination of Daren’s and the original Atomic Pickle design design. Here’s a photo of my original HDPE mechanism on my aluminum frame.
I used some 1/4″ scrap aluminum that already had a few holes in it, so I had to work around them using the original bracket as the template.
I don’t have a mill at home so everything would have to be either cut on the band saw, Dremeled, drilled or worked by hand tools like a file.
I rough cut the piece on the band saw and drilled the initial holes
The long hole in the center was greater from three smaller holes I’d drilled and filed away to create one big hole.
One issue I knew I would have was making the center hole/cut-out large enough to accommodate the approximate 1″ wide lip/step on on the motor. Also note how the drive shaft is off center due to the gear box.
At first I thought I could file away enough material but it would be slow going. I could also have bought a $40 1″ drill bit but I had another idea. My Dremel has a router attachment that could create a lip on the long slot that I’d created.
I forgot to take photos of the next few steps, but the rest of the mechanism that bolts to the frame is pretty simple, and just two blocks with a few holes in them. I made the main support piece a litte wider than on the Atomic Pickle design as I wanted to add extra support to the main bracket that holds the motor.
Here it is installed in the frame. Note, in some of these photos the motor is flipped the wrong way and the shaft should be toward the center of the frame.
At the pivot point I drilled and installed a bearing like the original Atomic Pickle design, but on hindsight I probably could have just stuck with Daren’s idea of just having a regular hole with a bolt through it.
Here’s the motor position correctly in the bracket
Overall I’m happy with how it came out and given a template and basic tools anyone could make one at home.
Here’s a short video of the new mechanism in action
Back when I was thinking about stepping up my Pittman dome motor to 24V, and not wanting to have the extra complication of tying two batteries together to make 24V, I’d ordered some small Voltage Converters from ebay to experiment with.
I’m probably not going to install it for the new Dome motor as it’s working fine at 12V, but I do have a 24V supply in the dome for the rear logic lights. This will allow me to have a single 12V battery and save some weight. There is a loss of energy and battery life due to the conversion, but it’s supposed to be in the 90% range.
Update 5/1/08: Andy tipped me off that these board can also be found here.
Then new dome motor (a surplus Pittman GM9236) arrived today and I thought I’d share a short video to compare it to the original underpowered GM9413 motor I had been using.
It’s still runs at 12V, so no need to upgrade my electrical system to 24V. It spins a lot faster and definitely has a lot more torque.
Here’s the original GM9413 for comparison.
And finally, here’s a quick side by side shot of the two motors, the GM9236 is on the left. It’s a little bit longer and has a small shaft at the bottom which I think is to connect an encoding wheel to.
I’ve continued to do research and testing on my dome drive system, and it does look like I need to change the motor out for something with a bit more humph.
It seem that a few people are happy with driving the recommended GM9413 Pittman motor at 24V instead of 12V with good results, but I really want to avoid introducing the complexity of 24V into my system just for one motor. It will complicate charging the batteries at best and the shorten the system run time between charges, and at worse reduce the overall battery life unnecessarily.
In the last day or so I’ve got a lot of great advice from fellow builders, including a lot of links that helped me decide which new motor to go with. I wanted to stay with a Pittman if I could to hopefully avoid redesigning the dome drive system, and it looks like one of the variations of the Pittman GM9236 would work. It comes in a few different gearings and voltage ratings.
There was one specific 12V version I was shopping around for, the GM2936S018, which delivers about 3 times the torque, and about twice the RPMs of my current motor, but it was around $130, and I’d rather find something a little cheaper if I could. Then Bob pointed me at another GM9236 model that was very similar, but only $23 at a surplus store. They’re used/surplus but probably worth the risk so I ordered one to try. It has very similar specs delivering a lot more torque and hopefully went it’s installed, spin the dome a little faster 🙂
So here’s the links I used for my research and where to find motors
Pittman website – not very good if want to look up older models, and you need to register to download specs.
It does look like I may end up trying running the Pittman Dome Motor at 24 volts as a test going forward. I can’t see adding a gear at this point in time.
This leads me to my next problem, my battery sub-system is all 12 volts. I’ve had no need for 24 volts in the body as my main drive motors the NPC-2212’s can only run at 12 volts, and everything else is stepped down from 12 volts using Dan’s distribution board.
I know how easy it is to create 24 volts from two 12 volt batteries, which I have and can do with what I have on hand, but configuring in series and then tapping off for 12 volts adds a level of complexity for charging the batteries w/o removing or rewiring each time. There’s also the problem of one battery draining faster.
For those unfamiliar with creating 24 volt from 12 volt batteries, this is a really good site explaining how and some of the problems.
One of the problems he talks about is charging unbalanced batteries in parallel and something called a “Battery Equalizers / Balancers” which can fix this problem. The unbalance comes from the 12 volt tap on a 24 volt system, and the two batteries discharging at different rates.
I’m hoping someone has tried an equalizer in their droid before. I’ve found a lot of information on equalizers, mostly large clunky boxes for RV’s and buses, but there are smaller version out the like this one.
What I’m hoping is to use the 3 12 volt 7amp batteries I’m using right now in parallel, and then combine two of these in series for 24 volts. For charging have a switch/circuit to disconnect/isolate them from anything in the droid and reconfigure just in the parallel state to charge all 3 batteries together.
Tall order I know without causing a short circuit or fire 🙂
Tonight I’ve started to work on some dome stuff, including the dome drive system which has been bugging me for months.
I’m using Darren’s dome drive kit (based on Atomic Pickle’s) and it uses the Pittman GM9413-2 geared motor. Spec’d at 12v, 19.7:1, 142RPM.
To me it seems sluggish and rotates slowly compared to a lot of other droids I’ve seen, especially when I change direction. It could be the weight of the dome, which is around 17lb with all the accessories. Including all alu parts, like HP’s, surrounds, radar eye, dome ring, and periscope, plus the batteries, smoke/fire-extinguisher etc. etc. But I’m sure I’m not the only one with a heavy dome.
I’m wondering if others are experiences sluggish domes who are using this Pittman either from Darren’s kit or from AP’s? Especially if your dome is heavy?
According to the GM9413-2 data sheet on the AP site, the motor is rated at 12v, which is what I’ve been running it at, but on the motor itself the label says 12/24v.
I’d rather not run it as 24v if I can help it – but if it would help I could re-configure some of my batteries as a last resort.
The only other thing I can think of is the slight sagging in the mounting, but the motor is making full contact with the rockler bearing. Maybe it’s not doing a great job of holding on or maybe pushing too hard against the rockler bearing?
I know many Sennaites are using the Saturn motors at 24v for the dome, but I’d rather not switch class of motors right now. I’ve posted to the Yahoo! Group to see if anyone has some suggestions specific to the Pittman motors.
Edited to add some videos. Here’s the Pittman running at 12v directly off a battery, no speed controller so startup is instant and it looks okay right now which I don’t understand. I’m wondering if it’s a speed controller issue, or maybe even a problem when he’s angled in 3 legged mode. mmm.
And here it is running at 24v – super fast I know, but I’d rather the top speed be fast and I can slow it down with the transmitter/speed controller commands.
Now to troubleshoot why the 12v spin seems okay.
I did some further testing tonight, and held the body at an angle while the dome spun. Man, is there a lot of force whipping around while it turns, even at the slower 12v. I can feel it struggle on the up swing as the two heavier side of the dome comes around. 24v helped a lot but I need to go back and put the speed controller back in the loop to confirm my test and see how snappy it is looking left and right.