Saturday, December 12, 2009

Drive System Upgrade

I've been researching and planning a new drive system for the R2. Right now, I've got some E150 Razor scooter motors in there, with 5" caster wheels. I used the chains and gears from the scooter. That system has gotten me around pretty well, but I think I pushed one of the motors too hard early on and its performance is pretty poor now. I can't make quick tight turns, especially with the left motor, and he won't spin in a small radius. Overall the motors just aren't that responsive and don't have a lot of torque. So it's a bit like driving an old worn out car around.

The motors of choice for builders in recent years have become the NPC 2212s. These are in the JAG drive systems, and they are what Mike Senna uses in his drive system (See the Senna files under Drive System in the Yahoo Groups page.)
The Surplus Center has some versions of these motors much cheaper. NPC wants $150, The Surplus Center has them for $50.

Several club members have investigated and here's the short story: the SC motors have a very short shaft on them, maybe 1/2". Apparently, however, it is possible to get this short shaft to work on a Senna style drive. That could save you another $200 overall. It is possible to use a press and some machine shop tools to make a new longer drive shaft, press out the old one, and put the new one in. But the consensus seems to be that this is difficult to do yourself without the right tools, and expensive if you get a shop to do it.

I bought a set of NPC motors. They are hard to get ahold of, and their website doesn't work. But if you call them persistently, they'll help. And they will give R2 Builders a discount to $135 if you mention the club, although this hasn't worked for everyone who tries. I got it and the motors look great.

I've seriously considered just building one of those two systems. Mike's has four wheel drive so that drive system is really rugged and doesn't run into any trouble getting across door thresholds or even going on grass. That's really appealing. I also found that if you get as many matching parts as you can from The Surplus Center you can save $200-$300 on the price that Mike projects in his Bill of Materials. I like the idea of building it up out of parts I collect or build more than buying a ready made system. Even if I had the cash to get the system already made, I wouldn't prefer that.

The only draw back that I can see, and this isn't much of one, for the 4 wheel system is that when you do tight turns, there's no differential gear to proportion the power to the different wheels, so one set of wheels will turn at a slightly wrong rate since they are tracking through a different radius circle, and they will chatter or slide laterally across the ground. I don't think it's a big problem, and as Victor Franco said to me it's a perfectly acceptable trade off for the killer drive system. I find that pretty compelling. And I may still go to a Senna type drive.

The JAG system has been very reliable, lots of builders use it, and it only has one wheel, so there's no chatter problem. And it has a belt drive. My experience with scooter motors is that the chains and sprockets are noisy. You can hear the chains spinning on the gear. So having a belt drive is pretty appealing too. I have spent a lot of time lately trying to learn about gear, sprockets, and timing pulleys and trying to decide which sort of system would be the best to build. Some of the good websites for parts for these things, besides The Surplus Center, are McMaster-Carr, and I have been searching through these sites and others looking for just the right timing gear pulley to do what I want, but I can't find them. A timing gear pulley will have grooves that accept a grooved drive belt. I'd like to use them on a single wheel drive system and get the best of both systems.

That search led me to the question of just what gears do I need to run for the R2. I don't know much about this aspect of engineering, so I've been trying to educate myself.

The NPC motors turn at 285 rpm at top speed. That's pretty fast. If you hooked those straight up to the wheels, it would be 4.2 mph. Here's how I figured that out, and I hope it's right. The standard wheel is 5" in diameter. Nothing much bigger will fit in the shells. So that times pi gives you 15.7" for the circumference. Turning at 285 rpms, means 4474 inches per minute. Multiplied by 60 gives you 268470 inches per hour. Divided by 12 inches per foot, equals 22372 feet per hour. And divide that by 5,280 feet in a mile, equals 4.2 mph. I figure a very fast walk is about 4 mph.

So that suggests that the speed needs to be geared down some. If there is a gear on the motor shaft and a gear on the wheel and a chain or belt connecting them, then the way to figure out the speed is a function of the ratio of the sizes of those gears. That is, if there is an 18 tooth gear on the motor and a 10 tooth gear on the wheel, then for every full revolution of the motor, the wheel will make 1.8 revolutions. This set up would speed up the wheel faster than the motor. What I need is to slow it down.

The JAG system has a 20/32 gearing. 20 on the motor and 32 on the wheel. So the wheel turns .625 of a full revolution for a turn of the motor. That makes 285 rpm into 178 rpm on the wheels at about 2.6 mph. The Senna drive runs at 18/11 for 2.6 mph, or 18/14 for 3.3 mph.

So I figure a speed somewhere in the range of 2.6 to 3.3 mph will be good. The problem for the timing gears and belts idea is that I can't find just the right size timing gears to give me that gearing. The limitation is the .5" axle. There's a .5" shaft on the NPC drive and I want to have one on my wheels too. But there's only a certain range of gears that are typically produced for a .5" bore. That range starts up high, so if I got them I'd have these huge honking gears on the wheel and motor.

And I think that as gear size goes up, the necessary torque to turn the gear increases. So stalls are more likely for the motor if you get close to its stall torque. But I'm no engineer. No doubt it would really help to talk to one at this point.

Long story short, I'd like to get the 30/22 or 18/14 gear ratio with gears and belts, but I can't find the stock parts for that. I could have some custom made up, but that would be a lot more money and time to wait. And I don't have enough confidence at this point to launch into that.

But there are cheap readily available chains and sprockets for these gear ratios at The Surplus Center. They have sprockets with keyed .5" shafts for $2 and $3 for example. That's a deal. I can afford to experiment a bit at those kinds of costs. They also have different size chains: #25, #35, #40, and so on. I think #35 will be suitable. That's cheap from them too.

So I think I can put a whole drive system including NPC motors together using chains and sprockets from SC for less than $500. I'll have an itemized list up here shortly. Compare that to $1000 or so for for the Senna or JAG systems.

I'm going to try to reuse the 5" caster wheels I have, and use the aluminum channel and sheet that I picked up here locally. The wheels were $13 a piece, and I have spent maybe $40 on aluminum. And then I'm going to buy parts from the Surplus Center and maybe McMaster Carr, and construct these myself. I'll document the build and give a parts list, prices and sources as I settle on final options.


Matt McCormick said...

Chris James has corrected me on something. The Surplus Center motors will work. Jason S in the group has done it. And the tolerances are no different. They are exactly the same motors with shorter shafts.


Rick said...

Super cool project! This was the first time for me on your site, what else have you done? I hope this isn't too many questions but could you help me figure out exactly what vibration isolators do? Thanks!