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Thread: Pneumatic motor drive?
03-29-2006, 12:23 PM #1
So we need a motor y'see, to drive a relatively low-speed (300-400 RPM or so) light load, more or less continuously for six-to-eight hour periods. It's a long story, but picture a rotisserie roasting a chicken at the supermarket.
We can't use motors with brushes (fire hazard, this is in the food industry), but there is plenty of compressed air available.
So I was thinking, that one of those low-speed pneumatic drills does around 400 RPM. Maybe I could use one of them. Clean, no brushes, no electricity either. Ought to be enough torque. Maybe put a bleeder valve on there to tune the RPMs... Okay.
Question is, is there any reason I shouldn't do this? Like the sticker on the side that says, OIL DAILY-- well, what if I don't?
There will be a certain radial load on there, by the way-- bag the drill chuck and replace with small belt pulley. Belt tension will be amount of radial load. No idea what sort of bearings those things have.
03-29-2006, 12:27 PM #2
03-29-2006, 12:33 PM #3
What do those run efficiency wise, like 5%?
You can put an inline oiler near your drill and feed it foodsafe oil. Who knows what the longevity on it would be. But why don't you just get an explosion proof motor?
03-29-2006, 12:37 PM #4
Indeed, Jon-- why not? (mostly because I just hadn't thought of it, but also because this is only the experimental prototype stage)
And David, yes- an encapsulated electric motor would be the next step. We might run into problems generated by an electric field though. This seemed like a good enough place to jump in, that's all- Easy way first. Efficiency-wise, I thought they were a lot better?
Okay, those rotary actuators are for less than one full rev. The motors are a little fast... Why are those motors so darn expensive? As compared to pneumatic tools, which are dirt cheap? (yeah, I know-- pay dirt, you get manure)
Will look more at those though. Thanks, both.
03-29-2006, 12:39 PM #5
Air motors must have a drip lubricator upstream in the airline. If you don’t the motor will be toast in short order. You probably could use some food grade mineral oil to lubricate it.
03-29-2006, 12:42 PM #6
If gearing is an issue, maybe an air-powered ratchet rather than a drill...
03-29-2006, 12:53 PM #7
Look at his second link. Those speeds look more suited to your application.
Build yourself a Mu metal faraday cage for the magnetic field. It's only money.
Since it's only a proof of concept, i'd go with the off-the shelf air tool. You can take the load off the motor bearings easy enough. You'll want to install a good filter/coalescer upstream to remove any water/oil from the airline then inject your lubricant of choice.
03-29-2006, 12:58 PM #8
if it's just a proto
to check a process
just rub goldburg the thing with
a couple old pullys
an air flow valve
and a fan blade
03-29-2006, 01:40 PM #9
Work the numbers. If this gadget has to run ofr any length of rime the cost of aair becomes significant. An air motor is about 30% efficient so take it from there.
If your load is small (like a rotissery) and you gear down an air tool sufficiently you may find that your air consumption over a year might pay for an explosion proof (misnomer is ever there was one) and a gas tight electrical feed back to the motor control.
The speed of an air motor is difficult to control. You can throttle them but the speed regulation is lousy if your load changes much over time.
If weight and portability was not an issue, I'd take another look at elctric motors. Three phase motors produce no sparks and if properly protected and sensed they can be shut down before any spark producing casualties occur. An explosion proof, sealed three phase motor with a temperature switch on the stator, run from a VFD is as fire safe as any sir motor and is reliably controllable to near zero RPM. Further, there are a large number of speed reducer options and high tech motion control options available and they interface easily with PLC's and process controllers.
I'd use an air motor only if weight was a major factor, you already had a large source of compressed air, and speed regulation and controllability was not a significant factor
03-29-2006, 02:12 PM #10
Hmm, yeah... Words well worthy of thought, Forrest. As it happens, yes-- weight is an issue for the time being. (thing fits midway along an unsupported overhead capsule feeder tube. We could of course construct a solid support structure, but really would rather not. Yet.) Also, yes-- this device will in future, final stage run for long time indeed. Months. Years, maybe.
Wippin' boy, that looks a like a great song... can sorta picture you strummin' it out on the gi-tar...
David, there used to be a giant roll of mu-metal in the shop. Used it for all kinds of handy fixin's. Shims, earplugs to keep the alien invader brainwaves out...
Railfan, was looking at those ratchets too. Plenty torque. How do they work; is there an extended shaft in there with bevel gears down at the end or what?
03-29-2006, 03:53 PM #11
03-29-2006, 04:04 PM #12
Hydraulic motor with vegetable oil.
03-29-2006, 04:12 PM #13
The noise of those things is oppressive over time, and as stated, waste gazillions of CFMs.
Doesn't a TEFC motor meet the non-brushed criteria? Of course there is TENV (non-ventilated) at higher cost.
You might need to brew up a long driveshaft to get the motor to "somewhere else".
03-30-2006, 12:13 AM #14
Hydraulic motor running on vitamin e oil- the exhaust goes straight into the capsules and then sold to the cosmetics industry. Ok, I'll settle for .5% of the take. [img]smile.gif[/img]
Food industry standards- what kind of belt are you having to use? What about direct drive using a stepper motor and controller- or a linear induction motor. Guess maybe not too appropriate for a prototype. The linear induction motor might greatly simplify the drive if it's suitable.
03-30-2006, 12:42 AM #15
Why not use a stepper motor - no brushes. Or for that matter - a three phase motor. Also no brushes and no starting switch.
03-30-2006, 10:28 AM #16
OK- roll back the clock for an idea which might work well. Use a small steam engine run on air. It was done regularly for running mixers and process equipment in explosion proof applications. For the oil free aspect, use some Teflon for the surfaces of the slide valve and for the piston rings. Or, use a piston valve engine and put teflon band sont he piston valve as well as the actual piston.
Munitions plants and chemical processes which had a great danger of explosions used smaller steam engines for all sorts of applications instead of electric motors. These were fully enclosed engines like Troy-Engberg built. They were built insizes from as small as 3" bore on up.
A steam engine, run on air, will run at very low speeds. You could get castings for a larger model engine with sealed ball bearings on the mains and crank. If you use Teflon or one of the self-lubricating bronzes (Lubrite comes to mind) for the crosshead shoes, piston and valve, the thing ought to sit there and run for at least a year.
There are a number of castings kits out there for building engines with fairly good sized bores and strokes. Other advantages: It's a heck of a nice shop project so can justify the machine shop and the machinists in it, it's classic, and certainly going to be quieter than a screaming air motor.
Years ago, an older engineer told me that he had known the founder of the Autocar truck company. The founder of Autocar truck had become quite wealthy and had a prototype machine shop behind his house. Some other person in th his circle of acquaintences was doing some sort of research in a chem lab. Whatever he was researching needed constant mixing and was explosive. The Autocar guy made up a small steam engine to run on air and drive the lab mixer.
Learn from history on this job- build a "steam engine" with modern materials.
03-30-2006, 10:45 AM #17
I am all with Mr. Michaels on this one. Build a small steam engine just because you can.
03-30-2006, 04:31 PM #18
03-30-2006, 04:42 PM #19
What power/torque do you need?
Can you do the job with a remote motor & flexible shaft drive?
You could use a modern induction motor with a sealed case & no brushes?
More details on exact requirements of application without actual application would help,