If you go out to buy a waterproof phone, radio, or any other item, you're going to run into two kinds of salesmen. One will say, "sure it's waterproof". The other will say something like "It's rated to IPX67", or "it can be submerged in a puddle or a few feet of water for half an hour". Avoid the first salesman.
Industrially, people are very reluctant to say waterproof because they want a definition to test their products to. For example, consider the term waterproof camera. It means something different to someone going out on a boat and wants to make sure their phone's camera will work if it gets hit by water drops than it does to someone who wants to go SCUBA diving in 60 feet of water.
The manufacturing world has largely gone over to IP codes: the International Protection Marking, IEC standard 60529, sometimes interpreted as Ingress Protection Marking. On a product, the codes are denoted by the letters IP and two digits. The first denotes being sealed against solids or particles, and the second denotes being sealed against liquids. The first digit can run from 0 to 6 and literally runs from entry of something larger than a hand down to dust particles:
The second digit can run from 0 to 9 with a special suffix:
Level sized Effective against Description 0 — No protection against contact and ingress of objects 1 >50 mm Any large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part 2 >12.5 mm Fingers or similar objects 3 >2.5 mm Tools, thick wires, etc. 4 >1 mm Most wires, slender screws, large ants etc. 5 Dust protected Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment. 6 Dust tight No ingress of dust; complete protection against contact (dust tight). A vacuum must be applied. Test duration of up to 8 hours based on air flow.
As you can see it's rather specific in what it says the product is protected against, and there's an additional column that I omitted which talks about how these things are tested!
Level Protection against 0 None — 1 Dripping water Dripping water (vertically falling drops) shall have no harmful effect on the specimen when mounted in an upright position onto a turntable and rotated at 1 RPM. 2 Dripping water when tilted at 15° Vertically dripping water shall have no harmful effect when the enclosure is tilted at an angle of 15° from its normal position. A total of four positions are tested within two axes. 3 Spraying water 4 Splashing of water 5 Water jets Water projected by a nozzle (6.3 mm) against enclosure from any direction shall have no harmful effects. 6 Powerful water jets Water projected in powerful jets (12.5 mm nozzle) against the enclosure from any direction shall have no harmful effects. 6K Powerful water jets with increased pressure Water projected in powerful jets (6.3 mm nozzle) against the enclosure from any direction, under elevated pressure, shall have no harmful effects. Found in DIN 40050, and not IEC 60529. 7 Immersion, up to 1 m depth Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time (up to 1 m of submersion). 8 Immersion, 1 m or more depth The equipment is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer. However, with certain types of equipment, it can mean that water can enter but only in such a manner that it produces no harmful effects. The test depth and/or duration is expected to be greater than the requirements for IPx7, and other environmental effects may be added, such as temperature cycling before immersion. 9K Powerful high temperature water jets
As I alluded to above, many VHF radios are rated to IP67, sometimes called IPX7. There's no rating for dust because it's assumed that anything sealed against water is going to be sealed against dust and particles as well. I've seen ads for smartphones that imply a certain level of waterproofness, the commercial showed them being washed in soap and water, but no IP ratings on them.
A practical application of this subject that's important to me right now is waterproof stepper motors. Mine aren't. In my last couple of years at work, I got involved with evaluation and testing of stepper motors (the radar antennas were moved by stepper motors) which were rated to be extremely resistant to water spray and corrosion. For this to work, the motors had to designed to meet tough specifications from the start. It wasn't possible to take a standard part and just dip it some sort of coating. The motors were rated to MIL-STD-202G, a salt spray test with varying test times. Their motors looked better at 10 days than the competition at 2 days and survived all of our tests without missing a step.
To complicate things, there are other standards, like the older NEMA (National Electronics Manufacturing Association) ratings. NEMA 6 looks like it would be a worthwhile level to try for, if a NEMA rating is all you have.
I designed moving systems using steppers for using in spraying water ultrasonic inspection systems. Nothing worked for sealing them. The slightest bit of water (or even high humidity – you wouldn't have any of that in Florida, now, would you? ;-) rusts the armature and bridges the small gap to the stator, thus locking them up.
ReplyDeleteThe solution we found was to enclose the stepper in a box, run the shaft out through an ordinary seal, and put the box under a slight amount of positive air pressure, like from an aquarium air pump.
Works like a charm.
Stepper motor manufacturers don't make them small just to make them small, the gap between the rotor and the coils sets the torque and affects overall performance. There's a strong incentive to make that gap small. It means they have to make the rotor out of corrosion resistant metal, or give it a surface treatment when they make the motor.
DeleteGood solution. When I see the number of stepper motors guys use that just have shields over them, I figure they can handle mist and small droplets of coolant, but I'm sure they can't handle being immersed.
The problems on an aircraft weather radar are rough because of no air seal. Imagine an aircraft cruising at 30 or 35,0000 feet. It's really cold up there, like -40C. When it begins its descent to land it gets into warm, moist air before the metal has warmed, and the condensation runs off it in streams. Worst part is that air gets into the motor and condensation occurs inside the motor, too.
Yep, the smaller the gap, the better the magnetic transfer. What I don't understand, since this is such a common problem, is why all such motors aren't already built with seals on the shafts and an air tube connector. Of course you'd have to use well-filtered air, or dry nitrogen, but dang it would be handy. You could completely submerge the things without trouble.
DeleteIn the meantime we are stuck with building small, form-fitting boxes.
BTW, I did an experiment where I stuck simple CPU coolers (available anywhere) onto one side only of my steppers (425 oz-in NEMA23). Lo and behold, without fans they kept the motors within ten degrees F of ambient, working all day. Highly recommended.
What I don't understand, since this is such a common problem, is why all such motors aren't already built with seals on the shafts and an air tube connector.
DeleteGot to be cost. People putting Steppers in printers, ATM machines, router tables or many of the bazillion other places they're used, don't care about them being water tight.
Still ought to be an option.
I noticed on that company's website (Moons motors) that they had 400 step motors as a standard product. My NEMA23 motors are 570 in-oz and their biggest was 330, but I'm using microstepping and that reduces torque. Maybe with a 400 step motor, microstepping stops being a big thing.
I'm not sure about cooling. It might be useful for them to get a bit warmer than ambient. If the motors get spray on them, it would help the spray evaporate faster.
When I said "within 10F" I really meant "about 10F", and I think that's a good temperature. It's enough to vastly decrease the internal case humidity, but keeps you well away from magnet degradation. I used some pretty hefty coolers, and I'm not using water nearby in this application. You can easy tailor the temperature to what you like by choosing the size of the cooler.
DeleteAs far as any cost increase is concerned, it would be ridiculously minimal. The cost of a couple of seals and a brass fitting, plus minimal machining. You could easily sell two versions, where a waterproof one would be perhaps double the cost (though the actual change in cost would probably be less than 2%).
What the heck, I should take one apart and see how difficult it would be to modify it. Perhaps I'm sniffing a side business, here ;-)
I understood you meant about 10F above ambient. No problems. My speculation that it's all cost is just that - speculation.
DeleteI would caution that once you take a stepper motor apart, it will never be a stepper motor again. Not that I've tried to use one after I took it apart, but I've heard that long enough that I don't want to do the experiment. You might contact a stepper motor manufacturer. There might be an option somewhere like that.
The corrosion resistant motors I tested from Moons motors did very well with condensation, melting ice, dripping water, and other non-pressurized tests. It looks like they apply those coatings to anything in their product line, if you order it.
I appreciate your admonishment about taking steppers apart ;-), but I've done it a dozen times. They go back together just fine. The end caps have a recess which fits into the body, and the shaft centers are kept very closely to tolerance.
DeleteIn the end, it actually is easier to just build a box that fits the motor closely. In general, now that I'm retired (it can't be said more clearly: HOORAY!!!) I don't have a lot of need to worry about water, so this is just an academic discussion (patents notwithstanding -- I probably shouldn't be discussing this).
I never tried to put one back together and measure torque. I've taken a few apart, though.
DeleteAfter reading the post, and the above comment, I understand now that the steppers that drive marble cutting CNC equipment and the steppers that drive CNC water jet machines are working in an environment that makes the average Navy machinery space seem desert dry.
ReplyDelete