NEMA 17 Stepper Motor Enclosure

[SolarMechE]

I work on a tracking solar concentrator that requires a motor to operate on a rooftop exposed to the elements for 15 years. I suspect that the only way for this to be possible is to seal the motor within an enclosure that mostly protects it from the environment.

Our current enclosure design is just a simple plastic box with an o-ring face seal between the cover and base of the box. Is that type of simple seal adequate?

The more complicated seal will occur on the motor shaft. Our motor is a NEMA 17 stepper that outputs around 100mNm of torque. It runs fairly slowly, around 50 RPM, but only for a few seconds at a time, once every minute or so (just enough to keep up with the sun). Cost is also a huge constraint for us. Anything over about $0.25 when purchased in quantities of several hundred thousand would most likely be out of the question. Are there any seals that meet these requirements?

The IP rating of the box is not something we're really specifying. All we care about is sealing it well enough to keep the motor running for 15 years. We've been under the assumption that there is no chance of sealing well enough to prevent moisture from entering the box, so condensation is something we are quite concerned about. We've been thinking about various solutions involving desiccant, but do not have a lot of knowledge of how to best use them.

Any thoughts or ideas on how to best implement this motor enclosure would be greatly appreciated.

 

[IRstuff]

Please do not double post:
thread237-156198

TTFN

 

[sreid]

There is some good news for you. Building a totally sealed box is almost impossible. If it leaks at all, in a high humidity environment, water will condense in the box when the temperature cycles (day and night). It is better to have a simple enclosure that tends to keep rain and dust out. Four small holes in the bottom four courners will allow pressure/water vapor equalization and allow any condensed water to drain..

Don't try to seal the motor completly for the same reason. A sealed bearing for the motor shaft should be used. If you can apply partial current to the stepper when stopped, this will keep the motor warmer than the environment and will tend to keep the interior of the motor dry.

To seal the motor shaft through the enclosure, take a look at V-Ring Seals (Chicago Rawhide). These can be put into a groove on the shaft and form a face seal to the enclosure. Simple and cheap. Also low drag.

 

[SolarMechE]

Sreid,

Thank you for the info. If it's impossible to seal an enclosure like this, is there any point in sealing the shaft at all? I've used V-ring seals on products in the past and not been particularly impressed with them, so I'm wondering if there's any point in using them if you've got drain holes in the bottom of your box anyways.

Also, applying current to the stepper is a great idea and one we've thought about, but unfortunately when the purpose of your product is to produce power, keeping the coils energized is not something we can do. We also plan on having the unit be self-powered, so when there's no sun out overnight, we don't have any power to use at all.

Would a shielded bearing on the motor have the same benefits as the sealed bearing you mentioned? Our current motor has a shielded bearing and I'm not sure what the cost hit would be to change to a motor with a sealed bearing.

Again, thanks for your help.

 

[sreid]

I guess that my thinking is that the clearance between the shaft and the enclosure side is a place for direct impingement of rain and airborn dust. A V-Ring would stop this and add little additional cost.

I think that keeping liquid water and dust off the shaft is important as the long term failure mode of the stepper is likely to be water and dust getting into the front bearing. This is the same reason I suggested a shaft bearing with a seal. Shields on bearings are primarilly to keep the grease in.

The very small vent holes on the botom could be covered with a filter material (felt?) to stop the direct entry of dust and water. The important thing is to let the partial pressure of water vapor equalise.

 

[SolarMechE]

Thanks again sreid. I agree with your thoughts on sealing the shaft. A v-ring may be too expensive for us. We might end up having to use an o-ring and taking a hit on torque output due to friction losses.

For covering the vent holes, we actually stumbled onto an interesting product made by Gore for this exact purpose. They make small patches that allow water vapor through but not dust. If you're curious, you can check it out at

http://www.gore.com/en_xx/products/venting/technical/problems/pressure_equalization.html.

 

[lgoochy]

SolatMEchE

Check out

http://www.labtecta.com

Looks like a similar product to Inpro, however it may solve your problem