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Considering the situation where:

  1. A non-waterproof motor is used inside a waterproof box
  2. Something must be rotated by that motor in a wet environment outside the box

What is the correct way to transmit the rotational power from the motor to the exterior environment?

Notes :

  • Rotation speed 1-10 rpm
  • Exterior environment has constant low power water spray
  • Low cost is a priority

NB : Apologies for using incorrect engineering language. I do not have an engineering background. I invite anyone to correct my words as appropriate.

Thank you Elliot

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    $\begingroup$ Ships & submarines have been using such a system for over a century. Look for water tight bearings. $\endgroup$ – Fred May 15 '17 at 9:34
  • $\begingroup$ Not only nautical vessels, but laundry machines and ICE (internal combustion engines). The latter keeps water out AND oil in. $\endgroup$ – Carl Witthoft May 15 '17 at 14:38
  • $\begingroup$ en.wikipedia.org/wiki/Stuffing_box $\endgroup$ – agentp May 16 '17 at 14:10
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A lot of bearings are sealed anyway, at least to some extent, to keep the grease in and water and grit etc out. Equally the inner and outer shells are typically a press or adhesive bonded fit to the shaft/hosing so tend not to pose any inherent difficulties for sealing.

Obviously the greater the pressure difference between the inside and outside the greater the demands on the seal but this is more a question of degree.

There also exist rotary seals which can seal a rotating shaft to a stationary housing, they are commonly used on things like the dive shafts of FWD cars where they exit the gearbox and fit in a similar way to a bearing.

For just waster spray rather than something submerged a standard sealed bearing may well be fine

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You could also consider a magnetic coupling if the box material is suitable.

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  • $\begingroup$ Depending on the amount of torque you need to transfer, magnets are a good way to go. This is used often in chemistry where a magnetic or steel rod is placed inside a decanter and spun with a motor under a plate via a magnetic flywheel. You could make your own by securing high power rare earth magnets (cheap on amazon) to a disk inside your box and to what you need to spin. The closer the magnets the more torque you can transfer. Keep in mind, your motor will need cooling inside the box, monitor the temp if you can to prevent overheating. $\endgroup$ – Gwydionforge May 15 '17 at 13:39
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For low speeds like what you have, you can use a flexible diaphram between a pair of rotating rollers. A bit like how a diaphragm pump works. Here's one called a "ball-diaphragm coupling"

https://www.google.com/patents/US3157054

It also describes using a crank instead of balls.

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