rov thruster seals

[galvic]

Hi there, I am to try to build a ROV for basic underwater missions (measurement, some deployment and collecting of samples) and am puzzled about the means to seal the thruster shaft. I will be using some small electric motors operated at 7-14 V, and want to know does anyone knows how to make seals on the motor shaft. I am considering lathing two or three grooves on a shaft (6-9 mm dia.) and placing o-ringhs into them. Is this type of sealing adequate for hydrostatis pressures up to 20 bar (150-200 m of depth), considering the rotation of the shaft (maybe 3-5000 rpm). Please help me anyone, thanks in advance, Niksa Glavic, Dubrovnik, Croatia.

[Geof]

O-rings almost certainly not. O-rings are not the best for rotating seals at any pressure.

Lip seals, like the oil seals on vehicle axles are used for rotating seals but they are limited intheir pressure capacity.

Face seals can handle reasonable pressures at reasonable rpm but I think the limit is somewhere around 300 psi at 1000 rpm. I have made face seals that operate in this region. With face seals there is a relatrionship between speed and pressure and if the pressure is lower the speed can b e higher.

But when you get to higher rpm and pressures than this you need to start looking at labyrinth seals. These can handle almost unlimited rpm, in other words it is not the labyrinth seal that imposes a speed limit; similarly, they can handle very high pressures. But labyrinth seals are designed to leak a tiny bit and have a means of catching and recycling the leakage; this is probably not practical in your application.

It is my understanding that the approach taken in ROVs is to have oil-filled motors with face seals between the oil in the motor and the external water environment. Then there is a mechanism that pressurizes the oil in the motor to a little bit below, or above, the external pressure. Whether it is below or above depends on what direction the seal is designed to work, in or out. If it is in and the seal fails you fill your motor with salt water; if it is out and the seal fauils you start losing oil. Whichever way it is the idea is to keep some pressure gradient across the seal so that it will seal, but keep the pressure gradient low enough that the seal can operate at the required rpm.

There is another entirely different approach: Do not have a rotating shaft passing through a seal; use a magnetic coupling between the inside and outside with a magnetically transparent but high electrical resistance housing separating the magnets, this can be stainless steel.

[digitalrust]

Contact www.johncrane.com they can solve just about any shaft seal problem you throw at them.