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Hi all,
I came across an article on oil compensating motors
on a ROV web site.
This involved a pressurized bellows system. I can't
remember this system
being mentioned on the Psubs site, so have posted
the extract below.
Have also added the link to the ROV site as
their is a lot of similarly good
information & links.
Alan
Sealing and pressure
compensating a DC brushless motor from a common appliance.
First ensure that the motor casing is watertight. You may need to replace the end plates to achieve this, or put the whole motor in an oil filled housing. The shaft will need to be sealed with lip seals. A lip seal will prevent oil leaking from the housing, but I would also suggest a double lip seal arrangement to help prevent water ingress when the shaft is rotating at high speeds. So 'back to back' the lip seals with one seal sealing against water ingress (Compensated or not this has been known to happen) and one seal sealing against oil egress. In other words both flat faces of the seals should be face to face. Install a bleed point on the top of the motor housing and a drain/fill point on the bottom. Then, using a length of low pressure tube, connect the housing to a pressure compensator (a simple rubber bladder will do) even a bladder from a football or similar. Use anything that has a reasonable wall thickness and can expand when pressurized. There are commercial spring loaded compensators available but, price wise, they may be out of reach of your budget. Fill the housing, bladder and tube with oil, through the lowest port ensuring that you have no air in the system. Open the port at the highest point to allow air to escape. Once the system is air free, close the bleed port (highest port) and over pressure the bladder with oil by about 14 psi (1 bar). The cheapest way to fill your sytem is with an adapted garden spray bottle of the type that you pump air into to pressurise. Fit an LP guage to the fill line, and mark it at 1 Bar as your maximum pressure. The way the compensation works is this.... because of the presence of the compensator, when the motor is submerged.... no matter what depth it is at, the internal pressure will always be about 1 Bar (14.7psi) over ambient pressure (i.e. The external pressure exerted by the water at any given depth). Should a leak occur, the 1 Bar (14.7psi) over ambient internal pressure helps ensure that water will not enter the motor housing and burn the motor out. This pressure will slowly reduce as oil is lost from the system. A camera with the ability to view the 'pressure compensator' is a good idea. Oil leaks normally occur as a result of worn shaft seals. If you test the unit before diving you should spot any leaks ahead of time. You can hook up more than one motor to the bladder (pressure compensator) and get away with using one compensator for all your motors by connecting them to a common manifold. Technically, with this system, depth is not an issue as far as the motors being protected and able to operate is concerned. Dan Fjellroth's website is a good resource for home builds. Go to http://www.ROV.net then ROV FAQ and look for 'Build your own ROV'. |