[PSUBS-MAILIST] MK 2 Brushless Thruster

Fri Mar 25 00:52:51 EDT 2016

Alan,

1.       The inside seal will still see full pressure and lip seals have PV ratings (pressure times Velocity)  They invented mechanical seals for that reason.

2.       The bearings in the motor are generally only for radial load not axial load.  You will need a thrust bearing probably.

3.       The solid nature of the end caps is going to make it too heavy possibly and you could drill a series of holes or other method of hollowing out.

4.       Those motors are reliant on a huge amount of air passing for cooling them so if possible ensure you have better contact area of the motor to the housing 

Hugh

From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org] On Behalf Of Alan James via Personal_Submersibles
Sent: Friday, 25 March 2016 7:34 a.m.
To: Personal Submersibles General Discussion
Subject: Re: [PSUBS-MAILIST] MK 2 Brushless Thruster

Hank,

thanks for the advice.

I thought that welding would be quicker & cheaper rather than cutting  an o-ring groove

& drilling & taping & recessing for 6 bolt holes. Also  a more smoother hydrodynamic

finish. I will be welding on the fins for the kort nozzle attachment so can do it all in

one shot.

    I will probably add a fill port in to the top of the seal compartment to aid air

removal. Once there is oil in there the only way you would get air in would be

running it out of water with some sort of reverse pumping action caused by the seal.

   The double seal arrangement I am copying  from that military document on

sealing & compensation. However they use face seals. This thruster is so small

that 2 face seals would take up too much space, so have doubled up on 2 low

pressure lip seals. In the pdf it makes a good case for the secondary seal & I

intend to do most of my diving in seawater where it is a more destructive environment.

All the Minnkotas I have seen have 2 seals, I think just lip seals.

Cheers Alan

  _____  

From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.org>
To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
Sent: Friday, March 25, 2016 1:31 AM
Subject: Re: [PSUBS-MAILIST] MK 2 Brushless Thruster

Hi Alan,

It wouldn't hurt to clamp the end cap tight but you do have a nice shoulder for the end cap to sit on.  Any time I make a part that can and usually does warp, I make it oversize and machine it true after welding.  I am curious why you want to weld it-that big shoulder has room for an o-ring.   I would also ditch the second seal-it is an air trap.    Great project!

Hank

On Thursday, March 24, 2016 12:27 AM, Alan James via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

This is my latest brushless thruster design for scrutiny.

The last thruster I made was 2 piece with one seal.

I machined the last version out of aluminum rod, which was expensive & difficult.

The end pieces in this design are from solid rod but the mid section is from a stock tube

that I intend to have welded to the propeller end. The back end is bolted on.

Am I likely to get distortion when welding the two sections together? There is a sleeve

press fitting to help give it some rigidity for welding. Should I make a jig to clamp it in

for welding?

The thruster is for 500ft depth & is oil filled with a slight over pressure above ambient. 

Oil filled mainly for cooling purposes, but also as a secondary defense against any leaks,

& for lubrication of bearings & seals. I am using low pressure lip seals & have a double 

seal redundant arrangement. The outboard seal is orientated to keep the oil in & with the

internal overpressure should keep the water out. The outboard seal should fail first as it is 

exposed to abrasives, especially in sea water. The inboard seal is oriented so that it keeps

any water out that may get past the outboard seal. 

The tube coming out of the seal cavity is for pressurizing it & for collecting any water that

may get through & come out of suspension. I am thinking of putting a water sensor in there.

Any comments welcome.

Alan

p.s. if image isn't big enough keep hitting ctrl & + TO ENLARGE.

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