[PSUBS-MAILIST] Minn-Kota Shaft Seals

[Alan via Personal_Submersibles]

Why do you need to do anything with the seal?
If they are both orientated so that they are keeping water out,
then if you air or oil compensate them they will fail from the inside out first.
First the inner seal will fail & air or oil will move in to the cavity between the
seals. When the submarine ascends the overpressure in the cavity will then
cause the outer seal to fail first. 
If you oil compensated, then if there was an air gap between the seals it 
would become filled with oil on the first dive that went beyond the seals
rating. Then you would have equal pressure in the cavity between, & no oil
leaking out on ascent.
The seals are designed so that additional pressure from the fluid they are
trying to seal, forces them down on the shaft. If theoretically the seal fails
at 15 psi (1 atmosphere) on the fluid (seawater side) then perhaps they
would fail at 7 psi on the reverse side. This means that when you dive a
set up with ambient compensation it would fail from the inside at 14ft depth
& either equalise the gap between the seals or at least increase the pressure
differential between them. In a system that was over pressurised by 4 psi
it would fail the same but earlier at 6ft.
These seals need lubrication on the seal face & I don't see how the inner seal
gets that. I would go with oil compensation for that purpose.
   With the current double seal set up at least with an oil filled motor you
have some redundancy to stop oil dripping out when in storage.
Alan




Sent from my iPad

> On 12/04/2017, at 2:27 AM, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> Greg, the MK RIPTIDE 101 doesn't have that "seal with shield".  It simply uses two identical seals.  My units came with the "weedless wedge" prop and I wonder if that's why they don't have the shield on the outer seal since the wedge is suppose to prevent things getting stuck around it.  The brass bearing is also different on your unit, oval whereas it is a flange bearing on the Riptide 101.
> 
> Jon
> 
> 
> On Tuesday, April 11, 2017 9:15 AM, james cottrell via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> Notice in this drawing
>  http://cdn.shopify.com/s/files/1/0191/7628/files/2002_AC_-_freshwater_motors_-_transom_-_specialtytransom_-_classic_28.pdf?793
> 
> That the outer seal is labeled "seal with shield".  I have not called Minn-Kota but assume that the purpose of this "shield" is to protect against fishing line wrapped around the shaft, ruining the seal and letting water in.
> 
> 
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