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Nothing is Impossible
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Join Date: Mar 2011
Location: Sydney . Australia
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Oil Film Thickness in Hydraulic Motor Port Plates

07/02/2014 10:31 PM

Can anyone advise what the typical oil film thickness is likely to be in a Hydraulic motor , where the following conditions exist.

It uses 68 weight hydraulic oil

It has a rotary port plate which is preloaded with springs to around 50 N of force , plus also additional preload from hydraulic pistons that increase port plate mating pressure proportionally to the working pressure in the motor circuit.

What i am trying to work out is typical oil film thickness between the steel port plates ( near mirror polish finish )

This is a photo of a worn port plate below. the oil galleries are around 12 mm diameter and those oval slots are approx 20 mm long.

who can give ball park figures on oil film characteristics ?

The amount of oil flow between the plates is related to the oil pressure through the system as the ports cycle between open and closed ( blocked ) , as the port plate rotates and closes , the closed part of the wear face is exposed to oil at system pressure

I have found that if the port plates are ground to the correct polished finish and have the correct preload between them , the motor will have very low case drain leakage rates ( <8.5 litres/min @ 350 bar ) , versus port plates with a machined surface which have shown test data of 50 - 60 litres/min case leakage.

Do we have any fluid dynamics people or hydraulic motor experts here to offer opinions about design characteristics in this area ?

thankyou
:)
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lyn
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#1

Re: oil film thickness in hydraulic motor port plates

07/02/2014 10:50 PM

50 N over how much area?

Is oil uncontaminated?

Etc. Etc. Etc.................................

What does the motor manufacturer, bearing manufacturer and oil manufacturer say? They have the fluid dynamics people and/or hydraulic motor experts on staff.
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Nothing is Impossible
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#2
In reply to #1

Re: oil film thickness in hydraulic motor port plates

07/02/2014 11:03 PM

The Oil is filtered to 10 micron

The port plate surface area is 114 square cm in total , less the port areas = 67 sq cm

The manufacturer think they are god on this earth and are not interested in answering emails .

I have found that the manufacturer claims performance and efficiency specifications for these units which are not matched by real world testing , however by tightening up internal tolerances i have obtained extended life by a multiple of up to 600 % , and decreased internal case leakage by 45 - 75 % lower than the minimum figures claimed by the manufacturer.
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lyn
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#3
In reply to #2

Re: oil film thickness in hydraulic motor port plates

07/02/2014 11:36 PM

You are to be commended for your efforts.

Manufacturers always test and specify their products under ideal, laboratory conditions. This is normal and to be expected. As an OEM, I did that, too.

But, we worked with our customers to solve their problems.

Good luck.
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Nothing is Impossible
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#4

Re: oil film thickness in hydraulic motor port plates

07/03/2014 2:35 AM

Thankyou :)
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roy hammy
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#5

Re: Oil Film Thickness in Hydraulic Motor Port Plates

07/04/2014 8:18 AM

One molecule thickness of oil any thing less leads to negative conditions.

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nick name
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#6

Re: Oil Film Thickness in Hydraulic Motor Port Plates

07/04/2014 11:07 AM

The film thickness is a result from a balance between all forces actuating on the barrel.

Reduction of leaks can be dangerous since the film is sheared (sliding of barrel on mirror) and thus heated. This leads to a loss of viscosity and of portance. Te thinner the film the higher the shear since the shear stress is proportional to viscosity AND velocity gradient. Thinner films lead to higher shear stresses and higher heat generation.

The barrel frontal area works as a hydrostatic bearing and the leak depemds on the distance between the 2 sliding surfaces.

For such pressures it is recommended to filter down to 1 ...2 micrometers. The distance is usually in the range of 5 to 10 micrometers which leads to the request for planeity and small roughness.

Wear between windows can be a follow up of cavitation due to a fast decompression of the oil in the cylinder.

Sorry to answer so late but I am out of house and have not access to the net every day.
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