Gear Pump Slip Calculation 1

[Dfirestine]

I am in the process of designing a gear pump for a specific application. The one area that I am having a hard time with is calculating the Slip of the pump.

Any suggestions on how to calculate Slip?

David

 

[curtis26]

Hmmm? We see slip in real world operation, but I have never found any equations for calculating theoretical slip. The only equations that I have ever seen for slip where empirical equations based on pump testing for a specific pump model.

Slip is the loss of capacity, expressed as a fraction or percentage of displacement, due to back-flow caused by delayed closing and leaks through the drive side of the pump (it does not however include fluid compressibility or leaks through the discharge of the pump)

IT is primarily due to the space created by the pump clearances. Some factors that increase slip are the presence of suspended solids in the pumping fluid and the discharge pressure of the application. One factors that can decrease slip is fluid viscosity.

Good luck. If you find a good way to model theoretical slip I will be interested in learning how this is done too.


The following is available on-line at Available on-line,

http://www.pumpworld.com/slip_and_efficiency_volumetric_e.htm

Slip & Efficiency
A major effect on positive displacement pump performance is the loss in flow due to slip. The expanding cavity on the inlet side of a positive displacement pump creates a low pressure area that asks to be filled with fluid. This cavity can be filled with fluid from the inlet line in normal performance. However, if slip occurs, the cavity will also be partly filled with fluid flowing back through the pump clearances from the outlet side.

Pump performance is dependent on the slip which occurs in a pump.

a) Slip Increases Directly with Pressure
b) Slip Increases Directly with Clearance
c) Slip Increases Inversely with Viscosity

 

[maytag]

Slip also will vary with RPM and vary greatly with different manufacturers. Gear pumps used for hydraulic systems are generally rated in the 80-85% eff. range @ a specified RPM and fluid @ a certain viscosity however Voith touts a gear pump at 95% eff. under similar conditions.

 

[electricpete]

I'm having a hard time getting a fix on which slip you are talking about.

If you have an induction motor, it has slip against sync speed.

If you have belt drive, there can be slip.

In some gear configurations, the speed of driven gears differs from driving gears by fixed ratio something like a gear ratio. For example GErotor shown at the link below the outer idler gear turns 4 revolutions for every 5 revolutions of the inner gear. I wouldn't call it slip... but I'm not sure what it is that you're calling slip.

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[electricpete]

Sorry, I didn't read Curtis' definition. I've never heard of the term slip used that way.

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[Dfirestine]

Thanks for all your input. You helped verify the conclusion I had come to regarding Slip Calculations.



David