Hydraulic Analogy

[horsefeather]

I'm creating a syllabus for a course I have to teach in June to hydraulics technicians regarding troubleshooting electro-hydraulic systems. It includes reading schematics, multimeter operation, electro-hydraulic devices and simple DC electrical theory. When teaching basic DC electricity one always starts with Ohm's Law - E = I X R. (Voltage equals Current times Resistance).

The analogy often used is the electromotive force (E) is analagous to pressure and current (I) is analagous to flow. There does not appear to be a analagous representation to resistance however. The closest thing to Ohm's law would be Poiseuille's law which while easy to understand, is not for the feint of heart in application.

I got to wondering why there is no identifiable flow resistance equivalent. After mulling over for a couple hours it struck me - there is no need for one. Fluid flow is dynamic with many variables, electricity is less so. Even more important from a technician standpoint, resistance is quantifiable, relatively static and easily measured.

Anyone with a couple hours of training can master a multimeter and make dynamic measurements of voltage and current or a static measurement of resistance (simple DC circuits) In fluid however, pressure and flow are easily measured, resistance to flow however requires dynamically powering up the cicuit making flow or pressure measurements and deriving the flow resistance - more trouble than it's worth. (Technically multimeter resistance measurements are performed the same way only the power is supplied by a battery and the current involved is minute).

Still it seems to me there should be a 'resistance to flow' value somewhere in the fluid power engineering.

Any comments?

 

[budt]

DeltaP?

There is lots of written information o Pressure Drop in Pipe, Tubing and Hose to size lines for minimum pressure drop in a circuit.

You can see a formula on this web site:

http://www.wainbee.com/prod_pneumatics/formulas.asp

I use charts from the Womack Data book available here:

https://training.womack-educational...vc?Screen=CTGY&Store_Code=WE&Category_Code=TA

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[horsefeather]

I would think deltaP would be equivalent to force differential in volts. What would be the fluid power equivalent to electrical resistance as expressed in ohms?

Incidentally, I have a copy of the Womack book at work. Worth its weight in gold.

rrv

 

[budt]

I am not electrical, tough I had high hopes at one time of mastering te field. It turned out there was so much Fluid Power work that I never found the time to take more than one class at a tech school. The other deerrent was the fact that 90% of the places I was asked to trouble shoot or design circuits already had an electricioan and often even an EE who did'nt need any help.

From what little I understand Voltage is equal to Pressure (PSI), Amperage is equal to Flow (GPM) and Ohms is equal to Pressure Drop or line and valve energy losses. (DeltaP).

To design a circuits Pressure and Flow requirement you need to know how much Force or Torque is required and what the Cycle Time will be to figure actuator size and pump flow. Then the Valves and Flow Lines can be sized that have a reasonable Pressure Drop so the least amount of energy is wasted to heat.

Normally, Pressure is arbitrarily decided by the circuit user if they have a spec or the circuit designer in many situations. Then the circuit designer can size the actuators, valves and flow lines to give the force or torque desired and keep pressure losses low.

BTW, since you will be teaching Fluid Power as part of your class, take a look at a book I use to teach Fluid Power at ww.hydraulicspneumatics.com and look at the ebooks. The Symbols in Chapter 4 are shown with a typical cutaway of the component which I find is helpful to persons who don't spend a lot of time working on hydraulics'


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[PNachtwey]

I would think deltaP would be equivalent to force differential in volts. What would be the fluid power equivalent to electrical resistance as expressed in ohms?

Q=Kv*sqrt(delta P)
or
P = (Q/Kv)^2
hydraulics and electric are similar but except for the square root of the flow or squared flow.
In electronics/electrical fields their is a concept of conductance which is the inverse of resistance. Conductance is measured in mhos. notice that that is ohm backwards. I am not kidding.

http://www.thefreedictionary.com/mhos

That is how hydraulic works. Hydraulics works in terms of conductance. Hydraulic valves are rated in terms of conductance or flow per unit of pressure drop.

 

[budt]

Is'nt this a good forum Peter????? Have you checked it all out?



Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[horsefeather]

I suppose that if fluid power engineering has got along this long without a unit of measurement for resistance to flow unit, it's not necessary. It was quite a revelation when Ohm published his paper and he was ridiculed for the idea.

Each discipline has different ways of looking at things that the entire system is built on. Early on I stumbled onto (not participated) a 'pumps make flow not pressure" debate that became very heated and very nasty. If you accept the correct view as a starting point everything else make sense.

I checked out the

http://www.hydraulicspneumatics.com

url - lots of good stuff, particularly in the ebooks and the online magazine. In the ebooks, I was particularly interested in the symbols chapter (another thread/forum) and in the magazine the "Fluid power electronic controls certification on its way" article caught my eye. The same thing I'm bringing my guys up on.

The national Fluid Power Association also has an elementary fluid power lesson I bought that I intend to use with animations for some of the components.

 

[budt]

horsefeather;

Speaking of "Ohm's" have a look at this site for what one company has done with flow resistance. They call it Lohms Law and give information on it. It just came back up in my old memory banks when Peter mentioned "mhos"

http://www.theleeco.com/LEEWEB2.NSF...60032b31476c28918525689b00654b37!OpenDocument

If that does'nt open on the right page go to the home page and look for Lohms Law.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[horsefeather]

Googling 'Lohm' indicates it didn't catch on, probably for the reasons I expressed in my first post - too many variables. I like the idea though.

Personally, I prefer the 'Fohm' unit of measurement. It represent the ratio of the head on a pint of beer to the liquid contents and is used to determine the number of glasses necessary to relax self-preservation inhibitions.

 

[PNachtwey]

Googling 'Lohm' indicates it didn't catch on, probably for the reasons I expressed in my first post - too many variables. I like the idea though.

I think it is the square root that makes the units messy. Should fohms be expressed as Q*F=sqrt(delta p) or Q^2*F=delta P. I also tend to work in units of cubic inches per second ( cuin/s )instead of GPM like most people.

In the first case the basic unit for fohms would be:

f=sqrt(delta p psi)/Q

Now how does one work with the square root of pounds force?

The Q^2*F=delta p is a little cleaner

Q has units of in^3/s
delta p has units of lbf/in^2 which results in lbf*s^2/in^8. That doesn't give me a warm and fuzzy feeling. Even if I substitute lbf=lbm*g or lbm*386.4in/s^2 the the result is still messy
F=386.4 lbm/in^7. This is still messy. I think is will back up and just leave it at psi*s^2/in^6.

Try teaching that to a maintenance person. It is no wonder the units for lohms or fohms never caught on.

Personally, I prefer the 'Fohm' unit of measurement.

I like the name too. Fluid ohms makes more sense than liquid ohms.

It represent the ratio of the head on a pint of beer to the liquid contents and is used to determine the number of glasses necessary to relax self-preservation inhibitions.

A maintenance person will make an effort to understand that.