Hopefully there is intelligent life here.

[PNachtwey]

Unfortunately there isn't an option for a poll.
1. How many of you think pressure is resistance to flow?
2. How many of you think flow makes it go?
Do you have an equation to prove either of the two statements?

 

[IceStationZebra]

Me thinks this is a set-up....

1. Obviously pressure can be caused by resistance to flow, but it can also be caused by trying to compress a trapped fluid. So is this taking "resistance to flow" to the extreme?
2. Flow makes what go? Work is a combination of pressure and flow.

ISZ

 

[PNachtwey]

Yes ISZ, this is a setup. You get brownie point for recognizing this.

I/my company, make hydraulic motion controllers. I/we know all about the math and physics of hydraulics. The problem we have is that ignorance isn't bliss. It is just plain ignorance and some how these problem become our problems.

So does any body believe in the the two statement?
So does any body have a formula to justify either?

Be careful and think about your answer.

 

[desertfox]

Hi PNachtwey

Well I'll say no with the exception that ISZ stated, firstly pressure is an analogous to voltage in an electrical circuit and secondly flow is analogous to current in the same sense, therefore likening it to Ohms Law
the resistance would be (pressure/flow). Again with ISZ I don't understand the "flow makes it go" perhaps now you might explain further.
No I have no formula to prove your above statements either.

regards

desertfox

 

[hydromech]

Peter...

I too struggle to understand the phrase "flow makes it go".

I feel that your request for an equation to quantify the pressure to flow ratio is a little simplistic.

I can give you the pressure loss coefficient, if you tell me the density of the fluid.

There is of course Bernoulli's equation.

I have a copy of the Handbook of Hydraulic Resistance, tell me what it is you want to know and I will see what I can do.

I spent a good 16 years in the the hydraulics "business". I could no longer stand the ignorance of the people who run it. The advent of electronic control was somewhat of a renaissance. However, the uptake of proportional control seemed to be very slow in the UK. That is with the exception of mobile hydraulics,which tends to be the complete opposite to industrial hydraulics.

I dont wish to labour the point, but there is saying that goes..."Power is nothing without control".

I have seen a few exceptionally good industrial hydraulic machines, but most people just seem happy to continue using big clunking hydraulic machinery as long as it keeps on bashing metal.

Adrian

 

[zdas04]

These are pretty easy terms, but the loaded questions are simply stupid.

[li]Pressure is an indication of the potential energy at a specific point within a fluid.[/li]

[li]Fluids will flow from a high pressure to a low pressure.[/li]

[li]The rate of fluid flow is a function of both the magnitude of the pressure differential over a measured distance and the total resistance to flow over the measured distance.[/li]

To say that "Pressure is resistance to flow" is nonsense. As everyone above has said, the second question/statement makes no sense.

David

 

[OTHGWayne]

After giving this a little thought I must admit that your statement "pressure is resistance to flow" is actually true from a certain perspective. In a situation were a downstream valve can be used to constrain the flow path causing a pressure build up that restricts flow.

That said, "without pressure there is no flow" is also a true statement. Flow is defined by the pressure differential between two points. Without differential pressure, there simply is no flow.

So pressure is both the cause and the restriction to flow. Definable by Q = Cv(Dp/Sg)^0.5

As for "flow makes it go" seems to speak to the concept that flow can do work which is clearly true. However, the relationship between flow and horsepower (work) is entirely dependent on the efficiency of the mechanism doing the conversion.

 

[PNachtwey]

Good there is intelligent life here.
I have heard those two sayings for years and they drive me nuts.

1. Pressure is just force / area.
2. Force makes things go. See Newton's laws of motion flow or oil wasn't mentioned.

 

[monkeydog]

It's going to be a long week.

 

[budt]

hydromech wrote;
"I spent a good 16 years in the the hydraulics "business". I could no longer stand the ignorance of the people who run it. The advent of electronic control was somewhat of a renaissance. However, the uptake of proportional control seemed to be very slow in the UK. That is with the exception of mobile hydraulics,which tends to be the complete opposite to industrial hydraulics.

I dont wish to labour the point, but there is saying that goes..."Power is nothing without control".

I have seen a few exceptionally good industrial hydraulic machines, but most people just seem happy to continue using big clunking hydraulic machinery as long as it keeps on bashing metal."

Where were you when all the post's were up about how I thouht there was a need for TRAINED/DEDICATED Fluid Power Engineers and Maintenance persons on the Fluid Power Forums???????

Sure could have used such great input.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

I am the one that got Peter started on this subject.

In all my hydraulic classes and in my Fluid Power traning books

http://www.hydraulicspneumatics.com/200/eBooks/

I point out to the students that "Pumps don't make Pressure , only Flow" and that "Pressure is Resistance to Flow."

It is my simplified way to get across the fact that a Pressure Gauge on a Hydraulic Circuit that is showing no pressure does not mean the pump is bad and conversely if the Pressure Gauge is showing full system prssure that the pump is good.

No prssure may only mean the Pump Flow is all returining to tank with little resistance and a Flow Meter in that circuit could be showing 10 GPM Flow from a 10 GPM pump.

OTH a 2,000 PSI pressure reading could be obtained from a pump producing 25% flow if there was enough resistance but the system would be operating slower than designed and a Flow Meter would show that reduced flow.

Unfortuanately few Hydraulic Circuits have a Flow Meter installed so many pumps are changed unnecessarily when a Pressure gaauge shows less than the schematics set pressure and no adjusting of the pressure controls will bring pressure back to normal.

At least that is the way I learned Hydraulics in the College of hard Knocks and the information has served me well.

This subject has come up seveal times on other Forums over the past 10 years but still has not been resolved. It actually started with my asking, Should There be TRAINED/DEDICATED Fluid Power Engineers and Maintenance persons.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

On "Flow Makes it Go" Peter has never answered my question on how fast a cylinder is moving when a Pressure Gauge at the port that is receiving oil that is moving the cylinder is reading 1,243 PSI and how much faster will it move if pressure suddenly jumps to 3,131 PSI.

That would seem an easy one to answer if "Force Makes it GO" and if you knew the Area of the cylinder.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[PNachtwey]

Anybody? I have answered this before but Budt doesn't like the my answer.

 

[hydromech]

I like to think of it like this...

A pump is simply a mechanical device for transfering energy from an electric motor and imparting the energy into the fluid.

In line with the the basics of Bernoulli's theory, there is a relationship between the velocity and the pressure of the fluid.

Let the the fluid move at high speed and the pressure will be low. Think of a pressure washer. High pressure fluid allowed to escape to low pressure will convert its stored energy into kinetic energy.

In a hydraulic system, the energy in the oil is contained by the pipework and the oils "energy" will carry the oil in the direction of the least pressure.

Thinking of cylinder moving with no load, the oil will obviously move towards the cylinder because that is where the pressure is lowest. When the cylinder reaches a resistant load its speed and therefore the speed of the oil inside will slow. The drop in speed will cause the pressure to rise and the pressure load across the area of the piston will create a force.

The ability of fluid to evenly distribute pressure means that when the oil is static the pressure is evenly distributed through the system.

Heat, vibration and noise all steal energy from the fluid and the net result at the cylinder is less power.

Flow makes the oil go because flow is kinetic energy. Resist the kinetic energy and flow will turn into pressure.

It is energy that make it go, energy in the form of pressure or flow.

All the of above is ignoring the changes caused by viscosity and reynolds numbers and laminar and turbulent flow and HOSES that seem to make a mockery of the basic laws of physics!

There...thats what I think.

 

[OTHGWayne]

Answer? Answer what? All I see is a seemingly never ending FLOW of trick questions, catch phrases and exercises in semantics. Quite franky I'm way too busy for this crap.

And for the record, I've been in hydraulics, pneumatics and flow control since 1983. And while I may not agree with everything I've seen over the past 25 years, I am still in awe at the sheer volume of advancement in the past 150 years of fluid power that has made this country the manufacturing giant it has always been and has put true meaning in the words "Git R Done".

 

[hydromech]

Wayne...

You are not that busy if you can post a reply..

What country are you in?

I think the 80/20 law can be aplied to your statement about 150 years of advancement. 80% of the advancement was done in the first 20 years. The remaining 20% was completed during the following 130 years.

As I said in a previous post, it is really the advent of electro-hydraulic control that really pushed the boundries of hydraulic applications.

Adrian

 

[PNachtwey]

I was hoping that some one would point out that Budt's question doesn't make any sense because there isn't enough information.

Things do not accelerate instantly to a steady speed. Budt wants to know the speed when? 1 micro second after increasing the pressure?

Pressure can not be changed instantly from 1234 to 3131 PSI.

Hydromech, I thought you were doing well until

It is energy that make it go, energy in the form of pressure or flow.

Newton didn't mention any of those terms in his three laws of motion. Hydromech, how does one calculate an acceleration from pressure or energy?

Force makes it go. F=m*a or a=F/m. Then you integrate acceleration to get velocity and integrate velocity to get position. However, the acceleration will change from instant to instant. Budt, didn't supply an area to push against so we don't know the force. He didn't supply a mass so there was no way to calculate an acceleration.

Also, Budt supplied no info about the pressure supply. Even if the first instant the pressure is 3131 PSI it will not be the next because as soon as the load moved the supply pressure would drop. No information was supplied as to how this pressure would drop as a function of flow so we have no idea what the delta p across the valve supplying the valve is.

Answer? Answer what?

Yes, it is a poor question.

All I see is a seemingly never ending FLOW of trick questions, catch phrases and exercises in semantics.

There isn't anything tricky about them.
The first two statements in the first post are 'jingles' that hear hydraulic people repeat over and over without and basis in math and physics.
Budt's question is incomplete. It is typical of many questions I see on forums are the person asking the question doesn't know what information is required to arrive at the answer.

Quite franky I'm way too busy for this crap.

Yes, I agree but as long as people are building crappy hydraulics I am afraid it is necessary.

No one has provided an equation for calculating the gain of the hydraulic servo actuator system that calculates the the steady state speed. If you can't do that then how do you know the system will go as fast as required?
There is nothing tricky about that.

Here is a hint. An object will accelerate until the sum of forces acting on it 0. When an object has a 0 net force acting on it will be at a constant speed.

 

[budt]

Peter answered this post:
Quite franky I'm way too busy for this crap.

With:
Yes, I agree but as long as people are building crappy hydraulics I am afraid it is necessary.

Peter, As long as the UNTRAINED are designing Hydraulic circuits the situation will not change. Just imagine where the Mechanical and Electrical field would be if they had been handled in a similar manner to how the Fluid Power field has been and is.

Just a thought.



Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[PNachtwey]

Consider this training.
The home work is to find the formula to compute the steady state velocity when the valve is open. In other words find a general solution to your question. You have seen posts where I have posted the solution.

 

[MikeHalloran]

A sudden jump in pressure at the cylinder's fill port suggests that the cylinder has stopped moving because it's hit a hard stop, the fill flow has gone to zero, and so has the pressure difference across the fill plumbing.

Such a large jump in pressure suggests that, instead, the cylinder has not necessarily hit a hard stop, but that a closed center directional valve with essentially zero internal leakage has stopped it, the stored strain energy in the cylinder has caused the cylinder to act as an intensifier, and we have been looking at the rod end, where the pressure has now risen considerably above the pump pressure, in proportion to the cylinder's area ratio.



Mike Halloran
Pembroke Pines, FL, USA