Speed of a hydraulic cylinder.

[GLK]

Gee guys I feel like I am back to teaching basic hydraulics again?
Let me try again.

Definitions:

Energy is a fundamental quantity that every physical system possesses; it allows us to predict how much work the system could be made to do, or how much heat it can exchange.

In physics, power (symbol: P) is the amount of work W done per unit of time t. This can be modeled as an energy flow, equivalent to the rate of change of the energy in a system, or the time rate of doing work, as defined by
P = (dxE)/(dxt)
where
P is power
E is energy or work W
t is time
The units of power are therefore energy divided by time.

Rsdoran quotes

Pump creates a pressure differential to produce flow. Answer:   Yes correct.

With flow you add resistance and you create pressure. Answer: Also correct.

FLOW just moves the oil, it does not transfer energy. Answer:  Not correct.  The fluid is the conductor of energy and does transfer energy from point to point.
 
Pressure transfers energy using fluid as the medium. Answer:  You have it backwards, fluid transfers energy converted from the pump, and it is called pressure.

Pressure is what creates force which creates motion. Answer:  Correct, but not complete.  Pressure acting against an area or surface will produce motion, if that area or surface can be moved with the force pushing on it.

Peter quote - So how does at pump convert power to hydraulic power with the pressure at 0?

Answer:  With the pressure at 0, the pump only creates flow with the potential to create power if and when the fluid flow meets a restriction.

How it really works:  A pump converts rotational force into fluid motion (flow) it really is pushing on one end of a column of fluid.  This fluid travels down its pathway until it meets a resistance to this flow.  So we have a pump pushing on one end of the column of fluid and another restriction pushing back against the other end of the column of fluid.  Because the fluid is near non-compressible it will see a pressure rise, which is actually the energy itself being transferred into the column of fluid.  This pressure can and will produce a force that acts upon the restriction, (actually it acts upon all surfaces regardless of direction).

If the is force is great enough to overcome the surface it is pushing against, that surface will move and you have motion.

So break it down, pumps create a fluid flow, fluids transfer energy if and when they meet restrictions, (we call this pressure), and this pressure acting upon a surface can overcome with force this surface to make it move causing motion.

So the pressure produces motion, however you must have flow to continue this motion.  Hydraulic fluids (and most fluids) are lazy, unless there are two forces acting upon this fluid, pump pushing at one end, restriction pushing backwards at the other end, the fluid will not see or transfer any energy in the form of pressure.

If this will not turn your light bulbs on, nothing will.

Best,
GLK

[rsdoran]

Gee guys I feel like I am back to teaching basic hydraulics again?

Thats the reasoning behind this site and forum. Its meant to be a place where people can learn from those that "KNOW" and not just from books.

Many of those here "now" will never go beyond the basic of fluidpower because it isnt their "field".

I am hoping having a fluidpower section will offer more information and opportunities to learn to those that only know the basics.

I will participate in discussions like this one because they are meant to be discussed/debated for the learning process. I will not try to tell someone how to design, build, or what components to use in a fluidpower system, that is beyond my capabilities.

I am hoping that more of the fluidpower people will come here to have discussions in the future.

[Steve Bailey]

If nothing else, I hope that the discussion has demonstrated that "Flow makes it go" and "Pressure is resistance to flow" and "Pumps produce flow, not pressure" are useful rules of thumb, but they are not all you need to know about hydraulic systems.

[GLK]

Well Steve, you are partly correct in your statement.

What I wrote regarding flow and pressure is not a rule of thumb, but can and is confirmed with science and physics if proper applied.  So I do not want anyone to think what was written was just a rule of thumb.

A rule of thumb is a generalized statement that may or may not be true, depending on how it is used.  My statements are true and correct regardless how you apply them.

People sometimes incorrectly believe that by using physics terms and units that any statement containing them must be true and factual, which is not always the case.  And any statement absent of these terms must be a rule of thumb or incorrect, which is not always the case.

Your statement of there is more to hydraulics is correct.  Many people under estimate the knowledge required designing hydraulic systems, and I could raise the level many times higher than what is being discussed here.  There is not much point in doing that because there are not enough people with the higher knowledge to keep it going that would make any sense.

Peter for the most part is correct in many of his statements, and his work does require a knowledge level a little higher than what is discussed here.  I think it is out of frustration that he wishes more people would engage at his level regarding servo valves, however there are few who can engage him.

Look how long this thread is just trying to figure out flow and pressure, which is basic, level one.  This forum can provide basic information to most that read this thread, but raise the level too high and people want to participate no matter what and then some really dumb stuff starts to appear in the posts.

There are just too many people that work around and use hydraulics that honestly believes that using hydraulics is the same thing as understanding hydraulics.  Well that is my two cents worth and all I have.  More work stacking up so I need to get back to it, I will let you guys debate on and will read the posts when I have more time.
Best,
GLK

[Peter Nachtwey]

Actually you did, on this page is one of the times you stated a pump produces pressure.
Overall this has been a very informative thread that has enhanced my understanding of fluidpower systems. Thank you.

I don't see 'pumps' and 'produces' as the same thing   Perhaps, I should just use the word 'adds' in stead of 'produce'.  Just to make things clear.  A pump pumps oil but it also adds energy to the oil.  Oil flows from a higher pressure to a lower pressure. If the oil does not flow then the pump will increase the pressure of the fluid.  This raises the energy of the fluid.

It is quiz time,  but on a different thread.   This one has been used for too many topics.  GLK should find this interesting too.

[Alaric]

Thanks for posting this Peter.

Now I underestand something that I have been seeing on a press with 21" pistons.  I plugged the numbers into the spreadsheet and it agrees very well with what we actually see.

Thanks again.