Hydrostatic quirk or total nonsense?

[KWentling]

I've too have often wondered why folks make that counter intuitive statement. It would be difficult to have pressure without a pump.
I realize that there also has to be resistance of some kind to create pressure.

I think technically it is a true statement as pressure is a reaction to a load/resistance. If it wasn't connected to anything it would not develop pressure. It's kind of a useless statement but I have read that very thing before.

 

[rScotty]

I've too have often wondered why folks make that counter intuitive statement. It would be difficult to have pressure without a pump.
I realize that there also has to be resistance of some kind to create pressure.

Well..... it is possible to have pressure without a pump. It just takes gravity.

But I agree that theoretically all pumps create flow, not pressure - the pressure is created by the resistance to that flow. But in the real non-theoretical world the inside surface of even the smoothest pipe or fitting has a slight resistance to flow... and so in reality there is always some pressure and some loss of flow just because the fluid is moving.

There was a discussion about pumps and RPM and lifting things by increasing RPM - and for that to make sense it's back to theory again. Any 100% positive displacement pumps - like a hypodermic syringe - don't create any more pressure when they go faster, just a greater flow rate......same flow in a shorter time.
But our tractors don't really have positive displacement type pumps for their hydraulic systems. The type pumps tractor have as hydraulic pumps have some internal clearances and those clearances get larger as it ages. So at low speeds even a new tractor hydraulic pump will lose some efficiency as the fluid slips past those clearances.

When we raise the RPM to speed the pump up, what is called a "fluid wedge" is created inside the pump surfaces and makes the pump more efficient. In an old pump with lots of wear it really becomes noticeable that the pump works better with more viscous oil and higher RPM.
So yes, it's not imaginary. Sometimes full throttle really does help to lift a load.

rScotty

 

[JWR]

There are different types of pumps. Positive displacement pump or a centrifugal type pump. They create pressure in different ways.

And they all create pressure...

 

[npalen]

Well..... it is possible to have pressure without a pump. It just takes gravity.

But I agree that theoretically all pumps create flow, not pressure - the pressure is created by the resistance to that flow. But in the real non-theoretical world the inside surface of even the smoothest pipe or fitting has a slight resistance to flow... and so in reality there is always some pressure and some loss of flow just because the fluid is moving.

There was a discussion about pumps and RPM and lifting things by increasing RPM - and for that to make sense it's back to theory again. Any 100% positive displacement pumps - like a hypodermic syringe - don't create any more pressure when they go faster, just a greater flow rate......same flow in a shorter time.
But our tractors don't really have positive displacement type pumps for their hydraulic systems. The type pumps tractor have as hydraulic pumps have some internal clearances and those clearances get larger as it ages. So at low speeds even a new tractor hydraulic pump will lose some efficiency as the fluid slips past those clearances.

When we raise the RPM to speed the pump up, what is called a "fluid wedge" is created inside the pump surfaces and makes the pump more efficient. In an old pump with lots of wear it really becomes noticeable that the pump works better with more viscous oil and higher RPM.
So yes, it's not imaginary. Sometimes full throttle really does help to lift a load.

rScotty

I totally agree with the higher RPM being capable of producing more pressure up to the point that it exceeds the relief valve setting. Then the pressure increase stops unless the pump output exceeds the flow rating of the relief valve but the increase would be on the pump side only of the relief valve. Pressure to the cylinders would cease to increase at that point.

 

[Bentrim]

Bentrim said:
"Hydraulics a very simple and confusing topic.
Open Center system. Pump = A positive displacement unit that draws oil from a tank and moves oil to a control device, note a pump does not create pressure it only moves fluid at a set rate per the rpm operated."

The inlet to a pump is at negative pressure causing fluid to enter the pump ("draws" it) from the sump which is at neutral pressure. The outlet of the pump is at a positive pressure which goes to a high number varied by things like relief valves and loads. Obviously the pump DOES create pressure and if it did not the fluid would never move. By definition, and normal specs, a pump creates an amount of flow at a pressure.

What is the point of trying to claim the pump does not create pressure?

Take a pump Using a tank, and hoses, How much pressure does the pump make? None It only creates flow. Now put a valve and cylinder on the outlet, move the valve to put oil into the cylinder The cylinder now creates the pressure because oil has to go somewhere so it exerts force on the piston. The load on the cylinder is what creates the pressure. If the cylinder is overloaded or reaches the end of stroke the relief valve prevents the pressure from rising too high and damaging the system.
Yes I argued the pump does create pressure but it was explained to me that a pump only creates flow not pressure a restriction or load is what creates pressure.

Does a Hydraulic Pump Create Pressure?

The topic of whether a pump creates pressure or fluid flow can generate quite a bit of debate! What do you think?

info.texasfinaldrive.com

How does a hydraulic pump create pressure? – Short-Fact

short-fact.com

Do hydraulic pumps create pressure? – QnA Pages

guillaumeboivin.com

 

[JWR]

Take a pump Using a tank, and hoses, How much pressure does the pump make? None It only creates flow. Now put a valve and cylinder on the outlet, move the valve to put oil into the cylinder The cylinder now creates the pressure because oil has to go somewhere so it exerts force on the piston. The load on the cylinder is what creates the pressure. If the cylinder is overloaded or reaches the end of stroke the relief valve prevents the pressure from rising too high and damaging the system.
Yes I argued the pump does create pressure but it was explained to me that a pump only creates flow not pressure a restriction or load is what creates pressure.

Does a Hydraulic Pump Create Pressure?

The topic of whether a pump creates pressure or fluid flow can generate quite a bit of debate! What do you think?

info.texasfinaldrive.com

How does a hydraulic pump create pressure? – Short-Fact

short-fact.com

Do hydraulic pumps create pressure? – QnA Pages

guillaumeboivin.com

Oh balderdash... ! All in fun.

 

[Bentrim]

Well..... it is possible to have pressure without a pump. It just takes gravity.

But I agree that theoretically all pumps create flow, not pressure - the pressure is created by the resistance to that flow. But in the real non-theoretical world the inside surface of even the smoothest pipe or fitting has a slight resistance to flow... and so in reality there is always some pressure and some loss of flow just because the fluid is moving.

There was a discussion about pumps and RPM and lifting things by increasing RPM - and for that to make sense it's back to theory again. Any 100% positive displacement pumps - like a hypodermic syringe - don't create any more pressure when they go faster, just a greater flow rate......same flow in a shorter time.
But our tractors don't really have positive displacement type pumps for their hydraulic systems. The type pumps tractor have as hydraulic pumps have some internal clearances and those clearances get larger as it ages. So at low speeds even a new tractor hydraulic pump will lose some efficiency as the fluid slips past those clearances.

When we raise the RPM to speed the pump up, what is called a "fluid wedge" is created inside the pump surfaces and makes the pump more efficient. In an old pump with lots of wear it really becomes noticeable that the pump works better with more viscous oil and higher RPM.
So yes, it's not imaginary. Sometimes full throttle really does help to lift a load.

rScotty

Most tractor hydraulic pumps are positive displacement pumps. They MUST displace the fluid entering the pump if they don't there are a few things that may happen. 1. Engine may stall 2. output hose or line may break 3. pump will break.
In a variable displacement pump the pump will "shut down" at a specified pressure.

Types of positive displacement pumps Positive Displacement Pumps: The Most Common Types | DAE Pumps
Examples of non-positive displacement pumps are Centrifugal pumps, Multi-stage pumps, Axial (propeller) pumps

 

[Bentrim]

Oh balderdash... ! All in fun.

It has taken many years to teach me certain things and yes this was one of them. All I try to do is help folks understand what I have learned so maybe we can all learn. (and hopefully get rich someday)
Have a wonderful day

 

[5030]

I disagree. Try lifting a heavy load at idle...

I kind of disagree with you disagreeing...lol At any engine speed, so long as the pump makes sufficient pressure and flow, what limits the lifting power is always the hydraulic bypass valve. I can put both my M's in bypass at an idle no issue. That may not hold true for others but with mine it does.

Actually, I prefer that as both mine are gear drive hydraulic shuttles so the slower the rpm, the slower the movement is. Which is a great advantage when making precise movements with a large tractor.

 

[4570Man]

I kind of disagree with you disagreeing...lol At any engine speed, so long as the pump makes sufficient pressure and flow, what limits the lifting power is always the hydraulic bypass valve. I can put both my M's in bypass at an idle no issue. That may not hold true for others but with mine it does.

Actually, I prefer that as both mine are gear drive hydraulic shuttles so the slower the rpm, the slower the movement is. Which is a great advantage when making precise movements with a large tractor.

It would still lift a little bit more at full throttle vs idle. Trying to push more volume through the relief valve will make higher psi than pushing a lower amount through the same valve.