hydraulic psi vs strenght

[alexis.marcil]

Hi, from what I know in hydraulics, flow represent the speed and pression is the force.

When I compare specs from different excavator, I see that sometimes, smaller excavator have mor hyraulic psi than bigger... is there's something I miss?

Also sometimes, mini excavator have psi almost the same as bigger one... so theorically, the only thing that is keeping them from lifting the same thing as a bigger one is the size of the boom (so the steel doesn't crack) Am I right?

thanks

 

[xcmark]

you also need to factor in the size of the piston bore and if there is angles that the piston is moving in . the total distance of the stroke vers how far the tool is moving is also a factor .

 

[Lee-online]

If a piston is 10 square cm with 1000psi it will put out 10,000 pounds of force. now if that piston is 20 square cm with the same 1000psi it will put out 20,000 pounds of force but, assuming the pump flows are the same it will only move at half the speed.

This is why a cylinder has more power extending but more speed retracting. The rod removes some of the pistons surface area.

The higher a system pressure runs at, the more likely for failures to occur. Hoses blowing, seals blowing etc.

 

[roadrunner81]

If a piston is 10 square cm with 1000psi it will put out 10,000 pounds of force. now if that piston is 20 square cm with the same 1000psi it will put out 20,000 pounds of force but, assuming the pump flows are the same it will only move at half the speed.

This is why a cylinder has more power extending but more speed retracting. The rod removes some of the pistons surface area.

The higher a system pressure runs at, the more likely for failures to occur. Hoses blowing, seals blowing etc.

You meant to say sq. in. not sq. cm. right.

 

[ILLICEMAN]

Probaly not where he is from

 

[alexis.marcil]

so it's true, the mini ex could power the same big piston but slower?

 

[Asphalt/Dirtman]

Just because the pressure is the same, doesn't mean if you add 5 tons of counter weight you will lift like a big excavator.

The lines are much larger on a big excavator, meaning much larger pumps, pushing larger volumes, to maintain same psi pressure.

AND NO it wouldn't have the same power

 

[Asphalt/Dirtman]

I mean, they size the lines on a mini to give it a certain psi that the cylinders like
Just as they size the lines on a 30 ton machine to give it a certain psi that the cylinder likes and the fluid flows properly and the pump likes and so on and so on

I mean I am being very vague because its been a few years since I was in a fluids class. But the grand point is you look at the GPM of the pump, not the psi of the system. Parasitic loss also needs to be looked at but that's another story.

1000 psi through a 1/2 inch hose is not the same as 1000 psi through a 6 inch hose

 

[John C.]

High pressure hydraulic systems are more efficient on fuel and produce a lot less heat primarily because they move less amounts of fluid to achieve the same production as a low pressure machine. Total lifting power is usually about the same per machine size range. A old 80 Drott would handle any load that a 300 size machine today can do. However it also burned through 80 to 110 gallons of fuel a day in order to do it.

 

[maytag]

so it's true, the mini ex could power the same big piston but slower?

You are correct, the smaller machine will have smaller displacement pump and smaller lines but it will run an oversized cylinder but slower-you could do the same with a hand operated port-a-power pump after you filled the circuit.
I've seen several large press applications where a centrifical pump was used to fill the circuit, maybe moving 500-1000 gallons of fluid, then a small piston pump comes online to build the pressure to do the work.

Tom

 

[roadrunner81]

Probaly not where he is from

I'm sure it was just an oversight, but 10 sq cm with 1,000 psi does not equal a force of 10,000 pounds.

 

[cutting edge]

Just because the pressure is the same, doesn't mean if you add 5 tons of counter weight you will lift like a big excavator.

The lines are much larger on a big excavator, meaning much larger pumps, pushing larger volumes, to maintain same psi pressure.

AND NO it wouldn't have the same power

I mean, they size the lines on a mini to give it a certain psi that the cylinders like
Just as they size the lines on a 30 ton machine to give it a certain psi that the cylinder likes and the fluid flows properly and the pump likes and so on and so on

I mean I am being very vague because its been a few years since I was in a fluids class. But the grand point is you look at the GPM of the pump, not the psi of the system. Parasitic loss also needs to be looked at but that's another story.

1000 psi through a 1/2 inch hose is not the same as 1000 psi through a 6 inch hose

This makes my head hurt.

1 psi is 1 pound per square inch.

1000 psi is 1000 pounds per square inch....big pump,small pump,large lines,small lines,no difference.

Pumps and hoses are sized based on flow requirements,as are cylinders.

A small excavator with its main relief set at 4000 psi will fill a large cylinder off of a large excavator,but at a reduced speed due to the flow limitations of its pump.It can also theoretically lift the same load as the large machine with its relief set at the same pressure.

Smaller machines may run at higher pressures due to space limitations on size of cylinders that can be installed and or number of cylinders.

For example, lets work with 1000 psi.

1000 psi applied to a piston surface area of 20sq/in. would produce 20,000 pounds of force.
1000 psi applied to a piston surface area of 200sq/in. would produce 200,000 pounds of force.

See how the bigger machines create more digging power?

 

[maytag]

maybe/maybe not

This is why a cylinder has more power extending but more speed retracting.

Flow depends on pressure differential and viscosity-
force will depend on pressure * affected area

 

[mxridernrth]

Force (cylinder) = Pressure X Area = Pressure X ( .7854 X Diameter^2)

So from the above you can see that the force is independent of time and is only a function of system pressure and cylinder diameter - not machine horsepower.

Power = Work/Time = Force X Distance / Time
plugging in the above
Power = Pressure X .7854 X Diameter^2 X Distance / Time
= Pressure X Flow rate

for Horsepower this simplifies to = psi X gpm /1714

So yes a mini excavator could do the same work as a big excavator if it had the same sized cylinder that was doing the work - albeit slower because of the reduced horsepower of the machine. Of course it would rip itself in half doing so./

 

[>HevyIndsMFGng<]

It doesnt matter what units he is using, all of the basic principles and ratios dont change. His units are consistent so the math works. For any of you who wonder what equation he uses its: F=SA*P

where

F = Force
SA = Surface Area. In this case, the surface area is circular (because we're talking about a cylinder) so SA or A=pi*r^2. r = inner radius of the cylinder
P = Pressure (of the hydraulic circuit)

 

[>HevyIndsMFGng<]

so it's true, the mini ex could power the same big piston but slower?

Yes, the mini ex's engine and pump could and like you assumed, the linear movement of the piston would be slower.

 

[>HevyIndsMFGng<]

Just because the pressure is the same, doesn't mean if you add 5 tons of counter weight you will lift like a big excavator.

The lines are much larger on a big excavator, meaning much larger pumps, pushing larger volumes, to maintain same psi pressure.

AND NO it wouldn't have the same power

Yes, it would have the same power... assuming the max psi of each excavator is the same. How do you argue?