What port size do I need ?

[Jamaz17]

Hey everyone, I don't do fluid calculations that often so any help would be great.

I have a 2.5" piston stroking .75" at 500 rpm pushing on a hydraulic fluid. I calculated the GPM at 7.95gpm how do I calculate the port size I would need to be able to push the fluid thru? Also is there a calculation that I could use to calculate the pressure build up if I had the exit port to small?
Thanks to all who reply all help is appreciated.

-James

 

[maytag]

J,you didn't say what press. this application uses. Medium pressure lines (500-2000)should be sized to keep fluid velocity to 15-20 fps. If I'm reading my pipe table correctly 3/8 or 1/2 SCH 80 pipe would do the trick for this pressure range and flow you calculated. This should give you an idea for port size-are you designing a manifold? Maytag

 

[Jamaz17]

Hmmm maybe I'm' not sure that's what I'm looking for. I was hoping to control the pressure by the size of the port that's why i wanted to be able to calculate what kind of pressure a given opening would create? Am I thinking about this the wrong way ?

As well there won't be any piping in my system so I don't think I have to worry about that.

 

[Montemayor]

James:

The problem you are presenting is one of describing what you are doing or hope to accomplish. If you don't level with the Forum and give us ALL the basic data at one time, you are going to go through a difficult and confusing experience explaining your problem piece-meal, one crumb at a time.

You haven't answered Maytag's question about the manifold. If you are using a PD type of pump, are you trying to maintain the pump's continuous and constant discharge pressure (at constant flowrate) with a fixed orifice? What are the pressures involved? The pump's discharge pressure and the downstream, post-orifice pressure? Or are you trying to maintain (fix) a constant pressure downstream of the orifice?

 

[Jamaz17]

Sorry for the confusion guy I really don't do fluid stuff very often so I'm not sure how to describe what I'm doing all that well. What I have is a cam pushing piston thru a .75" stroke every revolution (cam is rotating at constant rpm). The piston in turn pushing a hydraulic fluid thru a port which has an adjustable size. I was hoping that by changing the port size ie: making it smaller that the pressure acting on the piston would be increased which in turn would increase the force between the piston and the cam.
When I decrease port size which the oil flows thru will this not increase the pressure inside cylinder?

Please let me know if there are any more grey areas I can fill in, like I said I'm REALLY new to fluids stuff so I'm starting from ground zero here.. thanks a lot for the help.

James

 

[maytag]

One of fluid power's Ohm's law "Pressure Is Resistance To Flow" Maytag

 

[Jamaz17]

hmm cool.. how is resistance measured in the case of fluids ?

 

[maytag]

Jamaz,here in the U.S. pressure is measured in pounds per square inch for most fluid power applications and will be expressed PSIG with G standing for Gage. Maytag

 

[Jamaz17]

So resistance is measured in terms of pressure ?

The formula I have is Flow = Pressure change / resistance

but I am not sure how resistance is measured ?

 

[methcat]

"J,you didn't say what press. this application uses. Medium pressure lines (500-2000)should be sized to keep fluid velocity to 15-20 fps. If I'm reading my pipe table correctly 3/8 or 1/2 SCH 80 pipe would do the trick for this pressure range and flow you calculated. This should give you an idea for port size-are you designing a manifold? Maytag"

hey maytag(or anyone else), i'm a structures guy that's trying to to acquire some good reference material. where might i find a decent pipe table?

 

[maytag]

Womack Machine out of Texas has a good FREE small reference book. It contains pipe and tubing sizes,flow rates and pressure ratings-also has some basic theory and troubleshooting. There is also a Lightning Reference book but i don't think it will be free. Maytag

 

[DesignerMike]

So, you are using the hydraulic piston and cam to create a "resistance", and you are not designing a pump or manifold for a hydraulic circuit.

I have a couple observations
1) the piston speed will not be constant, thus the flow from the piston will not be constant and you should size the port for the flow when the piston is moving at it's highest velocity. In this case probably at the steepest section of the cam. The actual flow rate could be more than 3X more than the 7.95 you calculated

2) If you use a cam, how do you plan on getting the piston to return to the end of the stroke. Note that you would most likely want the cam to stay in contact with the piston all the time or it will be a very loud unit, and the vibration and impact will tear it apart. Also, the spring will need to do the work of drawing the oil into the piston (and past some sort of check valve). I would think that you would prefer a crank-shaft type design instead of the cam partially for simplicity in design calcs. You would also need to take into account the compressive forces required for the spring while you are pumping the oil out.

3) If you only intend to use one piston, the "resistance" will not be very consistent and you will have major torque spikes every revolution causing shaft fatigue.

4) If you only need a variable source of tortional resistance, you could easily couple the shaft to a simple hydraulic gear pump, and install a needle valve in the outlet line of the pump to create a constant backpressure. You should have a relief valve before the needle valve in case some closes the valve while the unit is shut off. (the pump will just continue to build up pressure until something blows, and it won't be a slow process). Some pumps come with an internal relief built in. You can pick up a very inexpensive hydraulic gear pump from many vendors and not have to worry about making pistons, seals, valves, cams, shaft support, and all the design work that would involve. You can get a pump for less than $100. Just determine the amount of horsepower you want to consume (or resistance torque) as a starting point.