Finding accurate restrictor size?

[deluxeparty]

Hi everyone.

I am currently working to replace a mechanical quality testing-simulator with a hydraulic unit. I have little expertise in this area, but we are trying to keep it in-house as much as possible, so I am hoping you can help me out a little bit. I am currently trying to find appropriate dimensions for the hydraulic unit.

The goal is to get two adjustable resistance peaks against a pneumatic force in one stroke, this resistance will be between 400 and 1400N. So I have come up with an idea in which we use a hydraulic cylinder acting as a pump into a reservoir. The piston will be driven by an external pneumatic force, and it is upon the piston that the resistive force will act. To get the resistance, I have decided to use a pipe at either end going to an on-off valve. To get the adjustable peaks, I want to use an adjustable restrictor valve on each side that will limit the flow thereby increasing the pressure in the cylinder/pipe when the on-off valve is closed. The restrictor size will determine what force peaks we can achieve.

My query is what sort of formulae to use to find the appropriate restrictor diameter? I am making an Excel spreadsheet in which I can input different values for the piston size and stroke, stroke time, pipe diameter, fluid density, etc. whereupon it will output the flow, fluid velocity, etc.

Most resources I have found seem to discuss pressure drop. What I want is pressure buildup, but I suppose it's the same thing? I have been pondering this for a few days now and I have found a few formulae online and in books that never give the same results. Color me confused!!

One of the first online resources I happened upon was this thread, and I downloaded the spreadsheet collection posted by hydroman247 (thank you for that by the way!):

http://www.eng-tips.com/viewthread.cfm?qid=332676

Using this gave me some nice figures but I don't trust it because I don't fully understand how it uses the inputs!

I hope I have explained my question properly and that you can help!

Cheers!

 

[hydroman247]

May as well post my updated spreadsheet.

 

[LittleInch]

I'm having difficulty working out your system. Can you sketch up a schematic and up load? I think what you're trying to find is the diameter of an orifice to provide a fixed flow? You may be much better looking for a suitable control valve which you can vary to get your force right. For that you need to work out your min and max valve CV, using flow and pressure drop. Flow comes from your piston size and stroke, pressure drop from your force calc. You probably need to fix some things like movement of the piston within a certain time and area of the piston and then see what sort of pressure it comes up with?

Alternatively you need a pressure regulator.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[hydromech]

I think you'll struggle to get repeat performance from the set up. Depending on the ambient conditions and also how hard you work the fluid, the pressure will change because of the different viscosity and fluid density.

The basic equation is 0.5 x density x velocity squared

As the work done on the fluid will generate heat and heat changes the density of the fluid, you will get a different pressure. That is unless you can maintain a constant fluid temperature.

The idea would work in principle, but you may find the pressure drifts.

Hydroman...you need to add pump efficiency to your spreadsheet, or advise users that efficiency losses are not considered. If someone uses you spreadsheet, they may find that they are short on power.

Adrian

 

[deluxeparty]

Thank you very much for your insight!

Repeat performance is very much what I am looking for. It is to be used for quality testing. So it will hopefully see 3 000 000 cycles between extensive maintenance. And the cylinder should be able to operate in between -20 to 120 degrees centigrade (though the fluid reservoir will be stored in room temperature. I was going to use brake fluid in the system because of the better variable temperate characterstics, but I would prefer to avoid the issues if possible.

I will upload a sketch of my proposed setup ASAP!

I feel now more than ever that I am fumbling in the dark a bit here! I really appreciate your help. It's my first real project in this company and I'd really like to do well!

 

[MikeHalloran]

Seal friction will kill your repeatability.

Find another project.


Mike Halloran
Pembroke Pines, FL, USA

 

[hydroman247]

Hydroman...you need to add pump efficiency to your spreadsheet, or advise users that efficiency losses are not considered. If someone uses you spreadsheet, they may find that they are short on power.

Yes, although anyone actually designing a circuit from scratch like that should know about that.

 

[deluxeparty]

Thanks Mike, I appreciate the blunt honesty. But could you be a little more specific and explain what would do it?

 

[kcj]

controlling FLOW means the pressure drop/force is a result, not the primary variable being controlled.
If the goal is just an approximate load, it may work ok.
For more accuracy, control pressure drops with pressure reducing or relief valves.
If the goal is repeatability and precision and quality control inpsection of parts, I think you will have to controll force or pressures with trasnducers and proportional valves, or a load cell after the cylinder. Depending on the forces and operating pressures and how much precision you want, I +1 that the seal frictions may hurt you.

 

[deluxeparty]

Thanks to your advice here I have written off the idea of using the restrictor valve. I really appreciate not wasting more time on a design that will not work, so THANK YOU!

I have decided to focus instead on using a pressure control valve. I have been in contact with a sales engineer from one of our subcontractors and he will investigate and give me advice on what sorts of products they can offer that will give the results we are after.

kcj, thanks for your input!

The forces necessary are not very big. With a piston diameter of 20mm the maximum pressure necessary will be less than 40bar (this will be reduced if piston diameter is increased) and a flow of 3.5l/min (which will obv. increase with piston diameter). Precision is a priority but it does not need to be extremely accurate, especially as long as it is adjustable.


Again, thank you all! I wish you all a nice warm fuzzy feeling for having helped me!

Have a great weekend!

K