Fluid dynamics 2

[Biltwell]

Good day, I am new here and also not much a hydraulic expert but I have recently designed a valve body to be used on a horizontal band saw, This valve replaces a singular needle valve attached directly to the cylinder and now mounted about 36" from the cylinder. In the valve body is a needle cartridge valve with a toggle cartridge valve. The idea is to be able to adjust flow rate while also being able to lock the system completely. And it does work (sorta). When the valve is barely cracked open I seem to be having a flow issue. The fluid will flow for a couple of seconds then slow way down. And Ill have to keep opening the valve more and more until the flow rate is too high and that at that point it'll have consistent flow. I stuck to the same principles as the OEM valve and port sizes, IE 1/8 npt and 1/8 ID line. I've tried using 0w20 oil and that doesn't seem to increase flow rate at all. I could really use some advice/input on this one from someone with more experience than myself. Thank you!

 

[LittleInch]

OK. What details do have of the valve you're using? Make, model, trim etc

Why didn't you use the old one?

I can't quite see how it works but this is your issue as others have pointed out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Biltwell]

I needed to switch to a cartridge valve as opposed to the OEM in-line valve.

 

[Biltwell]

Is there anyway to share a video on this forum? It would be much easier to explain it on video instead of photos.

 

[LittleInch]

Take a video and load it into some file share system or you tube or vimeo or load it as an attachment, but then people need to download so it puts some people off.

But it's the internals of that valve which are the key to this I think.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Biltwell]

Let me know if this helps[URL unfurl="true"]https://res.cloudinary.com/engineering-com/video/upload/v1697724835/tips/10000000_6695546077159581_2585644868945899474_n_ncjs77.mp4[/url]

 

[LittleInch]

That's very useful, thanks.

So lets concentrate on your issue -

You lift the saw up, flick the switch, do what you have to do on the saw then as once you flick the switch the saw starts to descend at a controlled rate depending on the set point of your valve.

You issue is that it starts off OK then slows to a crawl?

Some of this may be just expansion of your plastic tubing going from no or negative pressure to being under pressure, but the key aspect is that valve. I just can't see enough detail in what you've posted to see how it functions but it really doesn't look like a needle valve to me.

Can you tell us make and model no of the valve or post a more detailed sectional drawing?

Actually hang on a minute - I think there is a volume mismatch like composite pro says. You need an air gap so that when the saw is raised, there is somewhere for the oil in the section below the leather seal to go as the volume per inch of travel is higher in the bottom section than the top. According to your diagram as the cylinder descends, the lower half wants to move more oil per inch of movement than the top part due to the piston rod. Hence that brass valve at the top is presumably an air release / vent? Is it blocked?, screwed down?

Now your valve is higher than the piston I think you're getting air in somewhere. Have you tried taking that unit off the top and placing it lower than the piston?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Biltwell]

The brass valve at the top is simply a fill port and it is a plug, that's from the factory. I didn't add or change that. The piston never reaches above the top fitting or below the bottom fitting. There is definitely air that remains in the top of the cylinder, there is still a cavity above the fill port. I wouldn't see any possible way of getting that air out.

 

[Biltwell]

I just installed a bleeder valve as well as unbolted the valve body and lowered it below the cylinder opened both the needle valve and the toggle valve and cycled the hydraulic cylinder several times and no air seems to be present. It did not correct the issue.

 

[hacksaw]

When the saw blade is raised, and you flip the toggle valve, the blade swings downward (under its own weight) slowing down as it reaches horizontal position.

So your hydraulic cylinder is slowing down as it reaches end of travel.

 

[Biltwell]

Yes but the information I'm trying to ascertain is why.

 

[Biltwell]

Im starting to think that maybe the ID of oil lines is just too small. Ive ordered some larger ID tubing we'll see if that rectifies the issue.

 

[LittleInch]

Looking at that valve data sheet I think it's designed for a much higher pressure (380 bar). I doubt you have more than 2 or 3 bar.

Also the valve is noted as "course". So at say 10 percent open you will get very little flow as your pressure drop is too low compared to what it is designed for.

You need to look for a much lower pressure ( like 10 var) fine control valve.

Still can't work out how that piston works unless the piston rod goes all the way through

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Biltwell]

Replacing the valve isn't a great option at this point. Will larger lines increaser bar?

 

[LittleInch]

No. The issue is the valve IMHO.

Try taking it out or keening it full and using a variable clamp on your plastic pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

You can not have the larger volume of the head end exhaust completely to the smaller volume of the rod end. The total volume of the displaced oil will not fit. There has to be excess flow to tank as the head end exhausts it volume.

Ted

 

[Biltwell]

That made no change.

 

[Biltwell]

Hydtools, I don't understand what you're saying.

 

[Biltwell]

Hydtools, Are you saying the input and output lines cant be the same size?

 

[LittleInch]

This post might help.

https://www.eng-tips.com/viewthread.cfm?qid=508140

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hacksaw]

Don't forget, the pistion is only a "shock absorber" without a spring.
The toggle valve holds is inplace when flipped closed.
The only source of pressure is the weight of the blade arm.