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!

 

[Biltwell]

Hacksaw, It acts as a damper not a shock absorber. It is not suppose to provide any recoil. The saw itself has two coil springs located at the rear of the bow to counteract the weight of the bow, even with those removed, it provides no change. Im not entirely sure what the point of your comment was? Can you elaborate?

 

[hacksaw]

Your description states more clearly what I was trying to get at, basically no external hydraulic pressure sources.

The only elasticity I saw in the video was possibly due to tubing flexibility, as I'm sure you bled the air out on filling.

Looks like its close to doing what you intended.

 

[hydtools]

Biltwell, I'm not talking about the hose sizes. Your sketch shows oil flowing from the head end of the cylinder, through your valve and into the rod end of the cylinder. The volume of oil being pushed out ot the head end is greater than the rod end can accept, therefore can't work unless the excess oil goes somewhere else. You have in effect a hydraulic lock, valve or no valve.

Ted

 

[hacksaw]

With the valve open, when the piston moves down 1 cm pushing the displaced fluid around to the otherside of the piston to allow the movement.

With the valve closed the piston is locked inplace, unless manually raise or lowered, as allowed by the leather sealed piston.

 

[LittleInch]

OK, final go.

I'm with others here in that I just don't actually understand how this system ever worked unless the piston seal allows air in and out or that brass plug has breather hole in it somewhere.

So the issue I think is related to a few things.

1) The pressure available to move the hydraulic fluid is only from the weight of the saw which is limited by the counterweight springs or the weight of the assembly. This is relatively low in hydraulic terms so the needle valve appears to be very over rated and will not allow enough oil through unless it's fully open. Action - source a different valve.
2) At low speeds of fall is the leather piston seal binding and not allowing a smooth movement? The additional frictional losses in all that tubing, plus the over rated valve is enough to cause it to bind. Action - try larger tubing if action 1 doesn't work.
3) Oil viscosity may be too high. Could you use a much thinner oil or even inhibited water or glycol mix?
4) But for me it's still you have inserted a valve which has different opening and flow characteristics to your current valve. Action - Could you try fitting the old valve in line by your new device or in the plastic line coming from the bottom of the piston and then open up your new valve fully open?

If none of those work I'm stumped.


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

 

[Compositepro]

Gosh, you are given the answer to your question in my my first reply, as well as later by hydtools. You, and others, have to bother to actually read and understand the replies. You must NOT bleed all the air out of your system. There has to be an air pocket or vent plug at the rod end of the cylinder. A sealed air pocket will result in some air compression as the saw lowers, resulting in more resistance to lowering as the saw lowers. This is a desirable feature, as weight on the cylinder increases due to the change in lever arm of the saw. With no air pocket, the piston rod becomes the piston, and it squeezes on oil with no place for it to go.

A hydraulic bottle jack operates very similarly, with air at the rod end and a rubber plug in the oil fill hole. I don't understand why your video does not show the problem you describe. It seems to be working fine.

 

[Biltwell]

That's because as described there IS an air pocket at the rod end of the cylinder, I'm unaware of any way to implement a vent plug into this system without losing fluid through it. In my video I do explain the issue, It doesn't show it because the minute movements are difficult to show on video. Also, As stated I am not a hydraulics expert. I make shit out of metal that's it, that's why I'm here to get information so if you're incapable of responding in a manner that's easily interpreted by a layman then don't respond man. If you don't have the patience for this then you don't need to participate in a forum that exists only to help people.... If you're going to be a condescending jerk then I just frankly don't want your help.

 

[Compositepro]

Then stop wasting our time. This is a site for engineers to give tips to engineers.

 

[Biltwell]

I have tried every single suggestion that's been made. If your ego cant handle the fact that your suggestions are wrong, then that's a you problem my dude. I'm happy to provide any and all information I possibly can and I've done my best to do that. If someone feels there's information missing then let me know, but as stated earlier since I'm not a hydraulic expert I don't know what you don't know. So if you feel so inclined to help then as the expert you need to drive that conversation. This is how talking to humans works in all conversations. I'm an expert in my field. So when someone seeks advice from me, I have to ensure that I'm gathering all the relevant data so that I can provide proper advice. I don't know how you ascertained that I am ungrateful, it sounds like your making things up to justify why your feelings are hurt, But I'm certain its just your ego. You should go out into public and learn how to talk to people. If you don't want to help that's not a problem, no one is holding a gun to your head forcing you to. Had you not ever responded I would have never known... Now if you don't mind, there are others that are actually helpful in narrowing down this issue. I would like to respond to them and continue to converse with them. Have a wonderful day.

 

[hacksaw]

Is there a chance some debris in affecting the needle valve?
I guess you'll find out when you try the new tubing, good luck!

 

[Biltwell]

I did pull it apart to look and to make sure there were no burrs or machining swarf left in there but I didn't see anything. I tried to keep all the orifice sizes the same as the OEM setup but I overlooked the ID of the tubing itself and that's at .0625" ID while everything else is .188" ID. I'm assuming that's just too restrictive especially on the down stroke back to the cylinder. If that's not the direct cause of my issues I don't think I can properly diagnose the issue without at least ensuring those are equal or greater to the rest of the orifices. But the new tubing should be here today, so we'll see.

 

[hydtools]

Remove some oil to create more of an air bubble on the rod end. Make more space for the smaller amount of oil to flow into. It probably doesn't need much oil to work.

Ted

 

[3DDave]

hydtools - I agree. Too much oil would cause more back pressure and lower the flow as the saw lowers.

The tubing will not have a significant effect.

Removing the upper plug would be a way to test this as the air could no longer be compressed.

Were there maintenance instructions about filling the original system?

Does closing the needle valve completely stop the saw from dropping?

 

[LittleInch]

Well maybe try and show us what the problem is on the video.

The fall speed on the original video is quite fast - one to 2 seconds for full drop.

what sort of fall speed are you trying to achieve?

And try not to get too upset with some of the replies - it doesn't go down well round here.

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

 

[Biltwell]

3DDave, the fill procedure for the OEM application is lower the bow so the cylinder is fully down and just fill until fluid drips out of the fill port. Which I've kept the same since I've implemented this new valve but I will suck some of the fluid out and see if that helps. There is some drop even with all valves closed, But its a known issue with these saws due to the leather cup piston. That cylinder is brand-new and even with the OEM valve in it there is still some drop. I don't think it can be avoided and yeah, it makes diagnosing any problems with my system kind of a pain. I'm in the process of designing a rebuild kit that can replace the leather cup with a more traditional piston but I'm stuck at the one way seal for now. But that's an issue for another day haha

LittleInch, I will post a video here shortly of the actual issue. I appreciate your comment/warning however, I wouldn't tolerate being spoken to in the manner he did face to face and I wont tolerate it on the internet. I hope the rest of you can appreciate that. I am a nice, kind person that would do anything to help others but I draw the line when I'm spoken down to. I would never do that to someone else. If the moderators see fit to remove me for that or none of you see fit to respond and continue to help then it is what it is.

 

[3DDave]

What would be funny is this - plastic tubing will have an initial give until the molecules have shifted to maximum alignment. Try pulling a rubber band and note that under low loads it elongates easily, but gets to a point where it goes stiff.

So, when the saw is first released there is no pressure and the tube is relaxed, but as pressure builds it expands until it won't expand more; now there is a change in stiffness causing the rate of pressure increase to increase.

Same test - release the plug so pressure cannot build, after the saw is all the way up.

 

[Biltwell]

Is there a good method to reduce bar without changing out the valve just for testing? I tried clamping the line and left the existing needle valve open all the way and then slowly cranking out the clamp and that didn't result in any changes. I think LittleInch is correct about the valve being the incorrect size. But I cant just change the valve without serious machine work. So hoping there's a testing method I can use.

 

[hydtools]

Don't replace the leather cup. It is the check valve that opens to allow oil to go back to the head end when the saw is lifted.

The device, system, damper acts like a piston pump moving oil as the saw drops from the head end, through the flow control and oil returns to the head end by bypassing the leather cup when the rod is pulled out. So it seems.

Ted

 

[Biltwell]

Removing fluid or filling even more fluid and the new larger lines didn't make any change. It would seem that LittleInch might be right with the valve just not being the correct size, at least that's the only thing I haven't tested yet. Im not sure how I'm going to. Unless I can find a lower bar valve that uses the same cavity, that's going to suck haha

 

[hydtools]

The pressure rating of the valve shouldn't matter.

Ted