Orifices in hydraulic systems

[colar]

I am looking for some advice regarding a hydraulic circuit I am having a problem with. To try to summarise;

We produce an agricultural implement that has two folding sections on it. It is important that these sections fold in the proper order. Previously, the order that the sections folded was correct naturally, i.e. the section requiring lowest cylinder pressure was the one that we wanted to fold first. Now, we have done some design changes where extra weight has been added to the section that needs to go first (unfortunately we have produced 80 of these things). The pressure now required for each section is very close and in some cases the wrong section will go first.

Will adding an orifice to the section that we want to go second force the proper section to rise first?

Assuming we already had orifices in place, would smaller orifices help the situation?

Will the type of hydraulic system cause the circuit to behave differently (open centered versus closed centered)?

 

[WardHollowayPE]

An orifice is usally used to control speed in a hydraulic system. The pressure and force will remain the same. Your best bet is to change the diameter of one of the cylinders.
Either larger on the primary or smaller on the secondary. In this case a little bit may go a long way.

 

[colar]

Ward,

Thanks for the reply.

I agree that orifices (needle valves, flow control etc.) are usually used for flow control. However, won't the oil tend to flow to the path of least resistance? It creates a large pressure loss to try to pass oil through an orifice.

The question becomes a bit like the chicken and the egg scenario I guess. The greater the flow the more the pressure drop through the orifice and therfore greater tendancy to drop the flow. This is where the problem becomes a little more complicated.

 

[Hush]

Would a check valve work?

 

[Hush]

Never mins, I just realized why that won't work. How about a valve that's actuated by the first wing when raised?

 

[colar]

A valve would work (I think). We already use a valve in the circuit to control the reverse operation of the circuit (that is , lowering the wings on the implement). The valve alows the non natural section to go down first. It is a free reverse flow relief valve.

We could use this type of valve, but it would mean a somewhat costly retrofit. We may go this route if the orifice idea does not work.

 

[gbent]

There are priority valves which will allow the high pressure to side to work completely before opening and allowing fluid to pass to the low pressure side. Orifices may work, but they will be pump specific, and slow down cycle times. I don't know what size equipment you manufacture, or what the flow requirements are. Please remember though, sometimes your equipment may be pulled with an older tractor with a hydraulic flow of only 10 to 12 gallons per minute, flow compensating only. The same piece may then be hooked to a brand new tractor with 40 to 50 gallons per minute, pressure and flow compensating. I don't think the same orifice size will be proper for both. I think resizing the cylinders would be the best idea.

 

[MikeMilburn]

If you don't want to use a hydraulic sequence valve, how about this,

http://www.tpub.com/fluid/ch2g.htm...Mike

 

[MikeMilburn]

Sorry about that, geez I've never had my name as a link before, try this instead

http://www.tpub.com/fluid/ch2g.htm

Mike

 

[MikeMilburn]

Ok, I give up, when you get to the page I posted, type into the search box "sequence valve"...Mike

 

[WardHollowayPE]

Fluid will seek the path of least resistance. But it will not stop the flow. Both cylinders will move, the one with the orifice will move slower. If you want a real cheap solution bolt on some extra weight to your secondary platform. The lighter platform will, again, move first.

 

[coveter]

Playing with orifices is touchy and you'll be chasing unforseen problems as conditions change (temperature, dirt in the oil, age of the equipment, whatever). Although adding weight to the other arm wouldn't be written up in any journals as being the most elegant solution, you are starting with a system that was intolerant to change in the first place and this might be the best option.

I'd like to give it a shot, but I'm not promising anything. I understand the importance of retrofittability, which makes the problem a bit more interesting. I have some questions:

1. Are the cylinders push-pull, or are they gravity operated in one direction? If push-pull, how do you sequence each side of a cylinder (reversing shuttle valve)?

2. Where's the relative position of the components in the system? What components are tied together in manifolds, and what components are separated by hoses? Is a separate "patch" manifold feasible for retrofittability?

3. Is there any form of a hydraulic locking system, like pilot operated check valves installed in the cylinders? Are there any safety issues to consider or codes that have to be followed? Can someone be squished if a hose is cut or breaks?