proportional spool valves 2

[naphty]

Hi Guys

Need some tips on proportional spool valves.
What are the 3 changes in proportional spool valves unlike normal DCV's?

 

[pennpiper]

Where did you get this term (spool valve)?
In all my many years in the piping profession I have never heard of a "spool valve".

Help us out with more information about your question.


prognosis: Lead or Lag

 

[naphty]

Pennpiper

Sorry, it's proportional valve spools which is the same as normal DCV, but there are 3 changes in it. Need to know those 3 changes.

 

[Oldhydroman]

If you are talking about hydraulic directional control valves then the differences between the "proportional" and the "bang-bang" versions are:

1) The spool lands have notches in them, the notches are gradually revealed to the flow galleries as the spool strokes and this gives the relationship between stroke position and flow resistance.

2) Even when at 100% stroke, a proportional valve does not open up as much as a bang-bang valve of the same body size.

3) The spool operates at part stroke rather than hard over against an end stop. There has to be a balance between the operating force (e.g., from a proportional solenoid) and the centering springs. This makes the valve particularly sensitive to flow forces, accelration forces and installation orientation, i.e. anything else which exerts a force on the spool changes the position at which a balance is found and this changes the effective opening of the valve. (Some versions of proportional spool valve have a stroke transducer fitted to monitor the actual position of the spool. The electronics driving the solenoid(s) perform a closed loop position control scheme to actively hold the spool in the desired position.)

Why do you need to know THREE (not two and not four) differences? Is it a quiz or an exam question?

DOL

 

[naphty]

oldhydroman

Thanks for your tip

It is part of home assignment. I did get the first one, but I was struggling to get the other two. The latter half of the 3rd point is about LVDT.
Can I summarize that, it requires a balance between operating force and center springs which changes the effective opening of the valve unlike normal DCV.

Naphty

 

[EnergyMix]

Perhaps you should read the Eng Tips site policies found in FAQ731-376. Students asking for help on homework is a good way to find yourself ushered off the site.

 

[Oldhydroman]

EnergyMix
I kinda thought it was homework but naphty has been straight enough to admit it so I don't mind.

naphty...
Yes, the last bit of point 3 refers to the "LVDT" except that some of the devices in use aren't strictly speaking "Linear Variable Differential Transformers", but it doesn't matter - the thing is just a non-contact electronic distance measuring device that gives feedback to the driving amplifier to help the system hold the spool in the required position.

But here's an extra question by way of pennance for getting help with your homework. List seven advantages of using a non-feedback (i.e., force controlled) type of proportional DCV.

DOL

 

[naphty]

Oldhydroman

Thanks for your tips again.

Energymix

Apologies. I missed out on few classes to attend my mother's serious health condition resulting in death.

I will list 7 advantages of non-feedback (force controlled) later today.
Naphty

 

[naphty]

Oldhydroman

I got only 4 advantages so far as per my study material.
1. Smooth control of actuators is possible with the aid of prop.sol. and electronic ctrl.
2. Can be used in metering-in, out and bleed-off. flow control app.
3. It has several spool options.
4. It provides some degree of pressure compensation.

Going back to your 2nd point. So how does it open?

Naphty

 

[Oldhydroman]

Hi Naphty

I suppose the best thing to pick up from this discussion is that your study material isn't telling you all of the story.

Your first three points also apply to the feedback type valves (those with the "LVDT" spool position feedback) and the middle two also apply to bang-bang valves (point 2 is valid if your DCV has a stroke limiter). As far as specific advantages of the non-feedback type go, I would put on my list:

1) The [non-feedback] valves and amplifiers are cheaper and more reliable (less to go wrong)
2) You can use [non-feedback] amplifiers from any manufacturer to drive your non-feedback valve, you can get amplifiers from people that don't even make valves themselves.
3) Non-feedback valves are cheaper/easier to install because there is no feedback transducer to wire up (remember that the wiring to the transducer has to use special screened cable)
4) The [non-feedback] amplifier/valve system does not shut down in the event of a small wiring fault (loss of continuity in feedback circuit)
5) On a "3 position" DCV You can swap the solenoid plugs over to reverse the direction of operation (as long as you are using a symmetric spool)
6) The [non-feedback] valve operation remains stable when the solenoid(s) are not completely bled of air
7) - you did get this one - a non-feedback DCV will allow the spool to move back towards neutral under the effect of flow forces and this gives an approximation of pressure compensated flow control.
8) there may be more advantages but they get increasingly tenuous...

And to add a little more to your question about valve opening: a proportional DCV will have a particular flow "rating". Most (but not all) manufacturers give a flow rate for the DCV based on a 10 bar [total valve] pressure drop with the spool being at 100% stroke. Even the biggest flow rating proportional spool for a particular body size still has a lesser flow rating than the basic bang-bang valve. For example, the biggest spool in a Rexroth 4WRA6 valve is "30 L/min" which will actually allow 26 L/min for a total pressure drop of 10 bar (5 bar for P-A and 5 bar for B-T). By contrast, the bang-bang version of the same valve (4WE6 with a "Q" spool) will allow a flow of 46 L/min for the same pressure drops.

Good luck with your continued studies.

DOL

 

[naphty]

oldhydroman

Thanks for tip and for the effort and time you put to assist with members of this forum (5 stars). I'd like to keep in touch with you if that is not an issue.

Warm Regards
Naphty