(reveiw) Hydraulic System with tandem center solinoid for control

[Talo]

hi i am fairly new to the hydraulic business and need to create a hydraulic lift that has warm up loop that also raises the lift at the speed i choose based on the power given to the pump. and lowers slowly with a regulator.


i made a schematic of the way i think it is supposed to be done.

here's the visio project in office 2007

http://files.engineering.com/getfil...8-a70e-2f9c8249e12a&file=Hydraulic_System.vsd

also attached is the image of the same schematic.

 

[hydromech]

Hey...

First off the flow control will be better between the direcional valve and the cylinder.

Secondly, are you sure about the transitional flow paths of the directional valve. I am not sure that there are valves that block all ports as the move from the normal position to the working position.

Because I am lazy and because it will help you to understand I suggest you research the following...

1) Counterbalance valves
2) Overcentre valves
3) Pilot operated check valves

All of the aboove will hold the load while the directional valve moves. The spool in the directional valve will allow oil from the cylinder to escape to tank as the it moves and the cylinder will drop slightly and there will be a noticeable bang as the valve moves. Any of the above valves will do what you need, but with varying degrees of success.

As you have drawn the system, any pressure generated at the flow control will be seen at the directional valve and the leakage across the spool could mean that the cylinder extends during the warm up.

The directional valve would be better with the P line blocked and A & B lines connected to T in the normal position. This will keep stop pressure build up in the cylinder during the warm up.

The pump could then be run against a releif valve or orifice to warm the oil. The size of the orifce can be calculated as...(metric units)

Flow(LPM)/Area(mm^2)x 1.44 = square of the pressure drop(BAR)

The answer squared is the pressure drop in BAR.

The temperature differential(deg's C) across the valve/orifice is equal to...

(5.7 x Pressure Drop)/100

It is worth noting that warm up cycles only warm the reservoir oil. The oil in the line and the cylinder will remain cold until it reaches the reservoir. When the warm oil is pumped out to the cylinder it will cool rapidly unless the the oil is worked hard to keep the temperature up.

I hope that helps a little.

Adrian

 

[hydtools]

The schematic is nonsense. It is apparent you know little about fluid power and its control.
Please learn more before seriously hurting someone.
I would like to see your analysis of the system; loads, pressures, flows, fail safe to prevent dropping of the lift, etc. How heavy is the lift + load? How fast must it move? Acceleration?
Why open center instead of closed center?
Why do you want to warm the system? To what temperature?
The placement of the flow control in your schematic will continuously and uncontrollably cause heating while the pump is circulating fluid.
There is no relief valve for circuit protection.
I hope the filter in the suction line is a mesh strainer, otherwise you will cavitate the pump.
You need a filter in the return line to keep fluid clean.
You should have pilot operated load check valve to hold the lift.

This needs more work.

Ted

 

[hydromech]

Easy Ted you're gunna scare him off!

It is clear he does not know what he is doing, but is it not better that he asks rather than go it alone and risk hurting himself?

The schematic is about as far from workable as one could get, but at least it's only a drawing. I have know people spend a lot of time and money and then ask questions after it's all gone wrong.

Lets try to offer our expertise rather than give him a hard time for having a go. If we can offer guidance and try to educate people it will be better for everyone.

Adrian

 

[hydtools]

Adrian, I just wanted to get started with what he does know about what he wants for system performance and then we can guide him from there. Essentially erase the sketch and start with what is to be done.

Taking shots at what is a nonsense circuit is a waste of Talo's time and teaches nothing.

Ted

 

[stroupaloop]

I think you're both right, but the question is where do you begin? I haven't been able to view the circuit due to the lack of software available at my office, but if he's missing such things as relief valves and design parameters, where would someone like to begin?

The most direct approach is to start with fundamental training, but that requires equations, lessons and developing a series of educational values on a hydraulic basis that, in my opinion, would be sold not given freely. For instance, the most general equations for pumps are calculating volumetric displacement and developing a Q vs RPM chart, but then you go into efficiencies, which could take a day just to explain...and that's just for a mono-directional fixed displacement pump, preferably a gear pump to ease variables...

The underlying approach is direct him to useful information and that is sometimes difficult to find...

 

[budt]

He could start by looking at my Basic Book,"Industrial Fluid Power Basics" at this site:

http://www.hydraulicspneumatics.com/200/eBooks/

Chapter 4 Pages 11-18 gives all the symbols and points out the Crossover Conditions of all the basic valves that I could find in the Parker, Eaton/Vickers and Rexroth catalogs.

Chapter 10-12 go through Directional Controls of all types that I'm familiar with and gives some application information for them.

The Advanced Circuits book at the above link shows the valves in all the typical circuits I know of. However the chapter on Directional Valves is not up yet so it will be of no help to look there. H&P is putting up a chapter every couple of weeks so it should be up soom.

The books are my attempt at training persons in the Mechanical and Electrical field what Fluid Power is about and at least get them somewhat knowledgeable in component operation and application.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[stroupaloop]

Good. Bud i'm glad you came in and provided some direction. I was rather waiting on it, but haven't had an opportunity to review your publications so I did not know the specific location to point him towards. Thinks for the additional insight.

 

[budt]

BTW, the books wee meant to also be addedincom to supplement my Social Security checks. However, except for the ones I've used for classes I've taught sales have been Underwhelming. Just not much call for Fluid Power Training or Fluid Power Training books.

The thid book I wrote to teach "Industrial Hydraulics Trouble Shooting" wikk be on the same site soon.It covers my rendition of Trouble Shooting that I must say came from reading about how IRS agents in the states are taught to find Counterfeit Bills. I read they spend about 3/4 of their training studying noting but good bills until they are so familiar with how the real thing looks that the bogus bills stand out quickly.

To that end 2/3 of the book is studying all the valves in typical circuits so the syudent is familiar with their application. The rest of the book goes through identifying symbols, circuits that have wrongly applied or incorrectly set valves. An explanation of how the circuit was designed to cycle is given and then an explanation of what the circuit malfunction is. The student must then diagnose which componentts could cause the problem and what approach to take to fix it.

The training winds up with practice circuits of circuits I have worked on over the years. There is an explanation of a possible problem and a space to give an answer for the way to correct the problem.

At a local large company the classes are for mechanical and Electrical Engineers and Maintenance persons and the practice circuits are for actual machines in the plant.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[Talo]

thanks for the insight. i will look into it. yes hydtools i plan to have a mesh filter in the return line and i also have a relife valve on the actual solinoid as a addon subplate.

and as a addon im am not sure yet of the weight of the load. but what i am trying to do is lift a computer rack out of a closet underground. just some insight into the project.

 

[Talo]

sorry max load is 750lb

 

[Talo]

hydromech regarding your first post the blocking "p" meaning connecting the inlet to "a" and "b" remains the cylinder with "t" also remaining the return line.

also with that setup how do i control the in stroke?

 

[hydromech]

Talo...

I mean like this...see attached.

It's a very simple diagram, but is shows the basics.

The pumps pressure is controlled by a vented relief valve. The valve can be set to the required pressure and when the solenoid is off there is no pressure. If the solenoid is energised the pressure will rise to the level set on the relief valve. This would heat the oil very quickly. Any oil that leaks over the spool will always escape to the T line.

The direction of the cylinder is controlled by directional control valve and when the cylinder is lifted, it is held up by a counterbalance valve. To lower the cylinder the counterbalance valve is forced open by the weight of the load on the cylinder + hydraulic pressure inside the cylinder. The position cylinder will not change when the solenoids are energised. Another option is pilot operated check valves, but they can be a bit aggressive in their operation.

Speed of operation would be regualted by throttle check valves between the directional valve and the cylinder ports. Throttle check valve are valves that allow free flow in one direction and controlled flow in the other. I have not shown these on the drawing.

All of this can be very assembled in a very compact unit as the low load would mean a small cylinder which means low flow. Low flow = small valves = less cash.

This drawing is by no means complete, but it does give an idea.

Regards

Adrian

 

[Talo]

just a question, the D05 subplate size is equivalent to what europeian size? (6,10,16,22) or is this just the ORB size?

 

[Talo]

so for instance i would need a throttle check valve at the top port of the cylinder for going down meaning that would be the slower flow.

and another one at the bottom port of the cylinder for going up, this one should be a bit faster.

is that correct?

 

[Talo]

also the throttle check valve would be after the counterbalance valve?

 

[budt]

"just a question, the D05 subplate size is equivalent to what europeian size? (6,10,16,22) or is this just the ORB size?"

Look in the Basic book mentioned above in Chapter $ Page 4-14 to see Directional Control Valve Designations from NFPA, ISO, CETOP and NG.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

"also the throttle check valve would be after the counterbalance valve?"

Only if the Counter Balance Valve is Externally Drained.

Much better if the Flow Control is before the CBV.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

Throttle Valves are commonly called Flow Controls when they have a Bypass Check Valve and Needle Valves when they control Flow in both directions are unnecessary in your circuit unless there is a need for variable speed. Even then I would use a Variable Volume Pump and set speed by changing pump flow. A lot less energy waste and woiuld eliminate the need for a Heat Exchanger.

Also, you seem to indicate the cylinder only needs the 750# of force for lifting so a Regeneration circuit would set speed nearly the same in both directionsa with the same flow.

Anytime I am asked to design a Fluid Power Circuit I ask a lot of questions up front so the simplest, least energy wasting and less expensive circuit can be designed.

You can see some questions I always ask for a simple circuit design situation in my basic book in the "Formula" Section on pages 14-15. Pages 16-17 are for sizing a Pneumatic circuit.

These two sizing examples are not all inclusive but give a general way to approach a given situation. With comtinued experiennce and a full understanding of the components available you can do some real energy saving circuits.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[Talo]

ok according to you hydromech lets say i use another directional valve as the pressure control for the counterbalance valve, modifying my original drawing it would be like this.