Controlling 5 Hydraulic Motors with the same speed and different torque 3

[MoHabib]

Hello Everyone,
I'm Mostafa from Egypt.

I'm Working on designing a hydraulic circuit for agriculture machine.i'm not very good in hydraulic so maybe i'll ask some stupid questions!.
I want to drive 5 Hydraulic motors to give almost the same LOW speed and different torque.Well, the torque also maybe low regards of the motor specs.

i uploaded the schematic for the circuit which shows that i have 3 groups ( 1st chain - Cutter - 2nd chain ) the cutter and the 2nd chain work in the same DCV till now. anyway, how can i reach the same speed that will be almost 60 : 100 RPM to each motor and the same torque for 4 motors and different torque for the cutter.

I don't need to use any sensors or PLC systems for this. for example i'm thinking about using throttle valves for each motor or using flow dividers to divide the oil between the low torque circuit and the higher one.

So, any advises or help about this ?

 

[HPost]

Are you controlling the torque with pressure or motor displacement?

If you are using displacement, then you just need a 5 way flow divider.

 

[MoHabib]

HPost,
i think motor displacement will control the speed!

And if it work with the torque,the 5 way flow divider will give the same amount of flow and this lead to same speed and torque. and i need higher torque for the cutter.

 

[EngineerTex]

For the most part, the torque will be determined by the resistance to the motor turning.

In the circuit you attached, most of the flow will go through whichever motor has the least amount of resistance. In certain circumstances, all of the flow would go through one single motor.

If each motor pair is connected by a chain, then you could have one pair of motors turning and the other not turning, depending on how much resistance the motor pair experiences.

You are correct that you will need to have a flow divider to split the flow between the motors.

What is each of the four motors connected to?

Engineering is not the science behind building. It is the science behind not building.

 

[TimSchrader2]

HELLO

Gear type dividers are more accurate then Vane types.

Unless the motors are somehow mechanically linked then the fluid will follow the path of least resistance without any dividers. As just mentioned.

If they are directly or even indirectly mechanically linked wether or not to use dividers becomes a very good question. I did so in the past with 4 motors directlly linked and it has worked fine for years. I am not fully convinced that was the best way. There may be better load sharing without the gear divider, depending on the stiffeness of the how they may are linked. This may be a tangent to the OP's question. As he mentioned having different Torques.

 

[HPost]

The flow determines the speed.

The displacement x pressure (plus some math) determines the torque.

Selecting 5 flow divider elements that have the same displacements as the motors will keep them all at the same speed.

For a given torque requirement, select the appropriate motor for the pressure available.

Gear type flow dividers work best...

Try Slack and Parr

 

[hydtools]

Are all the motors really reversable? Do all the motors run at the same time, all the time? Your DCVs when centered block the motor ports. Do you intend to do that? You block the pump outlet when the DCVs are centered. Do you intend to do that without unloading the pump?

Ted

 

[hydtools]

Consider starting with two pumps instead of one. A tandem pump in place of the single pump. One pump for each chain circuit.

Maybe even three pumps in a tandem unit. One for each chain drive and a third for the cutter.

Ted

 

[PNachtwey]

The flow determines the speed.

This is so wrong. Speed determines flow.
Speed is the integral of acceleration. Acceleration is the caused by torque / inertia.
Steady state speed is achieve when the sum of forces or torques acting on the motor is zero.
If the torque is not enough to turn the motor there will be no flow.

Newton did not include flow in his laws of motion.





Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

Peter, that's absolutely correct. So what are the steps you use to size a hydraulic system if you start with acceleration?

 

[HPost]

So wrong?

Is it not the case that for a motor of given displacement in CC/rev, it will rotate at a particular speed, dependent on the flow of oil into the said motor at a rate of cubic metres/sec? Less any losses for volumetric efficiency...

Speed determines flow? Are you sure Peter? Is "determines" the right word?

Pumps drive motors, in a inertial frame of reference where the load on motor is zero, until the pump generates enough pressure to cause the load to accelerate. On this basis, how can speed determine flow? You turn the pump off, the motor will slow down and stop. No flow means no motion.

The steady state speed of the system being the point at which equilibrium is established.

Regarding Newton's laws, I would argue that the first law applies.

In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Flow being a result of a pressure differential, forces act on the fluid, causing it to move, at a constant velocity or steady state, where a constant pressure differential exists.

A 100cc/rev motor will rotate at 1000 revs/min when the flow rate is 100 litres/min

Halve the flow rate and the speed will drop to 500 revs/min.

With a fixed displacement motor, you cannot reduce the displacement and the speed requirement does not change. So the speed and flow are linked and cannot be separated. We therefore say that the flow dictates the speed.

Of course the equation can be swapped to use the same units to solve for input flow based on speed and displacement, but that does mean that to say, in operating motors, that the speed is dictated by the flow into the motor is "so wrong".

 

[PNachtwey]

So what are the steps you use to size a hydraulic system if you start with acceleration?

Since we are talking about hydraulic motors here....
α = dω/dt = (Δpsi*CIR-2*π*fricforce(ω))/(j*2*π)
Since Δpsi = Pa - pb
there needs to be two more differential equations
dPa/dt = (β/Va)*(Q(Pa)-ω*CIR/(2π)) // notice that turning the motor can add
dPb/dt = (β/Vb)*(Q(Pb)+ω*CIR/(2π)) // or subtract oil depending on the rotation
Controlling motor speed is easy until the speed is slow. Hydraulic motors have a lot of static friction so the stick/slip ( Stribeck friction ) ratio is high.

Speed determines flow? Are you sure Peter? Is "determines" the right word?

Yes 'determines' is a little sloppy. I didn't want to write a book.
I should have said that flow equalizes pressure. Pressure drops when the actuator moves so the flow increases. Flow happens because there is motion, not that other way around. If the motor stops turning flow would stop too.

Regarding Newton's laws, I would argue that the first law applies.

Me too.

In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Yes but where is the force/torque in your "flow makes it go"?
Also, the first law should be amended to say net force. There are many forces or torques acting on the motor at the same time.

I have a pretty good hydraulic motor simulation I did in 2010. It solves 8 differential equations simultaneously. It includes

Spool position
spool velocity
accumulator gas volume
A port pressure
B port pressure
angular velocity
angle

I didn't respond earlier because the OP said same torque and in then he said the same speed. One can control speed or torque but can't control both at the same time. Speed and torque can be limited at the same time.

What the OP wants to do will be a endless 'whack a mole' type of operation because he can't control anything without feed back.








Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[HPost]

Flow does NOT make it go...

I hate it when people say that.

None of us want to write a story. This is on the basis that most OP’s don’t read or respond to our free help.

I agree with all your points, except where you misunderstand my point and say that I am so wrong.

Whack a mole is a very good description of the whole thing. I see it every day...

 

[PNachtwey]

Flow does NOT make it go...

You said above

The flow determines the speed.

I hate it when people say that.

Me too. It will get me out of lurking mode.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[MoHabib]

@EngineerTex
The4 motors connected to 4 chains, the pairs of chain presents one chain.

The this photos.

 

[MoHabib]

@Tim

I didn't get it! Can you explain what you mean?

 

[MoHabib]

@Ted

I'm thinking in operating the 5 motors at the same time. Maybe sometimes stopping the 1st chain for moments and keeping the other 3 motors working. Anyway, I don't need it to be complex,just trying to make it work first.

About the symbol of the DCV maybe i selected the wrong one.

 

[tmschrader]

Hello. It does not look like the 4 chains are mechanically linked. So I am not sure how the pairs represent one chain.
In any case motors can be directlly mechanically linked if all 4 motors would be driving the same gear or say one chain. In my problem is was four worm gears driving one gear to rotate a crane. I would call that a high stiffeness example cause they are all linked by gearing. So all motors are at the same speed and could get better load sharing without the divider. But it works either way. I chose to use the gear dividers. But probally did not need them. Some say use some say do not in these cases.
If The 4 motors drive 4 gear boxes with different ratios the need for a accurate divider may be more if they are driving something that may flex or give in such a way that equal load sharing will not occur. This probally occurs where there is a lot of accelerations.
If the motors are driving chains that are not linked by shafts or other gearing then dividers or a speed feedback system with encoders on the motors would be needed to keep the speeds the same or at what they need to be. It does not sound like you want to spend money on a feedback system. Gear dividers do not control speed by 100%. There is some minor error depending on the pressure drops. I also used dividers when I had two independent cranes lifting a boat, with one crane takening more load it would still be a bit faster then the crane with less load. Even with the dividers. But it was not significant. If FULL syncronization is required a feedback system will be required. Or a direct mechanical link. I did this with a shaft connecting the gearboxes on another job.

 

[MikeHalloran]

I'm thinking in operating the 5 motors at the same time.

WHAT 5 motors? You show only four.

Which will be sort of synchronized if whatever the red dot represents is tough enough to resist being punctured or ruptured by all the teeth.




Mike Halloran
Pembroke Pines, FL, USA

 

[MoHabib]

One can control speed or torque but can't control both at the same time. Speed and torque can be limited at the same time.

Peter,

So now maybe i can say that the most important thing to me is controlling the speed and allowing all motors to drive with the highest needed torque in the cutter.

by the way the needed torque to drive the chains is 25 : 40 N.M for each motor in opposite in need at least 100 N.M for the cutter. so no problem in letting the chains motor work with torque more than ( 25 : 40 n.m ) .