# Thread: Selection of hydraulic pump

1. ## Selection of hydraulic pump

Hello,
In the following diagram, I want to synchronize two cylinders in parallel for my application, I would like to know how to choose the pump. For the cylinders have a dimension of 40*28mm and the flow is 30l/min.
Thanks

2. The following equations should help:

Determine the volume of fluid you need for both hydraulic cylinders to extend and retract.

Flow rate: Q = D (Displacement) n ( revolutions) / 1000

Next, Horsepower Required to Drive a Pump from the GPM needed based of extending and retracting the cylinders and the pressure required

GPM x PSI x .0007 = horsepower needed

Next the Pump Output Flow (in Gallons per Minute)

RPM x Displacement ÷ 231 = gpm

Pump Displacement Needed for GPM of Output Flow

RPM x Displacement ÷ 231 = gpm

3. Hello sir;
I have another question about the pressure, should I take the pump for a pressure equal to twice the pressure of a single cylinder, or I take a pump with the pressure needed to move a single cylinder.
Thank you very much for your help

4. Originally Posted by m6957341
Hello sir;
I have another question about the pressure, should I take the pump for a pressure equal to twice the pressure of a single cylinder, or I take a pump with the pressure needed to move a single cylinder.
Thank you very much for your help
You'll want a pump capable of a higher pressure than required for full function of the cylinders as operating a pump at capacity tends to shorten the life span of the pump. The pressure regulator should be rated for the flow volume required and provide the pressure regulation accuracy needed.

5. Both cylinders will see the same pressure because they are fed from a common volume. That means the pump only has to produce that pressure. It also has to produce enough volume (at that pressure) to move both cylinders at the desired rate.

I'm not disagreeing with Kelly. He's 100% right. I just thought you were asking a more basic hydraulics question.

6. Originally Posted by jboggs
Both cylinders will see the same pressure because they are fed from a common volume. That means the pump only has to produce that pressure. It also has to produce enough volume (at that pressure) to move both cylinders at the desired rate.

I'm not disagreeing with Kelly. He's 100% right. I just thought you were asking a more basic hydraulics question.
Hello,
If I have a 1450 psi pump, the two pumps will have a pressure of 1450 psi, or 725 psi for each pump.
Thank you very much for your help.

7. Please read your post again. I think you are confusing pumps and cylinders. In your original diagram you had one pump and two cylinders. But now you are talking about having two pumps.

Pressure is not additive or distributive. You cannot create more pressure by adding more pumps. Neither can you decrease the pressure in a circuit by adding more cylinders. It is what it is.

If a single pump, or a combination of pumps, creates 1450 psi in a circuit then every component in that circuit will see 1450 psi, regardless of how many components there are. Both cylinders will see 1450 psi.

Also in your original post you said you want to synchronize the motion of two cylinders. If they both see the same pressure (which they will) and are working against equal loads, theoretically they will move the same amount at the same speed. But that word "theoretically" is a big word. It rarely ever works out that way in the real world due to minute differences in internal friction, circuit design, mounting difference, etc.

If the two cylinders MUST be synchronized there are ways to do it, but that's for another post.

8. Originally Posted by jboggs

Pressure is not additive or distributive. You cannot create more pressure by adding more pumps. Neither can you decrease the pressure in a circuit by adding more cylinders. It is what it is.

If a single pump, or a combination of pumps, creates 1450 psi in a circuit then every component in that circuit will see 1450 psi, regardless of how many components there are. Both cylinders will see 1450 psi.

Also in your original post you said you want to synchronize the motion of two cylinders. If they both see the same pressure (which they will) and are working against equal loads, theoretically they will move the same amount at the same speed. But that word "theoretically" is a big word. It rarely ever works out that way in the real world due to minute differences in internal friction, circuit design, mounting difference, etc.

If the two cylinders MUST be synchronized there are ways to do it, but that's for another post.
Hello,
Thank you very much, I wanted to write two cylinders and I wrote instead two pumps, now I understood well.
For the synchronization I'm going to use rotary flow dividers, I find it's the easiest way than synchronizing by series cylinders which is very difficult to do.
I thank you for this precious help.

One comment - you said you plan to use rotary flow dividers - plural! You will only need ONE flow divider. It takes a singular input flow and distributes it equally to two or more output flow paths. It basically takes the place of the TEE fitting in the line.

10. Another consideration is that unequal hydraulic lines can cause unequal flow to the cylinders.

It's a good idea, to try to keep the line equal in length and connector fittings.