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Flow rate and pressure required for a cylinder

Great strides have been made in industry and production since the invention of the hydraulic system. Present in many forms, it enables work to be carried out involving colossal force. Hydraulic cylinders are essential parts of hydraulic systems. If you're reading this article, you're probably wondering: what flow rate and pressure should a cylinder have? Follow the guide!

Before getting to the heart of the matter, let's take a look at the hydraulic system.

The hydraulic system is a set of components that use the pressure exerted on a fluid to perform mechanical work. The difference in pressure between two elements of the system creates movement. Industry frequently uses this type of equipment for tasks that cannot be performed by human power alone. But what is a hydraulic system generally made up of? There are 4 main assemblies.

The first isthe power unit. It consists of a pump, a reservoir and a drive motor. The combination of these elements allows the fluid to circulate in the hydraulic system. The fluid is taken from the reservoir by the pump and sent to the motor. The motor recovers the hydraulic energy created by the pump and converts it into mechanical energy (thanks to the actuators).

Control, as its name suggests, is used to maintain harmonious fluid management by controlling the system's pressure and flow. It prevents serious accidents by redirecting excess fluid to the reservoir.

Distribution allows the fluid to pass through the circuit and control the direction it takes within the system. Distributors can have several flow rates. To choose the right valve, it is of course necessary to assess the desired flow rate for the hydraulic system in advance.

Actuators are added to the generating set to transform hydraulic energy into mechanical energy. They enable heavy parts to move. The energy is converted by rotary and linear movements. The hydraulic linear actuator has another name: the hydraulic cylinder!

As an actuator, our famous hydraulic cylinder receives hydraulic energy and converts it into mechanical force. Cylindrical cylinders are divided into two chambers by pistons. They also have a rod at their centre.

Flow rate and pressure required for a cylinder

To calculate the flow rate and pressure required, check the diameter of the piston, the diameter of the rod and the stroke in millimetres. From this, we can calculate the surface area of the piston, rod and annular surface. We can use this as a basis for defining the pressure and flow rate.

If Sf represents the surface area of the piston, St the surface area of the rod and Sa the annular surface, we can take the following references:

  • With Sf = 78.54 cm², St = 38.48 cm² and therefore Sa = 78.54 - 38.48 = 40.06 cm²
  • Cylinder surface ratio Sf/St of 1.96
  • Rod extension time 7 s
  • Speed: V(m/s) = stroke (m) / time (s) therefore: 0.5 m / 7 s = 0.071m/s

We calculate the flow rate using the following formula: Q (L/min) = 6*S(cm²)*V(m/s):

  • Flow entering at the bottom = Q(L/min) = 6 x 78.54 (cm²) x 0.071 (m/s) = 33.45 L/min
  • Flow leaving stem side = Q(L/min) = 6 x 40.06 (cm2) x 0.071 (m/s) = 17.06 L/min

We calculate the pressure using the following formula: P = F (daN) / S (cm²). In our case, the pressure will therefore be : P = 7000 daN / 78.54cm² = 89 bar.

These formulas are a little tricky to get to grips with at first, but it's like riding a bike, you never forget!

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