When operating a hydraulic press, many people struggle with the distinction between two different types of pressure: the hydraulic pressure applied to the cylinder and the pressure applied to the workpiece.
These two pressures are expressed in different units and serve different purposes, making them challenging to understand.
In this article, we will clearly differentiate between hydraulic cylinder pressure and workpiece pressure, explain how to convert between pressure units, and provide calculation examples.
By the end, you will be able to handle pressure settings and management with confidence.
Pressure Conversion: MPa ⇔ kgf/cm²
There are two main pressure units commonly used in hydraulics: MPa (megapascal) and kgf/cm² (kilogram-force per square centimeter).
- 1 MPa ≒ 10.2 kgf/cm²
- 1 kgf/cm² ≒ 0.098 MPa
Conversion Examples:
- 10 MPa ≒ 10 × 10.2 = 102 kgf/cm²
- 10 kgf/cm² ≒ 10 × 0.098 MPa = 0.98 MPa
(Tip: You can roughly think of 10 kgf/cm² as approximately 1 MPa for simplicity!)
Pressure Units by Country/Region
Different countries and regions use different pressure units. Below are some common conversions:
- 1 MPa = 10 bar
- 1 MPa = 145 psi (pounds per square inch)
- 1 MPa ≒ 1097.61 ton/ft² (tons per square foot)
Additionally:
- 1 cm = 0.3937 inches (in)
- 1 foot (ft) = 12 inches (in)
Here is a summary of the typical hydraulic pressure units and force per unit area used in different countries and regions:
| Region/Country | Hydraulic Pressure Unit | Force per Unit Area |
|---|---|---|
| Japan | MPa | kgf/cm² |
| USA | psi | psi, ton/ft² |
| Europe | MPa, bar | kgf/cm², N/m² |
| China | MPa | kgf/cm² |
| South Korea | MPa | kgf/cm² |
| India | MPa, bar | kgf/cm², N/m² |
| Southeast Asia | MPa, bar | kgf/cm² |
| Russia | MPa, bar | kgf/cm² |
As seen in the table, Japan and many Asian countries primarily use MPa and kgf/cm², while the U.S. mainly uses psi, and Europe frequently uses bar.
Avoiding Confusion Between Hydraulic Pressure and Workpiece Pressure
As mentioned earlier, operating a hydraulic press involves dealing with two types of pressure: the pressure inside the hydraulic cylinder and the pressure applied to the workpiece. This can often lead to confusion.
From our perspective as press machine designers and manufacturers, we focus on different types of pressure compared to those working with material processing. The units commonly used also differ.
To avoid confusion, we intentionally switch units depending on the situation. Below is the way we distinguish between them—this is simply our approach, so please consider it as one example.
Don’t worry! The press machine’s touch panel allows you to set the unit to the one you are most familiar with.
Hydraulic Pressure is Measured in MPa
For hydraulic pressure inside the cylinder, we use MPa (= N/mm²).
Workpiece Pressure is Measured in kgf/cm²
For force applied per unit area on the workpiece, we use kgf/cm².
This unit is beneficial when calculating pressing force in tons.
Example:
If a 50 cm × 50 cm workpiece needs 10 kgf/cm² of pressure, the required pressing force is:
50 cm × 50 cm × 10 kgf/cm² / 1000 = 25 tons
Thus, we can quickly determine that 25 tons of force is needed.
(Note: We use cm instead of mm in these calculations for convenience in hydraulic calculations.)
Example Calculation: Determining Hydraulic Pressure from Workpiece Pressure
Let’s go through a specific calculation example.
【Hydraulic Pressure Calculation Example】
A 100-ton press machine applies force using a single φ250 (diameter 250 mm) ram cylinder.
The workpiece is 50 cm × 50 cm, and the required pressure is 10 kgf/cm².
What hydraulic pressure (MPa) is needed?
Step 1: Determine the Required Force
We’ve already calculated this:
50 cm × 50 cm × 10 kgf/cm² / 1000 = 25 tons
Step 2: Calculate the Cylinder’s Pressure Area
The pressure area refers to the surface area of the cylinder where pressure is applied.
For a φ250 cylinder, the radius is 12.5 cm, so:
12.5² × 3.14 ≒ 490 cm²
(Side note: This 100-ton press has a maximum pressure of 20.6 MPa, so its maximum pressing force is:)
490 cm² × 20.6 MPa × 10.2 / 1000 ≒ 102 tons
Step 3: Calculate the Required Hydraulic Pressure
Using the formula:
(Force in tons × 1000) / Cylinder Area
we get:
25 tons × 1000 / 490 cm² ≒ 51 kgf/cm²
51 kgf/cm² × 0.098 MPa ≒ 5 MPa
Answer: The required hydraulic pressure is approximately 5 MPa.