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: psi ⇔ ton/ft²
There are two main pressure units commonly used in hydraulics: psi (pounds per square inch) and ton/ft² (tons per square foot).
- 1 psi ≈ 0.072 ton/ft²
- 1 ton/ft² ≈ 13.89 psi
- 1 kgf/cm² ≈ 0.2048 ton/ft²
- 1 ton/ft² ≈ 4.882 kgf/cm²
Conversion Examples:
- 100 psi ≈ 100 × 0.072 = 7.2 ton/ft²
- 10 ton/ft² ≈ 10 × 13.89 = 138.9 psi
- 10 kgf/cm² ≈ 10 × 0.2048 = 2.048 ton/ft²
- 5 ton/ft² ≈ 5 × 4.882 = 24.41 kgf/cm²
(Tip: You can roughly think of 14 psi as approximately 1 ton/ft² for simplicity!)
Pressure Units by Country/Region
Different countries and regions use different pressure units. Below are some common conversions:
- 1 MPa (N/mm²) = 145 psi
- 1 MPa ≈ 1097.61 ton/ft²
- 1 bar = 14.5 psi
Additionally:
- 1 inch (in) = 2.54 cm
- 1 foot (ft) = 12 inches (in)
Typical Hydraulic Pressure Units and Force per Unit Area by Region
| Region/Country | Hydraulic Pressure Unit | Force per Unit Area |
|---|---|---|
| USA | psi | psi, ton/ft² |
| Japan | MPa | kgf/cm² |
| 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 psi
For hydraulic pressure inside the cylinder, we use psi (pounds per square inch).
Workpiece Pressure is Measured in ton/ft²
For force applied per unit area on the workpiece, we use ton/ft².
This unit is beneficial when calculating pressing force in tons.
Example:
If a 1 ft × 1 ft workpiece needs 10 ton/ft² of pressure, the required pressing force is:
1ft×1ft×10ton/ft2=10tons1 ft × 1 ft × 10 ton/ft² = 10 tons
Thus, we can quickly determine that 10 tons of force is needed.
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 10-inch diameter (diameter = 10 inches) ram cylinder.
The workpiece is 1 ft × 1 ft, and the required pressure is 10 ton/ft².
What hydraulic pressure (psi) is needed?
Step 1: Determine the Required Force
We’ve already calculated this:
1ft×1ft×10ton/ft2=10tons1 ft × 1 ft × 10 ton/ft² = 10 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 10-inch diameter cylinder, the radius is 5 inches, so:
52×3.14≈78.5extin25^2 × 3.14 ≈ 78.5 ext{ in²}
(Side note: This 100-ton press has a maximum pressure of 3,000 psi, so its maximum pressing force is:)
78.5extin2×3,000extpsi≈235.5exttons78.5 ext{ in²} × 3,000 ext{ psi} ≈ 235.5 ext{ tons}
Step 3: Calculate the Required Hydraulic Pressure
Using the formula:
extForceintons×2,000/extCylinderArea(in2) ext{Force in tons} × 2,000 / ext{Cylinder Area (in²)}
we get:
10exttons×2,000/78.5extin2≈255extpsi10 ext{ tons} × 2,000 / 78.5 ext{ in²} ≈ 255 ext{ psi}
Answer: The required hydraulic pressure is approximately 255 psi.
By understanding these pressure conversions and calculations, you can efficiently manage and optimize your hydraulic press operations, ensuring accurate force application and material processing!