Hydraulic workholding components are designed to operate at specific pressure ratings. Understanding these ratings and selecting the appropriate operating pressure ensures optimal clamping force, component longevity, and system reliability.
Standard Pressure Ratings
Hydraulic workholding cylinders and clamps are manufactured to three common design pressure standards. The following table provides conversions between bar, MPa, and PSI for reference.
| Bar |
MPa |
PSI |
Common Name |
| 70 |
7.0 |
1,015 |
Low Pressure |
| 240 |
24.0 |
3,481 |
Medium Pressure (Legacy) |
| 350 |
35.0 |
5,076 |
Standard High Pressure |
| 500 |
50.0 |
7,252 |
Ultra-High Pressure |
70Bar (1,015 PSI)
Low-pressure systems are used for standard clamping applications and systems where air-over-oil intensifiers provide hydraulic pressure. These components are typically smaller and lighter than high-pressure equivalents.
350 Bar (5,076 PSI)
Traditional high-pressure systems represent the a common design pressure for industrial workholding. Components rated for 350 bar provide a balance of compact size, high clamping force, and broad compatibility with standard hydraulic power units.
500 Bar (7,252 PSI)
Ultra-high-pressure systems enable maximum clamping force from the smallest possible cylinder bore. These components are specified when envelope constraints demand minimum size, or when extremely high clamping forces are required. Ultra-high-pressure systems require compatible power units, fittings, and hoses rated for 500 bar service.
Design Pressure vs. Operating Pressure
The design pressure rating of a hydraulic component is the maximum pressure at which it is intended to operate continuously. Components are engineered with safety margins that account for pressure spikes, temperature variations, and fatigue over the service life of the component.
Operating a 350 bar system at its full design pressure of 5,076 PSI is appropriate when the system requires maximum clamping force. Modern hydraulic seals, fittings, and power units are designed to deliver reliable performance at rated pressure.
A Note on Legacy Practice
Some facilities operate 350 bar systems at approximately 70% of their rating rather than the full 5,076 PSI rating. This practice originated from earlier generations of hydraulic equipment where seals and fittings benefited from conservative derating, and from air-over-oil power units that could not reliably deliver higher pressures.
With modern components, this derating is generally unnecessary. Operating at 70% of design pressure means paying for 350 bar capability while receiving lower performance. Either specify components rated for the actual operating pressure, or use the full capability of higher-rated components.
Recommendations for Fixture Design
- Design fixtures for the full rated pressure of the hydraulic components. This maximizes the clamping force available from a given cylinder size.
- Size clamping elements based on required holding force at design pressure. If calculations show that operating at full pressure provides more force than needed, consider specifying smaller (and more compact) components rather than derating larger ones.
- Verify that all system components (power unit, hoses, fittings, manifolds) are rated for the same design pressure. Mixed-pressure systems must be limited to the lowest-rated component.
- Label fixtures with the design operating pressure for future reference. This ensures that maintenance personnel and future users operate the system within its intended parameters.
- For delicate workpieces requiring reduced clamping force, use a pressure-reducing valve to control force rather than arbitrarily operating high-pressure components at low pressure. This provides precise, repeatable force control while maintaining system flexibility.