What is the difference between a servo-hydraulic and an electric fatigue tester?

Servo-Hydraulic Fatigue Tester

• Force & Dynamic Performance:​ The king of high force (up to 1000+ kN)​ and high dynamic performance. Capable of very high loading rates and frequencies (up to hundreds of Hz for smaller strokes/loads). Ideal for large, high-energy events.
• Stroke & Stiffness:​ Typically offers long strokes​ (e.g., ±100mm or more). The system itself is very powerful, but the framework​ must be extremely rigid to react against the immense forces.
• Waveforms:​ Excellent at generating complex, high-amplitude waveforms. Well-suited for random spectra and large block loading sequences.
• Key Components:​ Hydraulic power supply, servovalve, hydraulic actuator, high-pressure lines, hydraulic oil.

1. Extremely High Force Capacity:​ The only practical choice for testing large structural components (e.g., aircraft wings, automotive chassis, bridge sections).
2. High Power Density:​ Delivers massive force from a relatively compact actuator.
3. Proven Durability:​ The technology is mature, robust, and can withstand harsh test environments (e.g., in climatic chambers).

1. High Operating Cost:​ Requires significant electrical power to run the hydraulic power pack, and the hydraulic fluid requires maintenance (filters, changes, potential for leaks).
2. Noise & Heat:​ The power pack is very noisy and generates a lot of waste heat.
3. Complexity & Cleanliness:​ The system is complex to maintain. Oil leaks are a fire hazard and a cleanliness issue (not ideal for clean-room applications).
4. Lower Precision at Low Forces:​ Can be less precise than electric systems at very small forces due to seal friction and system compliance.

Electric (Servo-Electric) Fatigue Tester

• Force & Dynamic Performance:​ Best for low to medium forces​ (typically up to 300-500 kN) and high frequency for small displacements. Dynamic performance is excellent for high cycle fatigue (HCF) with small amplitudes.
• Stroke & Stiffness:​ Stroke lengths are more limited compared to hydraulics. The mechanical drive provides a very stiff system​ with minimal compliance, leading to superb control.
• Waveforms:​ Excellent at generating precise, high-frequency sinusoidal waveforms for HCF testing. Can handle complex waveforms within its force/displacement envelope.
• Key Components:​ Servo motor, ball/roller screw, crosshead, precision load frame.

1. High Precision & Accuracy:​ Exceptionally clean, smooth, and responsive control due to the direct mechanical drive. Excellent for low-force and strain-controlled tests.
2. Energy Efficient & Clean:​ Uses power only when moving. No hydraulic oil, so it’s clean, quiet, and has very low operating costs. Ideal for lab environments.
3. Low Maintenance:​ Essentially just the motor and drive screw, requiring minimal regular maintenance (like lubrication).
4. Ease of Use:​ Often simpler to set up and program for standard test routines.

1. Force Limitation:​ Ultimate force is mechanically limited by the motor torque and screw strength. Cannot compete with hydraulics for mega-newton applications.
2. Speed-Force Trade-off:​ The servo motor’s maximum speed and torque define a power envelope. High force and high speed cannot be achieved simultaneously (unlike hydraulics, which can store energy in accumulators).
3. Heat Generation in Motor:​ At very high duty cycles, the servo motor may require cooling.

Comparison Summary Table

How to Choose?

• Choose Servo-Hydraulic if:​ Your test requires forces above ~500 kN, involves very long strokes with high load, or you need to simulate high-energy, high-rate events​ (like impacts or vehicle curb strikes). It’s the standard for large-scale structural durability.
• Choose Servo-Electric if:​ Your test is within its force/speed envelope, and you prioritize precision, cleanliness, low operating cost, and ease of use. It is the dominant choice for material specimen testing, biomedical devices, and most R&D labs.