Fatigue-dependent load cells 101

In contrast to standard load cells, which are designed for static or infrequent dynamic measurements, the fatigue-resistant load cells from Interface specifically designed to withstand millions of repeated loading cycles without compromising their accuracy or structural integrity.

Interface's history began with fatigue-resistant load cells. We've been a leader in this technology since 1968. Our fatigue-resistant load cells are designed to withstand more than 100 million fully reversed load cycles at full capacity. This remarkable endurance is due to the robust mechanical deflection of the load cell and the sensitive strain gauges that form the core of the sensor.

The advantage of fatigue resistance lies in design and construction

What makes a fatigue-resistant load cell? This includes detailed design, careful material selection, and rigorous manufacturing processes. Here are some key mechanical and design considerations:

#1 Reduced Stress Values ​​– During fatigue-tested bends, stress values ​​are approximately half that of a standard Interface LowProfile® load cell. This significant stress reduction minimizes the risk of fatigue crack formation and propagation during extended use.

#2 Optimized geometries and surface finish – During production, internal points of high stress concentration, such as sharp corners and edges, are carefully polished to eliminate potential sites for crack development. This micro-level attention to detail significantly increases the lifespan of the load cell.

#3 Reduced sensitivity to external loads – Fatigue-rated load cells undergo special modifications to ensure lower sensitivity to external loads than standard LowProfile® models. This is critical in dynamic testing scenarios where off-center forces and moments can cause errors and accelerate fatigue.

#4 Controlled Mechanical Deflection – Fatigue-tested load cells typically exhibit a mechanical deflection approximately half that of a standard, non-fatigue-tested load cell for the same applied load. Consequently, the output signal is also approximately half as large. This reduced deflection contributes to the longevity of both the mechanical structure and the strain gauges under cyclic loading.

All Interface LowProfile® fatigue-rated load cells are specified for a lifetime of 100 million load cycles with full reversal of the measurement direction, a testament to their robust design and manufacturing.

Mechanical considerations for fatigue testing

Two main types of fatigue failure are high-cycle and low-cycle fatigue. High-cycle fatigue refers to low stress loads acting on a mechanical component that can lead to fatigue failure over millions of load cycles. Low-cycle fatigue refers to higher stress loads that lead to failure over fewer cycles. Because fatigue testing is very demanding, it is even more important that the mechanical components are correctly adjusted and installed. During installation, avoid applying torque or excessive force directly to the sensor body.

Mechanical capacity

Using a load cell with insufficient capacity can lead to mechanical failure or permanent damage. Selecting a significantly oversized load cell can result in mediocre performance and reduced resolution, especially when measuring small dynamic load changes.

Mechanical fastening

Proper fastening is critical during fatigue testing due to the repetitive loading involved. Understand the effects of tensioned and untensioned mounting positions on load application and the potential for side loading. Route and secure cables properly to prevent them from exerting unwanted forces on the load cell. Ensure all connections are securely fastened to prevent movement and potential stress concentrations. Applying preload can help maintain consistent contact and reduce the effects of dynamic forces on fasteners. Improper fastening can have up to 15 times the impact on static test performance during fatigue testing.

Mechanical fastening

Use appropriate fastening and isolation techniques to minimize the transmission of extraneous vibration and shock to the load cell. The test setup should be designed to prevent component separation or loosening under cyclic loading. Fasteners must be tightened to the manufacturer's specified torque to ensure consistent clamping force throughout the test. Using high-quality threads minimizes wear and the risk of fastener failure over millions of cycles. Ensure all fasteners are rated for the expected duration and load cycles of the test. For very long duration tests, consider scheduled replacement of critical fasteners.

Mechanical limits in fatigue testing

These limitations, which are due to the mechanical aspects of the test setup, can significantly affect the accuracy and validity of the test. These include the stiffness of the testing machine and the compliance of the grips, which can influence the dynamic behavior and force application to the specimen. Properly designed grips and fixtures ensure even load distribution and prevent premature failure at the gripping points. Temperature fluctuations, humidity, and other environmental conditions can affect the mechanical properties of the test setup and load cell. Consistent specimen preparation and knowledge of specimen geometry are essential for repeatable fatigue testing. Inertia and other dynamic effects at high test frequencies can complicate load measurement and control.

Advantages of using interface load cells with fatigue assessment:

• Long-term reliability: Guaranteed performance for more than 100 million fully reversed load cycles minimizes downtime and ensures consistent data over extended testing periods.
• Precise force measurement: Despite demanding cyclic loading, these load cells maintain high accuracy and provide reliable data for critical fatigue analyses.
• Durability in dynamic environments: The robust design and construction withstand the stresses of repeated loading and dynamic forces.
• Validation and liability protection: Critical for generating fatigue data to validate designs, comply with safety regulations, and mitigate potential liability issues.
• FEA Model Validation: Experimental fatigue data obtained with these load cells can be used to refine and validate finite element analysis (FEA) models to improve design accuracy.
• Identification of critical failure points: Fatigue tests with precise force measurement help to identify weak points in the design, optimize material selection, and determine assembly methods.

Interface's fatigue-resistant load cell offering

Interface offers a comprehensive range of fatigue-resistant load cells to meet a wide range of application requirements.

Technical Tip: Interface fatigue load cells are based on fully reversed loading cycles, meaning they are designed for bidirectional loading. This type of loading cycle is more severe than unidirectional loading and is the most common application for load cells. When a fatigue load cell is repeatedly loaded in only one direction, it can be loaded up to 133% of its bidirectional fatigue capacity without affecting its fatigue performance.

FATIGUE-RESISTANT LOW-PROFILE® TOP INTERFACE LOAD CELLS

• 1000 LowProfile load cell with fatigue rating – Versatile for tension, compression, or general-purpose applications with capacities from 1 lbf to 1 million lbf. It features a safe overload rating of 300% and extremely low torque sensitivity.
• 1000 High-Capacity Fatigue-Rated LowProfile Load Cell – Designed for higher force measurements while maintaining fatigue strength.
• 1500 LowProfile low capacity load cell – This cell is ideal for low capacity applications where low sensitivity to eccentric loads is critical.
• 1208 Standard Precision Universal LowProfile Flange Load Cell – Flange design with high capacities and low torque sensitivity.
• 1700 LowProfile load cell with flange mounting – Compact design with flange mounting options.

MOST POPULAR INTERFACE FATIGUE-RATED MINI® LOAD CELLS

• MBI Overload Protected Miniature Beam Load Cell with Fatigue Rating – Compact beam construction with overload protection.
• SSMF Fatigue Tested S-Type Load Cell – S-beam construction specifically designed for fatigue applications.
• SuperSC S-Type Miniature Load Cell – Fatigue Rating Option – Miniature S-beams with a fatigue rating option for space-constrained applications.

AVAILABLE MINI LOAD CELL STANDARD MODELS WITH FATIGUE-RESISTANT OPTIONS

• SM, SMA, SMTM, SMT, SSM, SSM2, SSM-FDH, SSMH, and SuperSC

Each fatigue load cell model group offers various capacities and variants, including additional bridge options. All product variants, capacities, customization options, and availability are available online. As with all Interface load cells, custom designs and customized solutions are also available.

Applications for fatigue-dependent load cells

The unique capabilities of fatigue testing load cells make them indispensable for a variety of demanding applications. Here's a brief overview of the types of fatigue testing.