Fatigue-Rated Load Cells 101

Unlike standard load cells designed for static or infrequent dynamic measurements, Interface Fatigue-Rated Load Cells are specifically engineered to withstand millions of repeated loading cycles without compromising their accuracy or structural integrity.

At Interface, our legacy began with fatigue-rated load cells. We have been at the forefront of this technology since 1968. Our fatigue-rated load cells are engineered for an operational life exceeding 100 million fully reversed, full-capacity loading cycles. This notable endurance applies to the load cell’s robust mechanical flexure and the sensitive strain gages that form the sensor’s core.

The Fatigue-Rated Advantage is Design and Construction

What defines a fatigue-rated load cell? It involves detailed design, careful material selection, and stringent manufacturing processes. Here are some key mechanical and design considerations:

#1 Reduced Design Stress Levels – In fatigue-rated flexures, the design stress levels are about one-half as high as those in a standard Interface LowProfile® load cell. This significant stress reduction minimizes the risk of fatigue crack initiation and propagation over extended use.

#2 Optimized Geometries and Surface Finishes – During production, internal high-stress concentration points, such as sharp corners and edges, are meticulously polished to eliminate potential sites for crack development. This diligence at the micro-level significantly enhances the load cell’s fatigue life.

#3 Lower Extraneous Load Sensitivity – Fatigue-rated load cells undergo specific adjustments to ensure lower sensitivity to extraneous loads than standard LowProfile® models. This is crucial in dynamic testing scenarios where off-axis forces and moments can introduce errors and accelerate fatigue.

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

All Interface fatigue-rated LowProfile® load cells are specified for a service life of 100 million fully reversed, full-capacity loading cycles, a testament to their robust design and manufacturing.

Mechanical Considerations for Fatigue Testing

Two essential types of fatigue failure are high-cycle and low-cycle fatigue. High-cycle fatigue refers to low stress loads acting on a mechanical component, which can lead to fatigue failure over millions of load cycles. Low-cycle fatigue relates to higher stress loads that lead to failure over fewer cycles. The demanding nature of fatigue testing amplifies the importance of proper mechanical setup and installation. Avoid torquing or applying excessive force directly through the sensor body during installation.

Mechanical Capacity

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

Mechanical Mounting

Proper mounting is critically vital in fatigue testing due to the repetitive nature of the loads. Understand the implications of live and dead-end mounting configurations on load introduction and potential for side loading. Properly route and secure cables to prevent them from exerting unwanted forces on the load cell. Ensure all connections are securely fastened to avoid movement and potential stress concentrations. Applying a pre-load can help maintain consistent contact and reduce the impact of dynamic forces on the fasteners.  Improper mounting can have up to 15 times the effect on static test performance in fatigue testing scenarios.

Mechanical Fixturing

Employ appropriate fixturing and isolation techniques to minimize the transmission of extraneous vibrations and shocks to the load cell. The testing setup should be designed to prevent any separation or loosening of components under cyclical loading. Fasteners must be torqued to the manufacturer’s specifications 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 test’s expected duration and load cycles. Consider planned replacements for critical fasteners in very long-duration tests.

Mechanical Limits in Fatigue Testing

These limitations imposed by the mechanical aspects of the test setup can significantly affect the test’s accuracy and validity. Considerations include the stiffness of the testing machine and the compliance of the fixture, which can influence the dynamic response and load application to the specimen. Properly designed grips and fixtures ensure uniform 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 the load cell. Consistent specimen preparation and understanding its geometry are essential for repeatable fatigue testing. Inertia and other dynamic effects at high testing frequencies can introduce load measurement and control complexities.

Benefits of using Interface Fatigue-Rated Load Cells:

• Long-Term Reliability: Guaranteed performance for over 100 million fully reversed loading cycles minimizes downtime and ensures consistent data over extended test periods.
• Accurate Force Measurement: Despite the demanding cyclic loading, these load cells maintain high accuracy, providing reliable data for critical fatigue analysis.
• Durability in Dynamic Environments: Robust design and construction withstand the rigors of repetitive loading and dynamic forces.
• Validation and Liability Protection: Crucial for generating fatigue data to validate designs, meet safety regulations, and mitigate potential liability issues.
• FEA Model Validation: Experimental fatigue data obtained using these load cells can be used to refine and validate finite element analysis (FEA) models, improving design accuracy.
• Identification of Critical Failure Points: Fatigue testing with accurate force measurement helps identify weak points in the design, optimize material selection, and determine assembly methods.

Interface Fatigue-Rated Load Cell Offerings

Interface offers a comprehensive range of fatigue-rated load cells to meet diverse application requirements.

Engineering Tip: Interface fatigue-rated load cells are based on fully reversed load cycles, which means they are rated for two directions. This type of loading cycle is more stringent than unidirectional loading and is the most common application of load cells. If a fatigue load cell is repeatedly loaded in only one direction, it can be loaded to about 133% of the bidirectional fatigue-rated capacity without degradation of its fatigue rating.

TOP INTERFACE FATIGUE-RATED LOWPROFILE® LOAD CELLS

1000 Fatigue-Rated LowProfile Load Cell – Versatile for tension, compression, or universal applications with capacities from 1 lbf to 1 million lbf. It features a 300% safe overload and extremely low moment sensitivity.

1000 High-Capacity Fatigue-Rated LowProfile Load Cell – Designed for higher force measurements while maintaining fatigue rating.

1500 Low Capacity LowProfile Load Cell – This cell is ideal for low-capacity applications where low sensitivity to eccentric loads is critical.

1208 Flange Style Standard Precision Universal LowProfile Load Cell – Flange-style mounting with high capacities and low moment sensitivity.

1700 Flange-Mounted LowProfile Load Cell – Compact design with flange mounting options.

MOST POPULAR INTERFACE FATIGUE-RATED MINI® LOAD CELLS

MBI Overload Protected Miniature Beam Fatigue-Rated Load Cell – Compact beam design with overload protection.

SSMF Fatigue Rated S-Type Load Cell – S-beam design specifically rated for fatigue applications.

SuperSC S-Type Miniature Load Cell – Option for Fatigue-Rated – Miniature S-beam with a fatigue-rated option for space-constrained applications.

AVAILABLE MINI LOAD CELL STANDARD MODELS WITH FATIGUE-RATED OPTIONS

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

Each fatigue-rated cell model grouping has various capacities and variations, including additional bridge options. All product variations, capacities, customization options, and availability are online. As with all Interface load cells, engineered-to-order products and customized solutions are possible.

Use Cases for Fatigue-Rated Load Cells

The unique capabilities of fatigue-rated load cells make them indispensable in various demanding applications. Here is a quick overview of the types of fatigue testing.

• Manufacturers of Test Machines: These load cells are the backbone of reliable, long-lasting fatigue testing equipment.
• Aerospace and Spacecraft Industries: These are critical for long-term structural fatigue testing of aircraft wings, fuselage components, and spacecraft structures, ensuring safety and reliability over their operational lifespan. Check out the Aircraft Wing Fatigue App Note.
• Automotive and Vehicle Industries: Used extensively in assessing the durability of chassis components, suspension systems, and other critical parts subjected to millions of load cycles.
• Consumer Products: From furniture stability testing to the fatigue life assessment of electronic devices, these load cells help ensure product longevity and safety. Review: Furniture Fatigue Cycle Testing App Note and Bike Frame Fatigue Testing App Note.
• Robotics and Industrial Equipment: This is essential for evaluating the endurance of robotic arms, actuators, and other machinery operating under repetitive loads.

When the longevity and reliability of your test setup and the integrity of your product designs are paramount, Interface Fatigue-Rated Load Cells offer the accuracy, durability, and endurance you can depend on. With a legacy built on this specialized technology, we remain committed to providing testing professionals with best-in-class force measurement solutions for the most demanding fatigue and cycle testing applications.

For additional reference, review Considerations for Fatigue-Rated Load Cells. If you are exploring fatigue-rated load cells, contact our application engineers to answer questions and ensure you get the right product for your testing needs.