High Capacity Force Calibration Above 4.4 MN (1,000,000 lbf)

Figure 1 Picture of Morehouse High Capacity Force Calibration 10MN System UCM Using Three Symmetrical Morehouse Load Cells

In previous posts, we have discussed ASTM E74 requirements for secondary standards.

However, we have not discussed what happens when primary standards cannot calibrate the secondary standards. Such is the case with High Capacity Force Calibration above 4.4 MN, as NIST is limited deadweight standards up to 1,000,000 lbf.

The ASTM E74 gives guidance that secondary force standards with capacities exceeding 1,000,000 lbf (4.4 MN) are not required to be calibrated by primary force standards because no higher direct deadweight calibration exists. Instead, multiple secondary force standards of equal compliance may be combined and loaded in parallel to achieve higher capacities.

The standard also addresses force-multiplying systems used as secondary force standards, the multiplying ratio must be measured at not less than three points over its range, with an accuracy of 0.05 % of ratio or better.

These systems must also be checked annually using elastic force-measuring instruments within their Class AA verified range to ensure applied forces remain within acceptable limits. Any deviations exceeding 0.05 % of applied force would necessitate reverification.

To break this down further, Secondary Force Standards (load cells):

• Secondary force standards (e.g., high-capacity load cells, proving rings) must be calibrated by primary force standards; however, an exception is made for capacities exceeding 1,000,000 lbf.
• If exceeding 1,000,000 lbf, a combination of multiple secondary standards may be used in parallel loading.
• The Lower Limit Factor (LLF) for such combinations is calculated using quadrature (root sum square) of the LLFs of individual instruments.

For our High Capacity Force Calibration System, Morehouse uses multiple secondary standards directly calibrated by NIST. We use three equal standards to achieve 10 MN or (2,250,000 lbf).

Figure 2 Picture of the Morehouse Morehouse High Capacity Force Calibration System 10MN Machine Being Assembled

High Capacity Force Calibration Build-up System Components to 2,250,000 

• Force Transducers: Three or more identical transducers are arranged in a symmetrical configuration (e.g., 120° apart) and loaded in parallel.
• Adapters:  Consisted of a base plate, load caps for each load cell that were ground to ensure each load cell was of equal height with the adapters on top, and a large load button.
• Load Transfer Mechanism: Ensures even force distribution among transducers. We built a custom UCM (Pictured Above)
• Calibration System: Each transducer is calibrated using a NIST deadweight before combining them together.

• Automation Control: Morehouse refined our Automation Control System to use a feedback loop from three 4215 indicators.
• Data Summation Program: Morehouse developed software to monitor each load cell individually and then sum all of the readings together so the technician can make sure each load cell is reading properly.

Additional notes on the high capacity force calibration system: The first time the system was loaded, each load cell was within 10 % of the expected output. Upon testing, we monitored each load cell's output and used shims to ensure each load cell read within 1 % of each other.

Meaning at 1,500,000 lbf, each cell read 500,000 lbf ± 1 %. After this was verified to work, we proceeded to grind and repolish the top caps.

All three load cells have been to NIST multiple times, and all load cells read within ± 1 % of each other. We can achieve about 0.025 % of the applied force from 200,000 lbf through 2,250,000 lbf and 0.01 % of the applied force from 1,000,000 lbf through 2,250,000 lbf.

High Capacity Force Calibration Above 4.4 MN (1,000,000 lbf) Conclusion

High-capacity force calibration beyond 4.4 MN (1,000,000 lbf) presents a unique challenge due to the absence of primary deadweight standards at these levels. To address this, calibration systems like the Morehouse High Capacity Force Calibration 10MN System employ multiple secondary force standards in a parallel build-up configuration.

Key Takeaways:

• ASTM E74 Guidance:

  • For forces above 1,000,000 lbf, secondary standards can be combined to extend calibration capabilities.
  • Force-multiplying systems must maintain 0.05% accuracy and undergo annual verification.
  • Lower Limit Factor (LLF) is determined via the root sum square method for combined load cells.

• Morehouse 10MN Calibration System:

  • Uses three identical load cells calibrated by NIST, arranged symmetrically.
  • Custom-built load transfer mechanism ensures even force distribution.
  • Automation control & data summation software enables precise monitoring.

• Performance & Accuracy:

  • Initial system setup achieved ±10% of expected output, refined to ±1% per load cell.
  • Achieves 0.025% uncertainty from 200,000 lbf to 2,250,000 lbf and 0.01% from 1,000,000 lbf upward.

This system exemplifies the effective use of a build-up method to achieve high-force calibration accuracy where direct primary standards are unavailable. Through rigorous calibration, precision adjustments, and advanced monitoring, Morehouse has successfully extended force calibration capabilities to 10MN (2,250,000 lbf) with exceptional reliability.

More Information about Morehouse 

We believe in changing how people think about Force and Torque calibration in everything we do. That includes letting people know what we did to achieve our high capacity force calibration system.

This includes setting expectations and challenging the "just calibrate it" mentality by educating our customers on what matters and what may cause significant errors.

We focus on reducing these errors and making our products simple and user-friendly.

This means your instruments will pass calibration more often and produce more precise measurements, giving you the confidence to focus on your business.

Companies around the globe rely on Morehouse for accuracy and speed.

Our measurement uncertainties are 10-50 times lower than the competition, providing you with more accuracy and precision in force measurement.

We turn around your equipment in 7-10 business days so you can return to work quickly and save money.

When you choose Morehouse, you're not just paying for a calibration service or a load cell.

You're investing in peace of mind, knowing your equipment is calibrated accurately and on time.

Through Great People, Great Leaders, and Great Equipment, we empower organizations to make Better Measurements that enhance quality, reduce risk, and drive innovation.

With over a century of experience, we're committed to raising industry standards, fostering collaboration, and delivering exceptional calibration solutions that build a safer, more accurate future.

Contact Morehouse at info@mhforce.com to learn more about our calibration services and load cell products.

Email us if you ever want to chat or have questions about a blog.

We love talking about this stuff. We have many more topics other than expressing SI units!

Our YouTube channel has videos on various force and torque calibration topics here.

# High Capacity Force Calibration