Measuring Preload Force Without a Test Bench: 3 Practical Methods for Production

How can you be sure your bolted joint actually achieves the required clamping force? The honest answer: on most production lines, you cannot - at least not directly. As we showed in our article Preload Force vs. Torque: Why You May Be Measuring the Wrong Quantity, around 90% of the applied torque is lost to friction. Only the remaining fraction generates real preload force - the parameter that truly matters.

It is possible to set up a classic test bench with a load cell, but in ongoing series production this is neither practical nor economical. The good news: there are three production-ready methods that allow you to measure preload force without a stationary test bench. Each has clear strengths and limitations - and the right choice depends on your specific application.

Method 1: Ultrasonic Measurement - High Accuracy, High Effort

How the method works

The ultrasonic measurement method uses the so-called acoustoelastic effect: when a bolt is under tensile load, the transit time of a longitudinal ultrasonic wave through the bolt shank changes. The wave travels along the full length of the fastener, is reflected at the end of the bolt, and then received again by the ultrasonic transducer.

From the change in transit time between the unloaded and tightened state, you can calculate elongation - and therefore the resulting preload force. In other words, you use ultrasonic measurement to directly measure bolt stretch.

Advantages

• Direct measurement of bolt elongation: The method measures a quantity that is physically very close to preload force and is independent of friction.
• High single-measurement accuracy: When using only the longitudinal wave (the so-called assembly mode), you can achieve an accuracy of about 3%.
• Validation of existing tightening processes: Ideal for experimentally verifying assumed friction coefficients.

Limits and disadvantages

The method has noticeable limitations in series production. With the single-mode approach, you need to know the reference transit time of every single fastener in the unloaded condition, as well as a material- and joint-specific calibration. That means: every bolt must be measured individually before installation - a significant extra effort in a series environment.

There are further limitations:

• The bolt surface must be accessible and prepared for acoustic coupling.
• Ultrasonic measurement delivers very accurate individual values, but it is temperature-dependent, relatively slow, and not suitable for large bolt populations - so continuous monitoring of all joints is not practical.
• The higher cost per fastener makes market acceptance of this sensor system difficult in many applications.

Conclusion Method 1: Ultrasonic measurement is the most precise method for samples, lab measurements, and validation work. For 100% inspection in ongoing series production, it is too complex and too expensive.

Method 2: Torque-Angle Analysis - The Most Economical Solution for Series Production

How the method works

In established tightening methods, preload force is not determined directly, but by controlling and monitoring process parameters such as torque, angle of turn, insertion depth, or insertion time. Torque-angle analysis goes significantly further than a simple torque check.

As explained in detail in our article Reading the "Fingerprint" of Your Bolted Joint, the full tightening curve provides a wealth of process information for torque analysis and angle analysis: a typical torque-angle tightening curve starts with a nonlinear zone where the components settle and align. This is followed by the linear, elastic region in which preload force builds up, the parts are pulled together, and the joint stabilizes.

The gradient in this elastic region correlates with the compliance (stiffness) of the joint - and thus with the actual preload force generated. The preload force is inferred from the applied torque or from a combination of torque and rotation angle. The calculation of the force is based on the assumption that a defined share of the torque is converted into elastic elongation.

Anomalies such as increased thread friction, side loading, or exceeding the yield point are clearly visible in the tightening curve - and cannot be detected by a single torque value alone.

The QUANTEC MCS® as an analysis tool

The QUANTEC MCS® from GWK is designed precisely for this task: with its angle sensor system for reference-free angle measurement (festpunktlose Drehwinkelmessung), it records the complete torque-angle tightening curve in real time - without an external reference point. The torque sensor achieves a measurement accuracy of ±1% between 10 and 100% of the nominal range, and the robust aluminum-titanium construction ensures long-term stability even under production conditions.

All measured data is transmitted wirelessly via WLAN and can be evaluated, archived, and documented for audits in the QuanLab Pro®, Ceus, or QS-Torque software suites. This makes the QUANTEC MCS® not only a torque analysis tool, but also a documentation platform for A-class safety-critical joints in accordance with VDI/VDE 2862 and comprehensive assembly testing and assembly quality assurance.

Advantages

• Ready for series production: No bolt preparation required; can be used directly in ongoing production.
• 100% inspection possible: Every single joint is monitored and documented.
• Process information instead of a single value: Anomalies, friction effects, and yield exceedance become visible in the torque-angle curve.
• Economical: One device covers all tightening locations within its defined measuring range.
• Flexible combination: Can be complemented by ultrasonic spot checks to validate friction assumptions.

By using torque angle measurement in this way, you significantly optimize assembly processes while maintaining high transparency over the actual behavior of every joint.

Limits and disadvantages

• The method is strongly dependent on friction: variations in lubrication, thread friction, or underhead friction lead to significant scatter in the resulting preload force. The friction coefficient must be known and stable so that the curve-based torque angle analysis can reliably indicate preload force.
• It is an indirect measurement - not a direct elongation measurement as in the ultrasonic method.

Conclusion Method 2: For series production, torque-angle analysis is the most economical and practical method - especially when process information and anomaly detection are required.

Method 3: Instrumented Bolts (Load-Cell Bolts) - Direct Measurement, Maximum Cost

How the method works

With instrumented bolts - also called load-cell bolts or sensor bolts - a strain gauge (SG) or a piezoelectric sensor is integrated directly into the bolt shank. These sensors capture the real bolt force and dynamic load components directly in the shank. That makes them the most direct of all three methods: the measured quantity is the axial tensile load in the bolt itself.

The full-bridge strain gauge circuit allows precise acquisition of axial bolt force. Depending on the variant, you can additionally measure bending moments or torsional components, which enables a detailed analysis of the load behavior of complex joints.

A key advantage over the other methods: instrumented bolts are particularly suitable for permanent monitoring of highly loaded bolted joints. Measurement is also possible in operation - that is, under service loads.

Advantages

• Most direct measurement method: No correlation, no assumptions - the force is measured directly in the shank.
• Operational monitoring possible: Force measurement even under dynamic operating loads.
• Very high accuracy when the sensor characteristics are well known.

Limits and disadvantages

The decisive disadvantage is price. With each installed bolt, an expensive sensor element remains in the assembly - the bolt cannot be reused after measurement. For series applications with hundreds or thousands of joints, this is simply uneconomical.

Additional limitations:

• Standard bolts cannot be used - custom-made parts are required.
• Limited availability in standard sizes.
• High integration and training effort.

Conclusion Method 3: Instrumented load-cell bolts are the right tool for development, prototyping, and highly critical individual joints - but not for economical series inspection.

Direct Comparison: All Three Methods at a Glance

Which Method Is Right for Your Process?

Use our interactive decision tool to find out which measurement method is best suited for your specific application and to choose the right torque sensor or angle sensor setup:

Why Torque-Angle Analysis Is Superior in Series Production

For the vast majority of industrial applications - automotive, aerospace, mechanical engineering, rail industry - the torque-angle analysis with the QUANTEC MCS® offers the best compromise between accuracy, cost-efficiency, and practical usability.

Cost-efficiency across the entire line

A single QUANTEC MCS® covers all tightening locations within its measuring range. There is no need for special bolts, complex preparation, or bolt-specific calibration. In contrast to ultrasonic measurement, no reference measurements in the unloaded condition are required.

100% inspection instead of sampling

Pure torque-controlled tightening is well suited to implement a defined tightening specification, but it does not tell you how bolt force changes in service. Torque-angle analysis goes further: every joint is captured in full, and deviations in the curve - for example due to friction anomalies or incorrect joint geometry - become immediately visible. This is especially relevant in the context of process capability analysis (PFU) according to VDI/VDE 2645-3, where scatter in the achieved preload force must be recorded and evaluated systematically.

With access to the full tightening curve, you can base your assembly quality assurance on robust torque analysis and angle analysis rather than on a single end value.

Anomaly detection as added value

Disturbances such as thread defects can produce a sufficiently high torque while the actual preload force remains far below specification. These are exactly the cases that remain undetected by pure torque control - but which torque-angle analysis reliably exposes. Side loading, excessively stiff joints (so-called hard joints), over-tightening, or settling movements during tightening: all of these leave a characteristic signature in the torque-angle curve.

The optimal combination strategy

The most effective practical strategy combines two methods:

• QUANTEC MCS® for ongoing 100% inspection in series production
• Ultrasonic spot checks for periodic validation of friction assumptions

This way, you exploit the advantages of both measurement approaches to measure preload force reliably - without having to accept their individual drawbacks.

If you want to support the Cmk and Cpk capability requirements of your tightening processes with reliable measurement data, the complete torque-angle tightening curve from the QUANTEC MCS® is the right foundation - not just a single torque value at the end of tightening.

GWK ToolRent®: Test First, Then Decide

If you want to evaluate torque-angle analysis and QUANTEC MCS® in your own production environment, you do not have to invest immediately. With the GWK ToolRent® rental system, you receive a calibrated QUANTEC MCS® on a weekly, monthly, or annual basis - including a current calibration certificate, ready for shipment and available worldwide.

This enables you to perform realistic assembly testing under true series conditions and to optimize assembly processes before making an investment decision.

Conclusion: The Right Method for the Right Application

All three methods for measuring preload force have their place - the decision depends on the application:

• Ultrasonic measurement -> Sampling, lab validation, prototype phase
• Torque-angle analysis (QUANTEC MCS®) -> Series production, 100% inspection, anomaly detection, torque angle evaluation
• Instrumented bolts -> Development, highly critical individual joints, in-service monitoring

For the vast majority of industrial series production environments, torque-angle analysis with the QUANTEC MCS® is the most economical, practical, and informative solution. It does not just deliver a single end value, but the complete "fingerprint" of every joint - and therefore the basis for well-founded quality decisions.

Accuracy by GWK - 30 years of metrology expertise in precision tools that reveal what is really happening inside your bolted joints.