Measuring the preload force of screws using ultrasound
The design of screw connections is not trivial; even with a wealth of experience and expertise, surprises can still arise in practice. Especially when it comes to dynamic, thermal, or mechanical stresses, the devil is often in the details. New materials and processes such as 3D printing, laser sintering, and the like can also create new conditions for screw connections that cannot be represented by previous calculation methods. The best solution would be to measure the actual preload force. However, conventional methods are either limited to the test bench, require different screws than those used in assembly, or require additional joints in the screw connection. Measuring the preload force in a real connection with original screws and threads on oil pans, transmission or gas pipe flanges was often not possible. A new approach changes this: by introducing ultrasound into the screw, the original screws serve as test specimens and the measured preload force then corresponds to the actual clamping force in operation.
Measurement without compromise
| Conventional measurements determine the preload force, e.g., by mechanically measuring the length of the screw to measure the force via the elongation, or the contact force is determined via intermediate pressure measuring discs. In both cases, the potential for error is quite high. Pressure measuring discs, for example, introduce two additional separation joints into the connection, the influence of which can only be estimated. ARNOLD BlueFastenig Systems offers the possibility of measuring screw connections in their original application with its intellifast® system ultrasonic measurement technology. Depending on the user's requirements, the system can be used not only in the measurement laboratory, but also for on-site measurements. In special cases, changes in prestressing forces can even be determined during operation over a period of months. This also allows aging processes such as seal shrinkage or temperature influences (summer/winter) to be recorded. Since the system is designed for multiple channels, not only are individual statements possible, but the screw connection can also be measured and evaluated as a whole. | 
Depending on the user's requirements, the system can be used not only in the measurement laboratory, but also for on-site measurements. |
Preload force measurement using the pulse-echo method
Just as the depth of a ship is determined by echo sounder, the ultrasonic method also measures the transit time of the sound pulse from its emission into the propeller to its return. |
Prepared screw heads with piezo system |
The way this method works is easy to explain: just as a ship's depth is determined using echo sounding, the ultrasonic method also measures the transit time of the sound pulse from when it is emitted into the screw until it returns. However, this simple principle requires extensive equipment and expertise to produce meaningful results. The comparatively short propeller length in combination with the high speed of sound propagation in the metal propeller material requires measurements in the nanosecond range. The original propellers must also be prepared. They are given exactly plane-parallel surfaces at both ends. The lower surface serves to optimally reflect the sound. A piezo element is applied to the flat surface on the screw head for sound coupling and echo pulse reception. There are two variants for this: either a prefabricated piezo element is glued in place, or a permanently mounted transducer system (PMTS) is installed. The inexpensive adhesive method is intended for quick, undemanding measurements, while the PMTS version can measure connections even under harsh conditions and remains functional for years.
What can the ultrasonic method do?
Temperature table for measurement and operating temperature ranges |
Table of possible drive sizes |
The method is suitable for aluminum and steel screws with metric threads or direct screw connections in metal with diameters of 4 mm and above and clamping lengths of 1 mm and above. The glued-in piezo version allows measurements in the range from -35 to +125 °C, the PMTS version from -40 to +165 °C. The ultrasonic elements can be attached to TORX®, TORX PLUS®, and internal 6KT/external drives (Figure 6); other drive sizes are available on request. Depending on user specifications, different measurement options can be selected: a single measurement at one or more screw locations, measurement of the preload force curve at one screw location over a defined period of time, or measurement of the preload force curve at up to eight screw locations simultaneously over a defined period of time. Various prerequisites are essential for reliable measurement, and ARNOLD's specialists provide detailed advice on this. The suitability of this measurement method is checked precisely and individually for the respective task. This requires the provision of detailed information on the individual components of the connection to be examined. The actual test usually takes place in our in-house test laboratory. Even large objects can be individually clamped and tested. For stationary systems or measurements during operation, mobile measurements on site are also possible. The measurement laboratory also carries out preparatory work on the original screws, such as machining a flat parallel surface on the head and tip sides and applying the piezo element.
The modern ultrasonic measurement method for determining the actual preload force on screws facilitates the creation of screw connections that are as reliable as possible. It supports AI design processes with real measurement values as training data, especially for innovative materials and manufacturing methods, and can help with troubleshooting. Since field measurements are also possible over longer periods of time, the preload force curve measured in this way corresponds to the natural aging processes and provides information about the long-term stability of the connections.
Technical box: How does it work?
A piezo element sends an ultrasonic pulse through a measuring device at the head of the screw, which passes through the fastener and is reflected at the opposite end. This echo passes through the screw again, is detected by the measuring device, and its transit time is measured. The transit time is specific to the screw and increases linearly when the screw is tightened in the elastic range due to its elongation and the so-called acousto-elastic effect. By comparing the transit times in the unclamped state (reference transit time measurement) with the time after tightening the screw, statements can be made about the actual preload force, taking other factors into account.
To measure the actual preload force in screw connections, ARNOLD UMFORMTECHNIK offers a measurement method based on ultrasonic measurement for screws ranging from 4 to 8 mm.
Further information on this topic can be found here
About ARNOLD GROUP – BlueFastening Systems
Since 1898, ARNOLD GROUP has been synonymous worldwide with intelligent and forward-looking fastening technology. Based on many years of expertise in the production of highly complex extruded parts and innovative fasteners, ARNOLD has long established itself as a comprehensive system provider and development partner for integrated fastening solutions.
For more than 30 years, ARNOLD has also been offering machine and processing technology for the automated assembly of fasteners – individually tailored to customer-specific requirements, efficient, and reliable.
The BlueFastening Systems philosophy underscores this holistic approach: The close integration of engineering, high-quality fasteners, precise functional parts, and state-of-the-art feeding and processing technology ensures maximum efficiency, sustainability, and global innovation – all from a single source.
ARNOLD has been part of the Würth Group since 1994 and is driving the future of fastening technology forward with the highest precision and innovative strength.
