Today on the Blog we explore the topic of controlled tightening in an interview with Marino Ferrarese, General Manager of Stahlwille Italy.
In this article, Marino answers our questions on torque control and explains what are the main differences that distinguish the torque tools available on the market.
With regard to the ISO 6789:2017 standard, Marino talks about the impact of the entry into force of this regulation on manufacturers and users of hand torque tools.
Finally, he explains what role Stahlwille plays in this important sector and which are the torque tools it produces.
M.F.: "Torque control is that activity aimed at ensuring that two mechanical parts are tightened by means of a bolted joint according to precise torque values."
M.F.: "In the past, when a component was tightened, for example the head of a car engine, the bolt was tightened as much as possible. However, with technological progress, heavy materials have gradually been replaced with lighter materials that could break or be damaged if tightened with excessive force. It is therefore necessary to apply a certain torque to ensure an effective tightening, not too much yet not too little.
Torque control has been introduced in industries where it is necessary to guarantee the tightening of a certain component according to a given prescription. It is a transversal activity that concerns different types of industries (for example, to tighten the wheels of a car or to tighten the bolted joints of a staircase to have the CE marking)."
When controlled tightening is done using a pneumatic or electric tool, it is often necessary to check with a hand tool whether the tightening has occurred at the correct torque. With pneumatic tools, this check is necessary because the size of the compressed air system affects the actual torque applied to the bolted joint. Electric torque tools are more precise but can still be subject to errors, making manual random testing necessary."
Mechanical torque tools are set to a specific torque and they click when this torque is reached, signaling the user that he must stop. In the simplest and most affordable mechanical torque tools there is a spring inside the wrench. The spring is compressed, charging the click mechanism at the set torque. Another type of mechanical torque tool uses the torsion bar mechanism. Although these tools are based on the same principle as spring-loaded locking mechanism, they are quite different for their accuracy and reliability. In fact, the torsion bar mechanism is more expensive to produce but guarantees greater reliability over time, while the spring mechanism is subject to greater wear and requires the spring to be discharged after each use in order not to lose its calibration. Since this is a time-consuming operation, most users do not discharge the spring which, being of harmonic steel, tends to lose its precision.
Electronic torque tools, on the other hand, have a load cell inside them. When the wrench reaches the set value, the load cell sends a signal to the wrench that emits a vibration, a light signal or even a simulated click. The electronic wrench is more expensive than a mechanical one, but it is more precise as it returns the effective torque value made by the user. In the case of the mechanical wrench, if the user who is performing a controlled tightening continues to tighten by inertia even after the wrench has clicked, the effective tightening value is not measured. This fact, although banal, is not normally understood by users of torque wrenches!"
M.F.: “The ISO 6789:2017 standard regulates the verification and calibration of hand torque tools, replacing the previous 2003 regulation.
With the entry into force of the standard in 2018, important changes have been introduced. The ISO 6789:2017 indicates to the manufacturer what the minimum constructive requirements of torque wrenches are.
For example, the correct positioning of the operator's hand must be indicated on the handle, since the position of the hand is one of the factors that could negatively affect the tightening and lead to a different result than the one set.
Other indications of the norm concern the realization of a graduated scale, discouraging the presence of more graduated scales in order to prevent users from getting confused.
Furthermore, the standard states that the calibration points of torque tools are 0%, 60% and 100% of the scale, while in the past these points were at 20% 60% and 100%. Many manufacturers played on this aspect, confusing the user and widening the scale of the key, knowing that values below 20% were not verified in any way. It is difficult if not impossible for a very large scale to pass the requirements for calibration."
MF: “The standard provides that every year (or every 5000 tightenings) torque wrenches need to be tested and certified in a calibration center. The introduction of measurement uncertainty adds another indication of the performance of the wrench. A torque wrench with high uncertainty will be less accurate and will lead to less accurate measurements. The standard includes a method for calculation of uncertainties, which must also be reported in the certificate supplied with the wrench. It also states that the uncertainty class must be verified every year when the tool returns to calibration and noted on the certificate."
M.F.: “The legislation has brought advantages to users, helping them to understand the different types of products on the market.
The tools are more compliant and therefore more precise. Many of the products that go into calibration every year do not pass the test, especially the poor-quality torque wrenches. This helps the user to understand which products are of good quality."
M.F.: “Stahlwille has been producing torque tools for several decades and it was the first manufacturer of to participate in the drafting of international standards, becoming an accredited member of the German national body that deals with the measurement of torque tools.
Stahlwille produces a wide range of mechanical and electronic torque tools that use technological solutions aimed at ensuring greater precision. In the case of mechanical wrenches, such as the MANOSKOP series 730, 730N and 721, Stahlwille uses the torsion bar machanism, while for electronic torque tools, such as the MANOSKOP series 730D or 714, it has created a patent for mechatronic torque wrenches, i.e. keys that combine the advantages of electronic keys with the "tactile" advantage of the click - a typical characteristic of mechanical keys.”