Day-to-day industrial processes frequently require the measuring of dynamic forces – unlike metrological traceability, which has yet to advance from its early stages.
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This article provides recommendations for “bolting force transducers, torque transducers and multi-component transducers” (referred to in the following as “transducers”), in order to make your work as a design engineer or user easier.
As a manufacturer of high-precision measuring equipment, we want to share our experience with you, in order to clarify questions and information regarding installation, and to provide you with tips on how to get the best and safest results when using our products.
Transducers are normally connected to peripherals using screw thread(s) that utilise friction or traction. The dimensioning of the bolted connection depends primarily on the absorption of process forces, which can occur both in the direction of measurement and in other directions (shearing forces, bending moments etc.). The bolted connection therefore must be able to transfer a combined load spectrum without the attachment parts slipping.
However, all assembly processes are subject to some uncertainties: irregularities, high roughness values or other varying tightening torques can have an impact on the zero signal, creeping or hysteresis, or result in loss of long-term stability.
The following proposals are based on our experience in the field of force and torque measurement technology and should help you reduce the effects of uncertainty when bolting together transducers.
In principle, the user is responsible for compliance with the correct process variables during assembly and for compliance with the applicable safety regulations. GTM does not accept any liability for the failure of components or tools or for physical injury, and retains the right to make changes to these recommendations. These recommendations are not a substitute for training and assume that the user has the necessary qualifications.
Transducers always have a load application area and a load discharge area. Between these there is an elastic area that can be used to measure the forces and torques by means of the resultant deformation when it is subjected to loads. In some cases, it is possible that the necessary tightening or releasing torques on the bolted connections are higher at the load application areas than the elastic area of the transducer can tolerate.
Example: For a torque transducer with a nominal load of 2 Nm, the bolts on the flange connections need to be tightened with a tightening torque of approx. 4 Nm. If the tightening torque is transferred through the elastic area of the transducer and an additional shear force is applied because a torque wrench is used, the transducer may be destroyed.
Our recommendation: In principle, do not transfer the forces and torques needed to tighten and release the bolts via the transducer; instead, brace against fixed attachment parts.
In general, cylinder head bolts with a hexagon socket, in accordance with DIN EN ISO 4762, are the most suitable choice when installing transducers. Compared to hexagon head bolts in accordance with DIN EN 24014, although the connecting surface is almost identical, the hexagon head bolts are often at a disadvantage in terms of space because larger counterbores are needed for the hexagon head and the necessary tools.
Avoid screw shapes for secondary purposes, such as slotted screws, Phillips screws, rounded head screws or screws with a built-in screw locking device, e.g. spring head screws or screws with adhesive bonded to the thread.
The size or nominal diameter of the bolted connections on the transducers is designed for the expected process variables. Only corresponding bolts in these nominal sizes may be used. Reduction to smaller nominal diameters, for example using threaded sleeves, may be dangerous from a safety perspective and may also lead to measuring inaccuracies.
Unless otherwise specified, bolts in strength category 10.9 should be used. Stronger bolts may have a negative impact on the fatigue limit. Bolts with a lower strength may make it impossible to reach the desired preload force. A specific strength category is recommended for bolted connections on transducers – the connection should not be stronger nor weaker than the recommendation.
In the case of transducers and attachment parts made of aluminium where steel bolts are used, note that thread in the aluminium is less secure than the steel bolts and is more likely to break away. Always observe the manufacturer’s recommendations in these cases and, as a rule, do not use bolts with a strength category higher than 8.8.
The bolt length must be selected according to the expected load and the material of the nuts. Do not use screw-in depths that are shorter than the recommendation. In particular, make sure for tapped blind holes in the transducer that the connection engineering and the bolt length are designed so that the bolt does not rest on the thread base, as this prevents the necessary preload force from being reached.
Expansion bolts feature greater elastic elongation than rigid bolts at the same preload force. Thanks to this elasticity, additional loads caused by dynamic elements of the bolt prestressing have less impact. Expansion bolts are therefore recommended for dynamic applications. In static applications, they generally offer no advantages.
When using expansion bolts, note that they need to be bolted using a lower tightening torque for the same nominal thread diameter.
To prevent warping of components with flange connections, we recommend tightening the bolts as follows:
To guarantee the technical measuring properties, the flange connections on the transducers must always be bolted using the specified number of bolts. Even if a smaller number of bolts would appear to suffice from a strength perspective, all the flange bores must be properly filled with bolts.
The thread and bolt connecting surfaces must always be free of contamination in order to ensure the best possible friction and to reach the maximum preload force. Paint and unstable coatings under the bolt head must be removed.
The thread pitches of the bolts could be coated with a little copper paste, for example, to further improve the friction in the thread. The force transmission surfaces on the transducers and the bolt connecting surfaces must not be moistened, however, in order to avoid sliding that would cause hysteresis.
For general load cases, the torque-controlled tightening method is the most cost-effective and time-saving method, and it can be performed easily using a torque wrench. Here, monitored application of the torque to the bolt axis generates an assembly preload force that is dependent on the friction between the connecting surface and the thread flanks that are engaged.
Depending on the friction, the preload force created in the bolts by this method varies by about ±15%. The bolts are loaded up to about 75% of the minimum yield point for the bolt material.
When choosing an assembly torque for this method, we recommend following the “VDI 2230” guideline, which specifies an average total coefficient of friction of µ=0.12.
If you wish to make better utilisation of the bolts in terms of higher preload forces, minimise the variations inherent in torque-controlled tightening, or possibly gain advantages for fatigue strength in dynamic load cases, the yield-point controlled tightening method may offer benefits. Here, the bolts are used to full capacity up until the onset of yielding in the plastic area, which can be detected by monitoring the torque/angle of rotation ratio. This requires special tools, such as a torque wrench with angle of rotation measurement. Plastic elongation is minimal in this method, meaning bolts can generally be reused.
As the expansion range of plastic deformation for a ductile bolt is significantly greater than the elastic expansion range, the preload forces generally vary much less with this tightening method than in the torque-controlled tightening method.