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    How to make hot-dip gal­vanised bolt sets even bet­ter

    Screwed fasteners are one of the most important components in wind energy units, bridges and steel construction. Alongside diverse factors such as material and tools, varying weather conditions also have an important influence on coefficients of friction and therefore the screwed fastening as a whole.

    But let us start at the beginning: there are different types of screwed fastening in use today: ISO bolt sets, friction grip bolt sets and bolt sets with calibrated pretension. All of these connections consist of a bolt, at least one washer and a nut. They are often made of high-tensile material.

    The coefficient of friction is a key element of a secure screwed fastener. This value describes the ratio of pretension and torque and is determined by two factors: thread friction and head friction. The load pressure that affects the surfaces is the pretension, which is also responsible for the elongation of the bolt and the corresponding spring effect. Torque, on the other hand, affects the surface of the thread and the contact area, it is divided into three partial torques: thread torque, head friction and the part translated directly into pretension. Consequently, the pretension ultimately achieved with prescribed torque is dependent on the coefficient of friction.

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    Yellow describes the head friction, purple the pretension and pink the thread friction.

    The friction µges and k-factor are influenced by numerous different factors: material pairing, surface, lubrication or wear. However, ambient temperature, humidity and rain also have a considerable influence on screwed fastenings. We are not only talking of elastic deformation here; in the worst case this may result in sheared bolts or failure to achieve the necessary pretension. The result: halts in construction with extensive costs and time delays due to repair work or the replacement of bolt sets.

    This problem has been addressed by Peiner Umformtechnik, a leading supplier of fasteners for wind energy units and steel construction, and Dörken, a producer of micro-layer corrosion protection systems. After a number of experiments and extensive testing over a period of nearly six months, a solution was ultimately found. The challenge lay in equipping the standard hot-dip galvanised bolt with washer and nut with an additional topcoat that means neither rain nor sunshine impact the stability of the fastening. Alongside the extremely demanding laboratory testing, the coating combinations have also proved their ability in various field trials. The results are impressive: no malfunctions have occurred in the nearly 3 million bolt sets delivered and installed. The co-operation between the experts at both companies has not only resolved a real problem, it also ensured that a previously unachieved level of process reliability could be reached on construction sites. Do the problems described sound familiar to you? Then come to us - we are happy to help!

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    The hot-dip galvanised calibrated pretension bolt set was equipped with an additional topcoat and is fastened in a secure and stable manner, regardless of weather influences.
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    Stable fastening properties: hot-dip galvanised ISO bolt set with additional topcoat.
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    Friction grip bolt set also display secure fastening properties with the new solution - irrespective of head, cold or humidity.

    Further information from the company Peiner Umformtechnik can be found here:  https://www.peiner-ut.com