Posted by Guy Avellon, Fastener World Magazine
Translation: marketing specialist LLC ZVK "BERVEL" Ignatova D.V.
Abbreviations:
RS - threaded connection;
Cr+6 – hexavalent chromium;
Cr+3 – trivalent chromium;
RoHS - (English: Restriction of Hazardous Substances) - a directive limiting the content of hazardous substances.
Most questions related to the use of fasteners have solutions developed through many years of experience. However, it is very difficult to find information about such solutions, since their scope is quite limited.
Often, manufacturers and distributors are unable to answer technical questions related to the operation of fasteners. At the same time, the service life of fasteners depends on the area of its application and operating conditions. An incorrect understanding of the purpose of the fastener and the conditions of its operation leads to serious consequences, because in the event of a broken connection or other types of failure, both parties, the buyer and the manufacturer, bear responsibility.
This article provides answers to the most pressing questions about fastener reuse and tightening torque.
1. Can fasteners be reused in critical connections?
Answer: No.
It would seem that the answer to this question is obvious, but still, the question is relevant. This question was once asked by a nuclear energy worker. In critical connections, fasteners are subjected to high loads, so proper assembly and installation of parts play a vital role. Improper tightening of connections leads to fatigue failure, which is the cause of failure of threaded connections in 90% of cases. Failure to timely detect damage in the bolt body can lead to rupture of the product. For example, microcracks on the body of a bolt are almost impossible to detect during external inspection, so testing can only be carried out in laboratory conditions (ultrasound testing, etc.). Therefore, the fasteners should definitely be replaced, because the cost of the bolt is not comparable to the cost of further costs for restoring damaged structures.
2. Can fasteners be reused under normal operating conditions?
Answer: Yes
The answer given in the article is conditional, since only fasteners made of carbon steel grades that have passed all the necessary laboratory tests and installed under controlled conditions can be reused. During the initial installation, it is necessary to monitor compliance with all technological operations (processing contact surfaces, assembling connections, installing bolts, tensioning and monitoring the tension of bolts), since in case of violation of the requirements, deformation and destruction of the products may occur during repeated use. If the conditions for using the fasteners are unknown to you, then you should definitely refuse to reuse them.
Fasteners made of special materials with permissible load must be used once. Here it is necessary to clarify that small fasteners (M8 or less) can be reused, but if the fastener was installed with maximum force, its mechanical properties may be reduced.
Therefore, if a threaded fastener made of carbon or special steel grades has been subjected to a load exceeding the proof load or yield strength, such a part should in no case be reused, since both its mechanical and physical properties will no longer meet the stated requirements.
3. Can the nuts be reused?
Answer: No
This is a simple answer to a rather complex question. The main explanation is the variable friction coefficient. When pre-tightening, the bolt shaft in the area between the head and the nut is exposed to torque, that is, when screwing the nut of a threaded connection, a torque acts on the nut, overcoming the friction moments in the thread and at the end of the nut. Under tension, the thread of the nut stretches. Force affects the threads of a nut and bolt differently. After the stress is relieved, the thread on the bolt shaft returns to its initial state, and the surface of the thread on the nut is deformed. When unscrewing nuts, a greater torque is required than when tightening. This is explained by PC corrosion, the mutual penetration of bolt and nut materials into the thread zone under the influence of prolonged load. In addition, the nut may have invisible defects, which subsequently lead to destruction of the RS.
Therefore, due to the unpredictable value of the friction coefficient for the reasons stated above, achieving the required tensile force is impossible.
Manufacturer: “But when reusing the part, the same tightening torque was used.”
When the nut is reused, the tightening torque values no longer meet the stated standards. The frictional force changes with each reuse as the internal threads of the nut are deformed and compressed. When tightening the PC using a torque wrench, it is necessary to accurately set the tightening torque and fix it in the specified position. But with repeated use, it is almost impossible to accurately determine the tightening torque, which reduces the reliability of the connection, and the likelihood of plastic deformation of the thread increases.
The most important is the friction in the thread between the nut and the rod fastener, as well as the nut and the surface of the part being connected, which depend on factors such as the condition of the contact surfaces, type of coating, presence of lubricant, errors in the pitch and angle of the thread profile, deviation from the perpendicularity of the supporting end and the axis of the thread, screwing speed, etc.
Friction losses can be quite large. With almost dry friction, a rough surface and shrinkage of the material, the losses can be so large that when tightening the connection to the direct tension, no more than 10% of the torque will remain. The remaining 90% is spent on overcoming frictional resistance and shrinkage.
When reusing nuts, the “parasitic” friction is greater. And, although the impact wrench will show the required torque, the required compression of the connection will not be achieved. When the PC is exposed to loads or vibration during operation, there is a high risk of self-loosening of the connection and, as a result, an accident.
The coefficient of friction can be reduced by using oil, but not excessively, since this risks reducing the resistance and exceeding the joint tension, which can lead to destruction of the rod fastener.
The friction coefficient values under real assembly conditions can only be predicted. As numerous experiments show, they are not stable.
The phenomenon of reusing nuts plays a significant role in the application of wheel nuts, since the removal and replacement of wheels is necessary for tire rotation, brake inspection, vehicle repair, etc. According to statistics, every car with a mileage of more than 120,000 km is at risk of broken studs due to weakening of the tightening force of recycled wheel nuts.
Consumer: “After tightening the bolts of your production, a breakdown occurred”
It is important to understand here that the declared tightening torque value should be used as a reference, since each finished part, before being installed in a connection, has certain physical properties that must be taken into account when determining the tightening torque.
Let's take electroplating as an example. Formulas for calculating tightening torque were developed decades ago, the main coating used was electroplating. At that time, cyanide electrolyte was used for galvanizing. As a result, the efficiency was 40-60%, the risk of hydrogenation remained high, the surface of the parts was porous and uneven. Therefore, the “K” factor (coefficient of friction) in such products was much higher than in products using alkaline and acidic non-cyanide electrolytes in the 70s. The efficiency of galvanizing with new electrolytes has become even higher. Therefore, the tightening force and friction coefficient have changed.
However, the reference data in the torque tables for galvanized RS has remained unchanged for decades. Today the situation is different. In 2002, the European Union adopted the RoHS directive, limiting the content of hazardous substances in electrical and electronic equipment. In this regard, the use of hexavalent chromium (Cr+6) was prohibited. Subsequently, toxic hexavalent chromium was replaced by trivalent chromium (Cr+3).
Coatings using Cr+6 provided higher corrosion protection than Cr+3 coatings. Cr+3 coatings have a thinner layer and the level of corrosion protection is much lower. To compensate for this disadvantage, it is necessary to apply a thicker layer of application or apply an additional layer on top of the existing one.
The variable tightening force is due to variations in the thickness of the Cr+3 coating. Recent torque control tests have shown that fasteners coated with Cr+3 require a 7-10% increase in torque values. Moreover, the accuracy of the tightening force determination is also influenced by the coating method.
Standardization of products by the manufacturer will control deviations and reduce potential consumer risk.
Date: 10/20/2017
Source: Fastener World magazine
