Posted by Nic Townsend
Translation: Ignatova D.V., marketing specialist LLC ZVK "BERVEL"
Which method is most effective when tightening a threaded connection?
This article will discuss in detail two methods of tightening a threaded connection. And wins...
Torque application method
Almost every one of us has had to tighten a nut to a certain point at least once in our lives. The principle of tightening a nut by applying torque is the most common and simplest for the average person. It consists of creating a torque on the nut (bolt) that provides the required tightening force. The main advantage of this method is that for its use there is a large range of professional tools with manual, pneumatic, hydraulic and electric drives: impact wrenches, torque wrenches, torque multipliers.
The torque application method is an economical method.
“With one torque wrench and a set of interchangeable sockets, you can tighten a significant number of nuts and bolts of various sizes. The tool is easy to use,” says Robert Nobel, chief engineer at Asset 55.
A torque wrench is considered to be effective in most applications. It is for these reasons that this tightening method is usually chosen.
However, this method has its drawbacks, the main one of which is the friction force, according to Nobel, “the main enemy of tightening a threaded connection.” In general, friction influences 90% of the amount of preload achieved at a given torque. This means that only 10% of the torque is converted into preload. Accordingly, it is difficult to accurately determine the magnitude of the load on the bolt. To highly accurately determine the preload value, it is worth considering many factors: the lubricant used, contamination on the surface of the parts, as well as the quality of the surface finish of the nut. The amount of preload is key because the connection can fail if the load on each bolt is too high, too low, or uneven.
Nobel notes that if all regulations are followed, qualified operators are present and calibrated, tested equipment is used, the tightening of the threaded connection is successful.
Method of axial drawing of a stud (bolt)
The axial bolt drawing method began to be used in the 1970s. The method consists of pre-stretching the bolt with a force equal to the required tightening force, then tightening the nut by hand until it stops against the supporting surface and removing the load from the bolt by releasing hydraulic pressure, after which the connection receives the specified tightening force. The fundamental difference of this method is that the force is applied not to the nut, but to the bolt using tensor jacks and hydraulic nuts. The pioneer of this method is considered to be the British engineer Fred Heaton, later the founder of the Hydratight and Boltight companies. Over the next 20 years, this method became increasingly popular. Today, the axial drawing method is preferred in connections with high-strength bolts in various industries (oil and gas industry, wind power, energy, etc.).
Compared to tightening with a torque wrench, using hydraulic tensioners is a more complex process. Under certain application conditions, the axial pre-tensioning method provides higher accuracy of the tightening force, uniformity of force application, and synchronized tightening of complex connections. The installation (assembly) speed is higher. In the case of flange connections, synchronous tightening is possible.
The next significant advantage of axial drawing is high accuracy, but, as Nobel notes, not everything is so simple. According to Nobel, modern industry often resorts to using universal installation rules, without giving due importance to such important characteristics as torque and tightening force. If all technical requirements are correctly followed, the precision of the pre-tightening force is as high as possible. The pre-tensioning method is especially effective when using long, stepped, or high-strength bolts to assemble heavily loaded joints. As for the use of short, lightly loaded bolts, in this case the axial drawing method is less accurate.
The axial drawing method also has disadvantages. First of all, this is a loss of load. Load loss is taken into account in the calculations in advance and added to the preload value to determine the initial applied load. Therefore, for greater accuracy, it is recommended to apply force to the bolt and screw the nut twice. During the first loading, compensation of gaps, deformation of micro-irregularities of surfaces and uniform distribution of load are achieved. The second load mainly ensures the required accuracy of the final tightening force of the connection.
Moreover, the axial drawing method is less economical. Assembly requires expensive equipment and special tools.
So, having compared two methods of tightening a threaded connection, we can conclude that it is quite difficult to determine which one is better. Each method has its own advantages and disadvantages. Robert Nobel notes that each case is unique and sometimes it is worth carefully considering which tightening method to choose, namely, consider the following points:
- scope of application of the threaded connection (for example, for foundation anchor bolts, the axial drawing method will be more suitable);
- cost of equipment;
- number of units and size of equipment;
- features of the object (area, location, etc.);
- delivery of equipment to the assembly site;
- equipment compliance.
Date: 02.12.18
Source: Nord-Lock group, BOLTED magazine
