Mechanisms: Tension control bolts | Hackaday

If there is a consistent idea of ​​how large steel structures used to be made, it is probably the hot riveting process. You’ve probably seen the grainy old black-and-white movies in which a gang of riveters—usually men in overalls with only a cigarette for protection—heat rivets red hot in a forge and toss them to the riveters with a pair of tongs. There, the rivet is caught with a metal funnel or even a gloved hand, pushed into a hole in the flange connecting the beam to the column, and hammered into submission by a pair of men with pneumatic hammers.

Although the work was dirty, hot and dangerous, hot rivets were a practical and proven way of joining elements into steel structures, and chances are that any commercial building built before the 1960s or so has at least a few riveted connections. . But times are changing, technology is moving forward, and in construction, rivet connections have largely gone out of fashion, giving way to bolted connections. Riveter crews of three or more people were replaced by a single machinist making hundreds of predictable, precisely tensioned joints, resulting in better joints at lower costs.

Bolted joints tightened to specifications with an electric wrench may not have the feel of hot rivets flying across the job site, but there’s definitely a lot of engineering behind them. And as it turns out, the secret to making bolt work a one-man job lies largely in the bolt itself.

Table with a view

My first encounter with tension control bolts began with the fact that I had a lot of luck at work. Back in the early 2000s, my department moved and somehow I managed to get a desk with a real window. The opportunity to see the world was amazing, but one day a company started building an extension right outside my window. It was a mixed picture for me; True, I will lose my view while the six-story building is built, but in the meantime I will be able to watch its construction from the comfort of my desk.

I watched in amazement as the steel frame rose and the metallurgists quickly and efficiently bolted the columns and beams together. One thing I noticed was that tightening the bolts seemed to be a one-person job, with one mechanic tightening the nut using an electric wrench without anyone having to prop up the bolt head on the other side of the joint. This puzzled me; How could the bolt simply not turn in the hole?

I got my answer when I saw something fall out of a wrench after a mechanic pulled it out of a tight connection. From my perch by the window it looked like the end of a splined shaft, and I could see that one end had clearly been sheared off. Then I noticed that all the still loose bolts had the same slot sticking out past the nut, and it all clicked: the key should fit into a socket inside the key, coaxial with the socket that tightens the nut holding the bolt. so that the nut can be tightened. Moreover, it was clear that this design could be used to automatically tighten a connection by designing the spline to shear at the required torque. Genius!

Stretch and Snap

Although I wasn’t quite right in my analysis, I was pretty close. It wasn’t until much later (like while working on this article) that I learned that the bolts used for the load-bearing frame are called tension control bolts, or TCBs, and that a lot of engineering goes into their design. But to understand them, we need to look at bolted connections and figure out how they keep everything from buildings to bridges from falling.

We took Detailed overview of bolted connections before, but for the TL;DR set the point is that the bolts are essentially very strong springs. When you tighten a nut on a bolt, the bolt stretches slightly, which provides clamping force to whatever is between the bolt head and the nut. The amount of stretch, and therefore the amount of clamping force, depends on the strength of the material from which the bolt is made, the size of the bolt, and the amount of torque applied. This is why most bolted connections have a specific torque for all bolts in the connection.

For structural steel, connections between framing members are carefully designed by structural engineers. For each connection, many calculations are carried out, resulting in detailed plans for bolted connections. Some connections have many bolts, sometimes 20 or more, depending on the application. The hole pattern for each member is determined before the steel is cut, and each frame member usually comes from the manufacturer with the exact number of holes specified on the plan. The plan also specifies which grade of TC bolt will be used in each connection (more on this below), as well as the diameter and length of each bolt.

When machinists build a frame, they first use a pin wrench to line up the bolt holes in the two pieces they are bolting together. A spindle wrench is a large open-end or adjustable wrench with a long handle that tapers to a point. The handle is used to line up the bolt holes while the machinist inserts the TC bolt into the other holes. The bolts are initially tightened by hand, but then comes an important part of the assembly process called tightening or pre-tensioning.

Clamping is somewhat defined as the tightness achieved with “a few blows” of an impact wrench, or “the full force of a machinist” using a standard spindle wrench. Everything about fit is very subjective, as the number of “ugga-arcs” that count as multiple strokes of an impact wrench varies from user to user, and mechanics can similarly apply a wide range of forces to a wrench. But the idea is to bring the frame members into “solid contact,” which is typically about 10 kilograms per second, or “kips,” which is 10,000 pounds per square inch (about 70 MPa).

Once all the bolts in the connection are pre-tightened, final tensioning is performed. The tool I saw those metalworkers use for TC bolts many years ago goes by many names, the most common being a “shear wrench” or a “TC gun.” It is also known as the “LeJeune gun” after the major bolt and tool manufacturer TC. Some shear wrenches are pneumatically powered, but most are electrically powered, with cordless guns becoming increasingly popular. The final tightening cycle begins by connecting the TC bolt spline to the inner seat and the nut to the outer seat. The outer socket tightens the nut with a predetermined torque, after which the anti-slip clutch switches power transmission from the outer socket to the inner socket, thereby changing the direction of rotation. This will apply enough torque to the spline to force it off the TC bolt at its weakest point – the narrowed neck between the spline and the threaded portion of the bolt. This will ensure that the bolt is properly tensioned and only the required amount of thread will be visible.

Tools of the trade

TC bolts typically come in two grades: A325 and A490. Both are based on international ASTM standards: A325 bolts cover a tensile strength range of 120 to 150 kps (830 to 1040 MPa) and A490 bolts cover a range of tensile strengths from 150 to 173 kps (1040 to 1190 MPa). Most TC bolts have a rounded head since there is no need to grip the bolt by the head. This provides a smoother surface on the head side of the connection, reducing the likelihood of damage during installation. Depending on the application, TC bolts can be treated to prevent corrosion by either galvanization or passivation.

If it is necessary to disassemble a TC bolted connection, the fact that the splined part is already broken off poses a problem. To get around this problem, a special wrench accessory known as a reaction bar is used. It is essentially an inner sleeve sized to match the nut, and an outer ring with a strong torque arm welded to it. The lever is pressed against the adjacent nut and provides the counter-rotation necessary to loosen the nut.

Batch testing is also very important to ensure compliance. This involves selecting random TC bolts from each batch to be tested on a Skidmore-Wilhelm machine that hydraulically measures bolt tension. Strict pre-tensioning and post-tensioning procedures for each bolt are followed and the results are recorded in the structural engineering records.