INTRODUCTION AND BASICS
AN OVERVIEW OF THE MOST COMMON ATTRIBUTES AND BASIC FUNCTIONS OF A THREADED FASTENER.
Threaded fastener definition and fastener application are both fields that need a common knowledge ground to enable effective communication; it is easy to mix up terminology and physical connections.
Threaded fastener definition – introduction
What is the definition of a threaded fastener? And what word is most commonly used to describe a threaded fastener?
There are two competing words: “screw” and “bolt”.
In Swedish, bolt and screw are easy to distinguish. The screw has a thread. The bolt has no thread. In English, there is no such clear distinction. Some say the screw is fully threaded and the bolt is partially threaded. Some say the bolt must be combined with a separate nut, whereas the screw can be entered in a threaded or unthreaded hole. Most threaded fasteners in the automotive industry are bolts, and this is the word we will mostly use through out the training.
There are two main categories of joints: non-disconnectable and disconnectable. The nondisconnectable joints are e.g. adhesives, rivets, and welds. These joints can be very strong and durable, but are hard to take apart. The disconnectable joints can be buttons, velcro and a number of other types.
One of the most useful and durable is the bolted joint. The bolted joint has three main subcategories: High-strength joints, low-strength joints, and sealing joints. We will focus on the highstrength joints in this training, since these joints benefit from the unique properties of a bolt-andnut combination. Low-strength joints and sealing joints prioritise secondary features such as making a connection airtight, or attaching a vulnerable part.
The bolt’s mission
Why use a high-strength, bolted joint? There are two main reasons: one is to keep assembled parts from separating or coming apart, the other is to keep assembled parts from sliding against each other.
Example of bolted joints (automotive)
Where can we find bolted joints in a personal vehicle? Almost anywhere. Some of the more obvious and important bolted joints connect the steering wheel, the wheels, the seats, and the seat belt. It’s easy to see that these joints are absolutely necessary to the function of a car. In total, thousands of bolts and screws are found in a typical private vehicle. Not many of these can fail before the vehicle loses critical functions.
Bolted joints around us
Other examples of designs with critical bolted joints are ski-lifts, wind turbines and roller coasters.
Force in bolted joints
Force is often mentioned in connection with bolted joints. What is force and how does it relate to screw joints?
Force is measured in Newton (N), in bolted joints often kN (1kN = 1000 N).
1 kg is affected by 10 N of gravitational force.
A person weighing 75 kg is, in the same way, affected by a force of 750 N (0.75 kN).
What force can an M10 10.9 handle?
A screw or bolt is enormously strong, for example, an M10 10.9 is strong enough to lift 4 medium-sized cars without breaking.
This equals 6 tonnes or 60,000 Newton. A force handled by steel bolts is often in the region of thousands of Newton, which is why the term kilo Newton (kN) is most common.
Torque is another commonly used term in bolt-joint technology. Torque is the result of a force acting on an object, causing it to rotate. The magnitude of the torque depends on the amount of force and the length between where it is applied and the center of rotation, i.e. the length of the lever. Torque is measured in Newton meter (Nm), another way of saying force and length. Torque = length of leaver (m) x Force (N).
Levers around us
Torque might seem abstract but is actually something all of us meet every day. Just try and open a door without using the length of the handle. Other examples are seesaws and wrenches with different lengths. For example, when assembling an IKEA piece of furniture, the different tools supplied have different lengths to help the user with different torques.
How can the assembly torque transform into a clamping force?
Remember how strong the bolt is. In the previous example, the bolt was strong enough to lift 4 cars or a weight of 6 tonnes. When the bolt is tightened using torque, the tensile force can be transformed into clamping force. Imagine the weight of four cars holding the assembled parts together.
The amount of torque required for reaching maximum clamping force depends on the friction in the joint.
What is friction?
“Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.”
Picture yourself pushing a wooden box over the surface of a table. As soon as you stop pushing, the box slows down and stops moving. This is due to the friction between the base of the box and the table surface. Without friction, the box would have continued to slide to the edge of the table.
Friction problems around us
In the same way as we all use torque and levers around us, we also need to deal with friction every day. There are scenarios when the friction is low which might cause problems. For example, if there is ice on the road and your car doesn’t have winter tyres. High friction is often a good thing when you need good traction. At other times, it presents a problem; imagine a sled on a slope with no snow.
With bolts neither high or low friction is good, it is important that the friction is just right.
Friction in the bolted joint
When applying torque to a bolted joint to reach a predefined clamping force it is very important to know the friction. There are two zones in a bolted joint that are affected by friction; one is under the head and the other is where the nut and bolt threads engage. When these surfaces move, the friction counteracts the movement. This means that only a part of the torque ends up as clamping force, the rest of the torque turns into heat. On average, only 10% of the applied torque is transformed into clamping force. The rest is divided between the under-head surface and the threads.
Learn more – take the full training
This is a shortened version of the Bulten Academy training course Basics. To take part of the full version, please contact Bulten Academy – firstname.lastname@example.org.