🟦 4.1 Overview of engineering connections
Are you designing a building, bridge, machine, rocket, or robot? You're going to have many components that must be properly connected.
When we talk about connections, we talk about their effects on the system:
Does the connection constrain translation? If so, it has the ability to develop a force reaction.
Does the connection constrain rotation? If so, it has the ability to develop a moment reaction.
For instance, think about a push pin. You use a push pin to hang a piece of paper from a bulletin board. Does the push pin constrain translation? Yes, it does. For that reason, it has the ability to develop a force reaction. Does the push pin constrain rotation? No, it does not. For that reason, it does not have the ability to develop a moment reaction.
❏ Overview of (2D) connections and reactions
The main types of engineering connections we use are the (external) pin, the roller (or pin-roller), and the fixed connection.
Joints in members generally fall into one of two categories. If force is transferred but moment is released, we model an internal pin (sometimes called a hinge). If both force and moment are transferred over the joint, we call it a continuous connection.
Each of these is discussed in more detail below. Three-dimensional connections are introduced in Lesson __.
🟦 4.2 A single pin
The most common type of engineering connection is called the pin.
One element (in this case, a yellow bar) has a hole. We insert a cylindrical element (in this case, a bolt) through the hole, and secure it (in this case, with a square nut).
Are you wondering what this pin is doing? Why is it there? What is its purpose? Well ... it's not doing anything; it's just here to show you what a pin looks like in real life.
❏ Flipbook: A single pin
🟦 4.3 The pin connection vs. the pinned connection
Now, let's build a structure out of two bars and a pin. I'll use the simple representation of a pin: a solid cylinder.
This concept is incredibly important for learning Statics and the classes that follow. You need to understand the difference between a pin and a pinned connection.
The pin transfers a force, but is not constrained from movement.
A pinned connection transfers a force and is constrained from movement. Whenever a connection constraints translation while allowing rotation, we model it as a pinned connection.
❏ Flipbook: pins and pinned connections
❏ Pinned connection
🟦 4.4 Rollers (pin-rollers)
The roller connection (or pin-roller)
prevents translation in one direction (perpendicular to the surface) and develops a force reaction in that direction
be very careful with the context! some connections can only transfer a bearing (pushing) force. Other roller connections can transfer either a bearing force OR a tension (pulling) force.
❏ Flipbook: rollers
🟦 4.5 Fixed connections
This one is like an “upgraded” pin connection. It does everything the pin connection does, except that it can also develop a reaction moment.
it develops force reactions in the x-direction and the y-direction (meaning that translation is prohibited with respect to the x-direction and the y-direction)
it also develops a reacting moment (meaning that rotation is prohibited)
❏ Flipbook: fixed connections
The
when to introduce qualitative equilibrium?
add section with the ladder and icy driveway - talk about
x
the pin connection / Forces on a bar in equilibrium / lever law / bearing contact of a normal force; the roller connection - bolts in holes - bearing, equal and opposite horiz. forces - modeling as a simplification of reality; the fixed connection
🟦 Summary
The pin connection
prevents translation in the x-direction and y-direction
develops force reactions in the x-direction and y-direction
The roller connection (or pin-roller)
prevents translation in one direction (perpendicular to the surface) and develops a force reaction in that direction
be very careful with the context! some connections can only transfer a bearing (pushing) force. Other roller connections can transfer either a bearing force OR a tension (pulling) force.
The fixed connection
This one is like an “upgraded” pin connection. It does everything the pin connection does, except that it can also develop a reaction moment.
it develops force reactions in the x-direction and the y-direction (meaning that translation is prohibited with respect to the x-direction and the y-direction)
it also develops a reacting moment (meaning that rotation is prohibited)
🟦 x
put this later?
In real-world applications, we securely tighten the bolt and nut assembly. In fact, we often make the connection so tight that we engage significant friction between the components.
In this class, we will never account for any friction in the connection. For this reason, you can imagine all pin connections (in this class) as loosely-fitted bolts.
➜ Practice Problems
coming soon