Statics

The main types of engineering connections we use are the (external) pin, the roller (or pin-roller), and the fixed connection. We use these types of connections to connect our design to the context.

The interactive provides an introductory overview of these three common types of engineering connections. This is how we connect our design to the context.

Inside our design, we often have to connect one member to another. Those connections can be complex, but 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 the three main connection types is discussed in more detail below. These are all considered two-dimensional (planar) engineering connections. Three-dimensional connections are introduced later in this course.

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 much of anything; it's just here to show you what a pin looks like in real life.

Of course, engineers need to understand how nuts and bolts work to make good connections. For now, though, it may be useful to think of the pin as a solid cylinder.

A pin can transfer a force. The force can be at any angle. For that reason, many texts will state that a pin can transfer two forces (one in the x-direction and one in the y-direction). The force points in the direction required for static equilibrium. It could be upwards, leftwards, or at a 30 degree angle.

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. (You can use your imagination if you want to think of it as a bolt.)

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.

In the flipbook, the orange cylinder is the pin. When the pins act in combination with the blue plates and the hands, we simulate a pinned connection.

If you connect two bodies with one pin, both tend to rotate about the pin.

Conversely, if you connect two elements with multiple pins, they tend to create a rigid body that acts like a single unit.

You can think of the roller connection (or pin-roller) by thinking about a skateboard.

The wheels of the skateboard can provide upward force reactions, but they do not provide resistance to the horizontal force the skateboarder supplies by pushing against the ground. Horizontal translation is unconstrained; vertical motion is impeded.

Since the roller prevents translation perpendicular to the surface, it has the ability to develop a force reaction perpendicular to the surface. It's a normal, compressive force.

Here is the tricky thing about roller connections. Different engineers use the roller symbol slightly differently.

Let's compare the skateboard type of roller to a wheel that rolls in a track. You will find the wheel-in-track connection in an overhead garage door, a dishwasher, and a "chest of drawers" ("dresser").

On this site, we will assume that unless noted otherwise, all of our pin-rollers act like wheels in a track.

The roller has a close cousin, called the rocker. The one depicted here develops a vertical force reaction, but does not provide a horizontal force reaction. Instead, if there is a horizontal force in the system, the rocker pivots to accommodate it.

This type of connection is used in bridges. It combats thermal contraction and expansion. This topic is studied in Mechanics of Materials.

Rockers aren't a focus of this Statics course, but it's good for you to know that they exist.

Let's begin with a thought experiment. 

Two bodies are supported by pushpins as shown. A single force is applied to each.

Can the pushpins create static equilibrium?

Pushpin A, acting in isolation, cannot create static equilibrium all by itself. The body will rotate clockwise about A due to the applied force.

Pushpins B and C both deliver normal forces to the body. Together, their force reactions impede both rotation and translation of the body. They prevent the body from rotating clockwise by applying an equal and opposite moment counterclockwise. They can create static equilibrium.