Cardboard Hydraulic Arm

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We will introduce fluid power and then you can construct your very own mechanical arm from a few simple materials! The overall construction of the mechanical arm is separated into several parts, each generating a different type of motion. Tips for constructing the mechanical arm and advice for troubleshooting common issues are included in Appendix C. Printable cardboard templates are included in Appendix D.

  • In Part 1, you will build a system that uses fluid power to generate horizontal motion for opening and closing a scoop attached to the mechanical arm.
  • In Part 2, you will build a system that uses fluid power to generate rotational motion for swinging the mechanical arm left and right.
  • In Part 3, you will build a system that uses fluid power to generate vertical motion for lifting and dropping the mechanical arm.
  • In Part 4, you will integrate the three systems you have built to create a working mechanical arm.

What is a fluid?
A fluid is any substance that flows and deforms to take on the shape of the container holding the fluid. Every day examples of fluids include the water you drink and the air you breathe. In general, most liquids and gases can be classified as fluids. The ability of a substance to resist flowing is known as its viscosity. The viscosity of a substance is determined by its molecular structure, which dictates the amount of internal friction experienced by the substance during motion. Honey is more viscous than water, which is why honey flows much more slowly than water. Most solids cannot be classified as fluids because they have very high viscosities.

What is pressure?
The molecules in a fluid are constantly in motion, and this motion generates forces on the surfaces enclosing the fluid. Pressure describes the amount of force per unit area felt by the surface, in units of Pascals (Pa). In a container, the pressure generated is equally distributed over the inner surface of the container.

How is pressure increased or decreased?
Adding fluid to a container will increase the pressure in the container since there are more fluid molecules moving around in the same container. The molecules are more crowded in the container, and bump into the container walls more frequently, generating more force and pressure. Similarly, removing fluid from a container will decrease the pressure in the container since there are less fluid molecules moving in the container. Fewer molecules are available to bump into the container walls, so less pressure is generated inside the container.

Compressibility
What happens to fluids during compression?
When a gas is compressed, the same number of molecules is pushed into a smaller space. This means there is less empty space in the container so the gas molecules will bump into the walls more often, generating more pressure. In order to reduce the pressure, the gas will try to find a way to escape from the container to re-expand.

In a liquid, however, the molecules are already more densely packed than the molecules in a gas. This means it will be very difficult to compress the liquid any further, which is why liquids can be considered incompressible in many cases.

What types of fluid powered systems exist?
The two main types of fluid powered systems are categorized based on the type of fluid used. A pneumatic system uses pressurized or compressed gas to produce mechanical motion. When the gases in a pneumatic system are replaced with liquids, such as water or oil, the system is called a hydraulic system. Both pneumatics and hydraulics take advantage of the potential energy generated from fluid pressure during compression to do work.

Does it matter whether you use pneumatics or hydraulics?
Pneumatic systems are generally quite safe and reliable. Oxygen is commonly pressurized for use in pneumatic systems, and so a leak in the system should not majorly impact the surrounding environment. Since gases readily expand, minor leaks in a pneumatic system do not significantly affect the system’s performance. Hydraulic systems are generally more precise due to liquid incompressibility and the components of the system are often kept well lubricated by the liquid used. Water and oil are typically used in hydraulic systems.

Templates and Procedures

Have fun building your own cardboard hydraulic arm! Please send us photos of you and your class building it : info@puebloscience.org