Building a Model Hydraulic Knuckle Crane *

Areas of Science Mechanical Engineering
Time Required Long (2-4 weeks)
Prerequisites None
Material Availability You will need to purchase syringes to build the model crane.
Cost Low ($20 - $50)
Safety Be sure to wear safety goggles when using tools like saws and drills. Minor injury is possible. Adult supervision is required.
*Note: This is an abbreviated Project Idea, without notes to start your background research, a specific list of materials, or a procedure for how to do the experiment. You can identify abbreviated Project Ideas by the asterisk at the end of the title. If you want a Project Idea with full instructions, please pick one without an asterisk.


A ship is a busy place. When out to sea, naval, research, and fishing vessels pull or push heavy loads, like research equipment and crab pots, into and out of the water. But equipment and pots can weigh hundreds of pounds. So how does a ship's crew lift loads efficiently and accurately? With a hydraulic knuckle crane, of course. But what is hydraulics and why does hydraulics make certain tasks easier?

Hydraulics is the study of liquids and their mechanical properties: how they move, resist movement, act when subject to pressure, and so forth. In engineering, one application of hydraulics is using liquids, like water and oil, to move things. Why use a liquid to move things, instead of, say, air? One important reason is that a liquid is incompressible, which means that if you press on it, you cannot change its volume. For example, if you have a cup filled with marshmallows, you can fit even more marshmallows into the cup by compressing (squeezing) the marshmallows together. But if the cup is filled with water, you cannot fit more water into the cup - no matter how hard you squeeze! Because liquid is incompressible, applying a force (for example, a push or a pull) to one end of a hydraulic system transmits (moves) the force through the liquid to the other end.

Pressure is a force spread out over an area, or a force divided by an area. Pushing on the top of a glass of water, for instance, puts pressure on all of the water in the glass. Another way of thinking about pressure is to imagine pushing on an object with your finger or with the palm of your hand. In both cases, you are applying the same force, but the pressure is different because the area of the tip of your finger is much smaller than the palm of your hand. Figure 1 shows a simple hydraulic system that uses the concepts of force and pressure.

Mechanical Engineering science project
Figure 1. This simple hydraulic system has two pistons in cylinders on the top.

The hydraulic system in Figure 1 is made up of a liquid-filled tube topped with two pistons that move up or down as the liquid moves. Applying a downward force on this closed system (in this case, pushing down on the left-hand piston) causes the liquid to move against the right-hand piston and, because the liquid is incompressible, push the piston up. The pressure in the liquid is the same at every point within the liquid, because the liquid is incompressible. When the energy from the force applied to one piston is transferred to the liquid and then to the second piston, thus moving it, we say that work has been done by the first piston. Hydraulic machinery, like wheelchair lifts and excavators, use liquids to do work. That work can be applied to big jobs, like lifting heavy loads.

An advantage of hydraulic tools is the concept of force multiplication. Force multiplication enables the force you apply to one piston to be multiplied, by a multiplicative factor, to make a larger force acting on the second piston. This only works if the first piston is smaller in area than the second piston. Taking the simple hydraulic system mentioned and altering it to use force multiplication, it will look like Figure 2.

Mechanical Engineering science project
Figure 2. Simple hydraulic system with pistons of different areas. The piston with the smaller area is called the master and the piston with the larger area is called the slave. The difference in areas between the two enables force multiplication.

When you push in the master (smaller) piston, the amount of force applied to the slave (larger) piston by the liquid is multiplied by the ratio of the area of the slave piston to the area of the master piston. Using Equations 1 to 3 derives this relationship.

Equation 1.
Pressure at slave piston = Pressure at master piston
  • Pressure is in units of newtons(N)/area
  • Area is in units of centimeters (cm) squared

Because pressure is force acting over an area, Equation 1 is transformed into Equation 2.

Equation 2.

  • Fs = Force applied by liquid on slave piston (in units of newtons, N)
  • Fm = Force applied to liquid by master piston (N)
  • As = Area of slave piston (cm²)
  • Am = Area of master piston (cm²)

In Figure 2, the area of both pistons is a circle and the area of a circle is πr², where r is the radius of the circle. Using this given, you can transform Equation 2 into Equation 3 to find the force the liquid applies on the slave piston.

Equation 3.
Force on slave piston = Force applied by master piston x (area of slave piston/area of master piston)
  • Fs= Force applied by liquid on slave piston (N)
  • Fm=Force applied to liquid by master piston (N)
  • rs= radius of slave piston (cm)
  • rm= radius of master piston (cm)
  • π= pi (approximately 3.14)

The ratio of the areas of the two different pistons in Equation 3 is called the multiplicative factor. Using Equation 3, you can see that the force on the slave piston in Figure 2 is four times the force applied by the master piston. So if the master piston applies 1 N of force to the liquid, the liquid's force on the slave piston is 4 N. But note that the master must be pushed down 4 cm while the slave rises only 1 cm.

Force multiplication is the reason hydraulic machinery is so useful. A small force from the operator on one end of the machine is multiplied and results in a much larger force on the other end. Construction equipment like excavators and forklifts use the force multiplication of hydraulics. Often these machines have hydraulic cylinders. A hydraulic cylinder is made up of a piston and a cylinder that houses the piston and the hydraulic liquid.

In this mechanical engineering project, you will apply the principles of hydraulics to build a working model of a knuckle crane that will pick up and move a load. You can use syringes to act as the hydraulic cylinders. The plunger of the syringe acts as the piston in the hydraulic cylinder.

One approach to building your model knuckle crane is to use 4-inch bolts to attach two or three lengths of 2 x 2-inch wood. If you take this approach, you will have to figure out the best way to attach the pieces of wood together. You may want to attach syringes and pistons to each arm of the crane to make each part move separately from the other. You will also have to determine where to attach the syringes, as well as the sizes of syringes to use and how you will use force multiplication. Finally, you will need to choose the best attachment for picking up a load. Will you use a hook or a claw?

If you would like to see an example of hydraulics at work, take a look at this Science Buddies project on building model hydraulic lifts, Jack It Up! Lift a Load Using Hydraulics.

Share your story with Science Buddies!

I did this project Yes, I Did This Project! Please log in (or create a free account) to let us know how things went.


Michelle Maranowski, PhD, Science Buddies

Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Building a Model Hydraulic Knuckle Crane." Science Buddies, 29 Mar. 2018, Accessed 20 Oct. 2019.

APA Style

Science Buddies Staff. (2018, March 29). Building a Model Hydraulic Knuckle Crane. Retrieved from

Last edit date: 2018-03-29


News Feed on This Topic

, ,
Note: A computerized matching algorithm suggests the above articles. It's not as smart as you are, and it may occasionally give humorous, ridiculous, or even annoying results! Learn more about the News Feed

Share your story with Science Buddies!

I did this project Yes, I Did This Project! Please log in (or create a free account) to let us know how things went.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Ask an Expert

Related Links

If you like this project, you might enjoy exploring these related careers:

Mechanical engineer building prototype

Mechanical Engineer

Mechanical engineers are part of your everyday life, designing the spoon you used to eat your breakfast, your breakfast's packaging, the flip-top cap on your toothpaste tube, the zipper on your jacket, the car, bike, or bus you took to school, the chair you sat in, the door handle you grasped and the hinges it opened on, and the ballpoint pen you used to take your test. Virtually every object that you see around you has passed through the hands of a mechanical engineer. Consequently, their skills are in demand to design millions of different products in almost every type of industry. Read more
female mechanical engineering technician

Mechanical Engineering Technician

You use mechanical devices every day—to zip and snap your clothing, open doors, refrigerate and cook your food, get clean water, heat your home, play music, surf the Internet, travel around, and even to brush your teeth. Virtually every object that you see around has been mechanically engineered or designed at some point, requiring the skills of mechanical engineering technicians to create drawings of the product, or to build and test models of the product to find the best design. Read more
Argonne National Laboratory automotive engineers testing plug-in vehicle

Automotive Engineer

Cars are an important part of our daily lives. We depend on them to perform everyday tasks—getting to and from school and work, sports practice, grocery shopping, and various errands—and also to keep us safe while doing so. Our cars can keep us cool or warm while we drive them, and they even help us find our way. The automobile is made up of complicated braking, steering, and electrical systems, in addition to the engine and drive train. All of these systems require a tremendous amount of engineering, which is the responsibility of automotive engineers. They develop the components and systems that make our vehicles efficient and safe. Read more
Female physicist working


Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to human-scale projects to bring people new technologies, like computers, lasers, and fusion energy. Read more

News Feed on This Topic

, ,
Note: A computerized matching algorithm suggests the above articles. It's not as smart as you are, and it may occasionally give humorous, ridiculous, or even annoying results! Learn more about the News Feed

Looking for more science fun?

Try one of our science activities for quick, anytime science explorations. The perfect thing to liven up a rainy day, school vacation, or moment of boredom.

Find an Activity