Abstract
Can you lift a car? No? You say you are not strong enough? True, our bodies are not built to lift heavy loads like cars. Fortunately, our brains are smart enough to harness the power of fluids, like water and oil, to create hydraulic lifts. By pushing a button on a hydraulic lift, a mechanic can easily raise a car with one finger. Lifts can also be used to raise lots of other heavy loads - even such massive things as steel girders to construct a skyscraper! In this mechanical engineering project, you will get a firsthand look at the power of a hydraulic system by building your own working model of a lift.Summary
Michelle Maranowski, PhD, Science Buddies
Objective
To apply the principles of hydraulics in building a working model of a hydraulic lift.
Introduction
What do a wheelchair lift, an excavator at a construction site, and a dentist's chair have in common? The ability to lift a heavy load. And in all of these examples, the ability is due to the power of hydraulics.
Hydraulics is the study of liquids and their mechanical properties: for instance, 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 glass is full of water, there is no way you can fit more water into the cup - no matter how hard you squeeze! Since liquid is incompressible, applying a force (for example, a push or a pull) to one end of a hydraulic system transmits (passes along) the force through the liquid to the other end.
A force spread out over an area, or a force divided by an area, is called pressure. Pushing on the top of a glass of water is putting pressure on the water. Another way of thinking about pressure is to imagine yourself 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.
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. 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. Since the liquid is incompressible, the pressure in the liquid is the same at every point within the liquid. When the energy from the force applied to one piston is transferred to the liquid and gives its energy to move the second piston, we say that work has been done to the liquid by the first piston. Hydraulic machinery, like wheelchair lifts and car jacks, use liquids to do work. That work can be applied to performing 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. If we take the simple hydraulic system mentioned and alter it to use force multiplication, it will look like Figure 2.
Figure 2. Simple hydraulic system with pistons of different areas. The piston with the smaller area is called the primary piston and the piston with the larger piston is called the secondary piston. The difference in areas enables force multiplication.
When the primary (smaller) piston is pushed in, the amount of force applied to the secondary (larger) piston by the liquid is multiplied by the ratio of the area of the secondary piston to the area of the primary piston. This relationship is derived in Equations 1-3.
Equation 1.
|
Since pressure is force acting over an area, Equation 1 is transformed into Equation 2.
Equation 2.| Fs | Fp | |
| —— | = | —— |
| As | Ap |
|
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 fact, Equation 2 is transformed into Equation 3, to find the force applied by the liquid on the secondary piston.
Equation 3.| Fs = Fp ( |
πrs2 —— πrp2 |
) |
|
The ratio of the areas of the two different pistons in Equation 3 is called the multiplicative factor. Using Equation 3, the force on the secondary piston in Figure 2 is four times the force applied by the primary piston. So if the primary applies 1 N of force to the liquid, the liquid's force on the secondary is 4 N. The only issue is that the primary must be pushed down 4 cm to rise the secondary by 1 cm.
Force multiplication is the reason hydraulic machinery is so useful. A small force from the operator on one end is multiplied and results in a much larger force on the other end of the machine. Construction machines 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 science project, you will build three model hydraulic lifts and demonstrate the concept of force multiplication using syringes with different radii. The syringes act as hydraulic cylinders. The plunger of the syringe acts as the piston in the hydraulic cylinder. A real life example of a hydraulic lift is a vehicle wheelchair lift, like the one shown in Figure 3. What will be the maximum weight each model lift can raise? Will the maximum weight change if you change the size of the secondary piston?
Figure 3. A handicapped child can easily and safely enter a vehicle using a hydraulic wheelchair lift, as shown in this image.
Terms and Concepts
- Hydraulics
- Incompressible
- Force
- Area
- Pressure
- Piston
- Energy
- Work
- Force multiplication
- Newton
- Radius (radii)
- Multiplicative factor
- Hydraulic cylinder
- Hydraulic lift
- Diameter
Questions
- What is force? What units is it measured in?
- How is force related to work and energy?
- What are some examples of liquids used in hydraulic systems?
- What is the most important property of a hydraulic fluid or liquid?
- Could air be used to lift loads instead of liquid? Why or why not?
- What is the difference between the primary piston and the secondary piston?
Bibliography
- Hodanbosi, C. (1996, August). Pascal's principle and hydraulics. National Aeronautics and Space Administration. Retrieved March 20, 2012.
- Educational Video Library. (2017, March 12). Basic Principles of Hydraulics Explained. Retrieved January 17, 2021.
- Bartelt, T. (2004). The multiplication of force by a hydraulic system. Retrieved March 20, 2012.
Materials and Equipment
- Wall plates, blank, stainless steel (3). You can purchase blank wall plates from a hardware store. Minimum size: 4.5 x 2.75 inches
- Phillips screwdriver
- Syringes, 12 cc, catheter tip (4). You can purchase 12 cc syringes from www.amazon.com.
- Syringe, 35 cc, catheter tip. You can purchase a 35 cc syringe from www.amazon.com.
- Syringe, 60 cc, catheter tip. You can purchase a 60 cc syringe from www.amazon.com.
- Optional: Hacksaw
- Optional: File. You can purchase a file from a hardware store.
- Utility knife
- Epoxy. You can use either Elmer's® superfast epoxy cement or Devcon 5 minute epoxy.
- Paper plates (3)
- Disposable spoons (3)
- Optional: Plumber's putty
- Vise, 4-inch. You can purchase a vise from a hardware store or online at www.amazon.com.
- Metric weight set or a variety of materials of varying weights that can be used for testing. You can borrow a metric weight set from your school's science laboratory. Borrow a 500-gram weight, a 200-gram weight, two or three 100-gram weights, and several smaller weights to start.
- Digital scale, if you are not using a set of standard weights. You can purchase a digital scale online at www.amazon.com.
- Silicone fuel tubing, 5/32-inch inner diameter (3 feet). You can purchase silicone fuel tubing from www.amazon.com or from a hobby shop.
- Glass of water
- Ruler
- Adult volunteer
- Lab notebook
- Graph paper
Experimental Procedure
Building the Model Hydraulic Lifts
- In this project, you will build three model hydraulic lifts using syringes with pistons of different areas and metal wall plates. The syringes act as hydraulic cylinders.
- First you will build the model lifts, which involves attaching the top of a secondary syringe's piston to the back of a metal plate with epoxy and then connecting the primary syringe to the secondary syringe with tubing.
- Every lift will have a 12 cubic centimeter (cc) primary hydraulic cylinder but each will have one of three different secondary hydraulic cylinders: 12 cc, 35 cc, and 60 cc.
- You will use water as the hydraulic fluid.
- Prepare the metal plates. The metal plates will hold the weights while the secondary piston will lift the plate and the weights. Using the Phillips screwdriver, remove the screws from both ends of all three of the blank wall plates. Discard the screws.
- Prepare the syringes to attach to the metal plates. Make sure that the tops of the pistons in these syringes are flat because they will be epoxied to the back of the metal plates, and they need to be flush against the plates.
- Larger syringes may have rings on the piston. If a piston has a ring on it, then use the hacksaw to remove it. File down the remains of the ring, so that the top of the piston is as flat as possible, as shown in Figure 4. Ask an adult volunteer to help you remove the ring.
- If the 12 cc syringes have a curved tip, then cut the curved tip off with the utility knife so that the remaining tip is straight. Cutting off the curved tip makes it easier to push into the tubing.
Figure 4. Two 60 cc syringes, one shown with the original ring (left) and one shown with the ring removed (right).
- Epoxy the top of one 12 cc, 35 cc, and 60 cc syringe each to a separate metal plate, as shown in Figure 5, to create half of the model lift. The syringes that you epoxy to the metal plates are the secondary syringes (or secondary hydraulic cylinders).
- Following the safety recommendations and directions on the epoxy package, mix up enough epoxy to make one lift at a time. Use a paper plate and a disposable spoon to mix the epoxy. You may want to have an adult help you with this.
- Place a small dollop of epoxy in the middle of one of the metal plates.
- Epoxy the top of one of the 12-cc syringe pistons to the metal plate. Follow the directions on the epoxy package, and allow the epoxy to cure for the recommended amount of time. Throw away the paper plate and spoon.
- Repeat steps 4a to 4c with one 35-cc syringe and one 60-cc syringe.
Figure 5. This image shows the three secondary syringes and wall plates used in the experiment. Each secondary syringe/metal plate pair comprises half of a model hydraulic lift.
- You will next use another 12 cc syringe and tubing to make the primary syringe (or primary hydraulic cylinder).
- Cut a 30 centimeter (cm) piece of tubing.
- Push the tip of a 12 cc syringe (that is not epoxied to a wall plate) into one end of the tubing.
- The primary syringe is now ready to attach to the secondary syringe (secondary hydraulic cylinder) to create the hydraulic lift.
- Fill the primary syringe and its tubing with water and prepare to attach it to the secondary syringe to create the hydraulic lift. Water will serve as the hydraulic fluid in the hydraulic lifts.
- Push the piston in the primary syringe all the way down so that it is fully depressed into the cylinder.
- Place the free end of the tubing into the glass of water and pull the piston up (while firmly holding the rest of the syringe). This will suck water into the tubing and syringe. Pull the piston slowly and smoothly so that you avoid getting bubbles in the tubing.
- Pull the primary syringe's piston up as far as it can go without falling out of the cylinder of the syringe.
- Depress the primary syringe's piston a tiny bit to remove any bubbles at the end of tubing.
- Insert the tip of the 12 cc secondary syringe into the free end of the tubing that is attached to the primary syringe.
- Confirm that the 12 cc hydraulic lift works by pushing and pulling the water into and out of the secondary syringe by pushing and pulling the primary syringe's piston. Note If the tubing pops off the syringes, simply refill the tubing and primary syringe with water, as in step 6, and reattach the tubing to the secondary syringe, sealing the connection with plumber's putty.
- End your practice by pulling on the primary piston and bringing the water from the secondary syringe back into the tubing and primary syringe.
- Repeat steps 6 and 7 using the 35 cc secondary syringe/metal plate assembly and a 12 cc primary syringe/tubing assembly, to create the 35 cc model hydraulic lift.
- Repeat steps 6 and 7 using the 60 cc secondary syringe/metal plate assembly and a 12-cc primary syringe/tubing assembly, to create the 60 cc model hydraulic lift.
Testing the Model Hydraulic Lifts
- You are now ready to test the 12 cc model hydraulic lift, the 35 cc model hydraulic lift, and the 60 cc model hydraulic lift (all with a 12 cc primary syringe, or primary hydraulic cylinder). All three model hydraulic lifts should have a primary and secondary syringe, and hydraulic fluid, or water, in the primary syringe and tubing.
- Make sure that the hydraulic fluid, or water, is in the tubing and primary syringe. The secondary piston must be depressed in its cylinder.
- Place the vise on a sturdy table. The vise will be used to support and hold the lifts while they are lifting.
- Use a set of light weights (such as the metric weights in the Materials list) or several types of materials of various weights to test the model hydraulic lift.
- For example, you can use books, bottles of water, or cans of food. You can combine a couple of different items to make heavier weights.
- To prepare the 12 cc model lift for testing, insert the 12 cc secondary syringe and its metal plate vertically into the vise. Mount the lift so that the collar of the syringe is sitting on the vise's jaws and the metal plate is above its jaws, as shown in Figure 6. Mount the secondary syringe cylinder high in the vise so that the piston does not stick as it moves up and down. Mount each secondary syringe that you test the same way in the vise.
- Tighten the vise enough to hold the syringe but not so tight that the piston cannot move up and down within the barrel.
- The lift should be held firmly enough so that when you place weight on the plate, the lift does not fall over.
Figure 6. Here is a completed 12 cc model hydraulic lift, where the lift is properly held and supported by the vise.
- The 12 cc model lift is now ready for testing. Use the weights that you collected and determine how hard it is to lift them with the 12 cc secondary syringe. Start with a small weight and try increasingly heavy weights. Record all data in a data table in your lab notebook.
- If you are using your own materials for testing (not a set of standard weights) then weigh the material on the digital scale before placing it on the lift. Record the weight of the material in your lab notebook.
- Make sure the primary piston is pulled out all the way, so the secondary piston is all the way down.
- Set the materials on the plate. Smoothly depress the primary piston, delivering all of the hydraulic fluid, or water, from the primary syringe to the secondary syringe. As the primary piston is depressed, and the hydraulic fluid is delivered, the secondary piston and the metal plate will rise.
- Pay attention to how hard you have to press on the primary piston in order to lift the weight.
- Optional: use a ruler to measure how far the plate attached to the secondary piston rises. How does this compare to the distance you depressed the primary piston?
- Pull the primary piston back out all the way, fully lowering the secondary piston.
- Increase the weight by about 100 grams, using different materials, and repeat steps 5a–5f until you can no longer lift the weight by pressing on the primary piston.
- You may find that with heavier weights, with the piston fully extended, the vise may start to tumble. In this case, have your volunteer hold the vise.
- Repeat steps 5a through 5g two more times for a total of three trials. Redoing your experiments ensures that your data is repeatable and accurate. Before each trial, pull the primary piston out and bring the hydraulic fluid back into the primary syringe and tubing.
- Repeat steps 4–5 with the 35 cc and 60 cc hydraulic model lifts. Remember to record all data in a data table in your lab notebook. Is it easier or harder to lift the same amount of weight with the different size secondary syringes? Does the metal plate move a longer or shorter distance when you completely depress the primary piston?
Analyzing the Data
- Calculate the force you applied to the primary piston in each trial. You can do this using Equation 3 in the background, but you will need to rearrange it to solve for Fp instead of FS. You know the force applied to the secondary piston (FS) because you know how much weight you put on the plate, and you can measure the radii of the two pistons using a ruler.
- Which primary piston required the most force to lift the same amount of weight? Which required the least? Which of your hydraulic lifts allowed you to lift the most weight? How does this relate to the principle of force multiplication in hydraulics?
- Optional: which lift moved the secondary piston the farthest when you fully depressed the primary piston? Which moved it the shortest distance? How is this related to the force multiplication?
- Make an x-y graph with weight placed on the secondary piston on the x-axis and force required to lift that weight using the primary piston on the y-axis, with different lines for each of your three hydraulic lifts.
- What does your graph look like? Does the trend make sense given the principle of force multiplication in hydraulics?
Ask an Expert
Variations
- Repeat the experiment using mineral oil as the hydraulic liquid instead of water. Does this change affect the maximum weight supported for each lift?
- What other types of models of construction machinery or fun contraptions can you build applying the principles of hydraulics? For example, can you build a barbershop chair or a toy to entertain your pet? Let your imagination go! The Engineering Design Process guide can help you get started.
- Make a lift with two secondary syringes and one primary syringe. What is the maximum weight this lift can support?
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