Island 2 Fischertechnik-Static Level 2 Heavy Loads
Mechanics & Structures
POP students are challenged to build machines that use leverage to lift heavy loads. They will need to gain sufficient mechanical advantage. Calculate the mechanical advantage and determine if they can redesign the machines to improve this ratio.
Island 2 Fischertechnik-Static Level 2 Lifting Heavy Loads
Engineering Design Process
Take a moment and think about the form and function of a car jack. What does it need to do? What does it need to look like?
These requirements are called design specifications and are a critical part of the engineering design process. Engineering design is really just a specialized form of problem-solving. When engineers design a machine, they start out with a detailed description of the problem they are trying to solve. In addition to the function they need the machine to perform, they need to think about things like size, weight, reliability, cost, etc.
How a Car Jack Works
Car jacks rely on a combination of levers and screws to provide mechanical advantage. There are many different kinds of car jacks. A key design element for most is a special kind of gear called a screw spindle.
The screw spindle passes through a threaded collar or nut. This screw system works on the same principle as a common nut and bolt.
The screw spindle gear is self-locking. In the model they are building, the spindle screw can move the nut attached to the lifting arm, but the nut cannot turn the spindle screw.
By turning the crank by hand, a rotational force is applied to the screw spindle. The spindle then converts the rotational force into a linear force that pushes the nut forward and backward along the spindle, depending on the direction of the rotation. One end of the main lever is attached to this moving nut, causing the lever to feel an applied force from the nut that moves the lifting arm up and down. It is this lifting arm that applies the force to lift an object, like a heavy car.
Depending on the location of the pivot point between the lifting arm and the arm of the applied crank force, the jack can either increase our lifting power (mechanical advantage) or the lift distance.
Mechanical Advantage of a Car Jack
The car jack is a compound machine made up of several connected simple machines. The geometry is somewhat complex, but we can calculate mechanical advantage through observation.
Count how many revolutions of the crank it takes to fully extend the lifting arm. You can easily measure the vertical distance traveled by the lifting arm. The distance traveled by the crank takes a little calculation since it travels in a circle.
The formula for the circumference of a circle is C = 2π r where r is the circle radius and π (pi) is approximately 3.14.
Multiply the circumference by the number of revolutions to find the total distance.
Just as with our simple lever, the ratio of the distance traveled by the crank compared to the distance traveled by the lifting arm defines the mechanical advantage.