Science Focus 8Mechanical SystemsTopic 2

Topic 2 – The Wheel and Axle, Gears, and Pulleys

Levers are not practical simple machines that can do all kinds of work. There are times that you need different machines.

* Turn to page 285 of your textbook and read the introductory paragraph of Topic 2.

A Lever That Keeps on Lifting

A WINCH consists of a small cylinder that has a crankor handle. The axle of the winch acts like the fulcrum, and the handle acts like the effort arm. By exerting a force on the handle to turn the wheel, the cable retracts the load. Because the handle is longer than the radius of the wheel, the effort force is smaller than the load. Using a winch is like using a short lever over and over again.

* Turn to page 285 of your textbook and read “A Lever That keeps on Lifting”. Also read the “Did You Know” located on the right hand side of the page.

QUESTION 1:

Provide two examples of winches.

The Wheel and Axle

The WHEEL AND AXLE is a machine that consists of two turning objects that are attached to each other at their centers. One of the objects causes the other to turn. The “wheel” does not even have to be round. The wheel-and-axle provides us with mechanical advantage. Just like a lever, a wheel-and-axle device can generate speed. In return, however, these machines require a large effort force and produce a smaller force on the load.

The bicycleis a good example of a complex machine because it is a system for moving a person from one place to another. Within the bicycle are groups of parts that perform specific functions, such as braking or steering. These groups of parts are called SUBSYSTEMS. Each subsystem in a complex machine contains a simple machine and usually has just one function. The subsystems of a bicycle are the wheel-and-axle, drivers and gears, frames and materials, brakes and steering, and aerodynamic design.

The subsystems in a mechanical device that produce motion, such as in a bicycle, play a role in how energy is transferred within the system. The subsystems are called LINKAGES and TRANSMISSIONS.

Linkages

The linkage is the part of the subsystem that transfers your energy from the pedals to the back wheel. In the bicycle, the chain is the linkage. In a car, the fan belt is the linkage from the engine to the cooling fan. This prevents the engine from overheating. Chains or belts form a direct link between two wheels. One wheel drives the motion and the other will follow in the same direction.

Transmissions

Machines that are more complex than a bicycle move much larger loads. A special type of linkage is needed. It is called a transmission. The transmission transfers energy from the engine to the wheels. A transmission contains a number of different gears. This enables the operator to move the object slowly with a large force, or quickly with a smaller force.

Gearing Up

Gears are essential components of most mechanical systems. GEARS are rotating wheel-like objects with teeth around their rims. When the gears rotate together, one gearwheel transfers turning motion and force to the other. Gears transfer energy in a mechanical system. Gear wheels work together in GEAR TRAINS of two or more wheels transferring rotary motion and force from one part of a complex machine to another part. The gear that supplies the energy is called the DRIVING GEAR (often called the driver). The gear to which the force is directed is called the DRIVEN GEAR (often called the follower).

Look at the gears above. A large gear (X) driving a smaller gear (Y) decreases torque and increases speed in the driven gear. Gears such as these are called multiplying gears. A small gear (Y) driving a larger gear (X) increases torque and reduces speed in the driven gear. Gears like these are called reducing gears. When the driving and the driven gears are the same size they are known as parallel gears.

When the driving gear has fewer teeth than the driven gear, the driven gear then rotates more slowly than the driving gear. A car or bicycle in low gear uses reducing gears.

* Turn to page 289 of your textbook and read “Going the Distance”.

A gear with teeth that fit into the links of a chain is called a SPROCKET.

NOTE: Gears (or sprockets) connected by a chain will turn in the same direction, while gears in contact will turn in opposite direction.

The relationship between the speed of rotations of a smaller gear and a larger gear is called SPEED RATIO. It can be calculated by dividing the number of driver gear teeth by the number of follower gear teeth. Below is the formula for calculating speed ratio:

Speed Ratio / = / Number of Driver Gear Teeth
Number of Follower Gear Teeth

Pulleys

A PULLEY consists of a wire, rope, or cable moving on a grooved wheel. One or more combinations of wheels and ropes can be fixed in place or moveable. Pulleys help you lift larger loads. A pulley is similar to a lever, but instead of a bar, a pulley has a rope.

There are two types of pulleys; a FIXED PULLEY (MA = 1) and a MOVABLE PULLEY.

A fixed pulley is attached to something that doesn’t move, such as a ceiling, a tree, or a wall. A single fixed pulley simply changes the direction of the motion and makes certain movements more convenient. A fixed pulley can be compared to a Class 1 lever. A movable pulley is attached to something else, often by a rope that goes around the pulley itself. The load may be attached to the center of the pulley. A movable pulley can be compared to a Class 2 lever.

* Turn to page 292 of your textbook and read “Pulleys”.

QUESTION 2:

Provide four examples of pulleys.

Supercharging Pulleys

In both fixed pulleys and in movable pulleys, the “load arm” and the “effort arm” are going to be the same length. So how do pulleys help us lift heavy objects? In this case, a combination of pulleys, including both fixed and movable pulleys is used. This is called a BLOCK AND TACKLE.

A block and tackle can have great mechanical advantage. By combining several pulleys together (COMPOUND PULLEY), heavy loads can be lifted. To calculate the MA of a compound pulley, count the number of number of ropes that lift the load. DO NOT INCLUDE THE ROPE WHERE THE EFFORT/FORCE IS BEING APPLIED!

* Turn to page 293 of your textbook and read “Supercharging Pulleys”.

QUESTION 3:

Turn to page 295 of your textbook and complete the following questions from Topic 2 Review: ______