Unit 4: Mechanical Systems

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

Topic 2 Key Terms

• Winch

• Gear

• Gear Train

• Driving Gear

• Driven Gear

• Pulley

• Axle

• Speed Ratio

Calculating work review

1. Calculate the force needed to drag a log 11m, doing 1000J of work.

2. In which situation would you be able to carry the bag farther? Situation A: Doing 800J of work, using 12N or Situation B: Doing 1100J of work using 4N.

Calculating work review

3. In which situation would more work be done, Situation A, using 40N to lift a bag of soil 0.75m, or Situation B, using 50N to lift a bag of soil 1.25m?

4. Calculate the amount of work done opening a paint can when the lid moves 0.5m and 3N of force was used.

The Wheel and Axle

• Wheel-and-axle combinations come in a variety of shapes and sizes.

• “Wheels” do not have to be round.

• As long as two turning objects are attached to each other at their centres, and one causes the other to turn, you can call the device a wheel and axle.

• A wheel-and-axle device can generate speed. These machines require a large effort force and produce a smaller force on the load.

• http://www.cosi.org/files/Flash/simpMach/sm1.swf

• Wheel and Axle

• Winch: consists of a small cylinder and a crank or handle.

• The axle of the winch is held in place and acts like a fulcrum.

• The handle is like the effort arm of a lever.

• Exerting a force on the handle turns the wheel.

• The radius of the wheel- the distance from the center of the wheel to the circumference- is like the load arm on a lever.

• The force that the cable exerts on the wheel is like the load on a lever.

• Since the handle is much 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.

Gearing Up

• Gear: a rotating wheel-like object with teeth around its rim.

• Gear Train: a group of two or more gears. The teeth of one gear fit into the teeth of another. When the first gear turns, its teeth push on the teeth of the second gear, causing the second gear to turn.

• Driving Gear (driver; or first gear): may turn because someone is turning a handle or because it is attached to a motor.

• Driven Gear (second gear; or follower): the gear turned by the driving gear.

Going the Distance• Think about the gears on a bicycle. One set of gears is

attached to the pedals and the other to the rear wheel.

• A chain connecting the gears allows the front gear to turn the gear on the rear wheel, some distance away.

• Sprocket: a gear with teeth that fit into the links of a chain.

• Speed Ratio: the relationship between the speed of rotations of a smaller gear and a larger gear.

• Speed Ratio= #of driver gear teeth / # of follower gear teeth

Speed Ratio Practice1) Calculate the speed ratio if the driver gear has 45 teeth and the follower gear

has 18 teeth

2) Calculate the speed ratio if the driver gear has 45 teeth and the follower gear has 16 teeth.

3) If you are trying to win a race and your driver gear has 55 teeth, should you use the follower gear with 5 teeth or 9 teeth? Which one would use?

4) If you want your speed ratio to be higher than 2, which of the following follower gears would you use: 11, 12 or 17? The driver gear has 60 teeth.

Pulleys

• Pulley: a grooved wheel with a rope or chain running along the groove.

• A pulley is like a class 1 lever. Instead of a bar, a pulley has a rope. The axle of the pulley acts like a fulcrum. The two sides of the pulley are the effort arm and the load arm.