Mechanical Power Systems Chapter 9 Mechanical Power Systems

Objectives Mechanical advantage of simple machines. Types of gears. Mechanical transmission devices.

Simple machines There are 6 simple machines used to control mechanical energy: the lever, the pulley, the wheel and axle, the inclined plane, the screw, and the wedge (fig 9-2, page 203). Complex machines such as washing machines, and drill presses use more than one simple machine to accomplish their tasks.

Levers A lever is a rigid bar that rotates around one fixed point called the fulcrum. The position of the fulcrum, load, and input determines the lever class (fig 9-3, page 203). First class levers have the fulcrum positioned between the input force and the load. Pliers are an example. Second class levers have the load placed between the fulcrum and the input force. A wheel barrow is a good example. Third class levers have the force between the fulcrum and load. Shovels and rakes are good examples.

Pulleys A single fixed pulley changes the direction of force. Flags are raised on poles with the help of fixed pulleys. Single moveable pulleys change the size of a force (fig 9-7, page 206). Several pulleys together make a block and tackle. Piano movers and construction workers use this type of system to lift heavy loads to great heights.

Wheels and Axles, Inclined Planes Wheel and axle: systems can be used to change the size or distance of a force. The steering wheel of an automobile is an example of a wheel and axle system. Doorknobs are another example. Inclined Planes: Loading ramps are an example of this simple machine. Wheelchair ramps use the same principle.

Screws, Wedges, & Gears Screws: Screws are commonly used as mechanical fasteners for wood, metal, and plastic. When used in car jacks, screws function as force multipliers. Wedges: Wedges are often used to split materials such as chopping wood with a hatchet. Gears: Gear sets are made so the gear teeth interlock and drive each other. Most common are the spur, helical, rack and pinion, worm, and bevel gears (fig 9-16, page 210). Gears control mechanical power by changing direction of power, speed, and torque.

Characteristics of Mechanical Power A scalar quantity represents a physical quantity specified by the magnitude. Examples include 750F, 65 mph, or 45 lbs. A vector quantity has both magnitude and direction. Examples include 100 miles east (displacement), 100 mph south (velocity), 250 lbs upwards (force). A force that produces a twisting or turning effect or rotation is called torque. Horsepower (hp): is the rate at which output work is performed. When forces are balanced they are said to be in a state of equilibrium.

Transmission of Mechanical Energy Clutches: A clutch is a mechanical device that connects the power source to the rest of the machine. This device is needed so vehicles can remain at rest with the engine running, start slowly without stalling, and shift gears while moving. Pulleys and Belts: Belts and pulleys control mechanical energy through any of 5 different arrangements: connect and disconnect power like a clutch, change direction, reverse rotation, change speed, and change torque (fig 9-21, page 215). Chains and Sprockets: Usually used as the drive system to bring power to the driving wheel of the vehicle in bicycles and mopeds. Shafts and Bearings: Used to transfer mechanical energy in many types of machines.

Mechanical Advantage Mechanical advantage is increased force, speed, or distance and the benefits of that increase created by using machines to transmit force. Using mechanical advantage usually results in less effort required to perform a task. Using Simple Machines to Gain Mechanical Advantage: Simple machines can make lifting and other strenuous activities easier by multiplying force. The tradeoff for gaining force is loss of speed or distance. Mechanical Advantage of Compound Machines: Calculate the mechanical advantage of each simple machine within the compound machine. Multiply the mechanical advantages to get the mechanical advantage of the compound machine. Actual Mechanical Advantage: is less than the ideal mechanical advantage because of energy lost through friction.

Summary There are 6 simple machines used to control mechanical energy: the lever, the pulley, the wheel and axle, the inclined plane, the screw, and the wedge. A lever is a rigid bar that rotates around one fixed point called the fulcrum. A single fixed pulley changes the direction of force. Flags are raised on poles with the help of fixed pulleys. Wheel and axle: systems can be used to change the size or distance of a force. Inclined Planes: Loading ramps are an example of this simple machine. Screws: Screws are commonly used as mechanical fasteners for wood, metal, and plastic. Wedges: Wedges are often used to split materials such as chopping wood with a hatchet. Gears: Gear sets are made so the gear teeth interlock and drive each other. Gears control mechanical power by changing direction of power, speed, and torque. A vector quantity has both magnitude and direction. Examples include 100 miles east (displacement), 100 mph south (velocity), 250 lbs upwards (force). A force that produces a twisting or turning effect or rotation is called torque. Mechanical advantage is increased force, speed, or distance and the benefits of that increase created by using machines to transmit force. Using mechanical advantage usually results in less effort required to perform a task.

Home Work 1. Name the 6 simple machines used to control mechanical energy? 2. What is the difference between scalar and vector quantity? Give examples of each. 3. What is mechanical advantage?