Chapter 13 - Mechanical Devices

What is the primary function of mechanical devices?

They change one form of force into another.

What type of motion moves something in a straight line?

Linear motion.

A bicycle wheel turning is an example of which type of motion?

Rotary motion.

A repeated up-and-down or back-and-forth motion is known as _________ motion.

reciprocating

What type of motion is characterised by a curved backwards and forwards movement swinging on a pivot point, like a clock pendulum?

Oscillating motion.

What are the two main parts of a cam mechanism?

A cam and a follower.

What type of motion conversion does a cam mechanism perform?

It converts rotary motion into reciprocating motion.

How does a circular or eccentric cam cause its follower to move?

It uses an off-centre pivot, causing the follower to rise and fall.

What is the motion pattern of a follower on a pear-shaped cam?

The follower remains stationary for half a turn, then gently rises and falls.

Which type of cam is used to make carousel horses rise and fall?

A pear-shaped cam.

Describe the motion produced by a snail or drop cam.

The follower is stationary for half a turn, gently rises, and then suddenly falls.

What is a key limitation of a snail or drop cam?

It can only work effectively by rotating in one direction.

A heart-shaped cam is also known as a constant _________ cam.

velocity

What is the motion pattern of a follower on a heart-shaped cam?

The follower rises and falls steadily with uniform velocity and no stationary period.

What are the disadvantages of a flat follower?

They are not very accurate and have a lot of friction, though they cope well under load.

What is a major drawback of a point or knife follower despite its high accuracy and low friction?

It is quick to wear away its pointed edge.

Which type of follower is accurate, has low friction, and can withstand a load, but is more costly to produce?

A roller follower.

What is a lever?

A rigid bar that turns around a fixed pivot point, called a fulcrum, to gain a mechanical advantage.

What are the three main parts of any lever system?

The effort, the fulcrum, and the load.

In a lever system, what does the term 'effort' refer to?

The amount of force applied by the user, also known as the input.

What is the 'load' in a lever system?

The weight or resistance that needs to be moved, also known as the output.

How is mechanical advantage (MA) calculated using forces?

Mechanical advantage is calculated by dividing the load by the effort (MA = load ÷ effort).

How can mechanical advantage be calculated using distances on a lever?

By dividing the distance from the effort to the fulcrum by the distance from the load to the fulcrum.

In a _______ order lever, the fulcrum is positioned between the effort and the load.

first

A pair of scissors is an example of which class of lever?

A first order lever.

Where is the load positioned in a second order lever?

The load is positioned between the fulcrum and the effort.

A wheelbarrow is a common example of which class of lever?

A second order lever.

In a third order lever, what is positioned between the fulcrum and the load?

The effort.

Why are third order levers less common than first or second order levers?

They do not provide the same degree of mechanical advantage and are typically used for precision tasks.

What is a linkage in mechanical systems?

A set of levers joined together to transfer and control motion.

What are the three main functions of linkages?

They can change the direction of a force, change the magnitude of a force, or transform one type of motion into another.

Which type of linkage changes the direction of the input so that the output moves the opposite way?

A reverse motion linkage.

A _________ motion linkage, also called a push/pull linkage, makes the input and output travel in the same direction.

parallel

A bell crank linkage changes the direction of motion by what angle?

90 degrees.

What type of linkage is commonly used in bicycle brakes?

A bell crank linkage.

A crank and slider linkage converts motion into motion.

rotary, reciprocating

Which linkage is responsible for the reciprocating motion of a needle in a sewing machine?

A crank and slider linkage.

What type of motion does a treadle linkage convert rotary motion into?

Oscillating motion.

The mechanism that operates windscreen wipers on a car is an example of what kind of linkage?

A treadle linkage.

What is a gear?

A wheel with teeth around the outside that can change the speed, direction, or rotational force of a mechanism.

What is a gear train?

Two or more gears interlocking together.

In a simple gear train with two gears, how does the driven gear turn relative to the drive gear?

It turns in the opposite direction.

What is the 'drive gear' in a gear train?

The starting gear that provides the input movement and causes all other movement.

What is the 'driven gear' in a gear train?

The gear that receives motion from the drive gear and provides the output.

How is the gear ratio calculated in a simple gear train?

Gear ratio = number of teeth on the driven gear ÷ number of teeth on the drive gear.

What is 'gearing down'?

A gear arrangement where the driven gear is larger and rotates fewer times (slower) than the drive gear.

If a drive gear has 15 teeth and the driven gear has 60 teeth, what is the gear ratio?

The gear ratio is 4:1 (60 ÷ 15 = 4).

What is 'gearing up'?

A gear arrangement where the driven gear is smaller and rotates more times (faster) than the drive gear.

If a drive gear has 60 teeth and the driven gear has 15 teeth, what is the gear ratio?

The gear ratio is 1:4 (15 ÷ 60 = 0.25, expressed as 1:4).

What is the function of an idler gear placed between a drive and driven gear?

It makes the drive gear and the driven gear rotate in the same direction.

What is the function of bevel gears?

They transmit rotary motion through a 90-degree angle.

A hand drill often uses ________ gears to change the direction of the turning handle to the drill bit.

bevel

A rack and pinion mechanism converts motion into motion.

rotary, linear

In a rack and pinion system, what is the circular gear called?

The pinion.

A pillar drill uses a rack and pinion mechanism for what purpose?

To easily raise or lower the heavy metal table for accurate drilling.

What is a pulley?

A wheel with a grooved edge that a cord or belt passes around to lift loads or transfer motion.

What is the mechanical advantage of a single fixed pulley?

It has a mechanical advantage of one, as it only changes the direction of the force.

How can a mechanical advantage be created using pulleys?

By adding more pulleys to the system, forming a block and tackle.

What is a block and tackle system?

An arrangement of two or more pulleys used together to make lifting a heavy load easier.

How is the mechanical advantage of a pulley system calculated?

It is equal to the number of sections of rope pulling up on the object.

If a pulley system has 3 sections of rope supporting a 300 N load, how much effort is needed to lift it?

100 N of effort is needed (300 N ÷ 3).

What is the purpose of a belt drive system?

It transfers movement from one rotating pulley (the driver) to another (the driven) via a belt.

In a belt and pulley system, how can the output speed be changed?

By using different-sized pulleys; a larger driven pulley will rotate slower than the driver pulley.

How is the velocity ratio of a pulley system calculated?

Velocity ratio = diameter of the driven pulley ÷ diameter of the driver pulley.

How can the output speed of a pulley system be calculated?

Output speed = input speed ÷ velocity ratio.

A driver pulley of 40 mm diameter turns a driven pulley of 120 mm diameter. What is the velocity ratio?

The velocity ratio is 3:1 (120 ÷ 40 = 3).

In a pulley system, how can the driver and driven wheels be made to turn in opposite directions?

By crossing the belt over into a figure-of-eight shape.

What is friction in a mechanical system?

The resistance encountered when two surfaces rub against each other.

Which cam shape would be most suitable for a mechanical toy that requires a character to rise gradually and then drop suddenly?

A snail-shaped cam.

On a bicycle, what is the effect of selecting a small front gear and a large rear gear when climbing a hill?

This creates a low gear ratio, making it easier to turn the pedals but covering less distance per pedal stroke.

A lazy tong linkage is a set of interlinked arms that allow a product to or

contract, expand

A folding clothes horse is a common application of which type of linkage?

A lazy tong linkage.