chap14 pps Automotive technology at University of Cambridge

Decimals are used to represent fractions of a unit by using a dot called a decimal point to indicate that the number is a decimal.. To obtain a gear ratio, divide the number of teeth on

start

After studying Chapter 14, the reader should

be able to:

OBJECTIVES:

decimal point • diagram • direct drive • drive gear •

driven gear

fractions • gear reduction • graph • overdrive

percentage • scientific notation • variable

KEY TERMS:

DECIMAL

Decimals are commonly used by services The placement of the

decimal point indicates the value of the number The naming of

decimals includes tenths, hundredths, thousandths, and higher

Decimals are used to represent fractions of a unit by using a dot

called a decimal point to indicate that the number is a decimal.

Tenths A decimal with one number to the right of the decimal

point indicates an accuracy of 1/10 or 0.1 For example, 0.7 is the

same as 7/10 and is pronounced “seven tenths” or “zero point

seven.” A decimal can also include numbers larger than zero, but

has a resolution or accuracy measured in tenths, such as in 14.7

Continued

Hundredth Decimals with two numbers to the right of the

deci-mal point indicate an accuracy to 1/100 or 0.01 For example, 0.47

is pronounced “forty-seven hundredth” or “zero point four seven.”

Thousandth A decimal with three numbers to the right of the

decimal point indicates an accuracy to 1/1000 or 0.001 For

example, 0.867 is pronounced “eight hundred sixty-seven

thousandths” or “zero point eight six seven.”

Adding and Subtracting Decimals When adding or subtracting

decimals, the decimal point has to be aligned This ensures that the

numbers are placed into the correct position of tenth, hundredth,

and thousandth For example:

0.020.167+

0.147

PERCENTAGE

Percentage is the relationship of a value or number out of 100

Using money as an example, three quarters (25 cents each) equals

75 cents ($0.75) or 75% of a dollar ($1.00) Many examples are

not that easy, for example, 70 is what percentage of 120? To

determine the percentage, divide the first number (70) by the

second number (120)

70 ÷ 120 = 0.58

To convert this number to a percentage, multiply the number by

100 or move the decimal point two places to the right (58) and

then add a percentage symbol to indicate that the number is a

percentage (58%)

SCIENTIFIC NOTATION

Very large and very small decimal numbers are frequently

expressed using scientific notation, which written as a number

multiplied by the number of zeros to the right or left of the

decimal point

number shown has 3 zeros plus the 8 to the right of the decimal

point Small numbers, such as 0.00068, would use a negative sign

beside the number over the 10 to represent that the decimal point

ADDING AND SUBTRACTING

Techs are often required to add or subtract measurements when

working on vehicles For example, adding and subtracting is

needed to select shims (thin pieces of steel) for adjusting valve

clearance or differential preload measurements

For example, if the valve clearance specification is 0.012 in and

the clearance is actually 0.016 in., and the shim that is in place

between the camshaft lobe and the valve bucket is 0.080 in thick,

what size (thickness) of shim needs to be installed to achieve the

correct valve clearance?

Solution: The shim thickness of 0.080 in results in a valve

clearance of 0.016 in The specification requires that the shim

needs to be thicker to reduce the valve clearance

Continued

Figure 14–1 Valve clearance allows the metal parts to expand and maintain proper

operation, both when the engine is cold or at normal operating temperature.

(b) Adjustment is achieved by changing

the thickness of the adjusting shim.

(a) Adjustment is achieved by turning the adjusting screw

FRACTIONS

Fractions, such as 1/2, 1/4, or 5/8 are commonly found in specifications

for hose inside diameter measurements A tape measure or machinists rule

can be used to measure the fitting or the original part.

Sometimes, fractions need to be converted to decimal units if the

replacement parts are offered by that measurement method

When comparing fractions to decimal units, think about the

number of cents in a dollar

1/2 dollar 50 cents - 1/4 (quarter) 25 cents

1/10 (dime) 10 cents - 1/20 (nickel) 5 cents

Other fractions, such as 3/8, 5/8, and 5/16 are harder to determine

If a chart is not available, divide the bottom number, called the

denominator into the top number, called the numerator

MULTIPLYING AND DIVIDING

Multiplying by a service tech is usually done to determine gear

ratios and to determine the total of many of the same items

For example, final overall gear ratio is determined by multiplying

the transmission gear ratio by the final drive ratio and is covered

later in this chapter

Dividing is commonly done when calculating total resistance of

many resistances connected in parallel In this situation, the value

of the resistance is divided by the number of equal resistances

For example, if four bulbs with a resistance of 0.4 ohm were

connected in parallel, the total resistance would be just 0.1 ohm

(0.4 ÷ 4 = 0.1)

MATHEMATICAL FORMULAS

A formula uses letters to represent values or measurements and

indicates how they are to be acted upon To use a formula, the tech

needs to replace letters with the actual numbers and perform the

indicated math functions

For example, a formula to determine engine speed in revolutions

per minute (RPM) is represented by the following formula:

This is used to determine the speed of the engine compared to the

gear ratio and tire size Wheel and tire sizes are changed at timed

and knowing this is helpful.

Continued

FUEL ECONOMY CALCULATOR

Step #1 Fill the tank until the nozzle clicks off

Step #2 Drive a reasonable distance Example—220 miles

Step #3 Fill the fuel tank and record the number of gallons used

For this example, exactly 10.0 gallons

Step #4 Calculate fuel economy—MPG Miles driven divided by

the number of gallons used

MPG = 220 divided by 10.0 = 22.0 miles per gallon

See the metric fuel chart on Page 106 of your textbook.

Outside the US fuel economy is measured in liters of fuel needed to travel 100 kilometers (62 miles), abbreviated L/100 km

NOTE: Try to use the same station and pump, if possible, to achieve the

most accurate results.

NOTE: Try to use the same station and pump, if possible, to achieve the

most accurate results.

GEAR RATIOS

When one gear turns another, the speed the two gears turn in

relation to each other is the gear ratio Gear ratio is expressed as

the number of rotations the drive gear must make in order to

rotate the driven gear through one revolution

To obtain a gear ratio, divide the number of teeth on the driven

gear by the number of teeth on the drive gear Gear ratios, which

are expressed relative to the number one, fall into three categories:

Direct drive

Gear reduction

Overdrive

Continued

Gear Reduction If one gear drives a second gear that has three times the number of teeth, the smaller drive gear must travel three complete revolutions in order to drive the larger gear through one rotation

Direct Drive If two meshed gears are the same size and have the

same number of teeth, they will turn at the same speed Since the

drive gear turns once for each revolution of the driven gear, the

gear ratio is 1:1 and is called direct drive When a transmission is

in direct drive, the engine and transmission turn at the same speed

Figure 14–2 The drive gear is attached or

is closer to the power source and rotates

or drives the driven gear.

gear by the number of teeth on the drive gear, the answer is a 3:1 gear

ratio (pronounced three to one).

This type of gear arrangement, where driven gear speed is slower than

drive gear speed, provides gear reduction Gear reduction may also

be called underdrive as drive speed is less than, or under, driven

speed It is used for the lower gears in a transmission.

First gear in a transmission is called “low” gear because output speed,

not gear ratio, is low Low gears have numerically high gear ratios A

3:1 gear ratio is a lower gear than those with a 2:1 or 1:1 gear ratio.

These three ratios taken in order represent a typical upshift pattern

from low gear (3:1), to second gear (2:1), to drive gear (1:1).

Overdrive The opposite of gear reduction condition is overdrive and occurs when

a driven gear turns faster than its drive gear As shown here, the driven gear turns

three times for each turn of the drive gear The driven gear is said to over-drive the

NOTE: Ratios always end in

1 with a colon between The

first number is less than 1 if

it is an overdrive ratio and

greater than 1 if it is a gear

reduction ratio.

NOTE: Ratios always end in

1 with a colon between The

first number is less than 1 if

it is an overdrive ratio and

greater than 1 if it is a gear

reduction ratio.

GRAPHS, CHARTS AND DIAGRAMS

Graph Reading A graph is a visual display of information, used

in auto service to illustrate trends or specs along with some other

variable A variable is a measurement that changes, such as

engine speed or time

Graphs displays two variables One changes from left to right on

the horizontal, or X axis

The other displays on the Y or vertical axis

A graph is created by making series of dots at various locations

and connecting the dots with a line

Continued

Figure 14–4 A graph showing

horsepower and torque Notice

that the curves cross at 5252 RPM

or a little bit to the right of the 50,

which is expressed in hundreds

times 10 (thousands) of RPM.

Interpreting a Graph To interpret a graph, select point along the

horizontal (X) axis, then look directly above the point where the

line appears Mark this spot and look directly to the left along the

vertical (Y) axis to see the value represented by the points on the

graph

Chart Reading A chart is used to represent data, such as

numbers or specifications, along with another variable, such as

model or year of vehicle

A chart is very useful for showing many different specifications

or other facts in an easy-to-read format

The next slide displays an example of a specifications chart,

which shows the transmission parts listed along the horizontal

axis (X axis) and gear of the automatic transmission along the

vertical or Y axis

Continued

Interpreting a Chart A chart can look complicated but if

studied, is easy to interpret Look along the horizontal or vertical

axis for information such as model year of the vehicle Then look

directly above the model year to determine the specs for that year

Figure 14–5 A typical chart showing what is applied in what gear in an automatic transmission.

Diagram Reading A diagram

is a graphic design that explains

or shows the arrangement of

parts

Diagrams are commonly used in

the auto service industry to show

how a component is assembled

and in which order the parts are

placed together

Figure 14–6 An exploded view showing how the

thermostat is placed in the engine.

Continued

Interpreting a Diagram A diagram shows the relationship of

many parts Lines are used to show the centerline of the part and

the identity of the part is often shown as a number or letter

A separate chart or area of the diagram needs to be looked at to

determine the name of the part Diagrams are most helpful when

disassembling or assembling a component, such as a transmission

For best results, use electronic information and print out the

diagram so it can be written on and can be thrown away when the

repair has been completed This process also helps prevent getting

grease on the pages of a service manual

SUMMARY

specifications

many automotive service procedures

if other values are known

on the driven gear by the number of teeth on the drive gear

end