GEAR RATIOS

OBJECTIVES To understand and calculate gear ratios To understand and demonstrate the

relationship of torque and rotational speed

To understand and properly express the relationship between the driving and driven gears

How do you get 4000 lbs of wheels and steel moving from a stop and keep it going without over revving the engine?

Gears! They are used to keep the engine in its ideal RPM range for any given condition

What can be done to get a race car to the end of the ¼ mile faster?

Use a set of low gears (numerically higher ratios)

What would you do to a vehicle to achieve a higher top speed?

Use a set of high gears (numerically lower ratios)

If you have 20 apples and 10 oranges, what is the ratio of oranges to apples ?

10/20=2:1

Money in the stock market (2004)

Write a sentence about the graph to the left.

The graph has a positive return. When you put money in, you get more money out.

What is the ratio of output to input? (Show Work!)

Money in the Stock Market (2009)

Write a sentence about the graph to the left.

What is the ratio of output to input? (Show Work!)

0 20 40 60 80 100 1200

50

100

150

200

250

Torque RelationshipTo

rqu

e O

utp

ut

(ft/

lb)

Torque Input (ft/lb) Under Drive

Direct DriveWrite a sentence about the blue and red line? What is the gear ratio of the blue and red line?

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 55000

500

1000

1500

2000

2500

3000

SPEED RELATIONSHIP

ENGINE INPUT RPM

EN

GIN

E O

UTP

UT

RP

M

Under DriveDirect Drive

Write a sentence about the blue and red line? What is the gear ratio of the blue and red line?

Calculating Gear Ratios and Torque Always divide the

driven gear by the drive gear

2 divided by 1 = 2:1 ratio

This ratio is used to calculate the output torque

25 ft-lb x 2= 50 ft-lb

Calculating Output Rpm Divide the input rpm by the

gear ratio 1000 rpm divided by 2=

500 rpm

Calculating TransmissionGear Ratios (multiple gear sets)

1. Calculate the ratio of the first set of gears by dividing the driven (output) gear by the drive (input) gear

2. Do the same for the second set of gears

3. Multiply the answer from the first calculation with the answer from the second calculation

Divide the driven by the drive gear

44 divided by 11= 4:1

•How many times will the drive gear turn to rotate the driven gear once with a ratio of 4:1?• 4 times

• 100 ft-lb of torque is input to a gear ratio of 4:1, how much is output?•400 ft-lb

• 1000 rpm is input to a gear ratio of 4:1, what is the output?•250 rpm

0 50 100 150 200 250 300 350 400 4500

50

100

150

200

250

300

Torque Input (ft/lbs)

To

rqu

e O

utp

ut

(ft/

lbs)

Torque Relationship (ft/lbs)

OverdriveDirect Drive

Write a sentence about the blue and red line? What is the gear ratio of the blue and red line?

0 1000 2000 3000 4000 5000 60000

1000

2000

3000

4000

5000

6000

7000

8000

9000

Speed Input (RPM)

Sp

eed

Ou

tpu

t (R

PM

)Speed Relationship (RPM)

OverdriveDirect Drive

Write a sentence about the blue and red line? What is the gear ratio of the blue and red line?

•If the input torque is 150 ft-lbs and the output torque is 450 ft-lbs, is this is under, direct, or over drive? What is the ratio•Under drive•3:1

•If the input rpm is 1000 and the output rpm is 400, is this under, direct, or overdrive? What is the gear ratio?•Under drive•2.5:1

Two or more sets of gears in mesh- calculate the first set, then the second and multiply together for final ratio

3.63 x 3.52=12.78:1

Gear Ratio FormulaDriven gear (A) divided by Drive gear (A)

multiplied byDriven gear (B) divided by Drive gear (B)

equalsFinal transmission gear ratio

Gear set A-drive gear has 20 teeth, driven has 40, Gear set B drive gear has 30 teeth, driven gear has 40. What is the drive ratio?

2:1 x 1.33:1=2.66:1