Gear Terminology KC1

Question 1

Base Circle

Answer 1

The gear diameter from which the involute tooth profile is developed.

Question 2

Pitch Circle

Answer 2

AKA “Pitch Diameter (d)”

The imaginary rolling circle of a gear. The pitch circles of mating gears intersect each other. It is also the dimension from which most other gear measurements are made.

Question 3

Line of Centers

Answer 3

The line connecting the pitch circle center of two mating gears.

Question 4

Pitch Point

Answer 4

The point of tangency of two mating gear pitch circles through the line of centers.

Question 5

Line of Action

Answer 5

A straight line tangent to the base circles of mating gears and passes through the pitch point. It is the path of contact of the two mating involute profiles as the gears rotate.

Question 6

Pressure Angle

Answer 6

The angle between the line of action and the line tangent to the pitch circle at the pitch point.

Question 7

Outside Circle

Answer 7

AKA “Addendum Circle”

The outside diameter of the gear.

Question 8

Root Circle

Answer 8

AKA “Dedendum Circle”

The gears root diameter and is tangent to the bottom of the gear teeth.

Question 9

Addendum

Answer 9

The radial distance of a gear tooth between the pitch circle and the outside circle.

Question 10

Dedendum

Answer 10

The radial distance of a gear tooth between the pitch circle and the root circle.

Question 11

Tooth Thickness

Answer 11

The thickness of one gear tooth measured along the pitch circle.

Question 12

Circular Pitch (p)

Answer 12

The length of the arc along the pitch circle between corresponding points of adjacent gear teeth.

Question 13

Face Width

Answer 13

The gear tooth width measured along the axial plane.

Question 14

What is the basic function of the transmission in a car?

Answer 14

The basic function of this system is to control the speed and torque available to the drive wheels for different driving conditions.

Question 15

What is more important to climb a hill - speed or torque? How does a transmission achieve this?

Answer 15

Torque.

By reducing the speed at the transmission we are able to achieve higher torque for the same power input. (The drawback of this slower speed)

Question 16

What is the relationship between power, speed, and torque?

Answer 16

Power = speed x torque

Question 17

What is the function of gear ratios in a transmission?

Answer 17

Different gear ratios in the transmission allows for different speed and torque outputs.

Question 18

How is an idler gear (also called an “idler”) used to drive a car backwards?

Answer 18

It is an intermediate gear that causes the driven gear and corresponding shaft to rotate in the opposite direction that they would if they were directly connected to the driven gear.

Question 19

Spur Gears

Answer 19

Have teeth parallel to the axis of rotation and are used to transmit motion from one shaft to another, parallel, shaft.

The simplest type of gear.

Question 20

Helical Gears

Answer 20

Have teeth inclined to the axis of rotation. Less noisy than spur gears because of the more gradual engagement of the teeth during meshing. The inclined tooth also develops thrust loads and bending couples.

Question 21

What type of gears are used in an automobile transmission? Why?

Answer 21

Helical Gears.

They operate more smoothly and quietly than spur gears.

Question 22

What is a thrust load? What is used to “take up” or support at thrust load?

Answer 22

A thrust load is one that acts axially along the shaft. A thrust bearing is used to support the thrust load.

Question 23

What is the function of the differential in a car?

Answer 23

Allows drive wheels to turn at different speeds while receiving power from the engine.

Two more functions:

1) speed reduction at the pinion ring gear assembly which results in torque multiplication.
2) to turn the power flow direction by 90 degrees.

Question 24

How to bevel gears help the differential do what it needs to do?

Answer 24

A bevel “spider gear” is mounted to two bevel “side gears” which are each mounted the shafts that connect to the wheels.

Question 25

Bevel Gears

Answer 25

Have teeth formed on conical surfaces and are used mostly for transmitting motion between intersecting shafts.

Question 26

Worm Gears

Answer 26

Consists of a worm (the straight gear) and worm gear (worm wheel) that are typically at right angles to each other.

Mostly used when the speed ratios of the two shafts are high.

Only a few materials are suitable because the worm is highly stressed and requires a hardened steel.

Question 27

Rack and Pinion Gears

Answer 27

Used to convert rotary to linear motion (or vice versa)

Can be back driven, so it requires a brake if being used to hold a load.

Question 28

How does a rack and Pinion configuration allow you to steer your car?

Answer 28

The rack is attached to the front wheels and the pinion is attached to the steering wheel.

Question 29

Diametral Pitch (P)

Answer 29

It is the ratio of the number of teeth on the gear to the pitch diameter. It has to be the same for mating gears in order for them to mesh correctly.

Only used for English units.

Question 30

Number of Teeth (N)

Answer 30

Total number of teeth on a gear.

Question 31

Module (m)

Answer 31

The ratio of the pitch diameter to the number of teeth. Only used for SI units.

Question 32

Involute Profile

Answer 32

The selected profile that is (with few exceptions) in universal use for gear teeth.

It is a profile that allows meshing teeth in two mating gears to have conjugate action.

Question 33

Conjugate Action

Answer 33

Maintaining a constant angular velocity ratio during the meshing of two gears.

Question 34

Pinion

Answer 34

The smaller of two meshed gears.

Question 35

Gear

Answer 35

The larger of two meshed gears.

Question 36

Interference

Answer 36

The contact of portions of tooth profiles that are not conjugate.

Occurs when the face of one gear tooth makes contact with the flank of the mating gear tooth.

Question 37

Flank

Answer 37

Noninvolute portion of the gear tooth contact surface.

Question 38

Face

Answer 38

Involute portion of the gear tooth contact surface.

Question 39

What is undercutting? Is it desirable? Undesirable? A trade-off?

Answer 39

Undercutting is when the interfering portion of the flank of a gear tooth is removed.

If undercutting is at all pronounced, the undercut tooth is severely weakened. Thus, the effect of eliminating interference is merely to substitute another problem for the original one.

It is a trade-off: strength for nicely meshing gears.

Question 40

Besides adding more teeth to your smaller gear, how else can interference be reduced?

Answer 40

By using a larger pressure angle or undercutting the gears.

Question 41

What is the main difference between hobbing and shaping?

Answer 41

Hobbing can only generate external gears whereas shaping can generate external and internal gears.

Question 42

Hobbing

Answer 42

Gear teeth are progressively generated by a series of cuts using a helically fluted cutting tool called a hob. Both the hob and the gear blank rotate as the hob is fed axially across the face of the blank. It is the principle method for producing spur gears and helical gears and it is also used to produce many special gear forms. The main limitation of hobbing is that is can only be used to produce external gears.

Question 43

Shaping

Answer 43

Generates gear teeth by rotating the work piece in contact with a reciprocating cutter. The reciprocating cutter may be a pinion shaped cutter, a multi tooth rack shaped cutter, or a single point cutter. Gear shapers are used to make internal gears, shoulder gears, and many other gear types that cannot be produced with hobbers. Shapers can produce noninvolute shapes such as a cam and gear on the same shaft.