Gear Terminology KC1 Flashcards
Base Circle
The gear diameter from which the involute tooth profile is developed.
Pitch Circle
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.
Line of Centers
The line connecting the pitch circle center of two mating gears.
Pitch Point
The point of tangency of two mating gear pitch circles through the line of centers.
Line of Action
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.
Pressure Angle
The angle between the line of action and the line tangent to the pitch circle at the pitch point.
Outside Circle
AKA “Addendum Circle”
The outside diameter of the gear.
Root Circle
AKA “Dedendum Circle”
The gears root diameter and is tangent to the bottom of the gear teeth.
Addendum
The radial distance of a gear tooth between the pitch circle and the outside circle.
Dedendum
The radial distance of a gear tooth between the pitch circle and the root circle.
Tooth Thickness
The thickness of one gear tooth measured along the pitch circle.
Circular Pitch (p)
The length of the arc along the pitch circle between corresponding points of adjacent gear teeth.
Face Width
The gear tooth width measured along the axial plane.
What is the basic function of the transmission in a car?
The basic function of this system is to control the speed and torque available to the drive wheels for different driving conditions.
What is more important to climb a hill - speed or torque? How does a transmission achieve this?
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)
What is the relationship between power, speed, and torque?
Power = speed x torque
What is the function of gear ratios in a transmission?
Different gear ratios in the transmission allows for different speed and torque outputs.
How is an idler gear (also called an “idler”) used to drive a car backwards?
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.
Spur Gears
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.
Helical Gears
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.
What type of gears are used in an automobile transmission? Why?
Helical Gears.
They operate more smoothly and quietly than spur gears.
What is a thrust load? What is used to “take up” or support at thrust load?
A thrust load is one that acts axially along the shaft. A thrust bearing is used to support the thrust load.
What is the function of the differential in a car?
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.
How to bevel gears help the differential do what it needs to do?
A bevel “spider gear” is mounted to two bevel “side gears” which are each mounted the shafts that connect to the wheels.
Bevel Gears
Have teeth formed on conical surfaces and are used mostly for transmitting motion between intersecting shafts.
Worm Gears
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.
Rack and Pinion Gears
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.
How does a rack and Pinion configuration allow you to steer your car?
The rack is attached to the front wheels and the pinion is attached to the steering wheel.
Diametral Pitch (P)
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.
Number of Teeth (N)
Total number of teeth on a gear.
Module (m)
The ratio of the pitch diameter to the number of teeth. Only used for SI units.
Involute Profile
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.
Conjugate Action
Maintaining a constant angular velocity ratio during the meshing of two gears.
Pinion
The smaller of two meshed gears.
Gear
The larger of two meshed gears.
Interference
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.
Flank
Noninvolute portion of the gear tooth contact surface.
Face
Involute portion of the gear tooth contact surface.
What is undercutting? Is it desirable? Undesirable? A trade-off?
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.
Besides adding more teeth to your smaller gear, how else can interference be reduced?
By using a larger pressure angle or undercutting the gears.
What is the main difference between hobbing and shaping?
Hobbing can only generate external gears whereas shaping can generate external and internal gears.
Hobbing
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.
Shaping
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.
