Gears Rotation
ROTATION – the direction in which a gear revolves while in operation – is one of the most important concepts in the power transmission.
• In a spur drive having two gears, the pinion and gear will rotate in opposite directions. (See Figure 2.8A)
• In a spur gear train having three gears, the pinion and gear will rotate in the same direction. (See Figure 2.8B)
GEAR RATIO the mathematical ratio of a pair of spur gears is determined by dividing the number of teeth on the larger gear with the number of teeth on the pinion.
Example: The ratio of a 72-tooth gear running with a 16-tooth pinion is 4.5:1.
Ratio: 72÷16 = 4.5
REMEMBER THIS!
When there is an even number of gears, the pinion and driver will rotate in opposite directions.
When there is an odd number of gears, the pinion and driver will rotate in the same direction.
Gear ratio is important because it determines the drive speed.
VELOCITY, or speed, is the distance any point on the circumference of a pitch circle will travel in a given period of time. In the world of gears, this period of time is always measured in feet per minute (fpm).
Example: If you have a gear with a 2-foot pitch circumference and a given point on that circumference takes one minute to travel around the entire circumference, the gear is moving at a velocity of 2 feet per minute.
You can also figure out the velocity using the following formula:
Velocity = pitch diameter (PD) x .262 x revolutions (of the gear) per minute (rpm)
Example: What is the velocity of a Boston Gear NO18B spur gear – which, as you will see in the catalog has a 6-inch pitch diameter – turning at 7 rpm?
Velocity = 6" x .262. x 7 rpm, or 10.999 feet per minute (fpm)
