Gears in a Nutshell: NASA Gearing

Gears in a Nutshell

Gears have a special place in the mechanical world, they make things go, move, or rotate.  They are also hidden from our view as they need a case to hold their distance precisely together while rigidly on their shafts in a bath of their favorite lubrication.

NASA Gearing - Download

The NASA Reference Publication simply titled "Gearing" is a very complete summary.  The sections range from History, Life, Lubrication, and end up with Transmissions.  It is a great reference for anyone practicing the mechanical trades and is free to download here.  Some gears are great for transmitting torque, some are great for very high gear ratios or keeping track of angles.  All of the general and some very specific relationships are in "Gearing".

NASA Science and Technical Information Branch


Even if you are not designing or repairing gears, they have a very interesting history.  The NASA Gearing publication has one of the better history recaps.  We have used gears as the focus or our article and timeline 5000 Years of Gears where you can get a more in-depth review of the people that are part of the story of gears.  Early uses for gears include navigation, timekeeping, pumps, and calculators. Longitude was conquered using a geared clock mechanism.

Gear Relationships

When designing a transmission there are many factors to consider.  The relationships of different gear types are spelled out.  Gear ratio ranges for the various types of gears, amounts of torque that can be transmitted, and more is discussed.  No need to re-invent these well known relationships when you are feeling innovative.  The crossed-helical pair shown below allows the motion from one shaft to be turned at 90-degrees and transmitted into another.

Crossed Helical Gear Pair
Depending on the gear ratio your design requires, there is a type of gear for the job.
NASA Gear Ratios and Geometries

The Nomenclature for a Spur

The fundamentals of gearing are pretty clear, but due to their special rolling characteristics, the use of geometry, number of variables and equations gets complicated in a big hurry.  The image below shows the basics that are considered the minimum to describe the most basic spur pinion and gear combination.

Spur Gear Pinion and Gear with Nomenclature

Gear Reduction in your Car

If you have driven or ridden in a car, you have experienced the gear reduction of a transmission.  Cars engines have an RPM capability that is too high for the gear train and rear drive to the wheels,  so the transmission reduces this RPM significantly using first gear.  For most transmissions like a manual 3-speed, 1st and 2nd progressively increase the RPM to the rear wheels and then the final 3rd gear all the engine RPM is transmitted to the rear wheels and used for highway speed which is a 1:1 ratio with the motor.

The Pumpkin or Rear End of your car axle

The engine of your car lines up with the transmission and then the driveshaft to the rear-end.  The torque and power transmission needs to simultaneously transmit to your right and left rear tires.  The big pumpkin shaped round part at the center of your rear-end is really another transmission that accomplishes this task and it is called a differential.

Used Auto Rear-Ends on a rack
Differential gears inside an auto rear-end


Many Gear Types, Many Possibilities

Given all the different gear types, there are many ways to transmit, turn or rotate mechanisms.  Knowledge of gears and how they are designed will help you repair, design or maintain machinery.


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