Understanding Gears


(This article is from our recent blog post, found here: http://www.servocity.com/blog/)

At ServoCity we use gears all the time.  We love how something so small can make a huge difference in a build.  For example, our Servo Power Gearboxes.

Most people would not realize how frequently gears are utilized in items all around us.  From watches, vehicles, can openers to drills and even that office printer, gears are drive components found in our everyday life.

Gears can achieve three end goals:

  1. Gears are used to change the speed of rotation, also altering the available torque.
  2. Gears transfer rotational motion to another axis.
  3. Gears are used to reverse the direction of rotation.

There are five basic types of gears:



 Spur- Spur gears are the most common gears.  They are mounted on parallel shafts and have straight teeth, but tend to be noisy at high speeds






  Bevel – Bevel gears are used when the plane of the of the shaft’s rotation needs to be altered.







Planetary – Planetary gears consists of the sun gear, planet gears and a ring gear used to meet a range of speed-ratio.





Worm – Worm gears are useful when you need a large gear reduction.  With worm gears, the output shaft is perpendicular to input.  Within a worm gear the worm turns the gear, but the gear cannot turn the worm.  An example of this is our Vertical Shaft Worm Drive Gearbox which uses a 30:1 ratio worm-drive reduction.  This is great for applications that require extremely slow and smooth rotational movement.  Worm-drives not only minimize backlash, but also eliminates back-driving the gearmotor so a position can be held even when power is not applied.




Rack-and-Pinion – These gears are used to convert rotation into linear motion.





Not only do you need to consider the type of gear you need, but also the material of the gear.  For example, servo gears come in various materials such as nylon, Karbonite, metal and titanium.  Nylon gears are typically found in servos with relatively low torque output.  Nylon wears well, but cannot handle the abuse that other make-ups can.  Karbonite gears wear even better than nylon and can withstand abuse better.  Metal gears do not wear as well as the nylon or Karbonite gears, but are less prone to stripping out when used in demanding applications.  Titanium gears are found in the high end servos and have the best wear characteristics and are nearly indestructible.

Rate of Rotation/Gear Ratio:
The rate of rotation is determined by the number of teeth on both gears.  For example, if you have a 20T gear and a 40T gear, the ratio would be 2:1.

The 20T gear rotates twice per one rotation of the 40T gear.

When two gears are mated, the gear with a fewer number of teeth will rotate faster than the gear with a larger quantity of teeth.  Like above, the 20T gear will rotate two times in the time it take the 40T gear to rotate.  The 20T gear is spinning twice as fast because of the ratio between the gears, also described as the gear ratio.  If the gears were the same in diameter, the gears would rotate at the same rate.

Gear Reduction:
As mentioned above, one of the important jobs of gears, is gear reduction.  Gear reduction is important when you have a motor spinning fast enough to provide the power, but not enough torque for your application.  By utilizing a gear reduction, you can decrease the speed while increasing the torque.  There are two key advantages of gear reduction.  It allows you to bring the speed of your motor down to a speed that is more manageable, in turn putting less strain on the motor since it is not having to work as hard.

You can achieve this through a number of ways including belts, sprockets, chains, pulleys and of course, with gears.  We will be talking about gear reduction in reference to gears.  Gear reduction is necessary when the speed of a motor is too fast for your application.  For many applications, you need to reduce the speed to have more control for your application.

Let’s take a look at our Servo Power Gearboxes.  These are good example of using gear ratios to achieve the right balance between speed and torque.

These “Power Servos” are for superior strength and durability.  Their design and gear ratios allow them to meet a wide variety of applications from UAV’s (unmanned aerial vehicles) to radical robots.  The gearboxes are offered with various servos and in five gear ratios to find that right balance between speed and torque.

For example, the SPG7950 Channel Mount Gearbox is capable of achieving the following based on the various ratios shown.




4.8V Power

6.0V Power

7.4V Power

4.8V Speed

6.0V Speed

7.4V Speed

HS-7950TH (2:1 ratio)


5.6 oz.




.36 sec.

.3 sec.

.26 sec.

HS-7950TH (3:1 ratio)


5.6 oz.




.54 sec.

.45 sec.

.39 sec.

HS-7950TH (3.8:1 ratio)


5.6 oz.




.684 sec.

.57 sec.


HS-7950TH(5:1 ratio)


5.6 oz.




.9 sec.

.75 sec.

.65 sec.

HS-7950TH (7:1 ratio)


5.7 oz.




1.26 sec.

1.05 sec.


 Gears, so tiny, yet can make a huge difference in what your are trying to achieve in your applications.