Electric screwdrivers are neat because they combine batteries, motors, gears and a switch into one extremely compact and powerful package. Let's take one apart and see what's inside! Here's the screwdriver we will be dissecting today:

When you take off the cover you find all of the basic parts of the screwdriver lined up from one end to the other. You will find:

  • A rechargeable battery
  • A switch
  • An electric motor
  • A dual planetary gear system (cool!)
  • A simple locking mechanism for the chuck
  • The chuck itself, which holds the screwdriver

One of the neat things about this particular screwdriver is the way the manufacturer designed it to lower manufacturing costs. There are no screws or connectors inside the case - everything is held in place by indentations and tabs molded into the case itself. When you take the parts out of the case they come apart naturally because there truly is nothing holding them together:

Switch Block
The switch block really has three roles in this screwdriver:

The contacts for the motor sit in a gap in the middle of the switch contacts. With the switch in its neutral position nothing touches the motor contacts. In this picture the locations of the motor contacts are shown with green squares:

When the user presses on one side of the switch, current can flow from the battery to the motor contacts. Current flow in this picture is shown by red arrows:

Electric Motor
The electric motor is a standard DC electric motor. See
"How Electric Motors Work" for a description of how an electric motor works, and "Inside an Electric Motor" to see the parts inside an actual electric motor.

On the end of the motor is a small 8-tooth gear. This gear fits into the center of the planetary gear system, as shown here:

This gear system is the heart of any electric screwdriver. An electric motor by itself is a pretty weak device. You can grab the axle and stop a small motor's rotation very easily. But if you gear the motor way down, it can have enough strength to drive a screw into a piece of wood effortlessly. In this screwdriver the dual planetary gear system has a 68:1 reduction ratio. With this reduction ratio, the motor will turn 68 times for the chuck to turn once. This means that the chuck moves very slowly relative to the motor, but that the chuck has a great deal of torque (it takes 68 times more strength to stop the motor from spinning because of the gear ratio). The gear system is actually in two layers, and you can barely see one of the gears in the second layer in the following picture:

The top layer of the gear system turns once for every 8.25 revolutions of the motor. The second layer is identical and turns once for every 8.25 turns of the top layer. That gives a total gear ratio of about 68:1.

For more information on how electric motors work, please click here. For more information on how gears work, please click here.