Howstuffworks.com's "Inside an Eletric Screwdriver"

Inside an Electric Screwdriver
by Marshall Brain

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 electic 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:

The switch block really has three roles in this screwdriver:

In the center position it prevents electricity from flowing to the motor.
It can supply power of either polarity to the motor, so the motor can spin in either a forward or reverse direction.
On the bottom of the switch are the recharging contacts, so the switch block also provides recharging power to the battery.

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:

The electric motor is a standard DC electric motor. On the end of the motor is a small 8-tooth gear. This 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 36:1 reduction ratio. With this reduction ratio, the motor will turn 36 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 36 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 six revolutions of the motor. The second layer is identical and turns once for every six turns of the top layer. That gives a total gear ratio of 36:1.

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

Back to top