The bottom bracket is kind of like the kingpin in a bicycle frame, holding the main tubing together, as well as the transmission system. A bottom bracket and the included bearing hardware are easy to maintain, repair and modify once you know the basics.
Figure 1 - A 3 piece bottom bracket and crankset
A 3 piece bottom bracket gets its name from the fact that there are 2 crank arms and an axle that make up the crankset. A 3 piece crankset is easy to identify because each crank arm will have a center nut or bolt that fastens it to the axle, whereas a single piece crankset has a solid S-shaped crank arm.
Figure 1 shows a typical 3 piece crankset and bottom bracket with a triple chain ring. Higher quality crank arms are made of aluminum, whereas inexpensive cranks are made of steel.
Figure 2 - Removing the plastic dust cap
To remove the crank arms from the bottom bracket axle, the two bolts (or nuts) must be removed. There might be a plastic or rubber dust cap over the crank center as shown in Figure 2. This can be removed by unscrewing it using a penny or screwdriver, or by popping it out with a blade (if it is just a rubber cap). On bikes that have had a lot of sun exposure, the plastic cap may break due to being brittle, so take care.
Figure 3 - Removing the crank axle nuts
The axle nuts are both standard right hand threads, so they are loosened using a counter-clockwise rotation as shown in Figure 3. Most of the department store variety cycles will have bolts holding the crank arms to the square tapered axle, so they can be removed using the appropriate sized socket wrench.
Some bolt heads may require a hex key as well. The crank axle may have a bolt or bolt hole, but both types work essentially the same way, working the crank arm onto the tapered axle where it is held securely by friction.
Figure 4 - A crank puller is a useful tool