The other common type of bearing found in fan motors is the ball bearing. This bearing is made up of a ring of little steel balls surrounding the shaft and in a typical fan motor there are two such bearings, one in front of the other. Compared to a sleeve bearing, ball bearings reduce the amount of friction that must be overcome to start and operate the fan motor. Ball bearing systems also help to address the problem of uneven wear and rotor tilt/wobble in sleeve bearings. This is because the two bearing rings are typically separated by springs that press them apart. The weight of the rotor rests on the inner bearing, closest to the rotor itself, while the springs help to mitigate any tilt to the fan blades that the weight of the rotor may cause. Having these springs all the way around the shaft means fans with this type of bearing can be used at any angle, making them suitable for portable devices. Less wear also equates to a significantly higher MTBF on average compared to a sleeve bearing.