The need for Shielded Bearings Vs. Sealed Ball Bearings is determined by the application they are used in. Ball Bearings in electric motors used to drive pumps provide an extremely low rotational friction by using a stationary outer race which is connected to the motor housing and an inner race which is connected to the motor armature and rotates.
These races have grooves in which balls ride, rolling as the inner race turns within the stationary outer race. This allows the armature of the electric motor to be held in a very precise position within the motor housing yet allows the armature to spin at high speeds.
Ball Bearings spinning at high speeds require lubrication and a clean environment in order to have a long service life. The lubrication required is only a small amount of grease, in fact to much grease can actually increase the rolling resistance and generate heat causing the bearing to fail. It is possible in the case of electric motor bearings to put enough grease in them at the time of manufacture to last the entire service life of the bearing, if contaminants are kept from getting into the grease.
When contaminants like dirt gets into the grease the service life of the ball bearing can be drastically reduced, the grease layer between the ball and the race is actually a very thin film separating the components. When dirt lands in this film the effect is similar to that of chocking a wheel to prevent it from rolling, the bearing cannot climb up and over this obstacle as the clearances within the bearing are too tight to allow that, so the dirt has to be crushed or, penetrate the ball and race in order for the bearing to continue to turn. In actuality all of those happen at once when dirt goes through the bearing and the dirt penetrating the bearing surfaces displace metal from them which in turn breaks off becoming additional dirt causing further damage.
Ball Bearing lubrication can also be affected by moisture contamination of the grease which can cause the lubrication properties of the grease to suffer resulting in increased wear and also corrosion of the bearing components. The metal from wear and rust particles also act as dirt causing additional damage to the bearing.
To combat the contamination of a bearing by moisture and dirt bearing manufacturers have designed shields and seals to keep the bearing clean from contaminants ensuring a long service life. These shields and seals at first look are similar in appearance but are very different providing differing levels of protection.
Shielded Ball Bearings
Shielded Ball Bearings have a metal or plastic shield that is secured to the outer race of the bearing on each side of the bearing. This shield extends from the outer race down the side of the bearing to the inner race much like a wall covering the balls of the bearing. This shield however does not contact the inner race which is turning but instead has a small gap of perhaps .005” which allows the inner race to turn without friction and reduces the chances of dirt entering the bearing causing contamination.
Sealed Ball Bearings
Sealed Ball Bearings while quite similar to the shielded ball bearings in having covers over the balls offer a greater degree of protection because the “Shield” on the sealed ball bearing actually comes into contact with the inner race. This seal in contact with the inner race provides the highest degree of protection against dirt and moisture for a ball bearing. This higher degree of protection does not come without a price, the high contact pressure of the seal against the rotating inner race increases the friction and torque losses and can cause a build up of heat as well.
Selecting the right shielded or, unshielded bearing for your pump’s electric motor can go a long way in ensuring a long service life. But the thing to remember is that once a ball bearing starts to fail the bearing never will get better on its own and the additional contaminants from the indentation, smearing, spalling or flaking, and corrosion of the metal surfaces of the balls and races accelerate the failure process. The time between the first noise the bearing starts making, to when the friction of the failed bearing starts increasing the motor loading and current draw can be quite quick. You should always have the bearings in the motor replaced at the first sign of bearing failure rather than wait until the increased loading causes the motor to burn out or, the sloppiness caused by the wear causes other damage to the motor components.