To preserve the smooth running of a bearing assembly an effective solution can be found by using shields, which, as their name suggests, shield the internals of the bearing from any contaminants.
As stated in the Deep Groove Roller Bearing article, not all the different types of bearings used in model car are fitted with seals.
Bearings need to be lubricated and in ideal conditions this is fed by either a mist or drip feed of oil; a perfect example of this would be the main bearings used inside internal combustion (glow or two stroke) engines.
Model cars, however, rarely offer these ideal conditions due to being run low to the ground on dirty surfaces. In these circumstances a means to provide a constant supply of fresh oil to lubricate, cool and wash away any contaminants that are attracted into the bearing is not feasible.
There are many different types of shields fitted to the bearings available to the model car racer, but these use one of the two retaining mechanisms shown below.
The c-clipped type of shield is probably the most common type of shield in use as it can be scaled down to suit the small sized bearings that used in radio controlled model cars.
As shown in the accompanying image, a thin wire c-clip (shown in red) is fitted to a groove on the inside of the outer race to retain the shield.
The sprung type shield, which is depicted in the accompanying image, is normally only found on some of the larger bearings and uses a coiled over lip that is sprung into a groove in the bearing outer race.
As stated previously there are many different types of shields used in model car and there will be variations that we haven’t covered, but the below represents the majority in use.
The most common type of bearing shield is made from metal and can be produced in either type of retaining mechanism. Plastic ones are less common but have the same benefits and deficiencies.
These inexpensive solutions offer reasonable protection against contaminants entering the bearing but, due to the gap by the inner race, allow fine particles to enter that can reduce the operating efficiency of the bearing assembly and lead to premature failure if the bearing is not cleaned.
The rubber shield is normally a variation of the sprung in type that has a rubber lip formed on the inside of the shield to bridge the gap that is normally left open when metal or plastic shields are used.
Whilst this seals the bearing to stop contaminants from entering, it also creates drag on the inner race and reduces the efficiency of the bearing. This is the reason (apart from cost) why they are not commonly used on radio controlled model cars.
The most common application of rubber shields is on the front bearing on a model car engine, but metal shields are also fitted by some manufacturers due to the efficiency losses.