Also referred to as internal radial clearance, it is the amount of play or looseness between the inner and outer ring or more specifically: average outer ring raceway diameter minus average inner ring raceway diameter minus (2 x ball diameter). Radial play should not be confused with any tolerance grades and is entirely separate.
Clearance measured along the bearing axis is known as axial play. Axial play is typically 10 times the radial play value.
DIFFERENT RANGES OF CLEARANCE
Radial play (or internal radial clearance) is an important consideration when choosing a ball bearing. The radial play in the bearing before it is fitted can be called the “initial” radial play. “Residual” or “operational” radial play is what is left when the bearing has been fitted. There should normally be a slight residual radial play in the bearing to minimize ball skidding and reduce axial play (end play). Correct selection of the initial radial play can avoid faster bearing wear and reduce unwanted play.
WHY CHOOSING THE RIGHT CLEARANCE IS SO IMORTANT?
A number of things can alter the radial play during the fitting process. A tight shaft fit where the shaft is slightly larger than the bearing inner ring (often called an interference fit or a press fit) will expand the inner ring. This reduces radial play by up to 80% of the interference fit. The same thing happens if the outer ring is a tight fit in the housing. This can compress the outer ring also reducing radial play. A difference between the shaft and housing temperatures should also be considered for correct radial clearance selection. If a bearing inner ring gets hotter than the outer ring, it will expand more and reduce radial play. The roughness of the shaft surface can also be important, the better roughness you’ll have (N5 or Ra 0,4), the less it will affect the radial clearance.
HOW TO CHOOSE?
In most cases a standard radial play is suitable and preferable as these bearings are usually more readily available and may be cheaper but there are certain conditions where a specific radial clearance is recommended, providing other conditions such as temperature or interference fit known. A tight radial play is better for greater rigidity and running accuracy and if the load is purely radial. This may be worth considering for very low noise, low vibration applications. This is why many small electric motor bearings are MC3 radial play. However, in other applications, a tight radial play may be highly undesirable. If there is a high axial load, a loose radial play is preferable as it increases the bearing’s axial play and its load capacity. Also, a loose radial play will better accommodate misalignment between the shaft and housing and cope better with heavy or shock loads.
When a bearing is overmolded with plastic, the definition of enough radial clearance is really important. When a complete bearing is placed in an injection mold to be overmolded, the whole injection pressure is applied on the outer ring. If your initial radial clearance was too small, you can mark and damage the raceways with the balls just by overmolding the bearing. After the overmolding process, a phenomenon called “shrinkage” is also to taken in consideration when choosing the bearing clearance.
LOOSE BEARING ≠ LOW QUALITY BEARING?
Finally, radial play has nothing to do with precision grades or tolerances. It is often believed that a loose bearing means
a low precision bearing and that, when there is too much play, a higher “precision grade” will solve the problem. In this case, the answer is often to use a bearing with a tighter radial play or use a tighter shaft/housing fit or introduce an axial preload to the bearing. Using a higher precision grade will make no difference to the “looseness” of the bearing as long as the radial clearance is the same. You can have a P4 (Abec7) high grade bearing with a loose radial play just as you can have a P0 (Abec1) bearing with a tight radial play.
We hope that we have answered some questions with this article based on bearing specific literature. Should you have more questions about this, please do not hesitate to contact us or comment on this article. It will be a pleasure for our technical team to help you in your choice.