What is radial internal clearance?
It is how far the inner ring moves from one side to the opposite side.
The radial internal clearance of a radial spherical plain bearing is the distance the inner ring can be moved in a radial direction relative to the outer ring, from one extreme position to the precisely opposite extreme position. It is measured on the bearing before mounting. More widely, bearing clearance means the radial and axial freedom of motion of the inner ring, and the axial clearance can be a multiple of the radial one.
Why does the amount of clearance matter?
Both too little and too much do damage, in different ways.
Too little clearance leaves no room for the things that take it up: an interference fit, a temperature difference between the rings, a little misalignment or a film of corrosion. The ball then binds in the race, friction and heat climb, and a steel-on-steel pair heads for galling while a liner wears quickly. Too much clearance lets the joint knock when the load reverses, which hammers the surfaces and the fit seats and shows up as noise and positioning error. It also shrinks the arc of the ball that carries the load, so the same force acts on a smaller area and wear speeds up. The right clearance is the one that stays positive, but small, under every condition the joint will meet.
What are the clearance groups?
The series standard defines a normal group and a tighter and a looser one:
| Group | What it means | When to use it |
|---|---|---|
| N (CN) | Normal clearance. The suffix is not written in the part number | With normal operating conditions and the recommended fits, this gives the best operating clearance |
| C2 | Less than normal | Where the bearing arrangement needs very little play |
| C3 | More than normal | For fits tighter than the recommended ones (for example, both rings with interference), or where there is a large temperature difference between the inner and outer ring |
The groups assume a housing bore that does not change the size of the bearing apart from small geometric errors. The micrometre ranges for each group depend on the bore and are listed in the maker's table, so look them up for the exact part. Maintenance-free bearings are different: they are built with a very small internal clearance, and a preload (a built-in tightness) may be present once the bearing is fitted.
Does the clearance change when it is installed?
Yes. The clearance that matters is the one after installation, and for that the housing and the pin must be made to the correct tolerances. The operating clearance is the internal clearance less what the interference fit and the temperature take away, and it is determined on a fitted bearing still warm from operation.
An interference fit squeezes the outer ring in or stretches the inner ring out, and part of the interference is lost as clearance (Q1-06); this is why C3 exists for tight fits. Temperature works the same way. A steel ball and ring grow by roughly 12 millionths of their size per kelvin, so if the inner ring and ball run 40 K hotter than the outer ring, a ball of 40 mm diameter grows about 40 × 12×10⁻⁶ × 40 = 0.019 mm, about 19 µm, more than the outer ring does. That is a large part of the play a bearing of this size starts with (illustrative figures; take the real play from the maker's table). Some cylinder makers specify normal group N for ordinary duty in hydraulic cylinder eyes and a reduced group for dynamic duty at very small amplitudes.
How do you choose a group?
Start from the fit and the heat, then from the accuracy you need.
| Your case | Group to ask about |
|---|---|
| Recommended fits, rings at similar temperature, ordinary duty | N |
| Both rings with interference, or a housing that squeezes the outer ring hard | C3 |
| Inner ring and ball clearly hotter than the outer ring (a hot shaft or pin) | C3 |
| Positioning accuracy matters, or the motion is small and dynamic, and fits are the recommended ones | C2 |
| Maintenance-free liner type | The maker's own clearance and preload data |
C2 and tight fits do not combine: a C2 bearing pressed into a tight housing can end with no clearance at all.
How do you check it?
For a new bearing, compare it with the maker's stated range for its group. The clearance is the distance the inner ring can be moved from one extreme to the opposite one while the bearing is unmounted. The way you measure can change the reading, so use the method the maker specifies, including its measuring force.
As a general approach, hold the unmounted bearing so the outer ring is fixed, set a dial indicator on the inner ring, and move the inner ring radially back and forth under a small alternating force, reading the difference between the two ends of travel. The force and the point of measurement matter, so treat this as a way to see how much play there is, and take the acceptance figure from the maker.
For a bearing in service, the number to watch is how much the play has grown. In a spherical plain bearing, wear raises the clearance, and the end of life is defined by the play you can accept (Q1-14). Decide that figure before you check.
What should you tell the maker?
The group you want (N, C2 or C3), the fits you will use, the temperature difference between the shaft and housing, and the largest play you can accept in service.
What clearance suits a suspension pivot?
Play in a suspension link shows up as a knock whenever the load reverses (braking, lifting off the throttle) and as drift in wheel alignment, because the lever from the pivot to the wheel magnifies it. That pushes vehicle builders toward small clearance. Mud, heat and shock push the other way, and the pivot has to stay free after assembly.
Three points carry over from above. A tight group (C2) only works with the recommended fits; pressed into a tight or squeezed housing it can end with no clearance at all. In an aluminium arm the fit changes with temperature (Q1-06). And where heat reaches the bolt and inner ring before the housing, for example along a hot spindle, the C3 case applies (Q1-12); heat on the arm itself does the opposite and loosens an aluminium fit.
After assembly, the ball should still turn freely by hand through its full angle before any weight goes on. Decide the play at which the bearing is replaced when the car is designed, not when the knock appears (Q1-14). The group and fit for a steering or suspension pivot are the vehicle designer's to set.