Miniature and instrument ball bearings are used in high-precision mating parts to control movement and provide rotational and oscillating functionality. Aircraft, pick-and-place robots, computers, flow meters, and medical instruments are just some applications. Often, part of the deal is that speeds run very high. For example, miniature bearings in dental handpieces travel at speeds approaching 500,000 rpm. Flow meter bearings travel at somewhat slower speeds but also face a unique set of pressure, corrosion, and environmental challenges. Others have to run in pulling, stressing manmade vacuums and the natural vacuum of space.

Tolerances, material, preloading, coding — basically all that affects axial and radial play, as well as play itself — can be modified for optimal performance in these applications.

Fits and axial play

Within the assembled mechanism, the fit of the ball bearing on its mating components is vital to maximizing bearing life. If the fit is too loose, bearings slide around on the shaft, eliminating the advantages gained by selecting a ball bearing in the first place. If the fit is too tight, the bearing integrity may be compromised by reduction of radial play in the assembly. The perfect fit enables bearings to run at peak performance for maximized end-product life.

There are three main types of shaft and housing fits. A loose fit is when the bore of the inner ring of the bearing is slightly larger than the outer shaft diameter. A line-to-line assembly is when the bore of the inner ring of the bearing and the outer diameter of the shaft are the same. With a tight fit, the bore of the inner ring of the bearing is slightly smaller than the outer shaft diameter. Tight fits are also called interference or press fits, because bearings in these assemblies are pressed onto shafts.

Slightly loose shaft-to-housing fits are suitable for most applications, while line-to-line fits often provide the best performance. If bearings are assembled on a shaft that is too large (creating a press fit) the inner ring may stretch slightly. When this occurs, the bearing's radial play can be diminished or even eliminated altogether. We'll cover how this can cause problems shortly.

Bearing radial play is a function of radial movement of the inner ring in relation to the outer ring. It is usually measured by holding the inner ring stationary and applying a radial load to the outer. Specified radial play should not be impeded, as this has an adverse effect on bearing life. When the radial play is pushed below its specified design, discreet bearing components can become overheated, especially under extremely high running torque conditions. When the components become hot, the bearing retainer can cause catastrophic failure of the bearing. If that doesn't happen, the retainer still may heat up enough to melt or become deformed. Then the retainer may rub on the bearing rings and begin flaking apart, causing debris and contamination to fall into the bearing components and result in premature bearing failure.

Some manufacturers and overhaulers try to reduce cost by choosing lower-priced shafting. However, it is imperative that the shaft outer diameter not vary too much; otherwise it creates varied fits against even tightly toleranced bearing bores. Where the shaft and housing fits may be slightly looser than desirable, it is actually best to err towards a looser fit rather than an interference fit. In these cases, an adhesive such as Loctite often overcomes slightly loose assembly. That said, caution must be exercised with any adhesive, as they can contaminate inner assembly workings. Even a small amount can cause catastrophic failure.

Radial play

Radial play is the measured value of total movement of one ring with respect to the other in a plane perpendicular to the bearing axis. This internal looseness between balls and races is not related to bearing class or quality; it is what allows the inner and outer rings to rotate in concert with the bearing balls. Factors that rely on proper radial play are speeds, loads, thermal conditions, mounting fits, along with axial motion and deflection rates. Improper selection of radial play can affect bearing torque, overall running performance and bearing life.

Any impact on radial play must be carefully considered when designing the bearing specifications. So what if your design requires tight fits? For example, interference fits are necessary to prevent one bearing ring from turning relative to its mating part under heavy loads or cycling vibration. (High-torque motors and vacuum pumps especially require an interference to prevent slippage.) Interference fits cause a 50 to 80% reduction in radial play.

The main thing to remember is that one ring in the bearing assembly must be free to move to prevent axial preloading. Axial play is proportional to bearing radial play; the contact angle of a radial bearing under axial load is related to the radial play remaining in the bearing after installation. A higher assembled radial play makes for a higher contact angle, the angle at which balls and raceway touch. The higher the load on a single bearing ring, the further from the center or raceway bottom the ball is likely to contact the raceway. A low contact angle is desirable for pure radial loads, while higher contact angles are desirable for loads with an axial movement.