Understanding What Limiting Speed of Bearings is All About

April 16, 2019

Equipment shafts, they can only go so fast. Beyond that point, the rolling elements on each shaft end start to cook. It seems like the laws of physics have something to say about an out-of-control rotating bearing. Bound by those laws, a speed and its additional friction causes a subsequent increase in operational heat. Left like that, a bearing can’t stop producing copious quantities of heat.
Velocity to Friction Correlations

The frictional energy is negligible at first. The rubbing effect increases as a bearing gains speed. Consequently, the speed increase causes a resultant rise in frictional heat. Imagine this effect without a limiting speed in place. As a bearing’s moving parts accelerate to a blur, the parts glow cherry red until the whole assembly flies apart in a cloud of sparks and hot metal chips. Knowing this latent possibility exists, bearing design engineers quote a speed limiting maximum.
Incorporating A Bearing Speed Limiting Mechanism

Technically speaking, this is the velocity at which the rolling elements will seize or fail catastrophically. At such speeds, alloy parts fail, contact zones experience micro welds, and entire shaft mounting assemblies become overwhelmed. Equipment shafting parts trains must be kept below this velocity maximum if they’re to remain operational. To calculate this limit, engineers assess velocity-relevant bearing parameters. They include the following design criteria:

  • Bearing alloy
  • Lubricating agent
  • Cage type and configuration
  • Race and element diameters
  • Load factors

Knowing the above device features and specifications, the limiting speed is calculated. Advanced engineering equations and dynamic loading tables are also employed as key numerical determinants when computing maximum radial velocities.
Can Maximum Limiting Speeds Be Exceeded?

Ordinarily, the answer would be, no, for the speed limit acts as a thermal damage inhibition measure. There are even tables and charts included with bearings, which are consulted by engineers when a shafting arrangement is upgraded. They tell designers how far a spinning bearing can be pushed before it accumulates unmanageable quantities of thermal energy. At that point, there’s too much friction hitting the device, and it’s starting its downward spiral. To lift the thermal limit higher, cooling systems can be installed around the heated parts. Oil-submerged bearings sometimes apply this solution, for hot oils can circulate while distributing excess thermal energies.

All the same, it’s rarely a good idea to exceed a bearing’s speed limit. Mathematics can’t lie, frictional and micro welding heat will generate as the rolling parts accelerate. As such, shaft and/or powertrain assemblages require a velocity governing influence, as established by an equipment designer.

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