Spindle bearings (for machine tool spindles, high-speed spindles) feature high rotational speed, ultra-high precision, extreme sensitivity to dimensional stability, and large alternating contact stress. Compared with general engineering bearings, the problems caused by residual stress are amplified. Typical spindle bearings: angular contact ball bearings, double-row cylindrical roller bearings, mostly P4/P2 precision grades.
Residual stress originates from forging → quenching & tempering → rough grinding / finish grinding. Among these, grinding residual stress is the most critical factor for spindle bearings. It is divided into residual tensile stress (harmful) and controlled residual compressive stress (beneficial).
1. Harm of residual tensile stress
Caused by grinding burn, large grinding depth per pass or insufficient cooling.
1. Unstable dimensions and spindle precision drift
Spindles require micron-level precision. Tiny deformation of bearing ring inner bore and raceway directly affects spindle runout.
Residual tensile stress releases slowly over time and temperature rise, causing micro deformation of rings: raceway profile distortion, gradual dimensional change of inner/outer diameter.
Bearing preload drifts(both constant-pressure preload and position preload are affected).
Phenomenon: New spindle passes factory inspection, but radial runout increases and vibration rises after running for a period. Workpieces machined show waviness and poor surface finish.
For ordinary bearings, clearance change is acceptable. Once preload of machine tool spindle bearings drifts, the machining accuracy of the whole spindle may fail.
2. Easy generation of microcracks under high speed, risk of sudden failure
At high rotating speed, the raceway bears high-frequency alternating contact stress. Surface residual tensile stress superimposes working stress:
Tiny grinding microcracks (only detectable by magnetic particle or X-ray inspection) expand rapidly under cyclic high-speed load.
Risk: High-speed spindles may suffer sudden peeling or seizure instead of gradual wear. It may damage workpieces or even destroy the spindle.
3. Reduced fatigue life, worsened noise & vibration at high speed
Spindles run continuously at high speed with extremely frequent stress cycles.
Residual tensile stress reduces contact fatigue strength, leading to premature pitting and peeling on raceways.
Micro deformation and microcracks on raceway surface amplify vibration and noise.
Phenomenon: Spindle generates louder abnormal noise after service, dynamic balance deteriorates.
4. Poor thermal resistance, forming a vicious cycle of temperature rise
Spindles themselves generate much heat. Layers with grinding burn and residual tensile stress have degraded metallographic structure and uneven hardness.
Bearing operating temperature rises further.
Higher temperature accelerates residual stress release, which further changes preload. This forms a vicious cycle: temperature rise → deformation → unstable preload → higher temperature.
2. Benefits of controlled residual compressive stress
Achieved by 5-pass finish grinding with proper cooling and grinding wheel parameters, which is the target state for precision spindle bearings.
1. Offset alternating working stress: Surface compressive stress on raceway counteracts part of tensile stress generated during operation, restrains crack initiation and propagation, greatly improving high-speed contact fatigue life.
2. Better dimensional stability: Proper grinding process reduces stressed layers. Combined with stabilizing tempering, stress relaxation is minimal. Preload stays stable and spindle runout maintains precision for a long time.
3. Better overload & impact resistance: Resists surface peeling under occasional cutting impact on spindles.