Understanding Bearing Preload
Bearing preload — also called preloading or initial load — is a controlled compressive load deliberately applied to a bearing to eliminate internal clearance and create a slight interference between the rolling elements and the races. By keeping every rolling element in continuous contact with the races under all operating conditions, preload removes the small internal play that would otherwise exist, producing a stiffer, more precise bearing system with better load distribution and greater resistance to vibration. It is essential wherever an application demands high rigidity, precise shaft positioning, or smooth running under varying or oscillating loads, and it is standard practice in machine-tool spindles, precision instruments, and high-speed machinery where preventing instability is critical.
1. Definition: Turning Clearance into Stiffness
Most rolling-element bearings are manufactured with a small amount of internal clearance so they can be fitted and lubricated. That clearance is helpful for assembly but harmful for precision: it lets the shaft deflect slightly before the rolling elements take up load, and it allows lightly loaded elements to skid rather than roll. Preload deliberately reverses this — it pushes the races together (or squeezes them radially) until the clearance vanishes and a defined contact force exists at every element even before the external load arrives. In effect, preload trades a little extra friction and heat for a large gain in stiffness and positional accuracy.
2. Purpose and Benefits
1. Increased stiffness
This is the primary benefit of preload:
- Eliminates the clearance that allows deflection under load.
- Keeps all rolling elements in contact, sharing the load across the full complement.
- Can raise bearing stiffness by roughly 2–5× compared with an unpreloaded bearing.
- Reduces shaft deflection and improves overall system rigidity.
2. Improved accuracy and precision
- Eliminates shaft runout arising from bearing clearance.
- Delivers precise, repeatable shaft positioning.
- Critical for precision machinery such as machine tools and measuring instruments.
- Reduces vibration caused by clearance-related impacts.
3. Prevention of skidding
- Ensures the rolling elements truly roll rather than skid.
- Particularly important under light loads or at high speed.
- Skidding causes rapid bearing wear and surface damage.
- Preload maintains enough contact force for pure rolling.
4. Noise reduction
- Eliminates the rattling that internal clearance produces.
- Gives quieter, smoother operation.
- Valuable near personnel or sensitive equipment.
5. Stability enhancement
In rotor dynamics, preload contributes to stability:
- Increased bearing stiffness raises the critical speeds.
- It improves damping characteristics.
- It helps prevent bearing-induced instabilities.
- It reduces susceptibility to external vibration.
3. Types of Preload
1. Fixed (rigid) preload
A constant preload, independent of temperature or speed:
- Method: spacers, shims, or lock nuts set to a specific position.
- Characteristics: high stiffness and precise control.
- Limitations: it can increase with temperature, risking overload.
- Ilovalar: machine-tool spindles and precision equipment.
2. Spring (elastic) preload
A preload held by springs, allowing thermal compensation:
- Method: wave springs, Belleville washers, or coil springs.
- Characteristics: accommodates thermal growth without overloading.
- Advantages: far more forgiving of temperature change.
- Ilovalar: equipment with temperature variation and less demanding precision requirements.
4. Preload Methods
Axial preload (most common)
Face-to-face or back-to-back mounting
- Two angular-contact bearings mounted opposed to one another.
- An axial force pushes the bearings together.
- Eliminates axial clearance in both directions.
- The standard arrangement for machine tools and high-precision applications.
Adjustable preload
- A lock nut or threaded retainer adjusted to set the preload.
- Verified by torque, axial force, or bearing temperature rise.
- Can be set during assembly or revisited at maintenance.
Radial preload
- An interference fit between the races and the shaft or housing creates a radial squeeze.
- The rolling elements are compressed radially between the races.
- Less common than axial preload.
- Used in some sealed bearings and special applications.
5. Selecting the Preload Magnitude
Light preload
- Force: 1–5% of the bearing’s dynamic load rating.
- Foydasi: improved stiffness with minimal added friction.
- Ilovalar: general precision machinery.
Medium preload
- Force: 5–10% of the dynamic rating.
- Foydasi: high stiffness and good accuracy.
- Ilovalar: machine-tool spindles and precision drives.
Heavy preload
- Force: 10–20% of the dynamic rating.
- Foydasi: maximum stiffness and stability.
- Limitations: high friction, heat generation, and reduced life.
- Ilovalar: ultra-precision work and low-speed, high-stiffness requirements.
Because the right figure depends on the bearing’s rated capacity, it helps to know that rating before specifying preload; tools such as a bearing L10 life calculator (ISO 281) put the dynamic load rating and expected life into context, so the chosen preload percentage can be weighed against its effect on service life.
6. Disadvantages and Trade-offs
Increased friction and heat
- Preload raises contact loads and therefore friction.
- Operating temperature typically rises 5–20 °C above an unpreloaded bearing.
- Higher temperatures accelerate lubricant degradation.
- Enhanced cooling or lubrication may be needed.
Reduced bearing life
- Preload adds to the operating loads.
- Bearing-life calculations must include the preload effect.
- Excessive preload can dramatically shorten life.
- The fundamental trade-off is stiffness and precision against longevity.
Thermal sensitivity
- Fixed preload grows with temperature rise through differential expansion.
- Unmanaged thermal growth can drive the bearing into overload.
- Spring preload absorbs these thermal changes.
- The design must account for the full operating temperature range.
7. Applications
Where preload is essential
- Machine-tool spindles: grinding, milling, and turning spindles needing precision and rigidity.
- High-speed equipment: to prevent skidding and instability.
- Precision instruments: measuring equipment and optical systems.
- Oscillating loads: applications with load reversals or varying loads.
- Moment loads: bearings subjected to tilting moments.
Where preload is not recommended
- High-temperature applications, where thermal overload is a risk.
- Very high speeds, where friction and heat dominate.
- Heavy shock loads.
- Cases where long bearing life takes priority over stiffness.
- General industrial duty where precision is not critical.
Preload also has a diagnostic dimension. A spindle that has lost preload through wear, or one driven into thermal overload, will change its vibration signature — often shifting critical speeds or raising the broadband level — so the effects of preload are visible to a vibration analyst long before a failure. A portable two-channel analyser such as the Balanset-1A can capture a spindle’s vibration spectrum and overall level at operating speed, providing a baseline against which any later change in bearing preload or condition can be trended — and, when the underlying issue turns out to be unbalance rather than the bearings, balanced out on the same machine.
Bearing preload is a powerful tool for raising bearing-system performance, delivering greater stiffness, improved accuracy, and protection against skidding and instability. It must, however, be specified with care, weighing those gains against the penalties of added friction, heat, and reduced life, so that the optimum balance is struck for each specific application.
