Optimizing bearing preload methods to cut power loss and extend life
Bearing preload is one of those quiet design decisions that decides whether an aerospace gearbox runs cool for thousands of flight cycles or cooks itself within a few hundred. The preload force applied across a rolling element bearing sets the internal clearance, the contact angle, and the stiffness of the support. Too little, and the bearing slips, vibrates, and skids during acceleration. Too much, and friction climbs, temperatures spike, and fatigue life collapses. The art is to land in the narrow band where the bearing is rigid enough to keep gear mesh geometry stable but light enough to avoid parasitic drag.
The relationship is non-linear, which is why preload optimisation has become a focused area of research within projects like the OPTIMIZE Clean Sky initiative. Power loss in a gearbox is the sum of gear mesh losses, windage, seal drag, and bearing losses. The bearing contribution alone can reach 20 to 30 percent of total mechanical losses in a high-speed accessory drive, and a poorly chosen preload can roughly double that figure. At the same time, bearing life follows an inverse relationship with internal load, so doubling the preload can reduce calculated fatigue life by a factor of four or more.
In Australia, engineers working on aerospace gearboxes contend with operating environments that swing from the heat of an outback airliner tarmac in Katherine to sub-zero conditions at high altitude over the Tasman. Local research groups, including the aerospace teams at RMIT and the University of Melbourne, have studied how ambient temperature swings influence thermal growth in bearing housings, which in turn shifts effective preload during operation. Add in the strict oversight of the Civil Aviation Safety Authority and the documentation culture demanded by Australian operators like QantasLink and the Royal Australian Air Force, and it becomes clear why preload is not a parameter to leave to default factory settings.
This piece walks through the mechanics of preload, the trade-off between power loss and service life, the methods available to engineers, and the practical steps to land on a setting that holds up in real service. Along the way it draws on insights developed within the OPTIMIZE project's broader work on power reduction gearboxes, including adjacent studies into lubrication chemistry and oil delivery that share the same efficiency goal.
The mechanics of bearing preload in high-speed gearboxes
Preload is the intentional application of an axial or radial load to a bearing before external working loads are applied. In a back-to-back or face-to-face angular contact bearing pair, preload sets the contact ellipse, the operating contact angle, and the internal clearance. A positive preload means the inner and outer rings are slightly displaced relative to the free state, putting the rolling elements under compression. A negative preload leaves internal clearance, which sounds gentle but leads to ball skidding, raceway wear, and unpredictable stiffness under varying load.
In geared aircraft engines, the bearings supporting the input shaft, the planet gears, and the output stage are all candidates for preload optimisation. Speeds can exceed 25,000 rpm, and DN values (bore in mm multiplied by rpm) routinely cross the 1.5 million threshold. At those speeds, the centrifugal effects on the rolling elements, the cage, and the lubricant all change the effective preload dynamically. A setting that feels right on the test bench at room temperature can shift noticeably once the gearbox reaches its thermal steady state at cruise.
The choice of preload strategy interacts with housing design. A rigid steel housing that expands minimally with temperature will keep preload closer to the cold setting, while an aluminium gearbox casing with a different thermal expansion coefficient will drift as the system heats up. Australian manufacturers, who often machine casings locally for prototyping speed, have noted that even the choice between Al-7075 and Al-2024 changes the preload-temperature curve enough to require a different starting value.
How preload influences power loss and thermal behavior
Power loss in a preloaded bearing comes from several sources. There is rolling friction, sliding friction at the contacts, lubricant churning, and seal drag. Sliding friction scales steeply with preload because higher contact forces widen the contact patch and increase the differential sliding at the edges of the elliptical contact zone. This is why a bearing preloaded above its optimum can show a step-change in torque, even if the rolling resistance stays modest.
Thermal behavior is the other side of the same coin. The heat generated by friction must flow out through the housing, the shaft, and the lubricant. If preload is too high, the bearing runs hotter, which thins the oil film, which increases friction further. This positive feedback loop can lead to thermal runaway in extreme cases. In contrast, an under-preloaded bearing may run cool but suffer from skidding damage and reduced fatigue life. The optimum preload sits at the elbow of the friction-versus-preload curve, where torque rises slowly with load.
Work on related systems within the OPTIMIZE project, such as squirrel cage bearing oil feed studies, has shown that even small reductions in bearing torque translate into measurable reductions in overall gearbox heat rejection requirements. Lower oil temperatures downstream also mean cooler gears, which keeps viscosity in the right range and protects lubricant additives from thermal degradation.
Preload strategies for different bearing types
The most common preload strategies are fixed mechanical preload, spring-applied (elastic) preload, and hydraulic or pressure-controlled preload. Fixed preload uses precisely machined spacers or shims between the bearing rings. It is simple, reliable, and easy to inspect, but it cannot compensate for thermal growth or manufacturing variation. Spring preload uses a calibrated wave spring or coil spring to maintain a roughly constant force, trading some rigidity for thermal stability. Hydraulic preload, sometimes used in large industrial gearboxes and increasingly in aerospace accessories, uses oil pressure to set the load and can be modulated based on operating conditions.
For high-speed aerospace applications, hybrid ceramic bearings (silicon nitride balls with steel races) are often paired with moderate elastic preload. The lower density of the ceramic balls reduces centrifugal loading at the outer race contact, which means a given spring preload translates to a more consistent operating preload at speed. In contrast, all-steel bearings at the same speed would require a much lower cold preload to avoid overloading at operating temperature, which risks skidding during transients.
Australian programs supporting local aerospace component manufacturing, including those based around Brisbane's aerospace precinct and the Advanced Manufacturing Growth Centre, have invested in bearing test rigs capable of running these comparisons under representative flight profiles. The rigs measure torque, temperature, and vibration across simulated climb, cruise, and descent segments, allowing preload optimisation to be validated against realistic mission cycles rather than single-point steady-state numbers.
Tolerance analysis and manufacturing variability
Even with the best preload strategy, manufacturing tolerances can scatter the actual preload across a population of gearboxes. Bearing bore and OD tolerances, housing bore roundness and finish, and shaft shoulder squareness all feed into the final preload value. A statistical tolerance analysis using Monte Carlo simulation can predict how many units in a batch will land outside the acceptable preload window.
The OPTIMIZE project has emphasized Design of Experiments (DOE) methods to quantify the sensitivity of preload-related metrics to each manufacturing variable. By running a fractional factorial or Latin Hypercube DOE, engineers can rank the contributors and target the most influential ones for tighter control. In one internal example, housing bore diameter variation was found to dominate the preload scatter, while shaft shoulder runout had a smaller effect than expected. This kind of result justifies tighter boring tolerances rather than grinding the shaft, which is the more expensive option.
For Australian manufacturers exporting into global aerospace supply chains, controlling this scatter is not just an engineering nicety. Customers in the United States, Europe, and increasingly the Asia-Pacific region require documented capability indices on critical features. A preload optimisation study that demonstrates Cpk values above 1.33 on the resulting population is a competitive advantage when bidding against suppliers in lower-cost regions.
Simulation, testing, and validation methods
Modern bearing preload optimisation relies on a layered approach. System-level gearbox simulations using tools like Romax, KISSsoft, or in-house MATLAB models set the macro loads and speeds. Detailed bearing models then compute internal load distribution, contact pressures, and lubricant film thickness. Finally, computational fluid dynamics or thermo-mechanical models predict temperature fields and the resulting thermal expansion that shifts preload during operation.
Physical testing remains essential. Bench rigs with instrumented bearing housings, torque transducers between the motor and the gearbox input, and thermocouples embedded near the bearing outer races provide ground truth. Some Australian test houses, including DSTG's facilities in Melbourne and private labs in the greater Adelaide area, run endurance tests that exceed 1,000 hours under simulated flight spectra. The data from these tests feeds back into the simulation models, refining friction coefficients and thermal boundary conditions, and complements separate work on lubricant additives screening for micropitting resistance in aerospace gears.
Validation also includes accelerated life testing, where bearings are run at elevated loads or speeds to generate failures in weeks rather than years. The resulting Weibull plots are extrapolated to the actual mission profile. When preload optimisation is done well, the characteristic life can extend by 30 to 100 percent compared with a conservative default setting, with no penalty in torque or temperature rise.
Practical implementation guidelines
For engineers ready to apply these ideas, the workflow starts with selecting the right preload strategy for the operating environment, then moves into tolerance analysis and validation. Once a strategy is chosen, engineers should follow a documented sequence from bearing selection through to production acceptance testing. This sequence becomes part of the design history file that certification authorities require.
For project members and partners, additional design-of-experiments templates and case study data are available through the project members area, where preload optimisation work sits alongside related studies on lubricant additives and oil feed pressure. Access to these resources can shorten the learning curve for teams new to formal preload studies.
The following checklist captures the key steps from initial sizing through to production release.
- Start with the bearing manufacturer's recommended preload range for the chosen bearing type and operating speed, then refine using application-specific thermal models.
- Decide on a preload strategy (fixed, spring, or hydraulic) based on the thermal expansion profile of the housing and shaft materials.
- Run a tolerance analysis to predict the scatter in actual preload across a production batch, and tighten the most influential manufacturing features.
- Build or update a bearing model that includes centrifugal effects, lubricant viscosity at operating temperature, and housing thermal growth.
- Validate the chosen setting on a representative test rig under a mission-cycle load spectrum, not just steady-state conditions.
- Document the procedure, the predicted preload window, and the acceptance test criteria so production staff can verify compliance on every unit.
- Re-evaluate the setting after any change to lubricant, bearing supplier, or housing material.
| Preload method | Stiffness | Thermal stability | Complexity | Best suited to |
|---|---|---|---|---|
| Fixed (spacer or shim) | High | Low | Low | Rigid housings, stable temperatures, cost-sensitive builds |
| Spring (elastic) | Medium | High | Medium | Aluminium housings, wide temperature swings, high-speed shafts |
| Hydraulic (pressure-controlled) | Variable | Very high | High | Large gearboxes, adaptive systems, monitored critical drives |
| Active (sensor-controlled) | Variable | Very high | Very high | Next-generation power systems with real-time health monitoring |
To bring this together, the goal of any bearing preload study is not to chase the lowest possible torque or the longest possible calculated life in isolation. It is to find the operating point that gives the right balance for the mission, documented well enough that production can repeat it, and robust enough that real-world variation does not push the unit out of its safe envelope. Engineers working on aerospace gearboxes in Australia and elsewhere now have access to a richer set of tools and data than ever before. The next step is to bring preload optimisation out of the specialist's notebook and into the standard design workflow, so every new gearbox benefits from the lessons already learned. Anyone working on an aerospace gearbox programme can explore the OPTIMIZE project's methods through the resources linked above or get in touch with the team through the project channels.