The impact of good bearing system design and what to consider

At IDC, our work within the area of robotics is increasing and one of the key areas of engineering within these projects is bearings. In this article, we’ll explore why it’s so important, take a look at the considerations for bearing system design and offer insights into how optimal performance can be achieved.

The what and why

Bearings are fundamental components of moving assemblies which allow relative motion between two surfaces whilst under load. They can be found in virtually any product with moving components.

The motion may be rotational or linear and may be facilitated by rolling elements, sliding contact or fluid films. A well-designed bearing system will meet the requirements for life, efficiency, load and speed but a poorly designed system can cause a range of problems that are often difficult to troubleshoot. Symptoms of a poorly designed bearing system include free play outside of the desired range of motion, early failure and excessive noise. These symptoms can be related to a number of underlying issues and all of them can contribute to the level of success of the product in the market.

Tolerances and fits

For a rolling element bearing to function up to its rated loads and speeds, it must be installed correctly. For example, if the hole which houses the bearing is undersize, and the press fit too tight, the bearing will feel rough when the inner race is rotated by hand and its life could be significantly reduced. Similarly, if the bearing is pressed onto an oversized shaft, deformation and stress of the bearing will change the internal clearances and cause poor performance and early failure. When using plain bearings (bushings), it is important to keep in mind that the shaft which rotates in the bushing is effectively the inner race, therefore not only should it be within a specific tolerance window but it must also have a sufficiently fine surface finish (often less than Ra 0.3) and be a suitable hardness.

If a shaft is to be supported by multiple bearings, the concentricity, cylindricity and alignment of both the bearing housings and shaft must have suitable tolerances. Poorly controlled tolerances can lead to assembly difficulty, uneven load distributions within bearings, excessive noise and premature failure of rolling element bearings and often immediate jamming of polymer bushings. Additional information on tolerances and fits can be found HERE.
bearing system design

Bearing selection

Bearings come in a vast array of configurations, from bronze bushes to rolling element bearings optimised to support forces in specific directions. The process of bearing type selection is closely linked with the design of the wider system with respect to understanding the magnitude and direction of forces. For rotating shafts with no axial forces, the bearing choice may be as narrow as ball bearing vs plain bushing. However, when there are significant moment loads, radial forces and axial forces the variety of suitable bearings types can grow significantly. More complex cases require greater emphasis on how certain bearing combinations can impact system function or performance.

Over determination of bearing arrangements

Complex bearing systems may include shafts, gears and motors which need to be adequately constrained to allow only the desired motion. In these cases, there is a danger of individual bearings becoming overloaded due to having too many fixed bearings in the system. The risk is that one bearing supports a higher percentage of the load than intended, or is stressed during the assembly process. For example, a misaligned motor and shaft could cause the motors internal bearings to experience unusually high loads. In some cases, this can be avoided with the use of flexible couplings between motors, gearboxes and shafts, but in some cases there is a need for tighter tolerance control and the use of fixed and floating bearings to allow for misalignment.

bearing system design

Bearing life calculations

Life of a bearing and service intervals can be estimated through calculation. A Basic Rating Life, associated with 90% reliability is specified in ISO281. For more complex bearing arrangements or to have more certainty over bearing life in critical applications there are a number of more advanced techniques which can be used to estimate life. It is common for real engineering systems to have more complexity than the simple cases covered by the Basic Rating Life, which means that the assumptions in these calculations no longer represent the real system. In such instances, it may be appropriate to consider the “modified rating life” which includes variables for factors such as lubrication condition, lubrication contamination, belt driven systems, fatigue load and variation in load cases and speeds. Alternatively, bearing life can be calculated according to ISO16281 which factors in load distribution within the bearing, therefore factoring in the influence of bearing clearance, tilting angle or centrifugal forces. A well-designed bearing system can be tailored to suit the needs of the product, whether that is:

  • Quiet operation
  • Maximum precision
  • Extended life on products where regular servicing is not possible
  • Reduced vibration
  • Removing the risk of jamming/locking of motion systems
  • Function in harsh environments
  • Ease of assembly
  • Safety
  • Reduced friction for higher efficiency
  • High speeds
  • Low cost
  • Serviceability

Examples of the value of good bearing design

  • Product too loud? We have taken loud motion systems and silenced them through correct bearing selection and appropriate part tolerancing.
  • Need reliable performance with minimal maintenance? We have designed, tested and proven bearing systems to work continuously in varying conditions without requiring maintenance for over 5 years.
  • Problems with accuracy and backlash? We have designed joints using rolling element bearings to support significant moment loads whilst having no free play, enhancing not only the precision and accuracy of the joint but also eliminating noise and improving life.

Thanks for reading. If you have any questions surrounding the area of bearing system design or would like support in this area please reach out any time.

About the author

Nick Brown is a Mechanical Design Engineer at IDC. With a Master's Degree in Motorsport Engineering, Nick applies his in-depth understanding of mechanical engineering principles and complex motion systems to solve a range of technical problems across a broad spectrum of projects including robotics, medical devices, consumer products and industrial equipment.

nick brown
13 May 2020

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