Frictional torque affects the free-running of the bearing
Frictional torque in a bearing refers to resistance that hinders its rotational motion. In most applications, the goal is to minimise friction and torque in bearings. This achieves smoother and more efficient operation.
The effort required to rotate a bearing depends on a few different factors. One is starting torque, the effort required to rotate a bearing from rest, is greater. Running torque, required to keep the bearing rotating, is typically lower.
Other factors include:
- Roundness — Better roundness and surface finish of the balls and raceways can reduce effort needed to rotate the bearing
- Load — The greater the applied load on bearing, the greater the deformation of the bearing components leading to increased resistance. Proper load considerations are crucial for optimal bearing performance and longevity.
- Lubrication — A bearing containing stiff grease will be difficult to spin because it has relatively high frictional torque. A bearing with no lubrication will spin freely, meaning it has low frictional torque.
- Closures — Certain closures such as contact seals may create friction, which impedes smooth rotation in a bearing assembly.
- Clearance — Tight or loose clearances between bearing components impact friction. Proper clearance considerations are essential for balancing stability, performance, and longevity.
Types of bearing
A precision bearing’s ball or roller type, cage design and precision all influence friction. All of these factors must be considered for efficient operation and load-bearing capabilities.
Reduced friction — How does lubrication affect torque?
Low torque is generally preferable because it indicates there is less resistance to rotation. This results in improved performance, longer service life and increased energy efficiency.
Torque levels for a greased bearing are briefly higher to start with. The bearing grease takes a short time to "run in", or distribute, inside the bearing. Learn more on how to choose your grease.
Instrument oils will often produce lower torque levels, especially at low speeds. Low torque greases have a similar effect as instrument oils, especially when applied in low quantities.
High viscosity lubricants, on the other hand, can significantly increase bearing torque because they cause greater lubricant drag. Read more about the different types of bearing lubrication.
Doubling the original bearing life
Wind speed measuring instruments need bearings that offer very little turning resistance if they are to react to very light winds. A standard grease would give too much resistance, so a light instrument oil is usually the preferred choice.
In this context, an SMB Bearings customer was experiencing issues with their bearing oil drying up in harsh conditions. Our specialists experimented with low torque greases and different grease fills. In doing so, we discovered a combination offering significantly lower resistance, which also doubled the original bearing life.
Operating conditions
Higher speeds can affect frictional torque. Lubrication and design must accommodate increased rotational forces to maintain performance. Extreme temperatures alter lubricant viscosity, affecting friction. That's why proper temperature management is critical for attaining optimal bearing function.
Contact seals in bearings are physical barriers that prevent contaminants, such as dust and moisture, entering into the bearing. Contact seals will greatly increase the torque figures because they introduce additional friction to the system. On the other hand, non-contact closures, like non-contact seals or shields, minimise direct friction.
Read SMB Bearing’s essential guide to seals and shields.
Speed applications — Radial and axial loads
You can calculate approximate figures for frictional torque using this simple formula:
Radial ball bearings: 0.5 x 0.0015 x radial load in Newtons* x bearing bore (mm)
Axial ball bearings: 0.5 x 0.0013 x axial load in Newtons* x bearing bore (mm)
Please note, the formula is only valid if the bearing:
- Has a standard fill or low torque lubrication
- Does not have contact seals
- Is subjected to low speed and low load
Radial vs axial load — For radial ball bearings, the bearing axial load should be less than 10 percent of the radial load for very thin-section bearings, to 40 percent or more of the radial load for heavy duty types. The load should be purely axial for thrust ball bearings.
*The above measurements are in Newton millimetres (Nmm). This is a compound unit of torque corresponding to the torque from a force of one Newton (approximately 0.1 Kgf). The force is applied over a distance arm of one millimetre.
Contact SMB Bearings today if you need more accurate figures taking into account the speed and the lubricant viscosity.