Bearings

It seems today that most people are so concerned with looptimes, gyro sync, airmode, betaflight, the latest trending gear, the newest tech, that they are overlooking one of the most basic and important parts of a quadcopter- the motor bearings

The Motor Bearings.

All motors have bearings, and most of the time the thing that separates the mediocre motor from the top performing one is the bearings.  If you don’t properly take care of your bearings in any motor, you will destroy them, and without proper maintenance, you might not be getting the top performance out of your motor..

Ball bearings are high precision devices compared to many mechanical parts. Good performance will therefore require treatment that takes into account their characteristics and operating environment.
A high percentage of bearing problems, including failures, are the result of improper handling procedures. The following information represents the results of our most common case studies. We hope you will find this information useful in the care and handling of precision ball bearings.

The Function of Ball Bearings

From small and large motors to car axles to electric fans, Quadcopters and hard disc drives (HDDs), ball bearings are used in a wide array of machines for rotary motion. Ball bearings support rotary parts and reduce friction to facilitate the smooth operation of machines. The size of ball bearings can range from smaller than a grain of rice — small enough to fit inside a wristwatch—to over one meter in diameter for factory and power plant applications.

Most often you will find that quadcopter motors specify NMB bearings-  this is good because the NMB company specializes in the manufacture of miniature and small-sized ball bearings with an outer diameter of 30mm or less.  These bearings are typically found in Quadcopter motors- We recommend purchasing motors with NMB bearings in them for the best performance.

Ball Bearing Components

Ball bearings are comprised of four major parts: a large ring (outer ring), a small ring (inner ring), balls between the rings (steel balls), and a cage to prevent the balls from hitting each other. The modern structure dates back to around 1500, when Leonardo da Vinci invented ball bearings to reduce friction against the axles of horsedrawn carts. For more than 500 years, the ball bearing has been defined by this simple structure.

The Precision of a Ball Bearing

The main function of a ball bearing is to reduce friction and facilitate smooth rotation of an axis. Modern ball bearings test the very limits of precision. As an example, one of the key elements to making an ultra-precise ball bearing is the degree to which the groove where the balls roll can be made into a perfect circle.
The most important performance measure in judging a ball bearing is the extent to which it reduces friction and facilitates smooth and easy rotation. Ball bearings may end up in quadcopters, racing drones, aircraft, automobiles, precision medical equipment and many other end products, but no matter the application, it is important to maintain smoothness and minimize friction. Changes in the times brings diversification and complexity in end products and machine types along with new performance requirements for ball bearings, including miniaturization, higher rotation speeds, durability and duration, as well as noise reduction.

Particle Contamination

The performance of miniature precision ball bearings is critically affected by minute particle contamination. Avoiding exposing the bearing to any environment where particles may be present is highly recommended.
Shields and seals are used to prevent contaminants from reaching the inside of the bearing. However, after assembly, there is still a small gap between the shield and the inner ring. This gap may permit particle entry.
Please observe the following procedures carefully:

• Keep your bearings handling room as clean as possible.
• Do not remove the bearings from their packaging until just before use.
• If you move the bearings to a container, be sure the container is clean. The lid should be kept close, and it should be cleaned every day to prevent particle accumulation.
• Never use a bearing that has been dropped. It may be brinelled (race track dented). In use, a brinelled bearing will generate a high level of acoustic noise.
• Before applying adhesive to a bearing, use a clean cloth dampened with an alcohol agent to clean oily materials such as anticorrosion oil from the inner and outer rings. Do not saturate the cloth excessively with the cleaning agent. The liquid agent itself could leak into the bearing, carrying particles with it.
• When applying a lubricant to the outer circumference of a bearing, take care to make sure the lubricant is not contaminated. You might inadvertently transfer the contamination into the bearing.
• Never use an applicator that will leave contaminants on or near the bearings. A cotton swab, for instance, may leave small fibrous particles behind. We recommend a mechanical dispenser, many are available, or a clean room type of applicator.
• Do not handle bearings in a place where they could be directly exposed to outside air. Airborne contaminants include dust, dirt, and humidity.

Rust Contamination

Since bearings are metallic products, they rust easily. Their treatment requires certain precautions:

• When handling bearings, use finger caps, tweezers or gloves that do not generate cotton fibers.
• When using unprotected fingers to handle a bearing, first make sure they are clean and free from perspiration and dust. Apply a quality mineral oil to the fingertips before touching the bearing. Do not use hand cream, as it may induce rust.
• If a shaft is dirty on the surface, rust may gather between the shaft and the bearing after they are fitted.
  It is important to make sure that the shaft is free from finger prints, perspiration, dust and dirt.
• Avoid storing bearings near air conditioners and direct sunlight.
• Bearings may rust when placed near an air conditioner outlet, or any place where wind or sunlight can enter directly.
• A great temperature difference may cause condensation to form on the bearings. In colder climates, allow the bearings to reach room temperature before unpacking them.
• Store bearings in centrally heated and properly ventilated environments.

Shock Forces

Bearings are easily affected by shock forces. Depending on the size of the bearing, a shock force from a 100 gram weight at 4mm away could cause brinelling.
Brinelling could also occur when bearings are automatically press-fitted to a rotor shaft, if the shaft and bearing bore are not kept accurately in line.
A typical example of shock causing brinelling is when motors are placed on a conveyor belt. As the motors moved through the conveyor, the movement causes the motors to hit the iron plate underneath the conveyor, resulting in shock which causes brinelling to the bearing. Holes made to the iron plate prevent this type of shock force to be generated.

Another obvious example of shock causing brinelling is when a quadcopter is crashed and a motor experiences direct impact.

Lubrication

Synthetic Lubricants are recommended for brushless motor bearings.

The esters, diesters and poly-a-olefins are probably the most common synthetic lubricants. They do not have the film strength capacity of a petroleum product, but do have a wide temperature range (-65º to 350 ºF) and are oxidation resistant.
Synthetic hydrocarbons are finding a greater use in the miniature and instrument ball bearing industry because they have proved to be a superior general purpose lubricant for a variety of speeds, temperatures and environments.

Conclusion:

Your motor bearings are the heart of your quadcopter and very often overlooked during the maintenance and service of racing drones.  It is important to keep your bearings lubricated and clean to prolong the life of your motors and to keep the entire system functioning properly.

Do yourself a favor and purchase good lubricating oil and keep it near your quads to keep your motors in tip top shape.

Sources:

A large portion of this document was borrowed directly from the NMB bearing website.  We recommend that you check out their site for more info on bearing care, and look for motors sporting NMB bearings in their assemblies.

http://www.nmbtc.com/bearings/white-papers/bearing-care-and-handling/