Handheld power tools are commonplace in a variety of industries from automotive to aerospace. In construction, they are used by professional builders and DIYers alike. Ball bearings play a critical role in the performance and reliability of these tools, and are responsible for handling combined loading conditions (radial and axial), high speed operation, low friction, low noise and vibration, and high reliability.
Power tools have a global market of close to $26 billion (approximately $11 billion in the US). Cordless electric power tools are the fastest growing segment, with the industrial and professional market outpacing consumer demand. Products produced for the consumer market generally have lower price points, lower quality and incorporate components that are “off the shelf” or fall into the “commodity” category.
In the industrial arena, power tools are highly engineered products that reduce worker fatigue and improve productivity and quality. They are routinely used in all types of operations such as, driving (fasteners), drilling, cutting, shaping, sanding, grinding, routing, polishing and more.
Typical industrial rotary tools include drills, screwdrivers, nut runners, impact wrenches and drivers, grinders, sanders and polishers. Electric motors and compressed air are the most common types of powers sources used in a factory environment.
Bearing Loads: Ball bearings can support both radial and axial loads simultaneously, making them the ideal choice for the demands of power tools. However, due to their internal construction having relatively shallow raceways, the predominant load component should be radial. In the event axial loads are excessive, thrust bearings can be used in combination with radial ball bearings to effectively support these loads.
Noise and Vibration: A noisy and vibrating tool will quickly cause discomfort for its operator, particularly in factory or production environments where motion and operations are repetitive. In the case of ball bearings, noise and vibration are directly related to the quality of the materials used for the rings and balls, the internal manufacturing specifications and tight process controls.
Bearings used in these applications should be produced to Electric Motor Quality (EMQ) noise levels. EMQ standards are not uniform (except in China where there is a national standard) and vary from manufacturer to manufacturer, but the EMQ rating indicates they are suitable for use in electric motors.
Historically, electric motors must run quietly with little or no vibration, and this terminology came to be as a result. Manufacturers produce bearing components to strict specifications and employ various techniques to produce EMQ bearings.
Great care is taken to produce rings with near perfect geometry. Geometrical tolerances control features such as roundness, concentricity, run out and parallelism. Bearings with rings that have poor geometry tend to produce lower frequency noise and vibration.
Raceways (the tracks where the balls roll) are super finished to produce mirror like surfaces. High quality balls are also used, meaning they have good geometry, such as being perfectly spherical. Balls or raceways with poor surface finishes tend to produce higher frequency noise and vibration.
Certain lubricants can contribute to noise levels as well. The smaller bearings used in these hand tools require “clean” grease.
Manufacturers who produce EMQ bearings typically perform 100% noise testing of the bearings using specially designed equipment prior to final lubrication and packaging.
Power tool manufacturers must properly specify and apply preload to the bearings used in a power tool to reduce noise and vibration. One of the final steps in the bearing manufacturing process is the assembly of the individual bearing components: the outer ring, inner ring, balls and retainer (or ball separator). When the bearings are assembled, it is necessary to have a controlled amount of internal clearance, or looseness between the rings and balls. This is referred to as radial play in most bearing catalogs.
In power hand tool applications, this internal clearance must be removed for a pair of bearings to operate properly. The application of an axial load across the pair of bearings – for the purpose of removing free internal clearances – is called preload.
The application of axial preload forces the balls into contact with raceways, establishing a contact angle which causes the ball set to rotate in a uniform circumferential plane. Preload is critical in high-precision and high-speed power tool applications where speeds can reach as high as 60,000 RPM.
Running Temperature: Power hand tools must be designed and manufactured so that they can be held by an operator for long periods of time. Bearings must be mounted properly, have the proper preload and have the proper lubrication type and amount in order to minimize the running temperature.
Bearing Materials: All bearings should be manufactured using components produced from high purity material. Bearing rings and balls are most commonly produced from SAE 52100 chrome steel. This thru hardening alloy has excellent fatigue strength and wear resistance, and is the most cost effective choice in a non-corrosive environment.
In hand tools, bearings are typically well protected within a housing or enclosure. However, if there is risk of exposure to moisture or corrosive attack, martensitic stainless steel, similar to AISI 440C, is recommended. This 400 series stainless steel offer the best combination of corrosion resistance and fatigue life. It provides good corrosion resistance and has fine, evenly dispersed carbides which result in lower noise and vibration levels than 440C. This material can be specified differently by manufacturers.
In some cases, balls produced from ceramic materials, such as silicon nitride, are very beneficial. Ceramic balls are very light weight, highly polished, non-magnetic, exhibit high hardness and are resistant to attack from most liquids and chemicals. Ceramic balls greatly improve the high speed capability of the bearing. Bearings made of steel rings and ceramic balls are commonly called hybrid bearings. While ceramic balls have an impressive list of beneficial features, they are not typically used in hand held power tools.
Lubrication: Lubricant selection could be the specification most overlooked by designers and engineers. Bearing life depends on proper lubrication in terms of both type and amount.
In most cases, the smaller bearings used in power hand tools are lubricated once for the life time of the device. Thousands of greases and oils are available that are designed to function in a variety of conditions and environments.
Operating temperature is the primary consideration when selecting a lubricant. Temperature directly impacts the base oil’s viscosity which in turn impacts the ability to support loads.
Due to the wide array of products along with price and availability, both a lubrication specialist and the bearing manufacturer should be consulted before making a final lubricant selection.
Harsh Environments: In many industrial environments, power tools (and possibly the bearings) are continuously exposed to liquids and various types of particulate debris. Sealed or shielded bearings should be used in these situations. Seals are the best option for keeping foreign debris out of the interior of the bearing and keeping lubricant in.
The most common bearing seal material is a nitrile rubber. This type of seal is typically Buna-N bonded to a steel insert, which is fixed into a groove in the outer ring. The maximum operating temperature is 240° F. This type of seal makes contact with the inner ring providing better protection in contaminated environments than a metal shield. However, this results in an increase in torque and reduces the maximum speed capability of the bearing, but in almost all cases is a design trade off that is made to improve life.
Certain lubricants and chemicals react with rubber. Aside from the Buna-n and nitrile rubber material, other materials are available for higher temperatures and chemical resistance. These include F Teflon fabric and Viton that has good chemical resistance and a maximum operating temperature of 400° F.
As evidenced, there are a wide range of bearing options to take into account for optimal hand tool performance. Engineers and designers are encouraged to consult a bearing applications specialist for assistance with calculating load capacity, bearing life, fits and component tolerances, as well as evaluating environmental factors in order to properly specify bearing attributes.