Ball Bearing Materials
The balls are a vital part of bearings. These rolling bodies are located between the outer and inner rings, and the retainer or the cage. The most common ones are the steel ball bearings, such as stainless steel ball bearings, but there are balls made of other materials too. They are in contact with the rings at a single point and make the bearing rotate with little friction. The balls are in different grades, with the precision standards being set by the ABMA (American Bearing Manufacturers Association).
They are made to a specific grade between 2000 and 3, defining to its geometric tolerance. Balls in smaller number have higher precision. Lower grades have fewer defects too, like cuts, soft spots, pits, and flats.
The balls are also available in different materials. There are steel ball bearings, stainless steel ball bearings, and also those made with plastic or ceramic. The most popular materials for these balls are
- Chrome Steel
- Carbon Steel
- Stainless Steel
- Si3N4 Silicon Nitride
- Delrin Plastic
Stainless Steel Ball Bearings
Stainless steel, which is sometimes also called Inox steel is a steel alloy with 10.5% chromium minimum. This is an extremely corrosion resistance material, which gets even better when molybdenum is added. So there are many stainless steel grades with different molybdenum and chromium content suitable for varying endurance needs. Stainless steel ball bearings are also low maintenance. They provide superior performance.
Stainless steel balls are used because they have higher chromium content, which makes them more resistant to surface corrosion. Chromium reacts with oxygen and a chromium oxide layer is formed on the surface.
The 440 series steel ball bearings are known for their antifriction properties, and corrosion resistance. So if the application is outdoor it is best to go for stainless steel balls – remember, they are resistant to rust. They are popular in applications where anti-corrosion, and hardness is required.
Hardness – Hardens to 58 rockwell "C" (58 HRC) minimum.
Density – .277 pounds per cubic inch ( 7.67 g/cm^3).
Magnetic – Strongly attracted.
Machinability – The material can be ground, honed, and lapped, or shaped and drilled with an electric discharge machining or ultrasonic cavitation.
Chrome Steel Ball Bearings
Many balls in bearings are also made with chrome steel like 52100 Steel and AISI52100. These balls are present in tools, drills, utensils, and bearings. Its chemical composition has 1.5% chromium and high carbon content. Chromium increases hardness penetration, and also the toughness and wear resistance of steel. There are also traces of sulphur, manganese, iron, silicon, and phosphorous, giving the balls their hardness and exceptional surface characteristics. Chromium on steel has the ability of resisting corrosion and staining. They are also resistant to deformations. However, chrome steel balls are less resistant to wear and abrasion than stainless steel.
It would be chromium steel if there is less than 12% of chromium in it. Above this, it would be stainless steel. Steel ball bearings made of chrome are popularly used in the making of mechanical components. BC Precision chrome steel balls used in different types of load-bearing applications like vibratory finishing, food grinding systems, linear motion components, plating baths, check valves, and others. You will find them used in drawer slides, spray can agitation, tumbling media, locking mechanisms, and writing instruments.
These balls have been made from the best quality chrome alloy electric furnace steel and have been hardened well so you get the maximum strength and long service life. Highest dimensional accuracy and surface finish is always achieved. Advanced heat-treating methods and controlled processing add high strength and anti-cracking abilities to these steel ball bearings. They become resistant to contact fatigue too.
Carbon Steel Ball Bearings
This is steel where the carbon content is 2.1% by weight. The American Iron and Steel Institute (AISI) says that in carbon steel, there should be no minimum cobalt, molybdenum, chromium, titanium, tungsten, nickel, niobium, zirconium or vanadium or anything else for attaining the alloy effect. Also, the copper content should be 0.40% or less. The maximum content of silicon shouldn’t be more than 0.60, and manganese should be 1.65 or less.
The balls made with carbon steel become stronger and harder when the carbon content is higher, and if they are heat treated. But they are less ductile. Higher carbon content also reduces weldability.
Carbon steel ball bearings are of two basic types – those made of low carbon alloy steel, and those that use medium carbon.
Medium Carbon Alloy – They are referred to as “commercial grade” or “semi-precision” bearings. It is a low cost variety with carbon content between 0.3 and 0.8%.
Low Carbon Alloy – There is usually a coating here for protecting the material from corrosion. The carbon content here is between 0.05 and 0.30%.
Carbon steel balls are used in less expensive anti-friction bearings, casters, trolleys and conveyors, steering columns, toys, drawer slides, and roller skates.
Si3N4 Silicon Nitride
This is a ceramic material, which provides a very smooth surface and hardness up to Rc78. It is niche and costs more than the stainless steel ball bearings. Ceramic bearing balls are either zirconium dioxide (ZrO2) or silicon nitride (Si3N4).
A chemical compound, silicon nitride is made of nitrogen and silicon. Si3N4 is more thermodynamically stable, and thus the balls made of this material are more popular. Si3N4 is also more economically relevant.
Bearings with balls of ceramics usually have races in steel. The use of silicon nitride makes them good at resisting shocks. They are also harder than metal, so there is reduced contact with the bearing track, which means there will be less friction, and higher speed. Service life of the bearing is also enhances by 3 to 10 times. Bearings with silicon nitride balls are useful where electric or magnetic fields and corrosion prevent the use of metals.
Delrin is a polyoxymethylene acetal plastic (POM). It is known for its tolerance, toughness, rigidity, and strength, which makes it a good material for the bearing balls. But traditionally, they are not as popular as steel ball bearings.
The material has excellent fatigue endurance as well, and thus the balls made with it are expected to last for a long time. There are other benefits too. POM, for instance, is resistant to moisture, neutral chemicals, and petroleum based solvents, and has good electrical insulating properties. Approved by the FDA, it has low water absorption. It can handle a working temperature up to 150 Deg C.
Delrin is useful where rigidity, hardness, and high strength are needed. The balls are usually pure white in color, but can also be orange, yellow, brown, blue, gray, and black, depending on the manufacturing. Material properties, though, remain the same, irrespective of the color.