A guide to ball bearing materials
01 March 2018
Ball bearings can be used in a range of applications, from medical and packaging technology to electronics and office technology.
Since these components are available in different types of materials, each with their own set of unique characteristics, weighing the pros and cons of a specific kind of ball bearing can become an important part of the design process.
In this article Robert Dumayne, dry-tech director at igus, surveys the most commonly used materials - steel, ceramic and plastic.
Steel ball bearings
Partly because they are an older technology, steel ball bearings are a trusted solution for many design engineers. Typically, these types of bearings are comprised of all-steel parts, but are available with different types of steel races and balls, or with a resin cage.
Steel ball bearings are ideal for robust applications handling extremely high loads and fast revolutions per minute (RPMs), and some feature a radial load capacity of up to 133446.65 Newtons or 13607 kgf. Another advantage of steel ball bearings is that they tend to be very precise due to the clearance that can be achieved during the manufacturing process.
There are also quite a few disadvantages to steel ball bearings. They are heavy and noisy, and, depending on the grade of steel, they lack chemical resistance. They require constant lubrication, which means time and money spent on maintenance. Steel is susceptible to corrosion in humid or wet environments. Due to the need for lubrication, most steel ball bearings must also be sealed to eliminate dirt and debris from getting in and lubricant escaping. Failure to seal them adequately can lead to bearing failure. Furthermore, in medical applications, their magnetic properties can cause problems.
Ceramic ball bearings
The most common type of ceramic ball bearing is often considered a ‘hybrid’, which indicates that the outer race, inner race, and cage of the bearing are comprised of steel, while the balls are made from ceramic. The ceramic material enables the bearing to run faster while maintaining a cooler operating temperature and simultaneously reducing noise, vibration and wear. Since they are a hybrid design and still contain steel races, hybrid ceramic ball bearings are as equally susceptible to corrosion as steel ball bearings.
Fully ceramic ball bearings tend to be more resistant to corrosion, and more rigid and lighter in weight than most steel ball bearings. Additionally, fully ceramic ball bearings are non-magnetic, which is useful for applications where this might be critical, for instance, in MRI equipment. Lower coefficients of friction and higher RPMs are also possible and, since they are non-conductive, ceramic ball bearings can be used in electrical applications. In addition, most ceramic balls bearings can operate in temperatures up to 1,000°C.
With these advantages, ceramic ball bearings are an attractive solution. However, they are very expensive, which is a strong argument in itself for seeking out an alternative solution when extremely high speeds and high temperatures are not needed.
Plastic ball bearings
While plastic ball bearings are a newer technology, they have advantages that are not offered by their steel or ceramic counterparts. Plastic ball bearings are comprised of all-plastic races and a plastic cage, and are typically available with a choice of three different types of balls: plastic, glass or stainless steel.
The most common ball material within a ball bearing is stainless steel. Stainless steel balls are the most cost effective choice, but they are heavier than both the plastic and glass alternatives. They are also magnetic, which can be a detriment to some applications. Glass balls are ideal when a metal-free solution is needed; they are also chemical resistant and weigh less than steel balls. Plastic balls are another ideal option. They weigh less than both the steel and glass balls and offer excellent wear resistance, while still being resistant to a wide variety of chemicals.
Whatever configuration you choose, plastic ball bearings are ideal for applications with normal to high speeds and have a number of additional attractive features. Due to their plastic construction, they are self-lubricating, corrosion resistant and operate both smoothly and quietly.
Still, there are instances where plastic ball bearings should not be used, including applications with high loads or long-term temperatures exceeding 150°C.
Every manufacturer offers its own versions of ball bearings with varying features, all of which have an impact on bearing performance and life. Manufacturers of steel ball bearings, for example, often offer their products in a number of different types of steel, including a 300 series (soft stainless), a 400 series or chrome steel. In the same way igus offers its xiros ball bearings in nine different thermoplastic race materials – each designed to operate under specific application and environmental conditions. By considering these conditions early on in the design process, eases ball bearing material selection and enables engineers to obtain the best possible results.
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