Stainless Steel Bearings: An Introduction
Stainless steel was a huge boon for countless industries and applications throughout its development during the late 1800s. Today an incredible array of specific stainless steel grades are available, not to mention stainless steel bearings – each with its benefits. The medical and food industries gained massive opportunities from this material, inevitably saving lives.
As the techniques and processes for creating stainless steels became more advanced, other industries and products were able to benefit as well. Highly specific grades were even developed for use in the nuclear industry or the vacuum of space. While basic bearing steel has had some significant developments during its evolution, stainless steel bearings changed the fate of many applications subject to corrosion.
Stainless steel is a steel alloy with a minimum of 11% chromium by mass. The additional contents of the steel dictate the grade of stainless. Increased carbon content, for example, provides increased strength. Increased molybdenum provides more corrosion resistance. Even minor changes to the composition affect the characteristics of each steel grade.
The power transmission and bearing industries utilize a relatively narrow range of conventional stainless steels but fill a critical role. To best determine what grade will suit an application, it’s helpful to know a little more about the material itself.
What are the common stainless steels?
There are four main families of stainless steel. For our purposes, we’ll limit our discussion to just two – Austenitic and Martensitic – the two commonly found throughout stainless steel bearings. Few of these steels are load-bearing. The ones appropriate for load-bearing saw precise development over the years and contain specific material compositions, striking a sensitive balance between strength and corrosion resistance.
Austenitic stainless steel is the most common steel of the group due to its high resistance to corrosion. It contains varying amounts of nickel, manganese, molybdenum, and chromium, depending on the objective of the alloy. Austenitic stainless steel, though easily formed, does not harden as most steels will. The most common grades in this family are 304 and 316 (or marine-grade) stainless steel.
Martensitic stainless steel has increased amounts of carbon, compared to the other families. Carbon lends these steels increased strength, but at the same time, lowers their resistance to corrosion. Martensitic stainless steel can also be capably hardened and machined to exact tolerances. The most common grade in this family is the 400 series.
Where is the Stainless Steel Used?
Each family of stainless will have a varying number of individual grades within it. When it comes to applying these steels to bearings – or any application – each family and specific grade will have its own strengths. Each component of a bearing has specific duties and requirements which cannot all be accomplished by the same material, though specific grades can be optimized to increase performance. The following is a good example of this.
Austenitic stainless steel
The 300-series Austenitic stainless steels are for non-load bearing parts. Grade 304 for housings and 302 for the bearing flinger. Grade 304 is highly resistant to breaking (high ductility) and has very high corrosion resistance. 302 has slightly more carbon – increasing hardness – making it a more appropriate fit for a component within the bearing itself. Manufacturing an entire bearing with 300-series stainless steel would give it about half the load capacity of a bearing made with 52100 bearing steel.
Some manufacturers make housings using 316 stainless, due to its hybrid attributes of having increased corrosion resistance and higher tensile strength. This grade is very prevalent in marine environments and applications that implement caustic agents, such as brines, acids, liquors, and other harmful solutions.
Martensitic stainless steel
Martensitic stainless steels are for rolling elements and bearing rings. This is one of the few stainless families that can be successfully hardened, giving it about 80% of the load capacity of 52100 bearing steel. 440C then, which is of the Martensitic steels, makes for ideal steel to be used for load-bearing. 440C doesn’t have quite the corrosion-resistance that other stainless steels do, but it’s important to remember that it doesn’t have quite the same exposure as the housing does. Proper lubrication and sealing will aid the rolling elements in combating corrosion.
With all these grades of stainless unique in their own right, is there a comprehensive solution?
Which Stainless Steel is Best for Bearings?
As you may have gathered from the information above – there is no single stainless steel that is appropriate for use throughout an entire mounted ball bearing. The housing, bearing rings, rolling elements, and flinger all have different requirements. Mounted bearing housings, for example, require more ductility than you would ever want to see in a rolling element.
Using the same material throughout an entire mounted bearing creates a definite failure at some point during operation. This is not to say that certain stainless steels aren’t good enough or shouldn’t be used. During the design of a bearing, engineers choose the best material possible for a given purpose. Price points and availability of the steels is also an important factor.
Some stainless steels have nearly the strength that bearing steel does but are also prohibitively expensive. Brands, manufacturers, and the market have decided which steel grades are the best fit. Unfortunately, there is not a “one-size-fits-all” option for stainless steel in this application. Like any good team, all the players are chosen for their strengths, bringing their best to the table.
Innovation continues to develop new stainless steels in addition to the many variants developed since its discovery. Sustainable recycling lends longevity to the industry and special coatings for certain grades are improving them even further.
The best stainless steel bearing will utilize multiple grades.
When choosing a stainless steel mounted bearing supplier, we recommend you research the grades used throughout the construction of the bearing. You want to feel confident in your supplier and in the parts you’ve chosen for your application.