WHY BURNISHING IMPROVES PRODUCT LIFECYCLES!

In manufacturing, we focus on a part’s dimensional accuracy and surface finish, but do we really understand what effect the finish has on the performance and, in turn, the life of the part?

Surface finish is a function of the manufacturing process, whether it be turning, milling, grinding, or honing.  However, one of the most competent and under-utilized processes for achieving consistent surface finish is the use of roller burnishing tools.

Most part drawings will state surface finish requirements in Ra or Rz values, which are one-dimensional averages that do not provide information on the nature of the surface texture or profile and its bearing capacity and abrasion resistance.Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.

A typical chip-removal process produces peaks and valleys like a sawtooth profile on the machined surface, with the peaks being worn away by a mating surface when in service. However, a roller burnished surface will have the peaks leveled out, producing a surface that has a much higher bearing capacity and abrasion resistance.

The problem now becomes:  how do you clearly differentiate these two surface profiles when a comparative Ra or Rz measurement often shows minimal differences?

The answer lies in the Bearing Area Curve (BAC) – originally known as the “Abbott-Firestone Curve” – which describes the surface texture of a part.

There are many papers explaining the theories and application of the BAC, so we will not delve into that here.  Suffice it to say, the resulting measurements allow for a clearer picture to be established of the surface profile that has been produced. From this, it is possible to determine which characteristics are needed to improve the life of the part and the limits to be stated on the part drawing.

So how can using the BAC make your parts last longer?

Common applications where a seal moves within bores (like a piston or plunger), or along shafts (like pneumatic and hydraulic cylinders) is a good place to start.

In these types of applications, the surface finish is typically specified as an Ra or Rz figure.  This means that the peaks of the surface finish are considered acceptable, even though they are effectively sharp edges that will damage the seal and lead to early failure.

However, by specifying a surface texture based on the BAC and using a burnishing tool in the processing of the part, the life of the parts is now extended since we have increased the bearing area by flattening the peaks and removing the sharp edges.

Seal wear is dependent upon the manufacturing method used and the resulting surface texture generated. The higher the Ra value, the shorter the seal life.  On the other hand, the lower the Ra value, the higher the cost to achieve and maintain this finish.  Conversely, a roller burnished surface will result in minimal seal wear and manufacturing costs will be reduced by eliminating costly secondary processes such as grinding or honing.

The other benefit of a larger bearing area is the reduction of noise. Rotating parts rubbing against each other generate noise.  By increasing the bearing area, and the resulting smoother surface, you reduce surface abrasion and the noise. An example would be in the manufacturing of electric motors, where the burnishing of moving parts has led to great reductions in noise levels.

Another example would be a bearing running on a shaft. The peaks of a ground diameter will wear down as the two surfaces make contact, resulting in increased clearance and potential failure of the bearing. Yet, if the surfaces are burnished, the peaks are already removed, a larger surface area is created over which the bearing load is dispersed, and the bearing life is increased.

The other benefit of a larger bearing area is the reduction of noise.
 Rotating parts rubbing against each other generate noise.  By increasing the bearing area, and the resulting smoother surface, you reduce surface abrasion and the noise. An example would be in the manufacturing of electric motors, where the burnishing of moving parts has led to great reductions in noise levels.

In conclusion, the type of surface finish required depends upon the part application.  But, when wear or seal life is a consideration, the use of the Bearing Area Curve (BAC) alongside the roller burnishing process brings major benefits in improving the life cycle of the product.

At Monaghan Tooling Group, we offer the advice and support you need to select and implement the correct roller burnishing process for your application.  Please contact us at www.monaghantooling.com or 
sales@monaghantooling.com to discuss your application.