Lubricant selection: Bearings, gear drives and hydraulics

Tribology & Lubrication Technology, Apr 2008 by Johnson, Mike

Make sure you're using the right product by following a systemactic approach based on objective, repeatable and widely recognized engineering practices and principles.

Lubricant selection is a pivotal starting point in the pursuit of precision lubrication practices. All the effort applied to clean delivery and handling, filtration, dehydration, alignment, balancing, etc., is lost if the lubricant selected for the application cannot support the machine's demands. Many criteria must be considered when selecting a lubricant for a set of machines or machine components.

Lubricant chemistry has an influence on the final decision. The basestock (synthetic or mineral) and the additive systems in use (EP, AW, R&O) exert tremendous influence on the performance qualities of the lubricant. It is common to have multiple choices for a given grade and type of lubricant within a product line (i.e., ISO 220EP/SAE 9OEP gear oil), with each example designed to perform effectively for a set of given conditions.

The machines themselves have an influence on final lubricant selections. OEMs design and build machines for general types of service. A gear drive manufacturer could not possibly consider each and every type of application for a given make and model when it begins the design process. Designers can build sufficient durability into the basic design that allows customers to use those products for similar uses within widely varying production environments. The nature of the production environment (wet, dry, hot, cold, abrasive dirt, harsh chemical exposure, steady state or intermittent operation, etc.) influences the degree of effectiveness for a given lubricant type and grade.

Plant maintenance strategy has an influence on lubricant selection as well. Where management is particularly forward thinking and willing to invest in modifications that improve lubricant management effectiveness (filter connections, continuous filtration, embedded sample ports, bearing isolators, etc.), the company is positioned to maximize the superior value that can be achieved through the use of high-performance lubricants, both mineral- and synthetic oil-based.

With the variety of factors that can impact lubricant film formation and effectiveness, it is to the practitioner's benefit to follow a lubricant selection process that is objective, repeatable and based on widely recognized engineering practices and principles. This article addresses a formal lubricant selection process that could be used to make technically accurate baseline lubricant selection decisions without using exacting mathematical expressions.

Parsing the machine

In the January TLT 1 proposed cataloging all lubricated components within a machine for evaluation and specification of a lubricant. The purpose of the process is to consolidate the collection of machine components into a concise set of types and catalog which components rotate, slide, pivot or have other dynamically interacting surfaces. There are relatively few unique types of components, but there are many permutations of each of the few unique options. Let's address these components by general type that represent most industrial machinery, including:

* Plain bearings

* Element bearings

* Gears

* Hydraulic systems

Plain bearing lubricant selection

Probably the most common manifestation of a plain bearing is a round steel journal riding on a conforming one- or two-part brass or babbit bushing. There are plenty other machine component types that could be categorized similarly, including machine tool gibbs (slideways), brass bushing, pivot pins, ball screws, worm gears, etc. These bushings are similar to plain bearings in composition, form (shape), and film characteristics.

Regardless of shape and form, all plain bearing components have a common requirement: a full-fluid hydrodynamic film to sustain component life cycles. Two types of decisions must be made:

1. Viscosity grade according to the machine's operating profile.

2. Lubricant chemistry/additive type (R&O, EP, AW, Compounded, Tackified).

The viscosity grade considerations for sliding surface interaction are rooted in a common set of physical realities. Total available surface area, linear surface speed, surface unit loading, lubricant viscosity grade and lubricant replacement rate all have an influence on the formation of a hydrodynamic oil film.

STLE member Bob Scott, manager of Lube-Works, Ltd., in Calgary, offers a method of lubricant selection for plain bearing applications. According to Scott, the minimum information required for the determination of the proper ISO grade for journal bearings includes:

1. Shaft RPM.

2. Temperature of the oil in the bearing.

3. Approximate unit loading pressure (PSI or Newtons /m^sup 2^).

Scott adds that it would be best to also have additional information, including:

* Bearing length, number of bearings and rotor weight (to calculate the bearing pressure to verify the load).

* Shaft diameter to calculate the shaft surface speed.

* Driving horsepower.