Selecting a lubricant that matches the seal

All motion machinery incorporates seals to confine the oil or grease to a given volume while allowing input and output shafts to protrude from isolated spaces. Within this subsystem, the interactions between lubricant and seal material significantly impact overall application success; a poor match can cause leaks, failure, and costly downtime, not to mention excessive warranty claims. Complicating matters is the fact that a compatible material choice is essential when applying today's increasingly complex lubricants that are often synthetic, formulated with additives, and tailored to specific application requirements. There's no ready-made solution to this challenge, but seeking the optimal sealing and lubricant match dramatically reduces failure risks.

Consider, for example, that when engineers choose to change the lubricant for a gear drive the focus is typically on minimizing friction and wear in the gears and bearings. Often, engineers fail to consider how such changes may affect the seal.

Lubricant serves two critical functions in sealing systems — facilitating a proper seal-shaft interface while transferring heat away from the area. However, many modern gear lubricants formulated to improve life and resist high-temperature breakdown complicate tribological interactions. In fact, approximately 40% of long-term gearbox performance failures can be traced back to poor interaction between the application's seal and lubricant.

Functions and incompatibility

Modern lubricants are complex fluid technologies for which additives and base oils are selected to meet or exceed

industry standard or OEM-specific performance requirements. In general, their function is to:

Lubricate to reduce friction and wear — by introducing a thin film between moving parts
Cool by dissipating heat away from critical equipment parts
Clean and suspend by removing and suspending by-products such as carbon, sludge, and varnish
Protect metal parts from damage due to oxidation and corrosion

Base oils often lack the key performance attributes required for proper function; therefore, additives are incorporated to deliver the missing performance characteristics. Though the base lubricants can degrade seals, it's usually the additives that more severely affect elastomers. These additives, which are more or less reactive components, can attack the seal material at all oil temperatures.
Detergents are generally seal friendly and keep acids minimized.
Dispersants are tough on certain materials (such as fluoroelastomers), but can keep oil from “gunking” the seal because they help dissipate substances (such as ash) out of the oil.
Anti-wear agents generally bond to — and protect — metal parts, but can degrade certain seal materials.
Extreme pressure (EP) agents, some corrosion inhibitors, and friction modifiers offer unique benefits to specialty applications, but can restart the cross-link process in seal materials — causing changes to the elastomers.
Viscosity-index enhancers stabilize oil viscosity over a wider temperature range, but can cause unpredictable seal interactions.

Modes of attack

As mentioned, the chemical makeup of advanced lubricants significantly impacts — and sometimes impairs — sealing ability.

Radial shaft seals can be chemically attacked (especially at the sealing edge), causing blister formation, filler erosion, and polymer breakdown or regression.
Lubricant additives sometimes deposit near the sealing lip and develop into hardened accumulations that result in excessive seal lip wear from even the slightest axial movements.
Carbon can deposit on the sealing edge of the radial shaft seal due to thermal overloading of the lubricant — caused by high circumferential shaft speeds, insufficient heat dissipation, poor lubrication of the sealing edge, or use of the incorrect lubricant or seal. These deposits can cause tears in the sealing edge and significantly alter the seal — or cause blistering during operation to create holes.
Radial shaft seals can wear rapidly despite an adequate lubricant supply. This may occur with synthetic lubricants based on polyalphaolefin or polyglycol. These base oils are designed to provide lubrication at higher loads and higher temperatures — but the chemical makeup that allows such performance conflicts with the overall lubricity as it relates to the seal.

Change workload — change seals

Consider that the typical warranty period for a transmission with limited operating hours is approximately two years, which equates to a general service life of the standard radial shaft seal of about 3,000 to 5,000 hours. However, over the last decade, operational efficiency has increased in numerous applications from single-shift operations to double and even triple-shift operations. Consequently, the number of hours per year required by some transmissions has effectively increased from 2,400 to 7,200 hours.

All of this has greatly impacted the useable replacement cycle of a shaft seal, dropping from as lengthy as 2.5 years (depending on requirements) to less than one year. The root of the problem is that when machinery is put to harder use, the lubricant is sometimes upgraded, while seals are not. However, in a dynamic seal, as friction increases, power loss and temperature also increase — and in turn, life is shortened. Thus, the use of seal material and lubricant must be coordinated.

More specifically, friction produced by shaft rotation (and the seal riding on the shaft) drives the temperature at the sealing edge higher than that of the oil bath; failure to address this heat difference accelerates degradation of the seal elastomer and oil. This problem is on the rise in applications with increased shaft speeds, more compact units, and the trend towards multi-shift or 24/7 operations.

Early in design: Test and analysis

As lubricants continue to evolve, customized designs based on specific parameters will become more critical to the proper function of sealing systems. Therefore, it's recommended that engineers involve both seal and lubricant manufacturers during initial design development. Lab tests supply only some of the information needed to determine a seal's service life; in-depth investigation into actual system dynamics more accurately demonstrates the effects of matching oil with an elastomer.

To this end, it is essential that manufacturers use a battery of tests and chemical analyses to provide well-founded service life statements.

These tests are intended to demonstrate the service life of a solution as compared to the current best practice; transparently show friction and wear; extend the service life in a reproducible way; minimize the error rate over the service life; and minimize early failures.

This type of extensive testing can solidify the vital importance of proper seal and lubricant interaction, as well as the effect an application's parameters can have on this relationship. As lubricants continue to evolve, customized solutions based on specific parameters will become more critical to the proper function of a sealing system.

Simrit and Klüber of the Freudenberg Group collaborate in the Lube & Seal project — a research initiative and analysis service that aims to better define the chemical and physical interactions of seals and lubricants, and improve tribological solutions. For more information, visit www.simrit.pl/web/public/solutions/lubeandseal.

Case in point: Trying transmission

Small and medium high-end industrial transmissions from one manufacturer were failing after 5,000 to 8,000 hours, though the goal was to have the transmissions reach 20,000 hours without issue. It was determined that the failures resulted from poor performance by the system's polyglycol oil, which was causing high seal wear, deep grooves in the shaft, and (in some cases) depolymerization of the elastomer.

The new tribological system has been in production seven years, with zero claims. Friction-optimized radial shaft seals made of chemically formulated and wear resistant 75 FKM 170055 are used with Klüber PETAMO GHY 133N grease between the seal's dust lips, and the fluid is now Klübersynth GH 6-220 polyglycol oil.

Case in point: Extreme drilling

The seals on one power-dense gearbox — within a drilling and diamond saw machine — were failing prematurely. Target seal life in this equipment is 300 hours, but these seals were failing in less than 50.

Analysis showed extreme shaft speed of approximately 20,000 rpm, poor heating transmission via the shaft, pressure of about 0.3 bar, 160° C oil bath temperatures, and critical interactions between the seal material and lubricant.

In addition to premature failure, the seals and shaft were demonstrating excessive wear and rubber depolymerization. Now, a new peroxide cure material (75 FKM 260507) in combination with a friction-optimized seal and a PAO oil Klübersynth GEM 4-32N helps the design reach 300 hours without issue.