Understanding seals: Pushing the boundaries of gearbox performance
January 2023 © 2023. Afton Chemical Corporation is a wholly owned subsidiary of NewMarket Corporation (NYSE:NEU). 01/23. The information in this bulletin is, to our best knowledge, sure and accurate, but all recommendations or suggestions are made without guarantee since the conditions of use are beyond our control. Afton Chemical Corporation and its affiliates disclaim any liability incurred in connection with the use of these data or suggestions. Furthermore, nothing contained herein shall be construed as a recommendation to use any product in conflict with existing patents covering any material or its use.
Understanding seals: Pushing the boundaries of gearbox performance
Healthy seals are critical for effective industrial gearbox function. Afton is leading the quest for a deeper understanding of the mechanisms behind seal failure modes, to enhance gearbox and lubricant performance and durability.
Afton is exploring elastomer degradation pathways that lead to NBR seal failure in the SEW dynamic seals test to identify formulations that can counter undesirable seal interactions and still meet all other OEM requirements –including friction capabilities for the Ortlinghaus clutch test.
Using bespoke testing to design universal gear lubricants, with seal longevity as a prime objective, directly contributes to greater productivity and lower costs.
Importance of industrial gears
Only a gearbox can deliver the range of input speeds, torques and directions of movement that different machines need for optimum efficiency and productivity. To transmit power from an energy source to machinery, gearboxes need four elements: gears, input/output shaft, roller bearings and seals. Applications vary from incinerators, crane hoists and crushers to steel forming equipment, conveyors and machine tools, not forgetting wind turbines and ship propellers.
Significance of seals
Keeping metal gears, shafts and bearings cool and protecting them from wear, heavy loads and corrosion requires a suitable lubricant. This lubricant also has an important role to play in looking after the seals, which are vital for keeping the lubricant inside the gearbox and preventing contaminants from getting in.
Compatibility with elastomer materials of shaft seals must therefore be a primary consideration when formulating lubricants. Poor chemical compatibility or leaching –whether caused by the base oil or additives – can result in swelling, shrinkage, hardening or other physical changes to the elastomer that will affect seal integrity and performance.
Seal elastomers compared
Most seals are made from nitrile butadiene rubber (NBR) or fluoroelastomer (FKM). Material choice depends on cost, gearbox operating temperatures, shaft speeds and direction of rotation, levels of external contamination and compatibility with any liquids or gases the seal may come into contact with.
NBR
A more economical material, NBR is used widely for the good balance it offers between wear resistance and natural chemical resistance to a range of petroleum oils, fuels and greases. These seals work well in operating temperatures from around -35oC to 110oC and are used often for larger industrial applications. Wetting the area between seal and shaft with lubricant is achieved relatively easily, helping protect the shaft from wear.
FKM
Priced at a premium to NBR, FKM seals offer exceptional resistance to most chemicals – although can struggle with fluids containing certain chemistries. Performing well at higher temperatures of up to 200oC, they can be used down to around -20oC, below which they start to harden and lose their flexibility. Popular for applications involving greater movement, high heat and/or chemical processing, FKM seals are less easily wetted so can result in greater shaft wear if not adequately protected by the lubricant.
© 2023. Afton Chemical Corporation is a wholly owned subsidiary of NewMarket Corporation (NYSE:NEU). 01/23. The information in this bulletin is, to our best knowledge, sure and accurate, but all recommendations or suggestions are made without guarantee since the conditions of use are beyond our control. Afton Chemical Corporation and its affiliates disclaim any liability incurred in connection with the use of these data or suggestions. Furthermore, nothing contained herein shall be construed as a recommendation to use any product in conflict with existing patents covering any material or its use.
Understanding seals: Pushing the boundaries of gearbox performance
Gearbox trends
The most significant gearbox trend in recent years is increased power density. Gearboxes must output more power without increasing in size, placing components under higher loads and stress. This increases wear potential and shortens lubricant lifetime.
Dynamic demands
Given the fundamental importance of gearbox seal integrity, OEM lubricant specifications include dynamic seal compatibility. OEM testing has undergone a revolution in complexity and challenge in order to keep pace with real world requirements.
Flender test:
Long considered by OEMs to be a core requirement, the Flender test is carried out using radial shaft seals set on a spinning shaft that is half-submerged in lubricant at a fixed temperature, for a specified time.
To meet growing productivity demands, gearboxes are operated for longer periods, even continuously. Having little or no downtime increases the thermal stress on both gearbox and lubricant while the financial penalties for a breakdown climb ever higher.
Gearboxes are running harder and faster than ever, on thinner fluids, yet must remain reliable. Lubricants that can deliver the right balance of performance are essential.
After the test the seal is visually examined, the shaft wear tracks measured and any lubricant leakage documented. For NBR seals, the test is shorter and done at a lower temperature. Flender is more challenging for FKM seals, likely due to wetting issues; if the oil does not reach the shaft surface where it meets the seal, wear becomes a problem.
SEW test:
As OEMs pay particular attention to equipment lifespan and lubricant performance, more stringent testing has appeared – most notably from SEW – that closely reflects conditions experienced by gearboxes in the field. SEW specialises in gearboxes for applications such as materials handling, roller shutter doors and airport conveyor systems. These must deliver rapid start-stop motion, both forwards and in reverse.
Reflecting these different conditions, the SEW dynamic seals test lasts twice as long as the Flender method for FKM seals, and nearly 3 times as long for NBR seals. Shaft rotation includes forward and backward motion at constantly changing speeds. These conditions are particularly challenging for NBR seals.
Passing dynamic seal compatibility tests is a top priority: should a seal fail, the volume of lubricant (and the protection it affords) can fall below ideal levels. Oil will also leak from the gearbox and into the local environment, which is highly undesirable. When lubricants can pass this stringent seals test, any failures that do still occur are more likely due to manufacturing issues or poor set up.
© 2023. Afton Chemical Corporation is a wholly owned subsidiary of NewMarket Corporation (NYSE:NEU). 01/23. The information in this bulletin is, to our best knowledge, sure and accurate, but all recommendations or suggestions are made without guarantee since the conditions of use are beyond our control. Afton Chemical Corporation and its affiliates disclaim any liability incurred in connection with the use of these data or suggestions. Furthermore, nothing contained herein shall be construed as a recommendation to use any product in conflict with existing patents covering any material or its use.
Understanding seals: Pushing the boundaries of gearbox performance
Deepening understanding
When the SEW test was first introduced, lubricants proved relatively capable with FKM seals but not NBR seals.
Afton has therefore undertaken extensive research to explore more deeply what happens to seals under these conditions. Lengthy rig tests have been carried out using seals from different OEMs such as Freudenburg and Trelleborg, revealing that there can be more than one pathway to seal degradation.
Understanding the different processes involved in these pathways is essential for developing successful formulation strategies and enabling lubricants to achieve multiple OEM approvals. Current research work includes developing better predictive screener tests for NBR seal longevity, as well as solving FKM wetting issues.
Ortlinghaus Clutch test:
Although not a dynamic seal test, a further test worthy of mention here is Ortlinghaus – a wet brake test used primarily for marine applications. Many industrial gear lubricants are used in marine applications, making it a desirable – if challenging – test to pass.
The relevance of and particular challenge for Ortlinghaus is being able to achieve required friction capability without negatively impacting dynamic seal compatibility. With it being a rather costly test with limited capacity and a long leadtime, Afton has developed a useful screener test with excellent predictive value, which can be run prior to investing in the full Ortlinghaus test.
Overcoming future challenges
Gearboxes are highly complex systems: multiple interactions occur between different components in parallel, some synergistic and some antagonistic; some beneficial and some harmful.
Seal testing will become ever more demanding as OEMs maintain focus on this area. Afton’s deeper understanding of seal failure is informing the development of additive solutions that build upon its leading expertise in gear and bearing protection.
Looking for the seal of approval? Ask your local Afton representative for more details.
© 2023. Afton Chemical Corporation is a wholly owned subsidiary of NewMarket Corporation (NYSE:NEU). 01/23. The information in this bulletin is, to our best knowledge, sure and accurate, but all recommendations or suggestions are made without guarantee since the conditions of use are beyond our control. Afton Chemical Corporation and its affiliates disclaim any liability incurred in connection with the use of these data or suggestions. Furthermore, nothing contained herein shall be construed as a recommendation to use any product in conflict with existing patents covering any material or its use.
