Winning the fight against corrosion
24 February 2021
IT'S A tough life for industrial bearings, and one of the most challenging applications bearings will encounter is wet environments, where their core purpose of reducing friction and keeping essential machine parts moving also includes a constant battle against corrosion
Bearings have to carry heavy loads at high speeds and work for a long time in a large variety of applications, temperature ranges and environments. The smooth surface of a rolling element bearing is key to its performance, and little can ruin that surface faster than corrosion.
Corrosion is a chemical and/or electrochemical reaction: Most commonly between a metal and something in its environment. The most common cause of corrosion in many engineering applications is the presence of water, and bearings operating in wet or humid environments present one of the most difficult maintenance challenges.
Keeping bearings lubricated with a suitable grease can decrease the occurrence or rate of corrosion: In systems where corrosion is critical, this can extend the service life. In greases, the base oil, thickener, corrosion inhibitor additives and even some physical properties (e.g. tackiness) can all play a role in reducing corrosion risk and improving moisture control.
Submerged or semi-submerged bearings – in the offshore industry, for example - operate in some of the harshest and most inhospitable environments. They must operate in salt water, with many other challenging stresses from the environment such as extreme temperatures and high load applications.
Complicating matters further is the fact that many bearings in these environments are very hard to access. This makes close inspection and maintenance time consuming and difficult.
Longer lubrication intervals are a fact of life in such environments. Selecting high-performance lubricants that can extend the time between application as far as possible is key to maximising bearing life and performance.
Avoiding water washout
Few environments are harder on mechanical components than food and beverage production facilities. Wet, humid conditions with regular deep cleaning routines constantly challenge maintenance engineers to keep equipment corrosion-free. Bearings never get a break from repeated exposure to corrosive elements and high-pressure water jets.
Intensive - often daily - cleaning regimes, particularly in meat processing facilities, can lead to complete grease washout. Left unchecked, this can cause premature wear, corrosion and ultimately, bearing failure. At best, this significantly reduces the lifespan of the machinery. At worst, it results in production line downtime and lost revenue.
Many food manufacturers are operating around-the-clock schedules, leaving little time for unscheduled maintenance. It is important to help bearings perform as required and to withstand the rigours of regular cleaning regimes.
Look carefully at the bearing greases used as part of maintenance programmes. Cheaper brands may not be the cost saving solution they appear to be . If they wash out under water pressure more quickly, the associated maintenance costs can increase.
Do your lubricants make the grade?
The quality of the lubrication products used on bearings has an impact on overall equipment lifespan. Look for products with high performance in international standard tests.
The ASTM B 117 Salt Spray test is one of the longest established corrosion tests. A sample is placed on steel panels, placed inside a fog of 5% salt solution. The sample is checked for signs of corrosion at regular intervals. The longer the sample withstands those signs of corrosion the better the grease is at protecting the components it is used in against corrosion.
The ASTM D 6138 EMCOR test – another well-established testing method - considers the corrosion of a dynamic bearing in a wet environment. Bearings are loaded with grease and mounted in plummer blocks with 10ml of water added to each side. The machine runs for eight hours followed by 16 hours’ standing time. This is repeated three times before the bearings are left to stand for four days. The test runs for a week before the bearing is removed from the machine and disassembled. The interior race is then examined for corrosion and given a score: 0 equals no corrosion and 5 notes severe corrosion levels.
Another rating to look out for is the ASTM D 1264 Water Washout Test. During this test, a 38°C or 79°C jet of water is aimed at a shielded 6204 bearing filled with test grease for an hour. The percentage weight of grease washed out of the bearing is calculated. In this test, the lower the weight the better. It means less grease was washed out during testing and therefore has remained in the bearing, where it is needed to lubricate and protect the bearing.
When selecting bearing greases for use in wet environments, look for products that carry these certifications, and manufacturers that offer these high levels of performance testing as standard. This will give confidence that the lubricants used are of the highest quality and will outperform inferior products in challenging applications.
Another factor to be aware of is the need for food safe products in some applications and production environments.
When dealing with waste water, a standard industrial grease will do the job. However, in the food or clean industries, a bearing grease that has been registered in the most relevant category with NSF International (e.g. H1 for lubricants with incidental food contact) is required. This precautionary measure mitigates against grease dripping from the bearing onto the production line. Look out for greases with NSF registrations and ISO 21469 quality standard marks for peace of mind on food safety.
When are dealing with drinking water, another level of safety assurance is required in the form of Water Regulations Advisory Scheme (WRAS) certification. A WRAS certificate demonstrates that the grease complies with the high standards set out by the water industry regulators.
Check it out
Regular maintenance checks can help spot early warning signs of corrosion and provide opportunities to act before bearing performance is compromised. Left unchecked, operators could be left with an expensive repair and unplanned downtime. Making regular corrosion checks part of ongoing maintenance regimes is time well spent.
Bearings in industrial machinery where corrosion is a risk during operation, be it from frequent washdown, humidity or water ingress, require specific maintenance to ensure continued high performance and maximum lifespan. Without regular proactive maintenance, operators face the bigger cost of bearing failure and downtime through unplanned maintenance – a cost many businesses simply cannot afford.