Bearing in mind that 50% of all bearing failures are due to incorrect lubrication or contamination, SKF highly recommends that best practices, or at the very least, standard practices be followed when it comes to bearing fitment, removal and alignment methods. This will contribute to massive time and cost savings through extended machine life cycle, optimised plant availability, improved uptime and productivity. And there are environmental benefits too.
The cost of lubrication products (oil or grease) is regrettably often disregarded as they are categorised as consumable products together with cleaning equipment and fluids. Even a small plant can consume as much as 80 tons of grease per annum. On bigger plants lubrication spend can easily run into the millions and if correct lubrication methods are not followed, wastage and related costs remain undetected. So there is enormous potential for savings here by simply applying best practices and controlling grease consumption. Smart lubrication procurement and utilisation can significantly reduce costs; in some cases as much as 50%!
There is an incorrect assumption that a bearing must be filled with grease until clean grease is pushed out the other side of the unit. In other words a bearing must be full of grease to be properly lubricated. Unfortunately this method drives up costs in two ways – unnecessary grease wastage and seal damage.
Another fallacy is that grease is grease. It is commonly believed that EP2 grease is suitable for every application, from door hinges to fan applications. Many operators reason that as long as a bearing or unit has grease in it, it is lubricated and will not fail and that universal grease will do. This is most certainly not the case.
It is important to determine how grease actually works and to understand terms such as ‘Thickener’, ‘Base Oil Type’, etc. In a simple test placing grease on paper, a film will start appearing around the grease. This is the base oil ‘leaking’ out of the thickener. If that grease is warmed up, the rate of ‘leaking’ would increase as the thickener loses its retention properties. Thus, as the heat increases, the faster the oil will leak. It is also important to remember that it is the oil and not the thickener that lubricates the bearing.
So logic dictates that the oil will ‘leak’ from the thickener at a much higher rate in a bearing designed to run at a high temperature. Eventually the bearing will fail because there will no longer be any oil to lubricate the unit. So SKF has designed a grease that will ‘leak’ at a slower rate under higher temperature, making sure that the oil leaks enough to lubricate but not so much so that it runs out or separates completely from the thickener.
All grease is designed to perform to a specific set of parameters and conditions. All greases may overlap in certain areas when it comes to operational temperatures, but it is important to keep within those parameters. So selecting the correct grease to suit a particular application is essential. To this end all the operating conditions that the bearing or unit is expected to perform in must be known in order to recommend the correct grease. It is not advisable to merely recommend a grease based on what a customer is currently using as they could be using an incorrect product. Greases are a mixture of chemicals that could react very differently to each other if mixed meaning that different greases may be incompatible. It is thus important to know which chemicals can be mixed safely and which cannot. We therefore recommend that customers and end-users consult a SKF expert.
Thickener, as the name implies, adds density to the grease. Some greases are thick and tacky whilst others are almost oil-like in consistency. All grease conforms to the international grading standard, NLGI (National Lubrication Grease Institute) consistency class, starting at 000 (thinnest), up 6 (thickest). Once the correct grease has been identified, the next consideration is how much grease should be in a bearing. As already mentioned, a common practice is purging all the old grease out and ensuring that there is only clean grease in the bearing. Excessive grease can have a massive negative effect on the life span of the bearing.
By simply looking at how a bearing works makes it is easy to see why purging is wrong. When a bearing rotates, the rolling elements also rotate. If the bearing is filled to capacity with grease the rolling elements lose their rotation ability and they begin to slide, causing friction and subsequent heat. This leads to wear and ultimately failure in a shorter time period. It is possible to do a calculation for every bearing in every operation to ensure the correct amount of grease is applied. This is known as the initial fill and most engineers can calculate this quantity.
Once the bearing is in operation the grease will need to be topped up; once again this can be calculated based on the operating specifications. However, if a specific bearing requires 4 grams of grease every second day, for example, how is it possible to know that the correct quantity is being applied? Most lubrication operations are carried out manually using grease guns, bringing in the human element. The stroke of the grease pumps’ actuating lever differs from person to person. Moreover, pump parts wear, no longer delivering the same amount of grease. This scenario also applies to pneumatically operated grease guns. Each operation of the air pump should deliver a certain amount of grease and therefore by simply counting the number of pumps of the air gun should determine the exact amount of grease that has been delivered. However, lubrication audits often determine that lubrication points have been under- or over-lubricated using pneumatically-operated units.
One solution is the use of grease meters on all the lubrication equipment. Lubrication points should also be clearly marked with the grease type, amount and frequency of lubrication.
Contamination, which is defined as “the action or state of making or being made impure by polluting or poisoning,” will cause a machine to fail if it remains unchecked. There are a number of ways that lubrication can become contaminated including cross-contamination of different types of grease, dust contamination and the ingress of water or other forms of liquid. This is where sealing systems play a critical role. However, even the ‘best’ sealing system will fail if it is being filled with contaminated grease or oil. When a bearing is filled with grease, the actual lubrication is being carried out by the base oil being ‘leaked’ into the space between the surfaces of the bearing. The lubricating film between the contact surfaces being lubricated, when the unit is at full operating temperature, is between 0,1 and 1 micron thick. A grain of dirt 10 microns in size, which cannot be seen with the naked eye, will push the lubricating film away with ease.
Incorrect fitment methods, over-or under-lubrication and contamination – these three factors relate to 66% of all bearing failures. Consider two identical bearings manufactured to the exact same specification, in the same factory with one critical difference – one will produce 1,000 hours of service. The other will provide 10,000 hours service owing to best practices in terms of fitment, removal and alignment methods, using the correct lubrication (the 5 R’s) and reducing contamination risk. This can lead to massive savings over the long term and contribute to maximised operations and ultimately business sustainability.
To summarise, remember the ‘5 Rs’ of lubrication: Using the Right lubricant, the Right quantity at the Right time, at the Right point, applying the Right method.