Proper installation and adjustment plays a vital role in the performance and life of bearings, and so does the lubricant. In most cases, the failure of bearings is not caused by improper installation or manufacturing defects, but by a lack of lubricant, an incorrectly chosen or a contaminated lubricant.
The lubricant, be it oil or grease, is spread between the moving parts of a bearing assembly and separates them, reducing friction and preventing wear. Depending on the operating conditions and on the chosen lubricant, a protective film will form on the bearing elements, the role of this film being also to dissipate the frictional heat, preventing the deterioration of the bearing and protecting against moisture, corrosion and contaminants.
A correctly chosen lubricant has the proper additives and viscosity to accomplish all the mentioned goals. The most common lubricants are oil and grease, the use of one or another being determined by the speed of the application and by the amount of load placed on the bearings.
In case of oils, the most important characteristic is the viscosity, the proper product being determined by the temperature and speed of the application. If an oil with insufficient viscosity is used, then the two rotating surfaces will come in contact and this will not only lead to wear but will also generate contact heat, and will cause the quick degradation of the bearing elements.
The most common oils for bearings are petroleum based and synthetic oils such as silicone, fluorinated compounds, diesters or PAO’s. Oils are usually chosen for bearings with higher speed capabilities and higher operating temperature, as they can carry the heat away from the bearings. In some cases, such as miniature bearings for example, oil-based lubricants only need to be applied once for the life of the bearing. In assemblies that use larger bearings, relubrication may be necessary as part of the regular machine maintenance cycle.
For grease-based lubricants, the most important characteristics are the temperature range, the penetration level, the stiffness and the viscosity of the base oil. Greases consist of an oil base in which a thickener is added, the most common thickeners being organic and inorganic compounds, as well as metal soaps like sodium, aluminum, calcium or lithium. Additives with antioxidant, anticorrosion and anti-wear characteristics can also be incorporated to increase the performance of the lubricant.
Alternatively, a solid non-fluid film can be applied like a coating on the bearing elements to reduce friction and prevent wear. These films are used in particular situations when oil or grease cannot survive, and they include options like graphite, silver, PTFE or gold films. For example, in an application with extreme temperatures or radiation, an oil- or grease-based lubricant may not offer proper protection, therefore a more durable one like a solid film may need to be used.
In most cases, grease is a good choice for lubricating bearings. More cost-efficient than oil, grease is easily retained in the bearing assembly and is easy to apply. However, it is not suitable for applications where heat removal via circulating oil is needed, nor in gearboxes where lubricating oil is required.
Also, if the operating conditions require the relubrication of bearings with grease at intervals that are too short, and this becomes too time consuming and expensive, or if the removal or purging of grease becomes too expensive and difficult to handle, it is better to choose a lubricating oil.
After a lubricant is selected, an important aspect is applying the correct amount on the bearing. If too much lubricant is used, this can lead to excess heat generation and bearing damage. The speed of the application, the loads and the noise level can all be influenced by the amount of lubricant used.
Depending on the chosen type of bearing and lubricant and on the application, manufacturers may recommend different levels of lubrication, which are given in percentages. The lubricant goes in the free space inside a bearing and in the housing. This space is important as it allows heat to radiate away from the contact areas of the bearing, so if too much grease is added, this can lead to overheating and premature bearing failure.
For this reason, the common recommendation is to fill in 20-40% of the free internal space of a bearing, a smaller percentage being usually specified for high speed, low torque applications, and higher percentage for low speed, high load applications. For the housing, filling in even 70%-100% of the free space may be acceptable if the application involves a low speed and the risk of contamination is high.
Keep in mind that the initial fill level is also influenced by the chosen relubrication method. The common methods for re-lubricating a bearing are manual relubrication, automatic and continuous relubrication.
Manual relubrication is convenient and enables uninterrupted operation.
Automatic relubrication avoids over- and under-greasing, and is commonly used in assemblies where multiple points have to be lubricated, or where access to positions is difficult. Also, it is the preferred choice where equipment is operated remotely and there’s no maintenance staff.
Continuous lubrication is used for applications where the relubrication intervals are too short, because of the adverse effects of contamination. In this case, the initial fill of the housing will be 70%-100%, depending on the operating conditions.
Handling and maintaining your bearings properly extends their service life and optimizes performance. Use this basic checklist to reduce maintenance time, labor and costs.
Bearing handling: Handle the bearings carefully to avoid scratching the surfaces. Always handle them with clean, dry hands, or use clean canvas gloves. Do not handle bearings with greasy or moist hands, as this can quickly lead to contamination.
Bearing storage: Keep bearings wrapped in oil-proof paper, in a cool and clean environment with low humidity, free of dust, vibrations and shocks. After you handle a bearing, place it on a clean and dry surface, to avoid contamination. Do not remove the bearing from its original package until you need to mount it, and store it flat, not standing.
Cleaning a bearing: Always use uncontaminated solvents or flushing oils, and avoid using cotton waste or dirty rags to wipe a bearing. Use separate containers for cleaning and for final rinsing a used bearing.
Bearing mounting: Use the correct technique and tools to mount a bearing. Around 16% of bearing failures are attributed to poor fitting, so make sure you avoid an excessively loose or excessively tight fit. Before mounting, check that all parts are clean and undamaged, and that the lubricant is correctly chosen. Do not wash the bearing before mounting it if it comes directly from a package.
Do not hammer or apply direct force on the bearing or its outer ring, as this can cause damage and misalignment of the elements. For small- and medium-sized bearings, cold mounting or mechanical mounting is generally recommended. Heat mounting is usually more appropriate for relatively large bearings, while for very large bearings, hydraulic mounting may be recommended.
Use proper tools: Specialized tools are available for mounting and dismounting bearings – bearing pullers, fitting tool kits, oil injector tools, induction heaters and hydraulic nuts. All these are customized to ensure proper fitting and a smooth mounting, to minimize the risk of bearing damage.
Inspect the bearings: To prevent bearing failure, it is necessary to inspect them both during and after operation. For inspections in operation, check the temperature, noise and vibration and examine the lubricant to determine whether it needs to be replaced or replenished. After operation, examine the bearing and each of its components to determine whether there are changes. The common causes of bearing failures and their solutions are discussed in the final chapter of this guide.