A life support system for bearings
10 October 2015
Bearings continue to fail prematurely - largely due to a lack of knowledge on the part of the user, or simple negligence. Look after your bearings and they will look after you, says Phil Burge.
Studies show that 16 percent of bearing failures are due to incorrect fitting, 36 percent to poor lubrication and 14 percent to contamination. Collectively, these problems contribute directly to the remaining 34 percent of failures, which are due to bearing fatigue occurring well before the operating life quoted by manufacturers.
Premature bearing failures matter. Bearings are very often treated as a commodity purchase, but unplanned downtime caused by the loss of a bearing in service can be costly indeed. Fortunately, thanks to recent developments in tools and measurement technologies, the actions required to reduce the incidence of in-service bearing failure are neither complicated nor expensive. They do, however, require attention, the right methods and appropriate tools to be applied across the full lifecycle of the bearing.
Mounting and dismounting
The process of mounting is a critical moment in the bearing’s lifecycle. Choice of the right methods here can be decisive in determining the performance and service life of the bearing, yet many approaches in common use today have been demonstrated to create problems in service.
Small and medium sized bearings are generally cold mounted, for example, and to do this the installer may reach for a hammer and length of old pipe. If that pipe isn’t precisely the right size, this approach can lead to installation forces being transmitted through the rolling elements rather than the casing, causing damage to the raceways before the bearing has even turned. Proper installation tools, by contrast, are designed to have an interference fit that ensures forces are only applied to the retaining ring of the bearing, which has been designed to cope with them without damage.
During the installation of larger bearings, heat is often used to expand the bearing housing prior to mounting. Here, the traditional approach is to immerse the bearing in a heated oil bath prior to installation.
Unfortunately, this can introduce contamination, shortening its life, not to mention the safety hazards associated with handling a hot, slippery component prior to installation. A better approach is the use of dedicated induction heaters, which, as well as avoiding contamination or damage due to excessive or uneven heating, also allow the bearing to be heated with greater efficiency, controllability and safety.
An increasing number of bearing applications are being designed specifically to facilitate easier fitting and removal, often using hydraulic techniques. In the SKF oil injection method, for example, shafts are manufactured with special oil ducts and grooves. During installation, oil is injected at high pressure through these ducts, creating a thin film between the shaft and bearing that greatly reduces the force required to push the bearing into place.
At the other end of the bearing lifecycle, removing bearings with the right tools doesn’t just save time, it also reduces the chance of damage to shafts and equipment during removal. SKF manufactures a range of hydraulic and mechanical bearing pullers, including special designs for ‘blind’ applications where bearings are installed with an interference fit on both rings. When used on tapered shafts, the oil injection method described above can even be used to force the bearing off the shaft without the need for a separate bearing puller.
Rotating shafts need to be correctly aligned in all three dimensions. Poor alignment results in greatly increased friction, driving up energy consumption in equipment and shortening the life of bearings and other components.
Traditional shaft alignment techniques include the use of a simple straight edge: quick but inaccurate, or manual dial gauges which are fiddly and time consuming to set up. Fortunately for installation and maintenance engineers, the latest shaft alignment systems are much more accurate, robust, user friendly and cost effective than those of the past.
SKF’s latest TKSA 11 alignment tool, for example, connects directly to a smartphone app that guides the engineer step by step through the whole alignment process. In addition, the device is the first on the market to make use of robust, accurate and inexpensive inductive proximity sensors, making it suitable for every budget.
Poor lubrication is the biggest single cause of premature bearing failure. Therefore an effective lubrication programme lies at the heart of good bearing maintenance. Lubrication management is easy to define: the right amount of the right lubricant should be delivered by the right method to reach the right point at the right time. Making that happen in a real production environment, however, requires a detailed action plan that encompasses the whole lubricant supply chain, from selection to delivery, storage, application and disposal at end of life.
Leading bearing manufacturers have developed a wide range of lubricants with characteristics specifically tuned to the needs of different applications, from ultra-low friction greases to high temperature, high viscosity products for the most extreme dutiess. Some industries, such as food manufacture or environmentally sensitive applications, require the use of non-toxic or biodegradable lubricants.
To help equipment owners manage their lubricant requirements effectively SKF has developed complete lubrication management services for customers, which include an audit of the customer’s needs followed by the design and implementation of a programme for the supply of appropriate tools and materials and a schedule of activities.
Traditionally, operators faced two choices when planning bearing replacement strategies. They could run bearings to failure, and accept the risk of unplanned downtime, or they could change bearings according to fixed schedules, and risk spending more than they needed replacing parts that might have considerable service life remaining. The third alternative, the use of in-service condition monitoring techniques like vibration monitoring or oil analysis, was deemed costly and only suitable for the highest priority applications.
Today, however, maintenance engineers have access to a wide variety of simple, portable measuring systems that can help them build a clearer picture of the performance of a machine in service, allowing them to schedule service interventions when they are needed most.
These basic condition monitoring tools include temperature sensors to identify the tell-tale signs of excess friction, hand held tachometers to spot deviations from desired operating speeds, and stethoscopes and vibration sensors to listen for changes that might indicate wear or damage in rotating components. The availability of robust, inexpensive endoscopes even allows engineers to look inside equipment without the need for disassembly or even shut down.
Economical self-contained sensors, like the SKF Machine Condition Indicator CMSS 200 can even be left permanently attached to non-critical machines, automatically monitoring basic parameters such as temperature and vibration and alerting operators via a simple LED display when a change in operating conditions suggests further investigation is warranted.
Phil Burge is with SKF in the UK
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