Five stages of the bearing life cycle
2017 January 31, 09:00 GMT
Computer modelling programs allow machine designers to determine, with some confidence, how long their bearing is going to last. But not every bearing achieves its calculated life. What other factors, then, are at play in determining the actual life of a bearing?
While modern desktop techniques have certainly eased the burden of bearing life prediction, there's no getting away from the fact that a bearing's initial installation, setting and subsequent care-in-service will ultimately determine its life expectancy. Indeed, a bearing’s life can be broken down into several distinct stages, each of which will define its requirements regarding the maintenance methods and tools that it will need in service; ignore these and a bearing’s chances of achieving its expected service life will be significantly curtailed.
Research shows that 16 per cent of bearing failures are due to incorrect fitting, 36 per cent to incorrect or poor lubrication and 14 per cent to contamination of the bearing surfaces. Collectively, these problems contribute directly to the remaining 34 percent of failures, which are due to bearing fatigue occurring well before the predicted life span.
Premature bearing failure can be costly in terms of lost production and consequent damage to other parts of a machine or process, but it is avoidable if the right methods and appropriate tools are applied across the full lifecycle of the bearing. These methods and tools cover five distinct stage of a bearing’s lifecycle, from installation through care-in-service to final dismantling - namely, mounting, lubrication, alignment, condition monitoring and dismounting. Let’s look at each of these in turn:
Mounting - This is a critical stage in a bearing’s lifecycle and choosing the right methods will have a big impact on its performance and service life. Small and medium sized bearings are generally cold mounted, and while a hammer and a length of old pipe may seem appropriate tools for this job, if that pipe isn’t precisely the right size, the installation forces could be transmitted through the rolling elements rather than the casing, damaging the raceways before the bearing has even turned. Proper installation tools are designed to have an interference fit that ensures forces are only applied to the retaining ring of the bearing, avoiding damage to other parts of the bearing assembly.
For larger bearings, heat is often used to expand the bearing housing prior to mounting. Traditionally, these bearings are immersed in a heated oil bath prior to installation, but this can introduce contamination, shortening their life as well as posing safety hazards associated with the handling of hot, slippery components prior to installation. Purpose-built induction heaters are more suitable; as well as avoiding contamination or damage due to excessive or uneven heating, these also allow the bearing to be heated with greater efficiency, in a controlled and safe manner.
An increasing number of bearing applications are being designed specifically to facilitate easier fitting and removal, often using hydraulic techniques. With SKF’s oil injection method, for example, oil is injected at high pressure through ducts and grooves machined into the shaft, creating a thin film between it and the bearing, greatly reducing the force required to push the bearing into place.
Alignment - Poor shaft alignment results in greatly increased friction, increasing energy consumption and shortening the life of bearings and other components. Shaft alignment has traditionally been achieved using techniques ranging from a simple straight edge to manual dial gauges, the first being quick but inaccurate, the second fiddly and time consuming to set up. Recent developments in shaft alignment technologies, however, have simplified the procedure. SKF’s TKSA 11, for example, is an affordable, inductive sensor-based alignment tool that connects directly to a smartphone or tablet to provide step-by-step guidance through the entire alignment process.
Lubrication - An effective bearing lubrication programme ensures that an appropriate quantity of the correct lubricant is delivered by the right method to reach the right point at the right time. Achieving this in a real production environment, however, requires close attention to the whole lubricant supply chain, from selection to delivery, storage, application and end-of-life disposal. SKF, for example, offers a bespoke lubrication management service, which includes an audit of customer requirements, lubrication programme development and the supply of appropriate tools and materials.
Condition monitoring – Once confined to the care of high-value, critical assets, condition monitoring techniques like vibration, speed and temperature monitoring can now be achieved using a wide variety of simple and affordable portable measuring devices that can help build a clearer picture of the performance of a machine in service, allowing service interventions to be scheduled as and when they are needed, rather that periodically, which can be wasteful.
Economical, self-contained sensors, like SKF’s Machine Condition Indicator CMSS 200 can be permanently attached to individual machines, automatically monitoring basic parameters such as temperature and vibration, and alerting operators via an LED display when a change in operating conditions suggests further investigation is warranted. For multi-machine production lines, SKF’s ‘@ptitude’ Asset Management System connects condition monitoring hardware – including CMSS indicators - seamlessly across the enterprise on one integrated, networked platform.
Dismounting - When dismounting, there is always the chance that a perfectly serviceable bearing will be damaged in the process, so if it is intended for reuse, here are some golden rules to follow: never hit the bearing rings (or any other part) directly; do not allow the bearing withdrawal force to be transmitted through the rolling elements; and do not heat the bearing with an open flame.
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 comprehensive 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.
Bearing life is, to some extent, a calculable quantity, but variability enters the equation if some common sense rules are not followed when installing and caring for these components. Attending to the five stages of a bearing’s lifecycle will ensure that the rewards of good service and longevity are reaped.
The above technical article was originally published in the January 2017 edition of Plant & Works Engineering magazine.