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    Bearings and their Lubrications

 Since everything in industry turns on bearings, it is necessary for those persons responsible for keeping equipment running to fully understand bearings. There are two types of bearings; namely, sliding bearings and rollerbearings. The majority of bearings used are roller bearings. Then let us now see what are the causes and cures for roller bearing when operating in higher than normal temperature in different applications.

Higher-than-normal operating temperatures, whether caused by ambient conditions or generated within the bearing itself, have the potential to harm roller bearings. Normal operating temperatures differ, depending on the application. The maintenance technicians should be aware of this and know the common causes of, and remedies for, bearings overheating.

Electric Motors

The ball bearings used in most electric motors are pre-greased, shielded ball bearings. Normal motor bearing operating temperatures range from 140oF (60oC) to 160oF (71oC). Overheating in electric motor bearings is generally lubricant related. For example, re-lubricating open bearings, users may inadvertently employ low-temperature grease that does not provide adequate viscosity, at the normal operating temperature. Or the user may over-grease the bearing, forcing bearing balls to push through excess grease as they rotate, leading to a sharp temperature rise. Another cause of overheating is mixing incompatible greases, which can reduce the consistency of the grease and possibly the overall viscosity.

Fans

Commercial fans generally utilise ball and roller bearings mounted in cast iron or pressed steel housings. Fans are exposed to a wide variety of ambient conditions, ranging from below-zero temperatures for rooftop fans to extremely high temperatures for fans used in industrial processes. Normal bearing operating temperature varies, depending on the environment and application. The standard grease in most fan bearings remains effective to an operating temperature of 180oF (82oC). If steady-state operating temperatures are higher than 180oF (82oC), consider using a grease with a synthetic oil does not vary as much with temperature as in a standard mineral oil, and the rate of oxidation is much slower. For operating temperatures above 200oF (93oC), a circulating oil system may be needed. These systems pump clean, cool oil through a bearing arrangement. In hot-gas fans, special measures must be taken to protect bearings from high temperatures. In virtually all cases, an aluminium disk or flinger placed on the shaft between the bearing and the fan casing can act as a heat shield. Often, a blower wheel or compressed air can be used to direct cooling air across the bearing housing or the shaft.

Pumps

Depending on the application, normal bearing operating temperatures in pumps range from 100oF (38oC) to 180oF (82oC), with most running between 140oF (60oC) and 160oF (71oC). Although grease is used in some vertical pumps, oil is the preferred lubricant in the majority of pump applications. Standard bearing oils in pumps remain effective to approximately 180oF (82oC). If normal operating temperatures are higher than 180oF (82oC), a synthetic oil should be used; if temperatures exceed 200oF(93oC) , a circulating oil system will probably be required. As in other bearing applications, bearing over lubrication can cause higher than-normal operating temperatures in pumps. Bearing misalignment or ball skidding within the bearing can also cause overheating. Specially designed bearings are available to eliminate ball skidding. Ideally, bearing temperatures in pumps, especially those in critical applications, should be regularly monitored.

Gear Drives

Bearings in gear drives normally operate at 160o F (71o C) to 180o F (82o C) and are lubricated with static oil systems. As improved technology permits reductions in the size of gear drives, there is a growing trend to transmit more power through a given size drive than ever before. This practice can cause bearings in gear drives to run hotter and may necessitate the use of alternative cooling methods.Summing this up, proper bearing lubrication is the primary concern in all high-temperature applications. That concern is heightened by the trend of running industrial equipment at higher speeds than originally intended, further increasing bearing temperatures.The general rule is to provide the minimum viscosity required at the expected operating temperature: 100 SUS (20cst) for roller bearings and 70 SUS (13cst) for ball bearings.In addition, the increased thermal expansion of the shaft must be accounted for both axially (to ensure that high thrust loads are not induced) and radially (to ensure that radial internal clearance is adequate to avoid preload). The solution may also entail using grease with a synthetic base oil or converting to a different lubricant delivery system, such as circulating oil.

 

 

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