FEATURE BEARINGS & LINEAR MOTION Bearing in mind the importance of lubrication
BEARING LOAD The key here is the relationship between the loads occurring during operation and the bearing’s load capacity. Bearings are already considered highly loaded at 15% of their dynamic load capacity. At this load level, the use of lubricants with EP (extreme pressure) additives should be considered. Impact loads and vibrations can also affect the choice of lubricant.
The operational reliability and durability of bearings can be
significantly improved by correctly assessing and adjusting the lubrication, which can also help
reduce both initial costs and those associated with premature bearing failure. Daniel Stöckl and Klaus
Grissenberger, application engineers at NKE Austria, comment
W
ith rolling bearings, the main purpose of a lubricant is to separate
the metal surfaces of the bearing’s components with a thin lubricating film to prevent wear. The lubricating film also reduces friction and therefore power dissipation, resulting in reduced energy consumption of the system. However, with around 40% of all premature bearing failures caused by lubrication problems, there are a number of considerations when designing the lubrication system and selecting a lubricant.
BEARING SIZE AND TYPE The type of bearing used generally affects the lubrication requirements. Tapered roller bearings or spherical roller thrust bearings, for example, exhibit a sliding friction between guide flange and roller end faces in addition to the rolling friction between rolling elements and raceways. Insufficient lubrication in this area quickly leads to permanent damage of the contact surfaces. Cylindrical roller bearings also experience sliding friction between the guide flanges and roller end faces; and sliding friction can occur between cage and rolling elements and – with ring-guided cages – between cage and guide ring.
34 SEPTEMBER 2016 | DESIGN SOLUTIONS
TEMPERATURE VARIATIONS For reliable bearing operation, the chosen lubricant must be of a viscosity that is suitable for the operating temperature. While many mineral-based oils have a tendency to age quicker when the continuous operating temperature exceeds +70˚C, this can be avoided by using partly or fully synthetic oils. Low operating temperatures may also
have a negative impact, with typical greases becoming stiffer; while oil (sump) lubrication generally results in increased losses due to splashing. Temperature peaks should also be taken into account, as should ambient bearing temperature. This is especially relevant for arrangements with an automatic relubrication system. The lubricant usually reaches a higher temperature at the bearing location than it has in the feed lines; and, at low ambient temperatures, the grease can stiffen and may no longer be fed to the bearing in sufficient quantities.
Inner ring of cylindrical roller bearing after operation with
excessively mobile oil (left); and inner ring after operation with sufficiently viscous oil (right)
BEARING SPEED RANGE Beside temperature, the bearing’s running speed also affects the lubricant’s separating action which, in turn, affects the choice of lubricant viscosity. As a general rule, the higher the speed the better the surfaces are separated, and the lower the lubricant’s viscosity can be. On the other hand, high speeds also result in higher losses and therefore higher operating temperatures. This may make a circulating oil lubrication system necessary which can, in addition, cool and filter the oil. At very low speeds a full separation of the surfaces cannot always be ensured,
so a lubricant with appropriate wear protection additives should be considered.
CONTAMINATION Dirt etc. can enter through seals and vents while wear particles from surrounding machine components as well as production residue may be present. The lubricant itself can also contain contaminants if, for example, it was not stored correctly. Moisture and chemicals may also be present. In addition to technical factors, the costs of both lubricant and complete lubrication system must be considered. To achieve the ideal balance between technical configuration and cost, the system requirements must be drawn up with care. As there are many different lubricants to choose from, it is advisable to consult the lubricant manufacturer when selecting a product.
THE EFFECT
So let’s take a look at the effect of lubrication on the function of bearings. As part of a theoretical consideration of the bearings in a power take-off (PTO) gearbox, the lubrication was investigated more closely. It was found that, under the given operating conditions, the lubricant’s viscosity was much too low to form a sufficiently effective lubricant film, which would have resulted in increased wear and a reduced service life. NKE therefore recommended the use of a more viscous lubricating oil. A practical test was performed in
which two identical gearboxes were filled with the lubricating oils and run for a period of 500 hours. In the subsequent survey, discolourations and initial signs of wear were clearly visible on the functional surfaces of the bearing run with the thinner oil. The bearings operated with the thicker oil did not show any signs of wear. The customer’s fear that the thicker
oil would result in higher power dissipation proved unfounded. In fact the gearbox with the more viscous oil exhibited lower losses and therefore a reduced operating temperature. This can be attributed to the better separation of the metallic surfaces in operation, which more than compensates for the slightly higher fluid friction.
NKE Austria
www.nke.at
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