Greases for ball bearings

Greases for ball bearings

Right grease has crucial importance for bearing productivity. Is provides thin film between contact areas in the bearing for friction reduction, heat spread and corrosion prevention on the balls and raceways.

The grease will effect on maximal speed and temperature, rotational moment, level of noise, and finally, operation life of the bearing. There is a range of greases, depending on the area of application.

Mineral and synthetic grease are most often used and oriented for general and high-speed application. The micro-filter editions are used for low-noise applications. There are water resistant types for low or high temperatures.

Silicon greases have wide temperature ranges and more stable viscosity when temperature reduces. They are also water resistant and safe for application with the majority of plastics and rubbers (sealed rings etc.), but they aren’t suitable for high loads and speeds. Water-resistant.

Perfluorochemical grease and grease PFPE are incombustible, compatible with oxygen and very stable to many chemicals. They don’t enter into a reaction with plastics and elastomers. Many of them have low pressure of evaporation and they are suitable for applying in vacuum. Some of them can endure the temperatures up to 300 ° C. They are often applied for surface protection from sticking and frost-up due to high hydrophobic features.

Dry greases are recommended to use where the standard grease can cause contamination, for example, in vacuum environments. Such grease as molybdenum disulfide and tungsten disulfide are applied on balls and raceways of bearings, provide smooth work and higher moving speed. These coverings are resistant to water and diluted acid. They can be powder-like, wax-like and bonded coverings, which are applied with the help of solvent.

Damping greases are widely applied in automotive components to prevent crackle and creaks. They also make switches and drives qualitative. They can be applied in slow rotating bearings, like in potentiometers.

Greases for food industry meet special sanitary demands. There are two types of greases for cases, where the contact with food is possible and the contact is absent. These greases are steady to washout and cleaning.

Grease viscosity

Oils with low viscosity and greases are applied, where the low grease performance is required, for example in sensitive tools. The greases with higher viscosity can be recommended for high-loaded, high-speed and vertical applications of shafts. The greases with lower viscosity are preferred for high-speed applications, as they generate less heat. Though, greases often provide much bigger stability than oils, majority of modern greases with low rotational moment can create the indicators of rotational moment similar to oils according to features.

Many greases support their consistence in wide temperature range and they are easy for applying. The light instrument grease is used in components with very low rotational moment. The oil also can provide more high speed of work, but, as such grease isn’t retained on one place, it should be applied like a stream, oil bath or oil fog, if the rotations are short.

The oil-impregnated phenolic catch or synthetical catch, made of material with very low frictional factor, don’t need continuous outer greasing. These catches are often applied in high-speed tooth bearings with low rotational moment.

Plastic greases are oils, which are mixed with thickener to stay inside the bearing. They are usually suitable for high loads and have an evident advantage in providing constant greasing for a long time without technical maintenance.

Grease quantity

It is necessarily to take into account that big amount of grease may be harmful for bearing. The filling with grease will mean bigger resistance to rotation (higher rotational moment), but still the risk of heating still remains. The free space inside bearing is important for heat emanating from contact area between ball and raceway. As a result the grease excess can lead to premature destruction.

The standard filling comes to 25% - 35% of inner space, but can vary if it is required. Small percent can be indicated for high speed component with low rotational moment, while much bigger filling can be recommended for using with low speed and high load.

Speed factor of grease

Speed factor («DN») is calculated as rotation speed (rpm per minute) x (outer diameter + inner diameter) ÷ 2. Let’s suppose that bearing rotates with a speed 20000 rpm per minute. Inner diameter of bearing is 8 mm and outer – 22 mm. The above mentioned formula gives DN 300 000, so the grease should have factor higher than this number. Many modern consistent greases are suitable for high speeds with nominal 1 mln. DN or more.

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