BORA KIRAZLILAR
Mechanical Engineer
Gema Oil Lubricants Technical and Private Label Products Compliance Manager
In the industrial world, keeping machinery running smoothly is an art. One of the most important materials of this art is undoubtedly grease. Choosing the right grease extends the life of the machinery and maximizes its performance. Grease, which has a wide range of uses from bearings, which are the heart of the machines, to heavy industrial equipment, creates a miracle when selected correctly. Based on his field experiences, Mechanical Engineer Bora Kirazlılar provides important information about this indispensable element of industrial maintenance, from the factors determining the quality of a grease to the ideal conditions of use.
A mediocre base oil does not make a good grease
Although there are many factors affecting the quality of greases, from the additives to the manufacturing process, it is important to remember that it is the oil in the grease that lubricates. All tribological rules that apply to oils also apply to greases. For example, a grease made from a base oil with a high viscosity index prevents the lubricating film from breaking between the contacting surfaces, as the viscosity of the oil will remain more stable with increasing temperature. The preservation of the oil film means that friction and therefore failure does not occur.
There is no perfect grease, but optimum one
There are 12 criteria for the evaluation of greases. It is chemically impossible to keep all of these criteria at the highest level. This is because when you maximize one feature, it retracts or destroys some other features.
For this reason, it is necessary to be careful when evaluating the products in the product data sheet that the highest value obtained in the tests does not always mean that you will get the best result in practice.
The EP additive is sometimes a friend and sometimes an enemy
Extreme Pressure additives are chemical substances that prevent the contact of two surfaces by forming an intermediate layer between the contacting surfaces when the oil film breaks under extreme load. However, EP additives should be used if the actual load on the bearing is more than 15% of the theoretical catalogue load rating of the bearing. As it is not always possible to calculate this, it is practically assumed that low-speed bearings need EP additives, which is not always correct. In a nutshell, EP additives should be used when the need arises. For example, the use of greases with EP additives in electric motors is not suitable because EP additives, which are corrosive, damage the electric motor windings.
The NLGI grade as an important parameter
The NLGI grade, which determines the degree of hardness and consistency of greases, makes a significant difference in some cases. In general, NLGI grade 2 is common in bearings. NLGI grade 3 can provide success in vertical shaft bearings under the influence of gravity, bearings with an inner diameter of more than 150 mm and bearing with caps greases that are highly affected by the shear effect. NLGI 1 is preferred in needle roller bearings and in centralized systems where grade 2 has difficulty in pressing.
Temperature evaluation determines the life of the bearing and grease
Another important issue is the selection of grease according to the temperatures at which the bearings operate. Normally, a bearing should operate between 40-70 C temperatures. Although there are factors such as process temperatures, high speed, heavy load and inadequate lubrication that increase the heat, users often demand greases that withstand very high temperatures and unnecessarily pay more. A significant part of the bearings are made of SAE 52100 bearing steel, whose density starts to decrease in the temperature range of 120-150 C, and the bearings will start to be stressed due to the loss of surface hardness at these temperatures. For this reason, the position and load of the bearings are designed by the machine designer so that they are not exposed to these temperatures. To sum up, it is important to determine the exact temperature at which the bearing operates. However, using a grease with high temperature resistance can also be considered as a ‘preventive’ maintenance practice.
Good mechanical stability in grease means long bearing life
In practical terms, the mechanical stability of grease is the extent to which the grease does not lose its form under the effect of agitation and shearing in the bearing. In the field, it means that it does not soften or harden over time. Lithium soap greases have good mechanical stability and are accepted as general bearing greases. Since the fiber matrix structure of lithium complex greases, i.e. the ‘spongy’ texture that holds the oil, is stronger than lithium soap greases, they are even more successful in mechanical stability. The mechanical stability of greases is determined by the 100000-stroke penetration application of the ASTM D 217 test. The results obtained in this test are expected to be not much different from the results obtained with 60 strokes.
About Bora Kirazlılar
Bora Kirazlılar, an Industrial Maintenance and Lubrication expert, started his career as a machine maintenance engineer at Zorlu Holding. In 2007, he published his book titled ‘Industrial Maintenance’, which was first published in 2007 and about 100 pages of which are about lubricants. After 8 years as Technical Manager of BP-Castrol Industrial Oils Türkiye, he joined the experienced team of Gema Oil.
