The difference between viscosity and consistency of fan lubrication

Author: Zhang Wei (Petrolube sent through China)

We have already discussed what is the kinematic viscosity and viscosity index of the oil. Let's talk about the difference between viscosity and consistency. Frequently encountered some customers confuse consistency and viscosity collectively as viscosity, there is a certain misunderstanding, here to clarify.
First of all, whether it is lubricating oil or grease, there is an indicator of kinematic viscosity. The previous article on the kinematic viscosity of lubricating oil has been thoroughly analyzed. Here we focus on the kinematic viscosity index of grease (ie, grease), which is known as the main fat. It is a certain proportion of thickeners (metal soap bases, such as lithium base and calcium base, etc., non-metallic soap bases, such as bentonite, lime soil, urea base), which are blended with base oils (mineral, semi-synthetic or fully synthetic base oils). Etc.) with a small amount of other additives (such as EP extreme pressure agent, AW anti-wear agent, AO antioxidant, LTEA low temperature improver and CSA co-solvent, etc.). The proportion of base oil to oil is at least 80%, thickeners only account for about 10% to 15%, and the remaining 5% to 10% are all other additives. It can be seen that the most important component of fats and oils is actually the base oil, and the most important indicator of oil is the kinematic viscosity, because the kinematic viscosity determines the friction surface of this grease in the state of the friction pair just starting and stable motion. The oil film strength and adhesion also determine the permeability and lubricity of the oil film in the small space under the main motion state of the friction pair. Therefore, the base oil kinematic viscosity of the grease directly determines the surface of the grease on the friction pair. In the time (that is, under hydrodynamic lubrication, it mainly includes the lubrication protection effect of the friction pair just starting and the stable running state under the rated working load). The consistency mainly refers to the thick state and softness index of the whole oil. According to the consistency grade of NLGI (American Grease Association), it is divided into 9 grades, according to the corresponding penetration range of the grease at 25 °C. It is 000#~6# grease. Therefore, it is often encountered that some masters will say that they want to add 2# butter. This 2# refers to the consistency of the oil (sometimes also called the “hardness” of the oil). The consistency is not related to the direct lubrication effect of the grease, but it is related to the retention of the grease in a certain frictional sub-area. The consistency of the grease is relatively high, and it is relatively suitable for some applications that are not completely sealed or sealed so well. The problem of preventing the loss of fat and oil has already occurred. In general, there are 5 tips on the consistency of the grease for your choice of grease:
1. The higher the consistency of the oil, the more stable the solid state before use, the lower the consistency of the oil, and the more the oil is biased towards the semi-fluid state. Therefore, the choice of the consistency of the grease is related to the sealing property and the space size of the application point using the grease. The better the sealing point, the more suitable the use of low consistency grease or even the direct use of lubricating oil instead of grease, so that the lubrication efficiency is higher;
2. The consistency of the oil is not related to the kinematic viscosity of the base oil. The oil with high consistency is only thicker and harder than the oil with lower consistency before it is lubricated (such as when viewed in a oil drum), but it does not mean When used, the kinematic viscosity of the base oil must be higher than that of the low consistency grease. That is to say, the oil film strength and adhesion of the oil with high pumping degree must be higher than that of the low consistency oil because of the strength and viscosity of the oil film. The attachment is not related to the consistency, but is related to the kinematic viscosity of the base oil. Therefore, do not mistakenly think that the oil that looks thicker in the oil drum is used. The lubricating and anti-wear effect of the oil film is better than that of the oil which looks thinner, because it can be determined after determining the kinematic viscosity of the base oil;
3. Under the same temperature condition, the grease with high consistency needs more pumping pressure than the grease with lower consistency when filling;
4. Regardless of the consistency, the grease will be stored in the oil drum for a certain period of time, and there will be a certain amount of base oil. The technical parameters of the grease will generally list the normal percentage of oil, commonly known as the oil rate. In most cases, the higher the consistency of the oil, the higher the oil slick rate will be higher than the low consistency oil, which is normal, as long as the oil is not used, its oil slick rate is not higher than 3%, generally this The state of the grease is relatively stable, but it does not matter. However, if the oil slick rate is high during storage, it indicates that the retention is not good. Generally speaking, we do not recommend to continue using this product unless the application point can be lubricated with lubricating oil (ie, thin oil lubrication). Lubricated with grease (ie dry oil lubrication);
5. Generally speaking, the lubrication point has a large load and low rotation speed. It is recommended to use grease with higher kinematic viscosity of the base oil (but not necessarily the higher consistency of the grease). Similarly, the application of light load and high speed is recommended. Use a base oil with a lower kinematic viscosity (but not necessarily a lower consistency grease);
In short: many times people will confuse the two concepts of consistency and viscosity of fats. In fact, viscosity is a key indicator of how much the oil film plays a role in the friction pair, and the consistency is really related to the actual effect of the oil at the application point. Key indicators that are not lost, these are two indicators that are completely irrelevant but equally important and easily confused. Of course, if the lubrication point can be lubricated by thin oil, please do not use dry oil lubrication, because the unit cost of the thin oil lubrication method is low and the lubrication efficiency is higher.