During the process of equipment design, installation, and usage, the selection of the temperature for bearing lubrication grease is extremely important. For motors, the commonly used lubrication medium is grease.
Typically, grease suppliers will mark some data on the technical parameters of the grease, including the drop point, the range of use temperature, and so on.
On the other hand, there are many temperature calculations in different technical materials.
Sometimes, the results of various calculations and the temperature limits of other bearing components do not quite match the parameters indicated for the grease. The multitude of concepts and calculations can leave engineers in a quandary when making a selection.
Let’s sort out these concepts to help engineers have a clearer thought process when choosing the temperature of the lubrication grease.
Nominal Data of the Grease
Grease suppliers sometimes provide some temperature parameters for the grease:
|Optimal usage temperature range|
|Calcium-sodium-based grease/Lithium-calcium-based grease||150~180||-20~120|
|Composite lithium-based grease||>230||-30~140|
|Composite sodium-based grease||>220||-30~160|
|Composite aluminum-based grease||>230||-30~160|
|Composite calcium-based grease||200~280||-20~150|
|Composite barium-based grease||>220||-30~140|
In the aforementioned common parameters, two temperature terms are specified: the drop point and the optimal usage temperature range.
The drop point is a standard parameter indicating the physical properties of commonly used greases at varying temperatures. As the grease is heated, it softens with rising temperature (decreasing in viscosity). The temperature at which the first drop falls from the grease cup of the instrument or when the column touches the bottom of the test tube under specified conditions is referred to as the drop point of the grease.
From the definition of the drop point, it’s clear that this temperature indicates the critical point at which the physical properties of grease transition from a solid to a liquid state.
Naturally, the lubricating performance of the semi-solid grease, now in a liquid state, must have altered. So, does the performance of the grease change with temperature before it becomes liquid? The answer is certainly yes. Then, after the properties of the grease change with temperature, does it still meet lubrication performance requirements? The answer is “not necessarily”.
From the above analysis, it is clear that using grease at temperatures exceeding the drop point is unacceptable. However, it’s not necessarily suitable at temperatures below the drop point either. Therefore, the drop point cannot be used directly to select grease.
Sometimes, grease suppliers provide an optimal usage temperature range. From the concept and given value of the drop point, it’s evident that the drop point merely specifies a temperature, the critical threshold from solid to liquid, acting as an upper temperature limit. The temperature range typically provided by grease suppliers considers this upper limit while also specifying a lower limit.
Based on the fundamental properties of grease, its viscosity decreases with rising temperature and increases with falling temperature. As the temperature rises, the grease softens (viscosity and base oil viscosity decrease); as the temperature falls, the grease hardens (viscosity and base oil viscosity increase).
The optimal usage temperature range of grease is a general temperature range, within which the basic viscosity and base oil viscosity of the grease likely provide certain lubrication performance within a specific range.
The question now arises: Does the lubrication performance within this temperature range satisfy the operating range chosen by the equipment designers? The answer is not necessarily! Therefore, the actual selection of grease temperature needs to be determined through specific calculations.
Selection of Lubricating Grease Temperature
The common reason for choosing lubricating grease is to sufficiently lubricate bearings under specific operating conditions. This requires validation of the “given operating conditions” and “fulfillment of lubrication performance”, a typical process in lubrication selection and validation calculations.
The essence of these validation calculations is to check the kappa value. When the kappa value ranges from 1 to 4, it signifies that the chosen lubricant satisfies the lubrication needs.
During calculation, it’s clear that many factors are influenced by temperature, such as the viscosity change curve, which is essentially a curve showing viscosity changes with temperature.
The core of the validation calculation is to check whether the selected grease can meet a kappa value within the range of 1-4 under the current temperature. If the answer is affirmative, the selection is appropriate; otherwise, adjustments are needed.
From the above, it may seem like the choice of temperature has no direct correlation with the nominal temperature. In fact, the nominal temperature does have a relationship with the choice of grease because the properties of grease, such as the dropping point, determine the viscosity curve, and the calculation of the kappa value also comes from the viscosity curve.
As this article demonstrates, it’s often inaccurate to directly compare the operating temperature with the nominal data of the grease.
So, why don’t grease suppliers provide a comparable parameter? The answer is, it’s impossible. This is because grease suppliers are unaware of the operating conditions chosen by the equipment designers, and the performance of grease fluctuates with varying operating conditions (temperature), therefore a fixed value cannot be provided.