On the Assessment of Mechanical Degradation of Grease Using Entropy Generation Rate

Abstract

A methodology for continuous monitoring of grease degradation subjected to mechanical shearing is proposed. It is hypothesized that the mechanical degradation of grease is akin to the running-in process in a tribo-pair with both transient and steady-state regimes. To validate the hypothesis, a series of mechanical shearing tests are performed on three grades of grease: NLGI 00, NLGI 1, and NLGI 2.5 by employing a rheometer. From the results, a more effective method using the entropy generation rate is proposed for continuous monitoring of grease degradation. The proposed method is extended to estimate the time for a grease subjected to mechanical shearing to degrade to a lower grade. The efficacy of this method is demonstrated via long duration testing in a custom-built ball-bearing test apparatus.

Appendix

Figure 19 shows the schematic representation of the rheometer and the control volume considered for the present work. The grease is placed between the stationary surface and vane and is sheared by rotating the vane at an angular speed of ω for shear rate $\dot{\gamma }$. The rotating vane imparts work W on the grease due to the friction and the heat is conducted outside through the contact surfaces. The system considered is a closed system, where the transfer of matter is across the boundary of control volume does not prevail. Further, it is worth observing that during the initial stage of shearing of grease, the rheological properties are transient and after certain time reaches a steady state. It is erroneous to assume that the process is completely at steady state; however, thermodynamic state changes with time can be assumed to occur along a quasi-static isothermal process [33] or in other words they are at local thermodynamic equilibrium (LTE). A system is said to be in LTE if the thermodynamic state of a tribo-system at a given time is completely defined by properties of the system. The degradation of grease is one such process where the variation in the parameters is well defined. In this regard, Lijesh et al. [33] has established an equation Eq. (6) for determining the entropy generation rate per unit volume $\dot{s}_{g}$ for tribo-pair experiencing a transient as well as steady state wear and is given as:

$$\dot{s}_{g} = \frac{{\sigma :\dot{\varepsilon }_{p} }}{\rho T} - \frac{{\dot{e}_{\text{mt}} }}{T} + k\frac{{\left( {{\text{grad}}\;T} \right)^{2} }}{{T^{2} }} - \dot{s}_{{g_{\text{mt}} }} - \sum\limits_{k} {\eta_{k} {\text{d}}N_{k} }$$

(6)

where $\sigma$ is the stress developed by the shearing process, $\dot{\varepsilon }_{p}$ is the rate of strain responsible for plastic degradation, suffix p indicates the plastic deformation of the surface, ρ is the density, T is the temperature, $k$ is the thermal conductivity of the material, J_q is heat flux, $\dot{e}_{\text{mt}}$ is the rate of heat removed with wear particles, $\dot{s}_{{g_{\text{mt}} }}$ is the rate of entropy flow of the matters from the control volume, η_k and N_k are chemical potential and number of moles of species k. Equation (6) is adapted for the present work for determining the entropy generation during the degradation of grease under mechanical shearing.

Fig. 19

Considered grease system with control volume

Since the present work focuses on the mechanical degradation of grease, the contribution of chemical reaction during the shearing process can be considered negligible. Also the system considered is a closed system, the transfer of the matter to the control volume does not prevail. Assuming that the permanent mechanical degradation of grease chain occurs at the shear of $\dot{\gamma }_{p}$ and shear stress of τ, Eq. (6) reads:

$$\dot{s}_{g} = \frac{{\tau :\dot{\gamma }_{p} }}{\rho T} + k\frac{{\left( {{\text{grad}} T} \right)^{2} }}{{T^{2} }}$$

(7)

Further, in the present work, the grease is sheared at room temperature the change in temperature can be neglected. Now, using Eq. (7), the entropy generation rate density inside the grease during the shearing process after time t can be represented as:

$$\dot{S}_{{g,{\text{vol}}}} = \frac{{\tau :\dot{\gamma }_{p} }}{T}$$

(8)

Now, given that permanent degradation of the grease can happen even at lower shear rate, it is logical to assume $\dot{\gamma }_{p}$ ≈ $\dot{\gamma }$. Therefore, the total entropy generation density can be determined by integrating the entropy generation rate till time ‘t’, and Eq. (8) yields Eq. (9).

$$S_{{g,{\text{vol}}}} = \mathop \smallint \limits_{o}^{t} \frac{{\tau \dot{\gamma }{\text{d}}t}}{T}$$

(9)