# Experimental Study on In Situ Storage of Grease-Lubricated Ball Screws
## 2 Test Scheme Design
### Performance Testing of Lubricating Grease  
Based on the results of pretesting and the summary in the literature, the main degradation types of lubricating grease can be determined. The main ways of lubricating grease degradation and failure are physical degradation and chemical degradation. Physical degradation refers to all physical changes that occur in lubricating grease during storage or use, which are highly sensitive to the shear effects under working conditions. The main failure modes include the destruction of thickeners (mechanical shear degradation), base oil separation, base oil evaporation, grease softening, and contamination by particles (water, metal). Chemical degradation refers to all chemical changes that occur during the lifespan, producing oxidation products such as ketones, acids, hydrogen peroxide, alcohols, etc., which are sensitive to aging and temperature. The main failure modes include the depletion of additives (antioxidants), thickener oxidation, and base oil oxidation. As shown in Figure 2, the degree of physical and chemical degradation can be characterized by indicators such as appearance, oxidation properties, friction properties, and rheological properties. The changes in appearance of lubricating grease can be preliminarily observed and compared through human eyes, and a more detailed microscopic morphology can also be obtained through an electron microscope scanner (SEM) (ATA Scientific Pty Ltd., Caringbah, Australia) [10], transmission electron microscopy (TEM) (JEOL Ltd., Tokyo, Japan) [11], and atomic force microscopy (AFM) (Oxford Instruments Asylum Research, Oxford, UK) [12]. The appearance of lubricating grease was compared through human-eye observation, and the microstructure of lubricating grease was compared and analyzed through SEM testing in this article. The oxidation performance of lubricating grease can be measured by the chemical composition of its components and the oxidation stability of the grease. Detection methods such as Fourier transform infrared spectroscopy (FTIR) [13], X-ray, gas chromatography-mass spectrometry (GCMS) [14], and chromatography (SEC) can analyze the chemical composition of lubricating grease, identify the consumption of additives in grease, and measure the oxidation level of grease. The oxidation stability of lubricating grease can be characterized by the oxidation induction period. High-pressure differential scanning calorimetry (PDSC) [15] can compare heat release and absorption and identify the oxidation induction time of lubricating grease at different temperature conditions. The friction performance of lubricating grease is mainly characterized by the following two indicators: friction coefficient and wear resistance, which can be measured using micro friction and wear testing machines, the German abbreviation of which is SRV: Schwingung (vibration in English), reibung (friction in English) and verschleiss (wear in English) [16]. The four-ball machine can test the extreme pressure performance and wear resistance of lubricating grease. The rheological properties of lubricating grease include viscoelasticity and flowability, which can be measured by a rotational rheometer [17]. Due to the small amount of lubricating grease extracted from the ball screw raceway and considering the testing cost, the preferred testing scheme selected in Figure 2 is the one \begin{table}
\begin{tabular}{c c c} \hline \hline
**Order Number** & **Project** & **Conditions** \\ \hline
1 & storage location & Beijing \\
2 & placement status & lay flat and stand still \\
3 & lubrication status & filling with lubricating grease \\
4 & packaging status & wrapped in plastic protective film \\
5 & air circulation status & good \\
6 & sunshine status & no sunlight exposure \\
7 & transportation situation & no handling \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Storage situation of ball screw. with less grease extracted and more reasonable testing costs.