# Experimental Study on In Situ Storage of Grease-Lubricated Ball Screws
## 1 Introduction  
Ball screws are one of the mainstream transmission mechanisms in the actuation system, and its lubrication method is grease lubrication. Lubricating grease can not only separate the rolling surface but also can absorb and transmit the frictional heat of the screw. The lubrication space of the ball screw is relatively narrow, and the lubricating grease agglomerates and tightly adheres to the contact surface after volatilization and deterioration (as shown in Figure 1), such as the bottom of the screw raceway and nut raceway, between the steel balls and in the inverter pipeline, causing an increase in the working of the starting torque, which can affect the working performance of the servo actuator. Figure 1: Agglomeration phenomenon of lubricating grease after long-term storage. Researchers at home and abroad have conducted research on the correlation between the failure of lubricating grease and its components. The composition of the base oil, the refinement degree of the base oil preparation process, the reactivity of the thickener, the type of metal ions, and the composition of the composite formula can all affect the antioxidant properties of lubricating grease [1; 2]. For the oxidation stability of lubricating greases with different metal-soap bases as thickeners, the higher the content of metal-soap-based thickeners in the same series, the stronger the oxidation stability of lubricating greases. Non-soap-based lubricating greases have better oxidation stability compared to metal-soap-based lubricating greases. Lubricating greases prepared from moderately refined base oils have better antioxidant capacity than greases prepared from unrefined or excessively refined base oils [3]. Different types of antioxidants have different antioxidant properties for the same lubricating grease. The synergistic use of phenolic amine and bisphenol antioxidants can make various types of lubricating greases exhibit good oxidation stability and thermal stability [4]. In addition, researchers also studied the relationship between the failure of lubricating grease and major external environmental factors, such as temperature, metal type and humidity. Temperature increases accelerate the oxidation of lubricants [5]. The thermal oxidation of lubricating grease not only damages its own lubrication performance, but its by-products also have an impact on the surface of the friction pair. Metal corrosion and lubricant oxidation are also interrelated. The interaction between lubricant oxidation byproducts and metal surfaces can lead to metal surface corrosion [6], and metal corrosion products can catalyze the oxidation of lubricants, leading to the formation of sludge deposits in lubricating grease. Different compositions of metal surfaces [7] and different valence states of metals [8] have different catalytic rates for lubricating grease. In addition, the presence of water also has adverse effects on the storage of lubricating grease. When lubricating grease is stored in an environment where water, the catalyst (copper) and hot air are present, as the storage time increases, the hydrolysis of the base oil becomes more severe, and the acid produced by hydrolysis further promotes hydrolysis, forming a vicious cycle [9]. Based on the above analysis, various components of lubricating grease are affected by factors such as temperature and humidity during atmospheric storage, which can easily cause base oil volatilization and oxidation deterioration, leading to performance degradation and decreased lubrication. Studying the performance changes in lubricating grease used for ball screws during in situ storage and its influence on the transmission performance of ball screws helps to explore the friction lubrication mechanism of lubricating grease under typical external factors (temperature, humidity, oxygen, etc.