# Experimental Study on In Situ Storage of Grease-Lubricated Ball Screws
## 3 Test Results and Analysis
### Influence of Deterioration of Lubricating Greases on the Transmission Performance of the Ball Screw  
(1) Transmission efficiency testing  
The curve of the transmission efficiency of ball screws with the change in axial force is shown in Figure 11. It can be seen that, at the same speed, the transmission efficiency first increased with the increase in the axial force. When the axial load reached 5000-7000 N, the transmission efficiency reached its highest point and then showed a downward trend. The transmission efficiency of ball screws filled with new grease is significantly higher than that of ball screws with retained grease stored.  
Figure 11: Transmission efficiency variation curve of ball screws at 4000 rpm.  
Figure 10: SRV testing results of Grease 7014.  
(2) Friction torque testing  
The curve of the friction torque of the ball screws with the change in axial force is shown in Figure 12. It can be seen that at the same speed, the friction torque increases with the increase in the axial force. Under certain rotational speed conditions, the friction torque of the ball screw with stored lubricating grease increases by about 0.1-0.2 Nm compared to the ball screw with new grease.  
(3) Temperature rise testing  
The temperature increase in the ball screws at speeds of 2000 rpm and 4000 rpm and axial loads of 3000 N and 6000 N are shown in Figures 13 and 14. As the axial force increases, the rate of temperature significantly increases, and the temperature increase rate of the ball screw with stored grease is significantly higher than that of the ball screw with new grease.  
When the speed is 4000 rpm, the axial load is 6000 N, and both sets of ball screws cannot reach thermal equilibrium before the lubricating grease begins to melt. The temperature of the screw rises to 145-152 \({}^{\circ}\)C in a short period of time, and white smoke and an irritating odor appear. At a speed of 2000 rpm and an axial load of 6000 N, the ball screw with new grease reaches thermal equilibrium at around 130 \({}^{\circ}\)C, while the ball screw with stored grease cannot reach thermal equilibrium before the grease melts. It was indicated that the heat dissipation capacity of the ball screw significantly decreased after the grease deteriorated.  
Under the same operating conditions, the ball screw with new grease runs relatively smoothly, with a gentle increase in temperature and a certain degree of curvature in the temperature rise curve. However, the ball screw with stored grease generates more intense heat, and the temperature rises sharply at high speeds and high loads, presenting a straight line in the temperature rise curve. In harsh working conditions, the ball screw with new grease reaches the melting point of the grease, and the time for smoking and irritating odors to appear is significantly later than that of the ball screw filled with stored grease. And, under the same operating conditions of 2000 rpm and 6000 N, the ball screw with new grease can reach a steady-state temperature, while the temperature of the screw filled with stored grease continues to rise and does not show a stable trend when it reaches the melting point of the lubricating grease. It was indicated that after the performance of the lubricating grease deteriorated, the ability to absorb and transfer heat decreased, which could not effectively help the ball screw pair dissipate heat and maintain the normal working temperature of the screw, so the damage was caused by the overheating of the ball screw. Therefore, the degradation of lubricating grease has a significant impact on the operation and lifespan of ball screws.  
Figure 12: Friction torque variation curve of ball screws at 4000 rpm.