###### Abstract  
The operating efficiency of the hydraulic system depends on the response speed and driving efficiency of the hydraulic components. Seeking various methods and technologies to improve performance and work efficiency is an important work for many scholars. In this paper, the development and performance improvement of four generations of hydraulic cylinders are analyzed to obtain the development of the drag reduction technology by examples such as sealing methods, microtextured hydrodynamic lubrication, manufacturing materials, drive methods, control methods, etc. The reviewed results show that the drag reduction and efficiency improvement of hydraulic cylinders are affected by many factors, and the new drag reduction theories, advanced drag reduction technology, and emerging materials point toward the direction of the acceleration of hydraulic cylinders. Finally, according to the higher requirements for future cylinders, four new ideas are proposed for improving the speed performance of hydraulic cylinders, including new material sealing ring with low friction coefficient or self-lubricating, internal leakage suppression from smart material, implementation of servo control techniques and algorithms, and digital hydraulic technology. These new viewpoints may provide some methods and references for the speed improvement of hydraulic cylinders. hydraulic cylinder; response speed; gap sealing; hydrodynamic lubrication +
Footnote †: journal: 25 February 2022  
At present, there are many servo-hydraulic cylinders in the world, such as Parker, Hanchen, Boatke, etc. These are widely used in many large and heavy-duty metallurgical fields, such as continuous casting machines for crystal vibration, electric vibration tables, structural fatigue testing machines, steel rolling mills, and other important places of metallurgical production. Some working parameters of servo-hydraulic cylinders in some metallurgical equipment are listed in Table 1.  
Although servo-hydraulic cylinders have a wide range of applications, similar to traditional hydraulic cylinders, they also suffer from leakage and friction, as shown in Figure 1b. The contradiction between the friction and leakage of the seal interface has always been a problem that restricts speed performance since the development and application of hydraulic cylinders. For example, under an oil pressure of 30 MPa, the internal \begin{table}
\begin{tabular}{c c c c c c} \hline
**No** & **Application** & **Maximum Load (KN)** & **Maximum Frequency (Hz)** & **Acceleration (m/s)** & **Speed of Piston (m/s)** \\ \hline
1 & Continuous casting machines [6] & 100 & 20 & / & 0.5–1 \\
2 & Six DOFs electrodynamic vibration tables [7] & 25 & 2000 & 1.3 & 1.78 \\
3 & Structural fatigue testing machines [8] & 10 & 100 & / & 0.5–1 \\
4 & Steel rolling mills [9] & 10 & 20 & 0.2–0.4 & 0.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Working parameters of servo-hydraulic cylinders in some metallurgical equipment. Figure 1: A hydraulic servo control system: (**a**) an actual diagram of servo-hydraulic cylinder; (**b**) working principle diagram of the servo-hydraulic cylinder. Figure 2: A servo-hydraulic cylinder with pump direct-driven system [4; 5]. leakage of the Hanchen-306 hydraulic cylinder can reach 350 mL/min [10]. Therefore, less leakage and good sealing performance are key to improving the response speed of the servo-hydraulic cylinder. Domestic and foreign scholars have carried out much research on the sealing and leakage of hydraulic cylinders including the technology of sealing methods, structure optimization, and manufacturing materials, which have made significant progress and greatly improved the response speed performance of servo-hydraulic cylinders [11; 12].