# Research on the Reciprocating Friction and Installation Force of Seals in Deep-Sea Samplers
## 1 Introduction  
Deep-sea scientific research and resource exploration depend on a variety of operational equipment, including submersibles, robotic arms, and samplers [1, 2, 3]. O-ring seals are widely used in deep-sea devices due to the advantages of simplicity, reliability, durability, and cost-effectiveness [4, 5]. Rubber becomes harder, stiffer, and less resilient with decreasing temperature of the deep sea [6]. An accurate evaluation of the influence of the deep-sea environment on O-ring friction is essential for seal design and improving the efficiency of deep-sea equipment. Scholars have conducted extensive research on the friction of O-ring seals through theoretical modeling, numerical simulations, and experimental testing [7, 8, 9, 10]. For O-rings in deep-sea operating conditions, [PERSON] et al. [11] employed polytetrafluoroethylene (PTFE)-coated O-rings in a deep-sea water sampler to minimize the friction of O-rings, and the sampler was successfully deployed at a depth of 3930 m. [PERSON] et al. [12] studied the static sealing characteristics of combined seals for deep-sea sediment samplers under high pressure by numerical simulation and confirmed the accuracy of the numerical results through pressure-holding experiments. [PERSON] et al. [13] investigated the volumetric shrinkage of O-rings caused by increased ambient pressure and analyzed the static sealing properties ofO-rings using finite element analysis (FEA) method. [PERSON] et al. [14] designed a non-standard O-ring sealing structure for ball joints used in deep-sea drilling apparatus and validated the reliability of their design through pressure retention experiments. [PERSON] et al. [15] conducted both simulation and experimental investigations on the extrusion failure of O-rings in deep-sea environments, deriving the critical failure curve of O-rings. [PERSON] et al. [16] studied the sealing performance of O-rings in deep-sea wet-mate electrical connectors under a fluid pressure of 30 MPa. The results highlighted that employing a pressure-balanced seal structure can significantly reduce both friction and leakage. [PERSON] et al. [17] investigated the factors influencing the sealing performance of submarine electrical connectors. They examined the effects of the friction coefficient, compression rate, and other variables on the distribution of sealing contact pressure under a hydrostatic pressure of 15 MPa. [PERSON] et al. [18] analyzed the aging phenomenon of NBR O-rings used in a 6000 m class deep submersible named Ifremer Nautile after 10 years of operation. They proposed a testing method to predict the long-term sealing performance of these nitrile O-rings. However, there are relatively few studies in the existing literature regarding the reciprocating friction of seals in ultrahigh-pressure and low-temperature environments. Most research on the sealing characteristics of seals in deep-sea environments has primarily concentrated on the static sealing reliability, while their dynamic friction characteristics have predominantly been investigated through experimental methods. Furthermore, there is a notable gap in the research concerning the installation force of seals. Figure 1 illustrates the source of the practical problem in this study. In a field test at the Mariana Trench, the sampling piston of the full-ocean-depth pressure-retaining amphipod sampler named PMTI (Figure 1a) [19] developed by the research team was not fully retracted into the pressure vessel (Figure 1b). Considering the temperature of the operation site is approximately 2 \({}^{\circ}\)C [20]. It is hypothesized that the increase in the modulus of elasticity, hardening, and brittleness of rubber at low temperatures are the primary factors contributing to this failure. O-ring 2 was drawn into the pressure vessel in a manner similar to the installation of a seal, leading to a significant increase in axial movement resistance. As a result, the motor stalled due to excessive load. This malfunction has also been observed in a sediment sampler [21].