Mechanical seals rely on the hydrodynamic effect to develop the liquid film between the rotating ring and the stationary ring in order to improve the tribological characteristics. However, when the speed is low during the start-stop period, the hydrodynamic action is weak, the load capacity of the film is decreased, and the effective oil film cannot be well developed, which will lead to a dramatically increased friction and wear between the rings. At present, the logarithmic spiral groove structure is one of the most effective methods to improve the load capacity of mechanical seals. It is simple in design but has good effect of enhancing the hydrodynamic action, and has been widely used. There are many researches focus on the geometric optimization of spiral groove, the majority of them are to reduce the leakage rather than to improve the load capacity, so there is a serious problem of wear. In other studies, intricate structures and shapes are proposed to improve load capacity, however, the inherent advantages of the original spiral groove geometry lie in its simplicity and ease of design and manufacture.

In order to further improve the load capacity, opening capacity and wear suppression ability of the logarithmic spiral groove during the start-stop period while keeping the original groove type, a new stepped spiral groove with a step along the circumferential direction was proposed. The analytical lubrication model of stepped spiral groove seal was established, combined with the Jakobsson-Floberg-Olsson (JFO) cavitation boundary condition to solve the cavitation problem that always occurred in the spiral groove, and the finite volume method was used to calculate the tribological properties with different geometrical parameters and operating conditions.

The traditional flat-bottomed spiral grooves with primary groove depth d₁=1, 3 and 5 μm and their corresponding stepped spiral grooves were systematically analyzed and compared under different working conditions, in which secondary groove-to-land ratio β=0.3, and the secondary groove depth d₂=2d₁. The load capacity, stiffness, cavitation rate and opening speed were obtained for a fixed film thickness, as well as the film thickness and friction torque at a fixed load. The results showed that the load capacity and film stiffness of the stepped spiral groove were much greater than that of the corresponding flat-bottomed one, while the opening speed and cavitation rate were smaller than that of the traditional flat-bottomed one under the same conditions.

Based on the results, the principle of load capacity enhancement of the stepped spiral groove was analyzed intensively. It could be found that, the circumferential variable depth stepped spiral groove introduced an additional Rayleigh bearing effect on the original flat-bottomed one. The additional hydrodynamic pressure enhanced the load capacity at the step, significantly reduced the area of the cavitation region that did not provide an effective load capacity, and provided an enhancement of the load capacity in the pressure peak area at the termination line of the groove. From the numerical comparison, under the working condition of film thickness H₀=1 μm and rotational speed ω=50 r/min, the peak pressure of the stepped spiral groove was increased by 7.14%, the cavitation rate was reduced by 93.42%, and the total load capacity was further increased by 19.83% on the basis of the optimized flat-bottomed spiral groove. It could be concluded that the additional Rayleigh bearing effect due to the introduction of a step at the bottom of the spiral groove was the main mechanism for the stepped spiral groove to effectively improve the load capacity and opening capacity.