The HY-I type constant-level oil cup is a type of oil level gauge designed to maintain a fixed oil level in the bearing housing. It also serves as a viewing window, allowing operators to determine whether additional lubricant is needed based on the oil level inside the cup.

The oil cup is connected to the bearing box oil chamber via the base, and the oil cup is linked to the base through hinges. This forms a communicating vessel. During operation, the operator should fill the bearing box by injecting oil through its vent hole. When the lubricating oil flows out from the holes in the base, the oil cup should be secured onto the base. Oil should be added to the box by placing the filled oil cup onto the base until the oil level in the cup stops dropping, maintaining it at two-thirds of the cup's capacity. Due to the principle of the communicating vessel, the internal gas pressure in the oil cup is lower than the external atmospheric pressure, allowing the lubricating oil to maintain a stable liquid level.

When the constant-level oil cup is in the replenishment state, the lubricating oil in the bearing housing decreases due to various reasons, causing the oil level in the housing to drop. According to the principle of communicating vessels, the oil level at the inclined surface of the oil cup falls below the working oil level point. Under the influence of atmospheric pressure, external gases enter the oil cup through the upper end of the inclined surface. This disrupts the pressure balance inside the oil cup, allowing lubricating oil to flow out of the oil cup and be injected into the bearing housing. The replenishment process continues until the lubricating oil level returns to the working oil level point.