In the early 1900s – around the time that naval vessels were first experimenting with diesel engines – another important innovation was emerging at the other end of the propeller shaft line.
Across the first half of the twentieth century the pump mechanical seal became the standard interface between the shafting arrangement inside the ship’s hull and the components exposed to the sea. The new technology offered a dramatic improvement in reliability and lifecycle compared to the stuffing boxes and gland seals that had dominated the market.
The development of shaft mechanical seal technology continues today, with a focus on enhancing reliability, maximizing product lifetime, reducing cost, simplifying installation and minimizing maintenance. Modern seals draw on state-of-the-art materials, design and manufacturing processes as well as taking advantage of increased connectivity and data availability to enable digital monitoring.
Before Mechanical Seals
Shaft mechanical seals were a remarkable step forward from the previously dominant technology deployed to prevent seawater from entering the hull around the propeller shaft. The stuffing box or packed gland features a braided, rope-like material that is tightened around the shaft to form a seal. This creates a strong seal while allowing the shaft to rotate. However, there are several disadvantages that the mechanical seal addressed.
Friction caused by the shaft rotating against the packing leads to wear over time, resulting in increased leakage until packing is adjusted or replaced. Even more costly than repairing the stuffing box is repairing the propeller shaft, which can also be damaged by friction. Over time, the stuffing is likely to wear a groove into the shaft, which could eventually throw the entire propulsion arrangement out of alignment, resulting in the vessel requiring dry docking, shaft removal and sleeve replacement or even shaft renewal. Finally, there is a loss of propulsive efficiency because the engine needs to generate more power to turn the shaft against the tightly packed gland stuffing, wasting energy and fuel. This is not negligible: to achieve acceptable leakage rates, the stuffing must very tight.
The packed gland remains a simple, failsafe option and is often still found in many engine rooms for backup. Should the mechanical seal fail, it can enable a vessel to complete its mission and return to dock for repairs. But the mechanical end-face seal built on this by boosting reliability and reducing leakage even more dramatically.
Early Mechanical Seals
The revolution in sealing around rotating components came with the realization that machining the seal along the shaft – as is done with packing – is unnecessary. Two surfaces – one rotating with the shaft and the other fixed – placed perpendicular to the shaft and pressed together by hydraulic and mechanical forces could form an even tighter seal, a discovery often attributed to engineer George Cooke in 1903 . The first commercially applied mechanical seals were developed in 1928 and applied to centrifugal pumps and compressors
Post time: Oct-27-2022