gland
A compressable cylindrical case and its contents around a shaft where it passes through a barrier, intended to prevent the passage of a fluid past the barrier, used for example around a ship’s propeller shaft or around a tap, valve or faucet.
gland: the seal that stops leaks around moving shafts
A gland is a mechanical seal assembly that wraps around a rotating or reciprocating shaft to prevent fluid escape where that shaft pierces a pressure barrier, wall, or casing. It consists of a cylindrical body, typically brass or ductile iron, with a hollow centre through which the shaft passes. Inside sits packing material, traditionally cotton rope soaked in graphite or tallow, now more often PTFE (polytetrafluoroethylene) or synthetic fibres, which compresses around the shaft as a threaded collar (the gland nut) tightens.
The packing deforms slightly under compression, conforming to the shaft's surface and creating a tortuous path that restricts flow. Pressure is applied axially: as the nut turns down, it forces a cone-shaped follower or stuffing box sleeve against the packing layers. The seal is not absolute, a small controlled weep or drip is normal and expected, both to confirm the seal is working and to allow the packing to stay lubricated and cool. Total blockage indicates over-tightening and will generate heat, accelerating packing degradation.
Common variants and applications
Marine propeller shafts use large bronze glands to seal where the shaft exits the hull, often with multiple packing rings and drain holes. Pump shafts employ smaller glands to contain process fluid or coolant. Valve stems use compact glands to prevent leakage around the handwheel spindle. Some glands incorporate a lantern ring, a grooved spacer inserted midway through the packing stack, which allows a lubricant or flushing fluid to be injected, improving life and controlling leakage rate.
Failure modes include hardening and cracking of aged packing (packing must be renewed periodically, typically every few years depending on service), shaft scoring from abrasive particles trapped at the seal line, and excessive leakage from under-compression. Over-compression causes friction heat, extrusion of packing material into the shaft-packing gap, and premature wear. The nut must be backed off slightly after tightening to prevent locking.
The term itself comes from its resemblance to a gland in anatomy, a small rounded body fitted into a larger cavity. Gland design is largely empirical, though packing materials and follower geometries have become standardized by industrial practice. Mechanical seals and cartridge seals have replaced glands in high-speed or sterile applications, but the traditional packing gland remains the standard for low-speed shafts, for repairs, and in environments where simplicity and low cost outweigh the need for perfect sealing.