The Type A PSS Shaft Seal is a mechanical face seal. The sealing surface is created between the flat surfaces of the rotating stainless-steel rotor and the stationary carbon flange. The stationary carbon flange is attached to the front side of the bellow with hose clamps, and the back end of the bellow fits over the Stern tube and is secured with hose clamps. The stainless-steel rotor is fit on the shaft in front of the carbon flange. The stainless-steel rotor is used to compress the bellow before the rotor is secured to the shaft with set-screws. This compression (pre-load) maintains constant contact between the two flat faces of the stainless-steel rotor and carbon flange, allowing the PSS to compensate for the variable fore and aft movement due to propeller thrust. The carbon flange is over-bored to the shaft diameter allowing it to “float” around the shaft and thus compensate for most misalignment and vibration problems. The stainless-steel rotor is sealed to the shaft by two O-rings that are recessed into the bore of the collar. These O-rings rotate with the shaft and stainless-steel rotor and do not experience wear during operation.
Temperature Limits: 5°F to 225°F (-15°C to 107°C)
Pressure Limits: 15 PSI (1 BAR)
Shaft RPM Limits: 10,000 RPM
Carbon Graphite: Lapped to 4 HLB (0.000044″ tolerance)
316SS / Nitronic 50: Faced to 9 Ra
Bellow Material: Molded Nitrile – PVC
HOW DOES IT WORK?
Two nitrile o-rings seal the stainless steel rotor to the shaft and will not experience wear.
A primary set screw and a backup set screw (2 sets) are used to secure the stainless steel rotor to the shaft.
A seal is created between the two flat surfaces of the carbon stator and the stainless steel rotor.
Water is fed into the PSS Shaft Seal for cooling/lubricating the seal faces on high speed vessels.
The stationary carbon stator is attached to the nitrile bellow, which is attached to the stern tube (shaft log).
The rotor is manufactured out of 316L stainless steel and machined to a 9Ra finish on precision CNC lathes. It is secured on the shaft using 2 pairs of set screws positioned at 90 degrees angle for maximum holding power. The carbon stator further polishes the rotor during the initial hours of operations. This rotor does not need replacement or maintenance under normal operating conditions.
The high density, resin impregnated carbon stator is manufactured from a space age composite which is first mixed and molded, then formed under pressure. The blank parts are then baked, machined and lapped to a measured flatness of 4 helium light bands (measured at 0.000044″ of variation over its entire lapped surface). The grade of carbon composite used in the PSS Shaft Seal has a maximum operating temperature of 500°F (+260°C) and cannot melt if the seal runs dry for a short period of time unlike a lip seal or a plastic face seal. The high density of the carbon greatly increases the longevity and wear resistance. Several commercial vessels have recorded over 40,000 (over 4-1/2 years of continuous operation) engine hours on the same, original components. The carbon should not need to be replaced under normal operating conditions.
The bellow on the Type A seal is molded out of a Nitrile compound. Nitrile is known for its good resistance to petroleum products. It provides the best combination of durability, strength and elasticity necessary in this application.
For high speed vessels (12+ knots), a positive water supply is required to be plumbed to the PSS Shaft Seal for the purpose of lubricating and cooling the seal faces. Please review the PSS Shaft Seal Installation Instructions for more information on pickup points for water supply. In most cases, a slow speed boat (less than 12 knots) that does not have a bearing the shaft log does not require positive water feed and can simply be vented. Ideally, the vent line will be run at least 2-3 feet above the water line and will be as close to the center line as possible. This will ensure the vent hose is never below the water line, even with significant heeling. PSS Shaft Seal Installation Instructions has more more details on venting the seal.