Maintaining and Troubleshooting PSS Shaft Seals

A dripless shaft seal eliminates the maintenance and wet bilge that comes with a conventional stuffing box. Though relatively trouble free, problems do arise. Here a professional offers the ins and outs of the PSS Shaft Seal, one of the more popular units.

—By Jan Mundy

The PSS Shaft Seal is a mechanical face seal, which creates a watertight seal by the contact of a rotating surface (rotor) against a stationary surface (carbon flange). A rubber bellows attaches this flange to the boat's shaft log. The stainless-steel rotor connects to the propeller shaft and is positioned during installation to compress the bellows. This compression applies the pressure necessary for the two faces to remain in contact during operation.

When installed correctly, a PSS shaft seal is, for the most part, maintenance and trouble free. This presupposes that no petroleum products were used during assembly, the exact number of setscrews fastens the rotor, you have properly tensioned the bellows and, on a sailboat, the hose barb (on new models only) connects to a vent hose to burp the bellows after launching and, on a powerboat, the hose feeds cooling water into the seal. The hitch is that boats are more diverse than cars and each one has a distinctive set up.

J.C. Milton of Spurs (www.spurs.com) based in Ft. Lauderdale, Florida, has for many years been instructing commercial and recreational boaters in the technicalities of engine components. A former commercial diver, licensed operator who has skippered private yachts on most of North America's waterways, aircraft aficionado and jack-of-all-trades with an “isn't much I can't do and haven't done” attitude, J.C. agreed to share his professional experiences on troubleshooting and maintaining the PSS dripless shaft seal.  

Problem: The boat is docked, the engine isn't running and you check the engine compartment to find that there's water dripping from the seal.

Solution: There are several places a seal can leak. Make sure the hose clamps securing the seal to the shaft log are tight. Foreign material even as fine as a one grain of sand, between the rotor and carbon graphite flange, results in seepage. To clean the faces of these surfaces without disassembly, force them apart by compressing the bellows, run a clean cloth around the unit once between the mating surfaces and then pull out the cloth so the seals reconnect. Incoming water flushes the debris and the leak should stop. Do this quickly to limit the quantity of water gushing out the seal and into the bilge.

Problem: After cleaning the mating surfaces water continues to drip from the seal.

Solution: Improper tension on the bellows also causes the seal to leak. Compression guidelines provided in the owner's manual may differ with each individual installation. Clamping the bellows to a smooth, shaft log with no lip for the back of the bellows to butt up against, for example, may push the bellows, when compressed, over the log and thus reduce bellows pressure. Excess vibration, in some instances, causes the rotor to slide up the shaft and take pressure off the bellows. If you suspect vibration, remove the top two rotor setscrews and then slowly back out the two bottom setscrews until the bellows relaxes. Note that there are 4 setscrews in 2 holes. Now, recompress the bellows to the distance recommended in the installation manual and then reset the rotor. If leaking persists, reface the rotor and carbon flange using 600-grit wet or dry sandpaper (see method in next Solution below). Check that the hose barb (if equipped) is not broken and that the hose clamp is secure. Tip: A shaft zinc installed just ahead of the rotor will give peace of mind to vibration problems.  

Problem: At high rpm, a fine black mist emits from the shaft seal.

Solution: A black mist occurring after the break-in period (one hour) could mean incorrect bellows tension. Recompress the bellows by squeezing it together to the chart recommendations in the installation manual. If black mist continues coming from the seals there might be some foreign material on mating surfaces. To clean the seals, take 600-grit wet/dry sandpaper, folded in half, and compress the bellows to open the gap be-tween the rotor and carbon flange. Slide the sandpaper between the two surfaces and run the sandpaper around the mating surfaces about 10 times. Alternatively, wait until your next haulout and, once on land, loosen the rotor and slide it up after thoroughly cleaning the shaft. Use a whitening style toothpaste, which contains baking soda, to clean the rotor face and carbon flange. Apply the toothpaste to the rotor then clean by lapping it over the carbon flange and turning to "grind" both surfaces. Wipe both faces with acetone and a clean rag. Polishing both to a mirror finish should stop the leak.

TIP: No Lock Solutions

Never use Loctite or other petroleum-based products when fastening the four stainless-steel setscrews in the rotor.

Problem: When running the engine, water sprays from the seal.

Solution: A drop of oil on the seal faces can result in the seal spraying water. As the rotor rubs against the flange it creates a chatter and water mist. To repair, sand with 600-grit wet/dry sandpaper as detailed above, scouring the mating surfaces about 10 times. Keep the seal as free of oil and grease as possible.

Problem: White deposits have built up along the edge of the rotor and carbon flange.

Solution: This is normal for boats run in saltwater. As the shaft spins, the salt crystals dry on the top edge of the rotor and carbon. Salt doesn't harm the seal. Just wipe off the residue with a damp cloth.

Problem: The engine is running and suddenly there's a high-pitched squeal coming from the seal.

Solution: Shutdown the engine immediately. Such sounds result from insufficient cooling water on a powerboat installation. A dry running seal becomes extremely hot, as high as 400°F (204°C), so never touch one for a heat check. You might detect a burning rubber smell as the carbon flange comes in contact with the bellows. Note that when connecting the cooling water hose, it doesn't matter whether it's teed into the hot or cold water side of the heat exchanger. What's critical is that the water supply comes from as low in the system as possible and before any pencil zincs. Pencil zincs erode and could conceivably plug the water intake hose or hose barb and destroy the seal. On twin engine installations, plumb the seals may be to draw water from either engine in case one engine fails.

Problem: How often do I need to replace the bellows?

Solution: Proactively, replacing the bellows at least every eight years is a good practice, regardless of condition. At least once a year, pinch the bellows while installed and using a good flashlight check for small cracks. Any deterioration and it's time for repla-ce-ment.  

Problem: Does a seal that's saltwater cooled require routine checking for salt deposits in the hose?

Solution: Cooling water flows under pressure so it keeps salt deposits from clogging the intake hose. Check that the clamp securing the hose to the hose barb is tight and there are no leaks. Be careful when working around the seal. Stepping on this hose barb fitting could break it.

Problem: When do the O-rings need replacing?

Solution: Nitrile O-rings in the rotor are static in relation to the shaft. As the shaft, O-rings and rotor all move at the same speed there’s no wear on the rings. Once installed, they require no maintenance even a shaft misalignment won't cause O-ring wear; however, always replace O-rings and setscrews any time the rotor is remo-ved from the shaft.

The PSS Shaft Seal is surely an improvement over the conventional stuffing box. But like any thru-hull fitting it's important to be aware of its idiosyncrasies. With periodic visual inspection this shaft seal will deliver years of trouble-free service.