Navigating drive shaft seal failures
Question: “Dear Ken, one of our Aquamaster 631 on board ship drive units is experiencing an ingress of seawater into the hydraulic gearbox unit while leaking lubricant into the sea, I believe this is due to the numerous drive shaft seal failures we have experienced in the past. I have included some photos of the oil and unit reservoir sight glass and would like some advice I can share with management to help resolve this occurring problem.
Shaft seal failures on hydraulically driven azimuthing “rudder-propeller” thrust drive/steering systems are commonly accepted due to their harsh operating environment.
There are numerous known reasons for this type of seal failure that include propeller blade pressure pulsing that can overcome the static head pressure on the oil forcing seawater into the system past the seal, tangled debris (fishing line, rope, plastics, etc.) wound around the propeller/shaft abrading the seal, aged and worn seal elastomer, solids contamination in the oil abrading the seal from within, fitting a new seal on a worn shaft. Although the seal is an inherent weak point in the system design, it is a physical maintenance item that can be managed successfully if inspected and attended to on a regular basis when showing signs of wear.
One of the biggest “tell tale” signs of seal compromise is the condition of the oil. Based on the “milky” appearance of your oil you appear to have water contaminated oil that if left unmanaged could result in premature failure of your internal drive system components and expensive repairs, downtime and towing/docking losses—especially if the failure occurs under sail!
Water diluted lubricants—especially those with salt—are very harmful for mechanical systems and can result in the following:
- Hydrolysis and Oxidation promoting physical change in the base oil resulting in the oil thickening and the formation of corrosive acid, varnish and sludge
- Aeration, promoting foaming and air entrapment leading to additive depletion (lubricant breakdown) and vaporous cavitation in pumps
- Corrosion through water accelerated rust formation on gearbox drive components and bearings
- Lubricant film strength loss that can cause hydrogen embrittlement and premature bearing failure, particularly in rolling element bearings
- Filter blockage due to silting causing poor filterability and valve stiction in engines or hydraulic spools
- Rapid wear of all ferrous (iron and steel) components
Based on a common sense approach I would recommend you consider performing the following:
- Ensure the drive shaft seal has no debris entanglement, and check for erosion wear – if wear has taken place, the new seal will leak just as much as the old and will require a shaft change or redesign of the seal arrangement.
- Ensure you are using the correct specification of oil
- Perform a wear particle oil analysis on current virgin stock oil and perform a comparative analysis on the milky looking oil sample presently in the drive unit. This will tell you what should and shouldn’t be in the oil, the current contamination, oxidation, additive depletion, viscosity and acidity levels. From there you can better ascertain potential failure points and time lines
- Check unit integrity and identify no other leakage areas are present, and that all clean outs, fill points, and lines are intact and in place so no saltwater can ingress or egress. Repair all compromised areas
- Check if system breathers are plugged or missing plugged as this can be a contamination source and cause positive tank pressure that could be the root cause of the leaking seal. Always use a desiccant style breather.
- Check for a plugged oil filter(s) and perform oil and filter change ASAP. Your drive unit will likely need to be flushed to evacuate all seawater. Contact your lube supplier to recommend the correct flushing oil and procedure, then change your oil on a more regular basis until the contamination problem is resolved.
- Investigate the installation of an active bypass (offline) filtration system using filtration media designed specifically for thruster oil and operate in continuous filtration mode.
- If your vessel operates in cold water, consider installing a preheater to enhance filtration and water separatio.
Although seal failure is inevitable, many things can be done to ensure this doesn’t happen prematurely, and to mitigate the damage when it does occur!
For more information on implementing a best practice maintenance management program contact Ken Bannister by phone at 519 469 9173, or email him at firstname.lastname@example.org.