Better lubrication practices key to extending bearing life
Lubricant contamination is the leading cause of bearing failure.
The main reason a pump leaves service is failed mechanical seals or worn-out packings and sleeves. The second most-common problem is failed bearings. Worn-out/damaged mechanical seals are usually caused by operations; failed bearings are not. Bearings are expensive and the process of changing them is more involved than many people realize, which is why one in every five bearings is either improperly installed or handled.
More than two-thirds of pump bearing failures are the result of improper operating conditions. The bearings installed on most pumps by the pump manufacturers are deliberately over-sized and have the capability of surviving for years in the very worst operating conditions, assuming proper installation and lubrication. The most common mode of bearing failure is not fatigue, but contamination of the lubricant. Properly-sized pumps, operated by well-trained people, do not fall into this category.
Bearings are designed as non-wearing parts. They are supposed to maintain a thin film of lubricant between all of their elements. The oil film varies in thickness depending on the viscosity of the lubricant, bearing size, and the speed of rotation. The oil film called elasto-hydrodynamic lubrication is measured in microns, and a true lubricating film must maintain at least a 1-micron thickness. Lubrication is an increasingly expensive and complicated product, and should not be viewed as a commodity. If your lubrication vendors are not a valuable a resource, they are a cost.
Improper lubrication is involved in one-third of all bearing failures. Keeping water out of the lubricant will help it reach its full service life, and must be a primary concern. A major oil company documented that 0.02% water cut the service life of petroleum-based oils by 48%. This study is ancient history, done in the 1970s and accepted for years, but knowledge is useless if not utilized. Consider how minute a value that is compared to all the possible sources of contamination: aspiration through worn lip seals, wash-down hoses, pumps exposed to the outside weather, etc. Bearing fatigue life is proportional to the third power of the bearing load.
Aspiration and contamination
As bearings work, they create heat. This causes expansion of the air inside the bearing housing that passes out through any possible gap, most commonly the worn-out lip seals. Once a pump is stopped and the bearings cool, the process reverses and outside air enters the bearing housing, along with its moisture. This process is called aspiration.
Unfortunately, most pumps-particularly older pumps-are not equipped with effective lubricant protection.
Lip seals were designed for water pumps around the time of World War II. They wear out in less than 4 months of continuous pump operation, and moisture and dirt in the environment surrounding pumps end up on the inside of the bearing housing. Lip seal manufacturers are well aware of the lifespan of the product. Lip seals are cheap for a reason.
The use of bearing isolators (labyrinth seals) will provide better protection for bearings, and they do not wear the shaft. They provide both a "tortured path" for moisture and create centrifugal force to keep moisture from entering your bearing housing. Lip seals should be upgraded as part of every pump repair. You should also make certain to install a bull’s-eye sight glass at this repair opportunity.
Many lubricants have additives to increase service life and performance. Improper selection of these lubricants can leach materials from castings and may cause sulfates to form in the oils, cutting short their service life. Zinc salts are commonly added to lubricants, included as antioxidant and anti-wear additives. These lubricants have a 6- to 12-month service life, after which the salts drop out of suspension from air and moisture contamination.
Oil analysis must be a fundamental task for any real maintenance approach to improving plant equipment availability. Ferrographic analysis of lubricating oils can identify water contamination, metal particulates from disintegrating bearings, particulates from oil-additive depletion, and other lubricant failures.
The proper oil sump level is at the midpoint of the ball in the 6 o’clock position, when the bearings are at rest. This ensures a flinging of oil throughout the housing, which distributes the oil properly and dissipates heat. An oil-level sight glass installed in the correct position ensures the oil level in the housing can be observed. Installation and replacement of sight glasses should be a part of your standard pump overhaul procedure. Over-lubrication can cause churning and frothing in the oil, leading to a sharp increase in oil temperature.
If a pump’s wet-end application is over 400° F, insert a cooling coil in the sump to cool the oil and not the bearings or housing. Cooling the frame increases the load on the bearing and will increase the heat generated. Placing a hose spewing cold water on a hot bearing housing makes the situation worse!
If you use automatic oilers, make sure your people know what they are doing when installing them and setting the levels. These units cause a lot of easily avoidable damage when they are improperly mounted, and proper installation is critical to performance.
Once bearings start to lose material, they significantly contribute to their own demise. The particulates spalling off a bearing may be as small as 5 microns to 10 microns in size. If these particulates get between the bearing elements, they disturb the film thickness and start to generate rapid temperature spikes consistent with the blackened and burnt oil seen when a pump is taken apart.
Dust in the lubricant works the same way. You cannot eliminate the dust, so you must upgrade the protection. Dust is typically a soft material, but those particulates can actually dent the stainless steel where they are trapped between metal surfaces when the lubrication film is not properly maintained.
Bearing failure analysis
Examine failed bearings the same way you examine failed mechanical seals. Most failures will give at least a hint as to their mode of failure. Five min spent by well-trained personnel may double or triple the life of the next bearing. Untrained mechanics can damage a shaft or pump housing when removing the old bearing or installing the new one. How a failed bearing is removed is just as important as how the next one is installed; removal can damage mating components and fits, and damaged surfaces under the inner race of a bearing cause high vibration and very short service life.
Look for physical damage on the bearing balls or rollers, on the cage that separates the rolling elements, and on the races. Bearing companies have excellent guides for failure analysis-all you need to do is call and request one. This information is usually provided at no charge and can provide a useful resource in evaluating the causes of failure.
Grease-lubricated bearings are essentially permanently over-lubricated. A bearing should be half full of grease, yet the normal method of adding grease is to pump it in until the old grease comes out of the sealed areas. This ensures that the bearing will have to plow through extra grease, creating more heat, especially at high speeds.
Automatic grease dispensers have great value in any reliability program, particularly where greasing bearings put humans at physical risk. They also keep dust out of grease-packed bearings by producing a positive pressure, oozing grease out, and preventing dust from getting into the bearing housing.
Vertical pumps suffer lubrication starvation as the oil drains from the thicker carrier in response to gravity, especially if the grease was not specified to have good nonseparation properties. Mounting a baffle plate in the housing below the bearing on vertical pumps will help to retain the grease where it is needed, but these are ideal applications for the automatic grease dispensers mentioned previously.
Other contamination sources Is the pump packed?
Some of the leakage through a packed stuffing box will travel down the shaft toward the bearing. Areas where packed pumps are located often are washed down with a spray hose. This represents another source of moisture that a worn lip seal cannot exclude.
Pumps often are located under leaking equipment. They may be placed in dikes that fill up when nearby equipment is overfilled. Processes at a plant produce moisture; some of this falls to the ground/floor as a form of condensation once the saturation point of the air is exceeded. Paper machines, steel mill caster/roll areas, and mining operations are notorious for this extra moisture. Pumps are often outside, fully exposed to the weather. If pumps are located under vessels or tanks, any spillage or overflow may fall on them. Leaking flanges or valves may also drip onto the pumps.
The most common ingress points at a pump are gaps around the shaft from worn-out lip seals and pump vents. Lip seals have a dynamic sealing surface that wears itself away. Most lip seals are worn out within 100 days of continuous operation, by their manufacturers’ own studies. The lip seal may not just wear itself away; it may damage the pump shaft as well, in an area that can have severe stress.
If a pump is operating in a dirty area, dust and dirt will accelerate this cutting action. If the pump is operating well off its Best Efficiency Point (BEP), is poorly aligned, or has pipe strain, the pump shaft may also bend where the lip seal is fretting the shaft. In this scenario, the seal is removing the passivated layer of the stainless steel shaft: that material is a form of ceramic, and that edge just keeps cutting until the lip seal no longer makes any contact.
Many pumps have a vent on top of the bearing housing. These vents were a good idea when our grandfathers were running equipment, but have outgrown their usefulness on most applications in today’s workplace. Upgrading to bearing housing face-to-face seals dictates installing an expansion chamber in place of the vent.
Alignment and vibration analysis
Too many plants rely entirely on vibration analysis to formulate predictive maintenance programs. Vibration readings are regularly taken on equipment that is not properly aligned, suffers from poor foundations or harmonic vibration, or has severe enough pipe strain that a forklift truck must be used to re-install the pump. Proper alignment is a difficult thing to do: it requires skills that many plants don’t have, so you must be willing to pay for contractors to do it right.
Parallel Horizontal and Vertical, Angular Horizontal and Vertical all should be within 2.5 mils at 1,800 rpm and 1.5 mils at 3,600 rpm. Couplings can handle a lot of misalignment: they simply transfer this destructive force from a $50 coupling into a set of $400 bearings and a $1,000 single cartridge seal. Alignment is critical to extending bearing life. Checking the oil on critical equipment is more important than vibration if you are not bothering to align critical pumps properly.
Every rotating assembly should be balanced as a whole before it goes back into the pump. No repair shop should be allowed to work on your equipment if they are not providing an "As Found/As Repaired" report showing you what you sent them and what they are sending you back. Balance is the key to long life for rotating equipment. The tire on your car makes a full revolution around 800 times each mile when you drive at 60 mph, and you’ll notice if you lose a 1-oz weight at 60 mph!
A closer focus on bearing installation, lubrication, and failure analysis has an immediate payback. Greater attention to installation and bearing-removal techniques will minimize damage to important tolerances. Five min of careful inspection of failed bearings often reveals the cause of failure. Better protection against lubrication contamination, and paying attention to alignment and balance of rotating components are the only real costs to a program that will dramatically improve your plant’s bearing life.
William "Doc" Burke works for Pumping Solutions in the Chicago area and has more than 25 yr of experience in the pumping business. He can be reached through Pumping Solutions in Blue Island, Ill., at 708-272-1800.