Low Speed Bearing Lubrication, Monitoring & Reliability - CM & Lube Newsletter Article Oct 08
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Low Speed Bearing Lubrication, Monitoring & Reliability
The typical failure mechanism of this type of plummer block spherical roller bearings is for wear to be initiated in the races and rolling elements via a number of possible causes. This wear eventually causes collapse or contact between the shaft and seal area causing excess heat, cooking the lubricant, initiating more friction and damage and then final failure.
Lubrication, Contamination and Sealing
Once a spherical roller plummer block has been installed its lubrication should be your dominant concern. This is because as well as addressing the lubrication requirements of the bearing it is often the main protection from contamination entry. One key requirement for the lubrication of slow speed plummer block bearings is that grease purge lubrication should be used. This means that the bearing housing should be fully packed with grease and that when grease is pumped in, some older grease will ‘purged’ out of the plummer block. The old grease will either be expelled from the bearing during greasing or on next start-up. Purge lubrication is not important for the bearing lubrication but is very important for contaminant exclusion from the plummer block. In higher speed bearings such as for motors or fans, purge lubrication would cause grease churning within the bearing housing, generating heat but at low speed temperature rise is minimal.
If we take the example of a 160mm diameter shaft running at 150rpm the grease republication interval calculation is shown in green on the SKF re-lubrication chart below. It shows that an upper limit for lubrication interval is 13,000 operating hours, which is 1.5 year for a 24hr 7d operation. This is why low speed bearing in low contamination environments that have never been relubricated can typically last more than 10years before failure becomes likely due to lubrication failure. The reason that Shane New from Tom Price mine in his presentation is strongly specifying 4 weekly lubrication of his conveyor bearings is purely for contamination exclusion.
When purchasing a new piece of equipment, larger plummer block bearings will typically come with a felt or a lip seal unless you specify otherwise. Many of these designs are not suitable for applications where there are contaminants in the operational environment or where long life is desired. Experience has shown that labyrinth seals for plummer block bearings are a good sealing solution, if the appropriate seal lubrication approach is taken. For regular lubrication of these plummer blocks it is the labyrinth seal that needs to be lubricated. As discussed above the bearing itself requires less frequent lubrication. The advantage of the labyrinth seal is that there is no wearing parts and so will never wear out. Wear over time is a problem for lip seals, especially where there are abrasive contaminants in the environment. Below are two figures that show two levels of complexity in grease labyrinth seals. The first figure shows a longer complex labyrinth path while in the second figure the labyrinth is only a simple grooved cylinder. Both designs require a greasing hole drilled into the labyrinth. The simpler the design the more often the labyrinth will require greasing to exclude contaminants but four to six weekly would be a typical frequency in arduous environments. The person doing the greasing must understand what a ‘wet’ labyrinth seal looks like. Some plants have used single point lubricators (eg permalube type) on their labyrinth seals to provide a small but constant supply of grease to reduce the lubrication workload.
Another approach to plummer block sealing is the TA V-ring seal with a protective collar, which combines a high quality lip seal that does not wear the shaft, with a collar that can form a type of labyrinth seal. The protective collar also gives some protection if a high pressure water hose is directed at the bearing sealing area. Some plants are subject to operator wash down and one of the dirtiest looking equipment items is the seal area on a purge lubricated bearing. People involved in plant cleaning need some training on bearing seals. Cleaning around plummer blocks can be vitally important on conveyors where spillage often occurs. If material is allowed to build up around the seal area, no seal type is likely to protect the bearing from contaminant entry. Lip seals have the advantage if flooding and submersion of the bearing housing is possible, such as when located in wet mines, sumps and basements. Lips seals if still in good condition tend to be much more water proof than labyrinth seals.
The next issue is where on the bearing the grease should be applied. We have already discussed the need to lubricate labyrinth seals. Plummer blocks designed to have integrated labyrinth seals will often have 5 lubrication drilling points. For plummer blocks with only one shaft seal area, which is most conveyor pulley bearings, should be mounted at the sealed cap end of the bearing (either point 2 or 4 below). This is to avoid a dead grease zone at the end cap where dried grease might build up. For plummer blocks with two seals and where the spherical bearing is set up for central greasing, the centre point 3 is the ideal greasing point. If the bearing does not accept centre greasing then greasing at either side of the bearing (points 2 or 4) is acceptable. If using lip seals then a double lip seal can be added at one of the seals to stop grease flow and grease is applied at this double seal end. Even when centre greasing is used it is good to specify the drilling a tapping of points 2 & 4 to allow the potential of used grease sampling (see section on condition monitoring below).
Another innovative solution to improve the reliability of sealing is use of spray on Enviropeel coating (an anti-corrosive oil impregnated polymer) over the plummer block and adjacent shaft. This effectively forms a much more efficient labyrinth seal. Enviropeel case study Media:Enviropeel.pdf.
Whatever the type of plummer block, bearing and sealing arrangement you have, there will be an optimum way to set them up for lubrication. You need to have a close look both on-site and in your spares area at what plummer block and sealing arrangements you have and how they are currently setup for greasing and any changes that need to be made. A key issue is access to bearings for lubrication, especially for conveyors that only have single side access or where guarding limits access. These require the running of grease lines and as grease is a high viscosity fluid, the longer the line is, the larger the diameter the line needs to be. The lines have to be plumbed in a way that minimises the likelihood of future damage. You need to review current greasing procedures and any training and support that is needed for the people doing the on-site greasing. Getting lubrication right on any plant is not a trivial exercise and needs the strong involvement and ownership of the people doing the front line work. One priority is finding the bearings that are currently not being lubricated at all, which tends to be common where a lube program has not been rigorously setup and maintained. “One good lube tech will easily replace at least 5 good fitters” was always the rule, which still seems to ring true.
The last point about lubrication contamination is that unless it can’t be avoided and for non clean environments it needs to be an extreme situation, the plummer block should never be opened while in service. The plummer block should be installed as part of the assembled unit and removed in the same way. Numerous sites have proven that once a plummer block has been opened, the likelihood of future failure is significantly increased due to either contaminant entry or from disturbances to the sealing. Avoid the desire to open up bearing to measure bearing clearances as there is lots of better and safer condition monitoring methods available. As an example Port Waratah has set a standard that they only change out conveyor main pulleys as an assembly and don’t try to change out bearing in situ.
Bearing and Plummer Block Assembly
- Incorrect adjustment of radial clearances
- Incorrect locking technique
- Poor sleeve contact due to shaft damage
- Incorrect fitting to allow bearing to float
- Inclusion of foreign material into bearing
Spherical roller bearings in Plummer blocks are designed to be attached via tapered sleeves. The largest assembly failure cause is incorrect setup of the tapered sleeves. A typical in-service failure mechanism is slippage between the tapered sleeve and the shaft, generating steel wear particles from the shaft. These particles make their way into the bearing initiating bearing wear and this wear accelerates with the addition of bearing wear particles. Without close examination this can appear to be caused by external contamination as when wear progresses plummer block seal effectiveness reduces and allows contaminant entry. Another possible symptom of poorly fitted tapered sleeves is axial shaft movement through the fixed bearing on the shaft but this tends to only be if there are axial loads on the shaft and does not always occur. Often axial shaft movement will stop once the floating bearing clearance is taken up. See the photo below of visual symptoms of tapered sleeve slippage and the initial bearing wear generated from this slippage.
Tapered sleeves allow easy installation of a bearing onto a shaft but are not designed to cater for varying or worn shaft diameters. Worn shafts should be properly repaired by metal spraying or other technique. Damaged shaft diameters can also cause point loading on the bearing in operation. Shafts should ALWAYS be “blued” after disassembly to check for >75% good contact between the sleeve and shaft. If there is inadequate contact, emery the shaft until there is or replace/ repair it. A poor contact between shaft and sleeve will guarantee early failure. Another issue that is often ignored is wear in the plummer block housing. Always check the housing ID to ensure it is within specifications. Don’t assume that your overhaul workshop, even if they have micrometers with calibration test pieces and procedures that include shaft diameter check etc, will be actually doing these checks. The only way to be sure is to talk to the tradesmen on the floor. With the resources boom, the availability of skilled tradesmen is drying up and many fitters have not regularly used micrometers since their days as an apprentice and have little confidence in there use but may try to fake it.
Having the required special tools available for adjusting nut tightening for setting the clearance is mandatory and should be checked when auditing/ visiting overhaul workshops. For smaller bearings special adjusting nut spanners (see below) are adequate but for bearing with internal diameters over 150mm, they will not be able to achieve the required level of tightening. Bearing 160mm ID and up should be fitted with fitted with hydraulic nuts and hydraulic pumps with calibrated pressure should be used (see figure below). Not remembering to apply the locking tabs or other locking mechanisms for the adjusting nut is another error that needs to be covered off in assembly procedures and QA. Don’t assume that because you are using a high profile organisation for original supply or overhaul of assemblies that this issue is under control. Bearings on sleeves NEVER come loose due to load, it is always due to incorrect fitting as evidenced by rusty fretting corrosion on the bore of the sleeve.
Cleanliness on the assembly is another key issue. Maintenance people are often shocked at the conditions they find when they visit overhaul shops. When you find bearings open and uncovered, dirty work environment or work areas open to wind or rain, then you should start to be suspicious of the care taken with assembly cleanliness.
Allways INSIST on 100% grease packing of plummer blocks with the same grease used on site by the original equipment supplier and overhaul shop. That way, when you apply lube on-site, you don’t have to fill the whole housing up and it will protect the bearing better if it is stored in a warehouse or outside for a long time.
An issue relevant for both workshop assembly and on-site installation is axial float in the bearings. With any two bearing housings on the same shaft there should be one housing axially fixed and one floating. This is to ensure axial forces are not trapped between the two bearings. Nothing will destroy the cage in a sperical roller bearing faster than excessive axial force. It is very easy to trap axial forces between bearings on installation. If plummer blocks are not to be opened on site the accurate final distance between the two bearing housings needs to be known and set in the workshop. Indelible marks need to be made on the shaft either side of the floating bearing to indicate the available float so the correct float can be confirmed after installation.
If for some reason a plummer block has to be opened on-site, then a plastic tent should be built around the bearing to create a clean dust free working environment to do the work.
Condition Monitoring (CM) of Low Speed Bearings
No matter how reliability your slow speed bearings are, if their reliability is critical to your business, there should be some level of condition monitoring. The minimum for most low speed bearings would be initial baseline checks to confirm the condition after installation and in its early service life.
There are lot of options for monitoring the condition of low speed rolling element bearings. Some of the techniques that are applicable are:-
- Use of simple listening rods, stethoscopes or electronic stethoscopes
- Touch by hand for temperature and vibration
- Temperature monitoring using non-contact temperature meter or installed sensor
- Thermal imaging
- Grease sampling with visual analysis
- Grease sampling with PQ and laboratory analysis
- Grease sampling with Patch Test & microscope examination
- Ultrasonic listening instruments
- Ultrasonic listening and high frequency parameter monitoring
- Vibration spectrum analysis using demodulation or similar technique
- Wear monitoring
Before we look the best monitoring techniques to use we should look at the typical failure pattern for low speed bearings. As mentioned in the first paragraph of this article, either slow speed bearings have very few failures or failure rates are moderately high. This is because unless there are design issues, if plummer bock bearings are initially assembled and lubricated correctly and the shaft sealing is maintained capable, then the first 5 years reliability is almost guaranteed. Reliability after that is dependent on the lubrication and sealing effectiveness being maintained and how highly loaded the bearing is compared to its load specification.
The failure pattern diagram below suggests the strongest condition monitoring attention should be applied to baseline condition checks and then for the first two years after installation, when any installed defects should become evident. Once a bearing has passed this initial stage of scrutiny, then the monitoring intensity can be reduced. Ongoing monitoring frequencies will depend on equipment criticality, estimated failure rate and the warning time (PF Interval) of the CM techniques chosen.
To make monitoring of any parameter worthwhile it is necessary to follow a rigours procedure so the same parameter is measured in the same way at the same location and where practical by the same person. Too often measurements are collected and when compared to previous measurements, there is so much variability, the results they are next to useless. Unless consistency in measurement can be achieved it is probably not worth the effort of doing the monitoring. Good initial documentation of the system is necessary. It is also useful to maintain an internal or external technical expert on each monitoring technique who owns the monitoring process and can occasionally audit compliance.
Use of simple listening rods, stethoscopes or electronic stethoscopes
Some form of listening technique should be always used on commissioning baseline condition checks.
Touch by hand for temperature and vibration - Monitoring slow speed bearing by touch is a useful technique. Your hand will detect temperatures that are significantly above ambient. One type of slow speed bearing vibration fault, typically called a ‘thumper’, can easily be detected by hand. This is where there is significant clearance in the bearing or between the sleeve and the shaft and the bearing makes a definite thump on every rotation.
Temperature monitoring using non-contact temperature meter - Non-contact temperature meters are a useful monitoring technique for monitoring slow speed bearings, especially for inaccessible bearings. To make temperature monitoring more sensitive the measured temperature should be compensated for changes in ambient temperature (T = Titem – Tambient). Ensure you always use the same instrument type and take the measurement with the same instrument position and use the laser function to point at a painted target on the plummer block.
Temperature increase in slow speed bearings is generally an indication of an impending failure and should trigger an urgent thermal imaging and vibration analysis to analyse the cause of temperature increase and estimate condition severity. If the condition is considered severe but there are problems scheduling an early repair, then injecting a special friction reduction grease such as a PRO-MA MBL should be considered to extend the life.
Thermal imaging - Thermal imaging cameras are becoming cheap enough to be used in routine equipment inspection. Conveyor monitoring is one area where the technology has been used successfully. Thermal image monitoring of slow speed bearings involves looking for unusual temperature patterns and recording an image of any anomalies for reporting and further follow-up. A thermal image of a plummer block will give an indication of where the load zone is and if the temperature is coming from a seal, as in the example below, or from the bearing itself.
Thermal imaging should be used as a part of any commissioning baseline monitoring checks.
Grease sampling with visual analysis - A simple and very effective monitoring technique for slow speed bearings is ‘used grease sampling and visual analysis’. A used grease sample from a bearing is observed for any changes in Grease Colour compared to the new grease and a Sparkle Test is carried out to observe if there is any bright bearing wear steel particles in the grease. Grease Colour is important. It is affected by contaminants such as water, dirt and process material. It is also affected by the friction, heat, wear processes and wear debris from within the bearing.
The Sparkle Test involves rubbing a sample of used grease on a cloth and viewing in the sun. Bearing wear material protected by the grease will be bright and sparkle in the sun or bright light. Housing and shaft wear material would also be observed. Both represent serious problems. Even extruded grease that is contaminated with dirt or process material can be Sparkle tested as the contaminant will be unlikely to include bright material.
If results from the sparkle test are not certain, then a larger quantity of grease can be placed in a small sealable non-magnetic container and a small magnet and kerosene or similar solvent added. After shaking the container well, the magnet is removed and the material on the magnet observed for bright material. Again the residue can be rubbed onto a cloth & viewed in the sun for a sparkle test.
Getting a sample of used grease from a bearing is often easy. One technique is to scoop up the sample from the side of the plummer block seal or from below the plummer block after purge lubrication. Ensure that the used grease does not come from labyrinth lubrication but is purged out the main plummer block compartment. Always do a written ‘Take 5’ safety analysis before putting any object into a guarded area on operating equipment. Consider ways that the object could be caught up or turned into a spear.
Grease sampling with laboratory analysis - For more formal quantitative monitoring of used grease an uncontaminated sample is required. Most oil labs can do a similar analysis on grease to that on oil and can usually do the analysis on a relatively small quantity of grease. The lab report parameter that is most of interest is the PQ index, which measures the quantity of magnetic material in the lube. For larger sites there are relatively low cost instruments that can measure an equivalent to a PQ index without sending samples to at lab.
The main methods for getting an uncontaminated used grease sample are:-
- Using a ‘grease sampling hole’ in the plummer block. This is usually just a grease nipple hole along site the bearing location in the housing (see section above on plummer block lubrication) or could be a specially drilled and plugged hole created before bearing assembly.
- Using purge sampling, which requires off-line cleaning around the plummer block seal area and purge lubrication of the bearing area of the plummer block until a sample of used grease is expelled trough the seals. Grease is collected with a clean spatula and placed in an oil sample bottle.
- For plummer block designs that have a grease relief hole an uncontaminated sample can usually be taken from this area.
Approaches from taking a sample through a ‘grease sampling hole’ are:-
- If a larger diameter sample hole is available then a larger diameter clean plastic tube can be inserted into the grease cavity and a sample can be taken with a suction pump. A reducer is required to connect the smaller diameter tube from the suction pump to the larger diameter plastic tube. The grease is only sucked into the plastic tube, not into the pump. A sample bottle length section of the tube is cut off and placed in the lab sample bottle.
- A larger diameter drinking straw can be used to take a sample by plunging it into the grease through the sample hole to get a sample. Again put the straw into the oil sample container.
- The best method for grease sampling is where used grease can be expelled through a grease sample hole during purge lubrication (see below). This can potentially be used during service if the plummer block is accessible. This technique relies on there being enough back pressure generated at the seals to force the grease out the sample hole, which is not always the case. The used grease is collected in a larger diameter clean plastic tube, which is placed over the sample hole nipple.
Used grease analysis should be strongly considered as a primary monitoring technique as it gives the longest warning time (PF Interval) of any of the slow speed bearing monitoring techniques. It should be the technique that is most likely to detect the greatest number of faults but is not as well proven as the advanced vibration analysis techniques.
Baseline plummer block bearing condition checks should be made with quantitative used grease analysis at 3 months and 12 months after installation to confirm that no wear metal or contaminates are present in the used grease.
Grease sampling with Patch Test & microscope examination - As with oil analysis, used lubricant Patch Tests and microscope analysis is an ideal monitoring technique for grease. The only issue with grease is that the solvent that is required to dissolve the soaps in the grease to allow the filtering of the debris is relatively dangerous and so should only be used by trained personnel or industrial chemists. Check if your oil lab can do grease Patch Tests.
Grease Patch Tests are ideal when you only have a contaminated grease sample as under the microscope bearing, shaft and housing wear can be distinguished from environmental contaminants. A good lab analysis will also be able to give you information about the severity of the bearings condition by examination of the temperature effects on the wear particles.
Ultrasonic listening instruments - Simple ultrasonic listening instruments are now readily available and can be used by lube techs during bearing lubrication. These instruments offer the same advantages of other listening techniques but have been shown to give better audible information about lubrication and contamination issues. Often you can hear the effect of contaminants in the bearing. If this monitoring can be done in parallel to lubrication it is an ideal monitoring technique where the grease is applied directly to the plummer block rather than through external grease lines.
Vibration using listening and high frequency parameter monitoring - This technique combines all the advantages of the above ultrasonic listening technique with a quantitative high frequency vibration parameter. It has proven to be a successful monitoring system and is relatively simple. It will probably not detect as high a percentage of bearing failures as the more advanced techniques but would be a good combination with simple used grease analysis as both techniques can be carried out by non-specialists. To have longer term security with this monitoring strategy using non-specialists it would require occasional auditing of the information collected and the on-site procedures used.
Vibration spectrum analysis using demodulation or similar technique - This has been the most widely used condition monitoring technique for low speed plummer block bearing over the last 10 to 15 years and has proved very successful. Some sites have had almost a 100% success rate using the technique but others still have a few in-service failures sneak through. It is not clear if the differences in success are due to the specifics of the technique use or if it is a limitation in the technology. Port Waratah report that on their older plant where the bearings are smaller diameter and the loading 'factor of safety' is lower get at lease 6 week warning of a failure. For their newer plant that has been installed with larger bearings, the warning time is up to 12 months from using the demodulation vibration analysis and they have had a very high success rate so far. The technique has a significant advantage for inaccessible equipment as vibration transducers can be installed to ensure complete monitoring of all bearings. This technique requires well trained CM techs and good technical support backup. It should also be combined with a listening technique, which can be easily setup by tapping into the signal from the vibration transducer used.
Vibration spectrum analysis using demodulation should be a standard part of any commissioning baseline condition checks on low speed plummer block bearings.
Conclusion & Recommendations
- Understand the bearing and sealing arrangement you have on your slow speed bearings
- Ensure all your low speed bearings are adequately lubricated in a way the helps to exclude contamination.
- Train and support your lube guys to try to carry out basic listening and basic used grease visual inspection for all low speed bearings where practical.
- If you have reliability problems with your low speed bearings, implement formal vibration monitoring and or formal used grease analysis to manage these problems.
- Ensure a high level of quality control is applied to assembly of spherical roller bearings and plummer blocks, with special focus on the tightening of tapered sleeves.
- Where sealing problems are suspected on slow speed bearings, consider upgrading the sealing arrangements when overhauling equipment assemblies.
Article by Peter Todd - Thanks for the inspiration from Shane New and the input from Kevin Fenwick and Terry Blackman.
