Failure of Cooling Fan Shaft Bearings
Tags: bearing lubrication
A facility manufacturing intermediate chemical compounds for plastics manufacture has a large number of air-cooled systems with numerous fans driven by electric motor and speed-reducing pulleys. Over the years it experienced many premature bearing failures on the fan shafts.
Machine Details
The process and service fluids are cooled through banks of finned tube air coolers. There are long banks of these coolers arranged as cells in an elevated steelwork structure. The cooling fans are installed vertically underneath, blasting air upward into the coolers.
Despite all the fans suffering premature bearing failure at one time or another, there were in fact two different types of arrangements installed. One arrangement used spherical roller bearings both above and below the fan, fitted to the shafts on taper sleeves, with the bearings installed in plummer block housings.
A second type of arrangement used deep-groove ball bearings both above and below the fan, manufactured integrally with the bearing housing. This is a lighter duty arrangement that is fitted to the smaller of the fan coolers.
Initial Investigation
This plant had a high number of coolers under maintenance for a long period of time. Comprehensive maintenance records were not available, but the operating and maintenance managements both confirmed that there were regular breakdowns. Historically, the situation had been managed due to the plant operating well below its maximum capacity so regular breakdowns were not a problem.
The fans typically would be out of service for 24 to 48 hours. The effect on production could vary from severe in the summer, necessitating a turn down in plant rate, to simply inconvenient in the winter. Typical cost of mechanical repair would be approximately $2,000 to $5,000 per event.
However, a change in the market led to the plant having to run at high capacity. This combined with a period of unusually warm weather meant that fan breakdowns suddenly became a high priority and were limiting production.
Inspection of failed bearings in the local plant workshop showed that the two arrangements suffered different failure modes, as could be expected from two quite different bearing types. However, there are certain similarities in how the problems arose.
The spherical roller bearings are 65 mm bore and are fitted to the shaft with taper sleeves. Failures of the bearings were characterized by overheating of the races and breakage of the plastic cages. The bearings are not located axially against a shoulder on the shaft. The axial position is set by measuring the correct position and tightening the taper sleeve. Axial location of the bearing is set in the top plummer block with a locating ring, while the lower bearing is allowed to float axially in the housing. The drive pulley was below the lower bearing outboard of the bearing housing. It was noted that the bearings were standard clearance.
The deep-groove ball bearings are of the type made with an integral housing so they could be easily bolted in position. The shaft diameter is 50 mm. The bearing inner race is extended axially and incorporates a grub screw for locking the inner race to the shaft. The failures of these bearings were characterized by overheating, but with fatigue spalling of the races as well. When the bearing was sectioned, it could be clearly seen that the wear track was offset from the center of the races. This condition indicated a high axial loading and the fatigue of the races showed that the bearing was suffering an overload in the axial thrust direction. The axial location of both the top and bottom bearings was set by the inner race butting against shoulders on the shaft. It was noted that the bearings are fitted with integral seals and are lubricated for life.
The author worked with the maintenance technicians while they were repairing some of the fan coolers.
First, he worked with the technicians on the units fitted with spherical roller bearings. The installation of these units was characterized by a large amount of grease spilling out from the plummer blocks. The bearings were lubricated by a grease gun from a remotely positioned grease nipple connected to the plummer blocks by plastic piping.
Looking at this, it was obvious that the operations team was resorting to regular greasing of the bearings in an attempt to keep them running longer. In fact, they were filling the bearing housings with grease and the excess was being ejected through the seals. What was also obvious from the spilled grease was that this was a high-performance, high-quality grease with solid lubricant additives because the freshly ejected grease was dark gray. The maintenance supervisor confirmed that they had upgraded the grease to a type with a high extreme pressure (EP) and solid lubricant additive in an attempt to lengthen the life of the bearings.
Inspection of the shafts where the bearings were located showed a high degree of scoring, which was affecting the diameter of the shaft. The technicians were aware of this and were also aware that the axial location of the fan assembly was secured purely by the friction of the taper sleeve onto the shaft. When fitting the bearings, the technicians made sure, as best they could, that the shaft assembly was located firmly by tightening the taper sleeves as much as possible. They also packed as much grease into the plummer blocks as they could. This was to make sure that the bearing had "plenty of grease".
When the fans were put back into service, the bearings immediately ran hot. The author was told that they always run like that.
Certain factors stood out during this exercise:
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The technicians understood the need to secure the shaft axially and that the scoring caused by previous failures to the shaft compromised this security.
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The technicians had been given no information on how to fit the spherical roller bearings properly.
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Due to these facts they overtightened the taper sleeves, reducing the bearing clearance to zero.
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They thought they were doing a good job by getting as much lubricant into the bearing housing as possible.
As a result of the above findings the following actions were taken:
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New shafts were obtained for the fans to prevent security issues with the taper sleeve. However, it is not good practice to rely solely on friction for axial security on a heavy-duty assembly. It was agreed to pursue this issue as a redesign of the shafts.
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The technicians were given a training course explaining the approved methods of fitting spherical roller bearings to taper sleeves. Two methods were discussed: the drive-up method which measures the distance the bearing is driven up the sleeve, and the internal clearance check method where feeler gauges are used to ensure that sufficient clearance is present in the bearing.
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A laminated card was issued to each technician explaining the methods with an instruction to use the drive-up method by counting the turns applied to the lock nut and then double check the internal clearance of the bearing with feeler gauges.
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The bearings were respecified to C3, increased clearance. This meant that the bearings could be secured on the shaft with the taper sleeve but there would be a definite amount of measurable clearance in the bearing to check for with feeler gauges.
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The importance of not filling the plummer block full of grease was explained to the technicians.
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The regreasing facilities were removed and blanked off. The grease was respecified as a NLGI2 Lithium complex grease without any of the extreme pressure additives or solid lubricants. In rolling element bearings where the lubrication is purely by hydrodynamic oil film, solid lubricants contribute little or nothing to the lubrication of the bearings. In some circumstances, their effect can be negative rather than positive.
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An automatic grease dispensing unit was installed with a setting to top-up the bearing housing over a one-year period.
The results of these changes were immediate and dramatic. The bearings settled down over few days running only about 10 to 15 degrees Celsius above the ambient temperature. The installations also were much quieter.
It was immediately obvious that there had been a step change in the running condition of the bearing installations.
Attention then turned to the smaller bearing arrangements fitted to the remaining fans. After the success of the larger spherical roller bearing installations, the maintenance team was eager to get to the root cause of the failures on the smaller deep-groove ball bearing installations.
The author worked with the technicians on several of the fan assemblies to look at the underlying causes of the axial overload condition causing failures of these installations.
On the shaft of the 50 mm bearing assemblies, the inner race of the bearings was located axially against a shaft shoulder. The bearings were threaded onto the shaft and the inner races were abutted against the shoulders and a grub screw tightened up to secure the inner race to the shaft.
The flanges of the bearing housing were then bolted to the steelwork frame. The pulley and fan were secured and the belts were fitted.
The assembly was simple and the technicians had been carrying out the work for years. However, there were a couple of fundamental problems with this arrangement. Both bearings were located against shoulders, giving a fixed distance between the bearing housing flanges. The shafts, naturally, were all slightly different giving slightly different distances between the flanges. In addition, the steelwork frames were all slightly different too.
So whatever happened, when you located the bearings against the shaft shoulders, secured the grub screws and then bolted the assembly to the steel frame, you were absolutely guaranteed to have an axial preload on the bearings.
Once this was pointed out to the technicians and their supervisor, they were eager to resolve the issues. The solution was simple:
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The locating shoulder was machined off the upper bearing location on the shaft.
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This meant that the axial location and axial load from the fan is carried on the lower bearing (the fan blows upward so the thrust is downward).
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The bearing housing flanges are then fastened before the inner race grub screws are tightened.
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These simple modifications and change in the fitting procedure were documented and a procedure given to each technician.
As with the larger spherical roller bearing installations, the effect was immediate. Results included quieter operation, less heat generation and a more satisfied operations management. The grease type also changed from expensive EP additive, solid lubricant-laden grease to a general-purpose NLGI 2 lithium complex grease adequate for the duty.
In both cases, installation of these fan shafts introduced an overload condition into the assemblies. The bearings really had no chance at all to perform correctly, yet looking at the shafts installed, it was a common yet misguided belief that the assemblies were so simple that nothing could possibly go wrong.
In the case of the spherical roller bearings, the maintenance team was focused on axial security. This was a focus given to them by the original designer not understanding that on a heavy-duty fan assembly it is simply not good enough to rely on shaft sleeve friction to secure the axial position of the fan. The maintenance team, aware of this shortcoming, did its best by tightening the bearing as tightly as possible. For many years they coped with this strategy until fan failures became a limiting factor of the plant. Once the root cause of the failures was explained to the maintenance team, the team was enthusiastic to implement the required changes.
On the smaller units, the original design of the shaft assembly provided axial location for the bearings. The designer had not understood that when the assemblies are built in the field, it is impossible to precisely match the center distance between the bearing flanges and the steel work locations.
In both cases, simple and straightforward modifications solved long-standing reliability problems. The cost of the solutions was close to zero, but the consequential savings in the maintenance budget, the gains from resources redeployed on more important tasks and the increased production are large indeed.
Follow Up
The plant is now operating these fans without breakdown maintenance. Relubrication of the spherical roller bearings is carried out as an annual planned maintenance task. The bearings are changed every three years, but presently it is being considered whether to make the PM change-out at five years.
The 50 mm deep groove ball bearing assemblies are sealed for life with lubricant and are changed every three years.
To date, there has been not a single breakdown of the fan drives and the memory of the unreliability once caused by these units is fast disappearing into history.