When Small Machine Issues are Symptoms of a Bigger Problem

ERIKS’ bearings and belts experts were called to the site when the mill classifier failed for the second time due to a collapsed bearing. Close inspection revealed that the bearing problems were only symptoms of larger issues. In fact the machine, only commissioned around 6 months earlier – had never been able to run at full output.    

This second bearing failure also only just stopped short of a health and safety disaster.

The 45kW motor – weighing over 300kg – was ripped off its mountings, but fortunately hung on by a thread instead of dropping 20 feet to the ground, where a large hole would have been the least of the outcomes. 

Bigger, not better

In discussions with the German OEM, it came to light that the asset was a larger version of an otherwise identical machine already in use elsewhere. However, the specification had simply been upscaled, without considering resulting increases in forces and stresses on individual components. 

Compounding the problem were a number of installation errors spotted by the ERIKS engineers.

Firstly, the spherical roller bearing at the lower end of the shaft, and the spherical roller thrust bearing at the upper end, were fitted at the opposite ends to where they should have been. Severe radial load on the spherical roller thrust bearing was causing its inner ring to “climb” up the outer ring, placing extreme edge loading on the outer ring and cup. At the same time, the spherical roller bearing at the opposite end was experiencing severe axial load to the top set of rollers, which was causing spalling and – ultimately – bearing failure. 

Meanwhile, because the bearings were not sitting properly in their seats, they had been over-greased: treating the symptoms rather than the cause. Flooding in grease resulted in an increased running temperature, made worse still by the incorrect positioning of the grease escape holes at the top of the bearing arrangement. 

Power mad

The last weak link in the chain was the power arrangement. Although the drive was only rated for 30kW, it was being driven by a 45kW motor, controlled by an inverter to run at half speed. The drive was also operating 8 budget belts which were under-rated for the power, though the OEM’s engineer was proposing to reduce these to 5.

With the current set-up of the asset, this would only have lead to more problems and an even greater risk of failures. 

After their inspection, and after running the specification through the Fenner® Drive Designer, ERIKS’ engineers came up with the following recommendations:

1. Swap the relative positions of the bearings, to ensure the right bearings were taking the right loads.
2. Place a radial bearing above the spherical roller thrust bearing to take the radial load, relieve the thrust bearing of belt tension, and stop it “climbing” out of its cup.
   
3. Fit the bottom spherical roller bearing as non-located, allowing the shaft to expand freely without affecting the top bearing arrangement.
4. Reduce the belts from 8 budget belts to 5 Quattro Plus belts – comfortably accommodating the drive power and speed, but also lowering the end loads to reduce stress on the bearings. Using standard wedge belt drives has the added advantage of reducing lead times for belts from two weeks to two hours (from stock).
   
5. Relocate the lubrication escape holes to prevent flooding of the bearings.
   

Finally, when a fault developed on a Saturday due to a warped nut, an ERIKS engineer attended immediately and set up sensors to determine if vibration had caused the nut to loosen. When the monitoring proved this was the case, affected nuts were swapped for replacements which actually tighten through vibration.

That the customer is now getting full production from their asset with no drive issues is one testimonial to ERIKS know-how.  The other is the fact that the OEM has changed the design of the machine, based on ERIKS’ recommendations.