Finding a Long-Term Solution to an Short-Term Patch Up

The waste water recycling plant with the sticking plaster solution serves a major conurbation and a sizeable area of the surrounding county. The problem which faced ERIKS’ Bearings and Drives engineers lay in the Orbal aerator tanks. And after an initial inspection, it was the ERIKS engineer who described previous attempts at repair in terms of a sticking plaster. 

The more you discover about the earlier engineering efforts, the more you can see he has a point. 

Stress under pressure

Orbal aerators use discs mounted vertically above football pitch-size tanks of waste water. The discs rotate to aerate the water, encouraging cleansing bacteria to thrive. 

However, with the aerator discs arranged in line, at this plant only the first was functioning and the others were failing to turn. 

It was clear that at least one of the urethane coupling elements had failed, but since a large crane was required to lift out the shaft, it wasn’t cost-effective to replace the couplings until all of them had failed. 

In addition, when the 125mm bearings in the arrangement had failed initially, the drive shafts were machined down and smaller bearings installed. Then when the drive shaft snapped, it was repaired with a weld. 

The result, identified by ERIKS, was stress cracks running right through the shaft, which was effectively supporting a 1½ tonne gearbox and motor. Now a complete plant failure, with associated water quality risks and potential penalties, was a real possibility. 

Going back to basics

It was clear another sticking plaster solution would not be good enough this time. Not only would it simply postpone inevitable catastrophic failure, but it wouldn’t resolve some basic design issues highlighted by ERIKS.

These included:

• the method of gearbox mounting, which placed unwanted weight on the bearings
• the need for a large crane to lift out the entire shaft to replace couplings 
• health and safety issues with access for lubrication inspection and maintenance

It was time for radical surgery.

Balancing act

One of the first steps was to redesign the configuration, which had the gearbox and motor balanced at the end of the shaft. 

The gearboxes were remounted independently on a new mounting plate, to remove the weight burden and stresses from the shaft and bearings, with laser alignment used to ensure the installation was perfect. 

Using Falk Wrapflex couplings eliminated the need to remove the gearbox in case of coupling replacement. Thanks to the “replace in place” wrap-around design, replacing the elastomeric  element between the drive hubs (the most likely component to wear) is simply a matter of sliding the hubs along the shaft to release it. A change can be made quickly, and certainly with no crane required. 

Naturally the fractured shaft was also replaced, then the whole assembly was fitted with a Simalube remote lubrication system for the bearings, sleeves and so on. This removed the need for manual lubrication maintenance, and the associated health and safety risk. Previously, a lubrication inspection of all four Orbals had taken half a day, and involved accessing the aerators from below. 

Now it could be carried out remotely and safely from a control panel mounted at waist height on a handrail, taking no more than 15 minutes. 

The final piece of the puzzle was to ensure bearing uniformity across the enter installation, with consistent bearing arrangements and same-size bearings – fixed, floating or internal – throughout. This reduced inventory, increased cost-effectiveness, and made maintenance and repair far simpler. 

After the eight-week refurbishment and upgrade project, the plant was back in operation, and 18 months on the Orbal process is still fully functional, with no failures reported. In other words, no longer limping along, but running at full speed.