In many industries, equipment runs continuously which means that provided optimum lubrication conditions can be maintained, wear can be minimized. But in some industries such as batch manufacturing processes, terminals where periodic loading and unloading occurs, or industries such as refining and petrochemical where stand by equipment is used, the deleterious impact of stop/start operations are very real.
There are also similar issues in industries where stop-start operation hasn’t traditionally been the norm. As companies struggle with less than full order books; shutting down a production line, an area of the plant or even whole plants has put an even greater emphasis on the need to maintain equipment that’s not even running so that when times get better and equipment needs to run, you’re able to start it up and operate the equipment normally without adverse effects.
In part one of this three-part post, we’ll reveal how shutdowns can negatively affect lubricant film.
Impact of Shutdowns on Lubricant Film
When an oil lubricated machine stops, the oil drains to the lowest point in the system. For wet sump applications such as pumps or gearboxes, this means that the oil will return to the sump. The impact is that gears and bearings may over time become dry. Without a film of oil covering them, steel surfaces have a tendency to rust as they come in contact with air and moisture. Rust is a very friable material, so when the machine is started back-up, the ability of the gear or bearing to support and distribute the applied load is compromised which can result in premature fatigue failure down the road.
This problem is further exacerbated if any vibration is present. With the shaft at rest, the weight of the shaft and anything that it’s supporting is transmitted to the contact points between the shaft and bearing. If any vibration is present, the shaft can literally “bounce” on the bearing surface creating a failure mode known as false brinnelling. False brinneling often occurs alongside another failure mode known as fretting corrosion. Fretting corrosion occurs when small amounts of free or emulsified water sit between two contacting surfaces. Its effect can often be seen on disassembled bearings from idled equipment as evenly spaced dark stains around a bearing raceway where the rolling elements have come to rest with the machine stopped.
One simple way to prevent rust, corrosion and false brinnelling is to install a desiccant breather on the oil sump. Properly selected desiccant breathers not only remove moisture when a machine is actively breathing, but with the right type of breather the silica gel is actually in contact with the headspace within the oil sump. Just like the small sachet of silica gel contained inside a new shoebox or consumer electronics that’s designed to keep the product dry during storage and transportation, a desiccant breather will keep the inside of the oil sump dry, helping to limit the impact of moisture on rusting and corrosion during shutdowns. It’s also advisable to periodically rotate shafts to distribute oil and change the contact points between surfaces to minimize the impact of false brinnelling and fretting corrosion.