When does preventive maintenance really begin? Learn about proper storage
Preventive maintenance should begin before a critical asset is installed — with the proper storage of the asset
- Understand why storage matters for critical assets.
- Learn which storage conditions are required to preserve bearings, gearboxes and conveyor pulley assemblies for the long term.
- Know the practical steps to improve critical asset storage.
- Consider three main asset types and the best storage practices of each to preserve their service potential.
- Preventive maintenance is a regular focus of most plants.
It is about 2:30 a.m. when you are startled awake by a phone call. Your night-shift team at the plant is calling to let you know that one of your critical assets has just failed. After the initial anxiety, you are relieved as you remember that you purchased a complete spare just a few years ago. After directing your team to install the spare, you relax again, smiling as you remember how you persuaded management to add that costly spare to the budget. However, a couple of hours later, the anxiety returns with a second phone call.
The crew reports that the new spare installed on that critical asset is making noise and running hotter than it should. Now, with the spare facing imminent failure, you are faced with production losses and an uncomfortable report on why that spare is not providing the insurance against downtime you promised. What went wrong? To answer that, I will relate an experience I had in the field as a technical representative.
I received a call from a customer at a remote location requesting me to help them identify a spare conveyor pulley assembly they had on-site. After arrival, I met with a warehouse supervisor who directed me to where the item should be. I searched for 20 minutes but could not find the pulley. I returned to the warehouse supervisor and apologetically asked for more precise directions. He told me I was looking in the wrong place, saying, “The location for that item is over there,” as he pointed out the door to an empty snow-covered lot across from the warehouse.
The critical spare was outside where it had been for two years and under 4 feet of snow. I found the pulley and forwarded the specifications requested to the warehouse along with a report on its condition. Several months later that spare was needed for a conveyor repair. But before installation, they discovered that water contaminated the bearing housings and rendered the spare unusable without significant repairs.
That may seem like an extreme example of poor critical-item storage. However, inadequate storage is more common than we may realize. An improperly stored asset can be compromised with damaged bearings, corroded gearing, leaking seals and contamination from environmental exposure, making that critical asset an enormous liability.
Preventive maintenance is a regular focus of most plants. Sometimes overlooked, however, is that preventive maintenance begins before a critical asset is installed with the proper storage of the asset. What is proper storage? Ask any classic car enthusiast or collector, and he or she will tell you how storage is directly related to maintaining environmental conditions such as temperature, humidity and cleanliness. Proper storage is about keeping those conditions within acceptable limits.
The biggest factor in maintaining condition is proper location. Poor storage is not always caused by negligence or a lack of knowledge; instead, it is often due to working with unfavorable circumstances.
For example, many plants are designed and built with proper spare storage as an afterthought. Even in some new plants, the space allotted for warehouse needs is insufficient. This forces warehouse personnel to improvise and sometimes store spares in undesirable locations exposed to extreme temperature changes, weather, equipment vibrations or contaminants.
What can be changed?
How to properly store assets
Every piece of critical equipment that rotates or is susceptible to corrosion is at risk. Here are storage guidelines for three main asset types that almost all industrial plants have.
Bearings: From the start, proper storage is critical. Bearings should always be stored in a clean, temperature-controlled, low-humidity environment free of dust, shocks and vibrations. Many top bearing manufacturers coat them with a preservative oil film to protect against corrosion. Long-term storage times can be achieved by storing bearings in their original, unopened factory packaging.
To maximize the bearing’s operating life, use the “first in, first out” inventory policy. If a bearing is open (unsealed and not lubricated), bearings can be stored for up to 10 years without compromising service life (see Table 1).
For capped bearings (bearings with seals or shields), the storage time is limited by the lubricants and should be stored for a maximum of three years to avoid degradation of the grease fill.
To prevent deterioration of stored bearings, consider these basic storage factors:
Store bearings indoors in a condensation-free area, maintaining the environment’s ambient temperature, ensuring a maximum of 105°F (see Figure 1).
Store in vibration-free conditions. Vibration can damage raceways, so storing bearingsdirectly on a floor should be avoided.
Store bearings horizontally (flat), if possible, to avoid damage caused by the bearing falling on its side from the upright position.
Do not open or damage the original packaging. If your maintenance personnel open a bearing package to identify it, they need more training.
Gear reducers: Storage location is also the primary consideration for a gearbox. As directed for bearings in general, store the gearbox in a place free of ground vibration from sources like heavy equipment (e.g., vehicles, large forklifts, cranes) or railway activity. While stationary, rotating elements do not have a sufficient film layer between the metallic surfaces, vibrations can cause false brinelling from rollers oscillating against the raceways.
False brinelling is a recipe for failure. To further deter bearing damage, the shafts of gear reducers should be rotated a couple of full rotations every two months to distribute the lubrication film and prevent false brinelling and fretting corrosion. Tag the reducers, noting the storage date of the scheduled rotations, then ensure someone is responsible for keeping them on schedule.
Temperature is a condition that must be controlled for proper gearbox storage. Depending on the season and the location of your plant, temperature variations can be severe. The goal with temperature is consistency; although 70-80°F is optimal, 65-100°F is acceptable. Keeping the gearbox sheltered and protected from weather conditions is also essential, as condensed moisture can accumulate within the reducer housing, causing rust and corrosion of gears and bearings (see Figure 2).
Gearmotors consisting of a gear reducer and motor combination may require special consideration for storage life if the motors use grease-filled sealed bearings. As noted above, grease has a storage limit, so know the grease-lubricated equipment’s shelf-life limitations and establish a regreasing schedule if needed.
For internal conservation, manufacturers recommend using vapor corrosion inhibitors (VCIs), which are applied inside the free space of the gear unit. Except for some food-grade lubricants, many VCI brands are specially designed for use with lubricating oils so that any existing oil may remain in the gearbox.
If an air vent is installed, replace it and close the air vent hole with a sealing plug. Before putting the gearbox into service, a drain and flush with the recommended lubricant is always a good practice. To prevent rust damage to the external machined surfaces such as shafts, flanges and threaded attachment points, apply a manufacturer-recommended corrosion preventive.
Seals can be coated with bearing grease to protect them from ultraviolet (UV) damage and contaminants. A high-strength polyethylene resin with UV stabilizers should then be sealed around the exterior housing to protect the ferrous metal of the gearbox from corrosion.Desiccant should be stored under the cover with the gearbox and replaced periodically.
Often, spare gearboxes will not be put into service for 5-10 years, so it is highly recommended to order a gearbox from the distributor/factory with long-term storage preparations included. Long-term storage packages consist of all the foregoing preparations. A properly treated gearbox — with a suitable location and periodic shaft rotations — should be in like-new condition and ready for service when needed.
However, you should always check the gearbox condition before installation. A good practice is using a noninvasive borescope to inspect the gearbox internally and ensure its functionality.
Conveyor pulleys: A conveyor pulley with worn lagging, contaminated or damaged bearing assemblies, or corroded shafts can make a simple changeout a material handling nightmare. Here are recommendations for storing conveyor pulleys longer than three months.
Pulley assemblies should be stored indoors at a consistent temperature and the atmospheric humidity of the storage environment should be no greater than 40% relative humidity. High-humidity areas may require additional measures like dehumidifiers to maintain the atmosphere. If a pulley assembly must be kept for any length of time outdoors, it should be covered with a water-repellent tarp. It is best to avoid using poly sheeting or other nonbreathing tarp materials because water vapor can be trapped, causing condensation and rust. Using such nonbreathable materials may require adding desiccant bags under the covering to absorb the collecting moisture (see Figure 3).
Protecting pulley surfaces from damage is a good investment. All pulleys with urethane, rubber or ceramic lagging should have all circumferential surfaces wrapped with a steel sheet securely banded around them. The purpose of this sheet is to protect these surfaces from mechanical and environmental damage — such as UV rays — during transport, storage and initial installation. All exposed and unpainted surfaces, such as the shafts and hubs, should be covered with an approved rust inhibitor.
Pulleys can be very heavy and that mass must be distributed to avoid damage to the lagging surface. All lagged pulley assemblies weighing more than 10,000 pounds should be supported in wooden cradles designed to distribute the pulley assembly weight over an area and eliminate pressure on the lagging surface.
If the pulley must be stored with bearings preinstalled, all bearing housings should be filled to the maximum with grease to minimize any air pockets where condensation could occur. Pulleys with bearings should be inspected and rotated regularly every 90 days to prevent sagging, puddle corrosion or other issues. This rotation schedule will maintain proper lube film between rollers and bearings.
Support for long-term storage solutions
Preventive maintenance begins when spare equipment arrives or even before that if you order the spare already prepared for long-term storage. Each plant’s storage circumstances are different and even if there is not ideal storage for each asset, start with the most critical items and go from there.
Distributors and manufacturers want you to get the whole, reliable service life out of your required spare components, so contact them for advice and specific guidelines to create your storage plan. A local industrial solutions provider may even provide services to inspect stored assets and assist with scheduled storage checks and maintenance. With a little advanced preparation, you can rest peacefully — even after a breakdown — knowing that spares are ready for service.