Special Bearing Selection and Care for Harsh Environments: Part 1

Introduction

Bearings are critical components in industrial equipment and their proper selection and maintenance are essential to plant operations. This is particularly true when the machinery operates in severe conditions. A cement plant is one of the harshest environments; factors including high and low speeds, heavy loads, temperature variations, extreme levels of contaminants and debris in the air and challenging weather conditions place unique demands on bearing and power transmission reliability.

Plant operators who order the right bearings, take precautions in handling and installation and perform the necessary preventive maintenance will be rewarded with longer bearing life, increased equipment productivity and lower total cost of ownership.

Selection and installation

It is important to review and fully understand the bearing specifications for a piece of equipment. Bearings selected for machinery operating in hostile environments or exposed to severe service often have special requirements for construction, materials, internal clearances and mounting.

Factors that influence bearing specification can include operating temperature, rate of temperature change, system deflections and misalignment, type and degree of potential contamination, operating speeds and acceleration, vibration and more.

The original equipment manufacturer (OEM) will have taken these factors into account during machine design. The observation of bearing performance over time in heavy-duty equipment can help identify performance issues that can be resolved with improved products and better operating or maintenance processes.

Proper installation is the first step to ensuring long service life. Record keeping is also important. Record all maintenance activity, noting the date, equipment model and serial number, bearing assembly and serial number (if applicable) and manufacturer. Include comments regarding housing or shaft reconditioning and other components that were replaced. These records can be essential in establishing trends in bearing performance, defining service intervals and planning future maintenance. Records are also the baseline for troubleshooting equipment performance issues and related bearing damage analysis.

Lubrication

Proper lubrication is critical to ensuring good bearing performance and is a key part of machine operation and maintenance. Lubricant type, cleanliness, quantity and frequency of oil or grease supply are all important considerations. Oil viscosity is selected to maintain effective film separation, helping to prevent wear and surface damage. The lubricant can also reduce operating temperature and minimise contamination.

The lubricant must meet all OEM and supplier specifications. Manufacturers or distributors should provide detailed recommendations on storage conditions, maximum shelf life, delivery systems, filtration levels and other precautions. Consultation is recommended if any conditions change during operation, even those that appear insignificant.

Proper grease pre-pack or oil sump level must be present to ensure a lubricant film develops in the bearing on start-up and continues throughout the life cycle of equipment operation.

Mixing different grades and brands of greases or oils can be disastrous. Lubricants must be completely compatible to function properly over time. Because of significant chemical differences in formulation (thickening agents, base oils, additives, etc.), some lubricants cannot be mixed with others.

Before implementing a lubricant change, qualify compatibility with the suppliers. This check may require testing. If compatibility is questioned, more tests might be required. Plan the time and expense to remove all old lubricant from the system. This is also an opportunity to review the condition of lines, connectors, distributors and pumps and seals and address corrective actions.

Consistent monitoring of bearing and oil temperatures is also important. Bearings operating under severe conditions that generate more heat due to rolling contact friction will place more demands on the lubricant, as will bearings exposed to high ambient or process temperatures. Oil service life can decrease drastically when operating over 180 °F (82 °C), even with oxidation inhibitors.

Avoiding contamination

Even a small amount of debris can disrupt the oil film, resulting in bearing surface damage and reducing useful service life.

Control of lubricant system cleanliness is important. This includes bulk storage tanks, central reservoir, plumbing lines and connections. Central tanks should be supplied with magnetic traps and routinely monitored to prevent significant buildup of wear debris and other contaminants. Avoid excessive agitation of the reservoir; it is often sized to allow sedimentation of larger particles. Pipes and connections must be cleaned thoroughly to avoid introducing contaminants.

When using a manual grease gun or mobile pump, fittings should be protected by using shrouds, dust caps or both. Also, when using a manual lube schedule, post signs and procedures, record dates and make note of any unusual observations. An operator able to spot a leaking seal or changes in grease consistency can help initiate diagnostic activity rather than emergency troubleshooting.

Do not rely solely on filters to handle contamination. In harsh environments, increased demands on filters are typical, requiring better planning for scheduled inspection and/or the replacement of filter elements. Reducing the level of filtration can generate maintenance problems if the flow rates are restricted.

When possible, establish routine collection and testing of oil or grease samples for consistent properties and contamination. Look for fluids or solid particles that may point to unseen problems.

Housed units

One way to protect bearings from hostile conditions is to utilise special housed units. Housed units offer proven contamination resistance and are beneficial to today’s industrial applications in demanding conditions and highly contaminated environments, which are increasingly dependent on these robust products.

Housed units come in multiple varieties designed for many bearing types, including tapered roller, spherical roller and ball bearings. They feature dozens of sealing options and multiple locking mechanisms and are offered in a wide size range to meet shaft centreline and equipment construction requirements. Options also include solid and split housings made of materials ranging from cast steel to ductile iron.

Careful selection of housed units can yield less downtime, reduced maintenance intervals and an overall lower cost of ownership. To choose the right housed unit, work with a supplier trained to analyse specific operating environments and recommend appropriate configurations of housings, locking mechanisms and seals for each application.

Read Part 2 here.

Written by Claudiu Popescu, Laurentiu Ionescu and Jerry Rhodes, The Timken Company, USA. This is an abridged version of the full article, which appeared in the March 2014 issue of World Cement. Subscribers can view the full article by logging in.

Read the article online at: https://www.worldcement.com/the-americas/03032014/special_bearing_selection_and_care_for_harsh_environments_part_1/