Machine Design, Environment Affect Lubrication
Ken Bannister | December 19, 2017
Bearing lubrication decisions, made at the machine-design stage, often dictate how well the machine will be maintained throughout its working life.
Most design engineers understand how to correctly size a bearing for load or how to choose the proper bearing class, limits, fits, and tolerances for an application. But very few are versed in the science of lubrication and basic maintenance practices. The problem is evident when a grease-lubricated bearing is mounted inboard of the machine side frames in a hidden or difficult-to-reach location with a simple grease nipple at the bearing point. This type of arrangement requires a full machine shutdown and lockout/tagout to lubricate a single bearing—should it get lubricated at all.
Listen to this podcast with Ken Bannister, in which he further explores how machine design and the working environment affect lubrication strategies.
Specifying new-equipment lubrication procedures is always a compromise and many factors go into the decision-making process. Being aware of the “in service” consequence can not only make the operational effort much more efficient when the machine goes into service, but can also increase the asset life cycle and reduce downtime incidences and associated costs for the life of the machine.
Depending on the machine’s design, bearing lubrication may be performed using oil, grease, or both. Oil is prevalent with closed gearboxes, high temperature, and high-speed bearing applications. Grease systems are usually less expensive and more common. Here are some pros and cons of a number of popular lubrication application methods:
Manual delivery: This is your 30-cent grease nipple attached to each bearing point. The pro side is its minimal cost and minimum design requirement. The cons are uncontrolled delivery—too much, too little; bearing access problems; and an increased chance of contamination/cross contamination from dirty grease-gun tips and non-identifiable grease tubes in the grease gun.
Single-point auto lubricator: Newer versions are programmable, refillable, can be used for oil or grease, and develop enough pressure to move an eight-point divider valve, all for an investment of just a few hundred dollars. These are well suited for existing applications with minimum point requirements and out-of-the-way single points, but are still considered “throw away” lubricators that, in the end, cost many times more than a traditional automated-pump system.
Centralized automated single-line and dual-line injector systems: Systems can be used for oil or grease delivery and require little up-front engineering while offering built-in control flexibility at each injector point. Unfortunately, good features can also be detrimental, especially when flexible also means tamperable, i.e., easy adjustments that allow non-experts, with the best of intentions, to starve or over-grease bearings.
Centralized automated progressive divider system: This is probably the most respected delivery system on the market, although it does require up-front system engineering that is usually included in the cost of the system.
Unfortunately, even with a lubrication-savvy designer who has specified a state-of-the-art fully centralized and automated lubrication-delivery system, all can be changed with the stroke of a pen. Lubrication systems, as inexpensive as they are, are much more expensive than grease nipples and are an easy sacrifice when spiraling project costs need to be reined in.
With any type of oil-lubrication system, ensure that oil reservoirs have easy-to-read site-level gauges installed, desiccant-style breathers, filters with go/no go change indicators, and a contamination-controlled fill system. If you already employ a lubrication standard(s) in the plant, ensure that the new design complies with your standard so as to minimize spare parts and training requirements.
Always try to install the lubrication pump and controller at or outside the working perimeter of the machine envelope. This allows the lubricator to be filled, tested, and maintained without the need to shut down and lock out the machine.
If you have any load on your machine bearings you will require a lubrication strategy and need to choose a lubricant-delivery method. Choosing one is much cheaper and easier at the design stage. An automated system is the smart choice because, when set up correctly, it can increase bearing life cycle by three times that of a manually lubricated bearing, while reducing your machine’s energy profile.
WORKING ENVIRONMENT FACTORS
Bearing longevity depends on the machine operator and the maintenance crew factoring in the working environment and making necessary adjustments. Ironically, poor working environments can be found even in a food plant that is washed down regularly with water and caustic soda at the end of each shift. The plant may be clean, but strong water spray pointed directly at a bearing can cause the grease to wash out. In such cases, the bearings must be re-greased immediately or a more permanent solution implemented, such as installing water shields or run-off protective guards.
Similar solutions are required for processes that use aggregate or sand (silica), such as in a sandcasting foundry, cement plants, or mining facilities.
Other working environments that are detrimental to lubrication include:
A plant that runs two or three shifts. Most original-equipment manufacturers base their lubrication recommendations on a single-shift operation (2,000-hr. annually). Double and triple shifts require lubrication delivery and reservoir fill frequency to also double and triple if line throughput is constant.
Aggressive production schedules. Running over design capacity will break down the lubricant much faster, requiring a ramped-up delivery schedule.
Multi-mode operation. Different lubrication strategies are required for machinery that may operate in three separate modes, e.g., full-operation mode, standby/idle mode, and/or spare/redundant.
Temperature extremes. Wide operating-temperature ranges dramatically affect the lubricant viscosity. Very hot conditions can make the lubricant very “thin,” causing it to lose its ability to stay in the bearing pressure, or load, zone, allowing metal-to-metal contact to occur. Extremely cold environments make lubricants “thicken.” A thick lubricant may stall out the pump, causing the system to dramatically slow its delivery or stop altogether. The correct viscosity is essential in extremes that may require a complete lubricant changeover as the seasons change.
In every case, the onus is on the end user to make sure he/she understands and make adjustments that compensate for the working environment. The best strategy is to specify exact needs at the design stage. Machine design and working environment can significantly affect how bearings are lubricated. EP
Contributing editor Ken Bannister is co-author, with Heinz Bloch, of the book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for Engtech Industries Inc. (Innerkip, Ontario), he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact him at firstname.lastname@example.org, or telephone 519-469-9173.