Keeping an eye on alignment
Ten times more? That number represents a flexible coupling's capability for misalignment compared to the tolerance of rotating equipment. That means system alignment should be based first on the minimum requirements of the driven equipment or the driver and finally the coupling. The benefits of proper alignment would seem elementary, but it is all too easy when running three consecutive shifts ...
Ten times more? That number represents a flexible coupling’s capability for misalignment compared to the tolerance of rotating equipment. That means system alignment should be based first on the minimum requirements of the driven equipment or the driver and finally the coupling.
The benefits of proper alignment would seem elementary, but it is all too easy when running three consecutive shifts to let the little things slide, which can lead to major problems. Misalignment is a leading cause of bearing and seal failures, vibration, oil leakage from bearing frames, broken shafts and coupling failures. Always align the equipment as close as possible, keeping within the economics and sophistication of the system to reduce potential stress, avoid unnecessary maintenance and extend equipment life.
Planes of flexibility
The plane of flexibility can be viewed as pivot points within the coupling. A full-flex or double engagement coupling has two pivot points with one attached to each of the connected shafts. A coupling with a pivot point on one side and a rigid shaft attachment on the other is called a single-flex, flex-rigid or a half-coupling. The pivot point can be in the loose fit between separate parts, such as the hub-tooth-to-sleeve-tooth interface in a gear coupling, in the bending of a continuous flexing element such as a disk or diaphragm, or link-coupling. The flex plane of an elastomeric coupling is within the elastomer itself.
Types of misalignment
There are three variations to shaft misalignment: parallel offset (radial) misalignment, angular misalignment and a combination of angular and parallel offset. Axial displacement is considered a form of misalignment dealt with by the coupling.
Parallel (radial) misalignment occurs when the driving and driven shafts are parallel but with some offset between their axial centers. Accommodating such offset requires either a full-flex coupling, with two flex planes, or two single-flex couplings in series. In either case, the greater the axial distance between the two flex planes, the greater the coupling’s parallel or radial capability. Typical full-flex couplings include gear, grid and dual-element disk or diaphragm types.
Although the elastomeric type has only one flex-plane, the elastomer can distort enough in some cases to provide significant parallel offset capability if it has sufficient resilience. Elastomeric couplings can also be made as spacer or floating shaft types to a limited extent.
Angular misalignment occurs when the axial centers of driving and driven shafts intersect. Flex-rigid or half couplings provide only for angular misalignment because there is only one flex plane. Single-element disk or diaphragm couplings provide for angular misalignment only. Single-element couplings are used on three-bearing systems and on one end of floating shaft systems.
Axial misalignment or in-out movement is often associated with thermal shaft growth and floating rotors. Thermal growth is the result of high temperature in rotating equipment causing an unconfined growth along the length of its shaft. Sometimes a thrust bearing at the coupling end of a shaft will direct axial movement or growth away from the coupling, or the thrust bearing can be at the other end of the shaft.
Another well-known source of axial movement is the rotor that seeks its magnetic center. The coupling must either accommodate axial movement or contain it by transferring the thrust to the bearing system of the rotor. Those that contain it are called limited-end float couplings. Sometimes axial thrust is deliberately transferred to another machine through the coupling. Limited-end float may or may not be used in such a case.
Gear couplings exhibit the best capability to handle axial misalignment because the hub teeth are free to slide axially within the sleeve while enmeshed.
Other types of couplings, such as diaphragm couplings can flex or stretch to allow some axial displacement. Disk couplings can also do this, but to a lesser degree than the diaphragm coupling. In both the disk and diaphragm coupling, axial movement is met with resistance that increases as the displacement increases.
A coupling manufacturers’ full line catalog is a good starting point for a comparison of misalignment capabilities. Coupling misalignment is usually given in terms of angular misalignment that can be converted to radial or parallel misalignment when two flex planes are used.
The conversion of angular to radial misalignment capability is a matter of plane geometry. The radial offset distance is the product of the tangent of the angle of misalignment and the distance between flex points.
It is true that misalignment, torque capabilities and coupling life are intertwined. The torque capability of a coupling is reduced when the coupling is misaligned. The reduction in life can come from higher wear and the reduction in torque capacity from high fatigue forces.
Misalignment causes fatigue forces. Some manufacturers publish their torque ratings as a maximum value and require that the user de-rate by some factor to determine usable nominal torque. Others publish their torque ratings at rated full misalignment. When the coupling is selected it should be able to carry the nominal torque while misaligned per the application.
A high-speed, high-power system requires accurate alignment. General-purpose equipment is aligned to a looser specification. For example, high-speed equipment, running at 3,000 RPM or more, needs alignment to 0.0005 inch or better per inch of flex point separation. General-purpose equipment can be acceptable at 0.001 inches per inch of separation. Smaller values will improve the operation but should be consistent with the equipment manufacturer recommendation.
These recommendations are usually used with close-coupled equipment. When spacers and floating shafts are chosen for their ability to allow radial displacements of two pieces of equipment, these rules-of-thumb would not necessarily apply.
It is sometimes necessary to have large amounts of parallel displacement built into the equipment installation. Special design alterations to the couplings and the connected equipment can also be required in those cases. The drivers and driven equipment must have sufficient design strength to deal with the increased reactionary load. Equipment with spacer couplings or floating shafts may be designed for ease of maintenance rather than for the increased misalignment capability that would be possible.
Exceeding acceptable misalignment contributes to vibration problems. Severe misalignment imposes a heavy vibratory force on equipment. Large amounts of parallel misalignment are acceptable only on machines operating at slow speed. High-speed machines, even those with spacers and floating shafts, must be aligned accurately to limit vibration and reactionary loads.
Reactionary loads from misalignment
Shaft-to-shaft misalignment causes couplings to impose reactionary loads on connected equipment. The greater the misalignment, the greater the reactionary load. Different types of couplings produce different reactionary loads.
In gear couplings, reactionary forces result from the sliding friction of the tooth-to-tooth movement. Sliding friction is high when dealing with metal-to-metal contact. There is some lubricant but it is a very thin film.
When a gear coupling is misaligned, the friction or drag forces become a bending moment on the system. The bending moment reactionary load can be 10% of the value of the torque transmitted. At that value it is many times the reaction load of a disk coupling.
While replacing a gear coupling with a disk coupling would seem wise in certain applications, any situation requires a careful assessment of the true amount of misalignment the coupling needs to handle. In a paper mill, for example, even after switching to a disk coupling from a gear coupling in the dryer section, the disk coupling was failing after only 90 days of operation. Close evaluation determined this particular disk coupling was not rated for the amount of misalignment needed to respond to the natural misalignment of the application, which caused equipment failures.
In addition, that particular coupling design was difficult to install and time intensive for the plant staff to maintain.
Another disk coupling proved to be a better choice because it offered superior misalignment capabilities, eliminating costly machinery breakdowns. Because the disk coupling has no moving parts and therefore no parts to wear out, the paper plant has no need to lubricate the part, saving thousands of dollars by eliminating downtime for maintenance.
Disk and diaphragm couplings use the flexing of thin metal elements to handle misalignment, both angular and axial. The thinner the element, the less force it takes to bend the metal. Link couplings and spring couplings also provide a reactionary force in proportion to the loading force.
Reactionary loads include the weight of the coupling. Because weight usually reflects size, the more power intensive couplings, those with higher torque capacity for smaller size, such as gear couplings, have the advantage of lower reactionary forces at comparable torque loads. Large couplings, less power intensive types, may also be made from light materials to reduce their weight and inertia.
Loads imposed by the coupling are related to the point of loading. The pivot point or flex plane is the location of the loading for a coupling. Couplings that move the pivot point closer to an available bearing are termed reduced moment couplings because they reduce the reactionary load imposed on the bearing.
Align at installation
Best practice is to align the equipment when the coupling is installed. When aligning the coupling, the installer must take into account where the equipment is at standstill and where it will move when in operation. Hot operating equipment grows when it is brought up to operating temperature. The coupling is aligned cold at one location with the expectation that movement will bring it to closer alignment.
In addition to temperature considerations, rotating equipment alignment can be affected by tolerance stackup, pipe loading, the foundation and conditions such as bent shafts or soft foot.
If possible, select couplings that will require only initial equipment alignment. Shutting down equipment to replace a failed coupling and then realigning equipment properly can steal hours from plant productivity and thousands of dollars from the bottom line.
Benefits of precision alignment
When machines are properly aligned they run cooler. It is generally accepted that for every 10-degree increase in motor temperature, there is a corresponding reduction in expected coupling life.
When machinery is misaligned excessive forces are generated. This can be measured by observing increased vibration amplitudes. These increased forces have a detrimental effect on the life of bearings. If all factors are held constant, as the force increases, the life expectancy of bearings is reduced by the cube of the increased force. A doubling of the force results in a reduction in bearing life to 1/8th of what was expected.
The Bottom Line…
There are three types of misalignment: radial, axial and combined radial and axial.
Tolerance for misalignment decreases as the speed of shaft rotation increases.
When machinery is alignment properly, temperatures and vibration are reduced and bearing life is increased.
Tolerances of various couplings
Always keep in mind that equipment should be aligned first and foremost to the rotating equipment manufacturers’ standards and requirements, not the coupling’s. When operating misaligned, a flexible coupling can transmit reactionary loads and vibrations that are within the coupling capabilities, but not the equipments capabilities.
|Coupling types||Angular misalignment, inches||Max. parallel offset, inches||Max. axial freedom, inches|
|Pin & bushing||5.0||0.010-0.050||no rating|
|Shear type donut||1.0||0.010-0.062||0.125|
|Bonded tire, urethane||4.0||0.188||0.125|
|Jaw in shear||2.0||0.030-0.047||0.031-0.063|
|Three link disc||6.0||0.120-0.350||0.090-0.250|
|Kevin Remark, director of product development for Lovejoy Inc. holds a BSME degree and has spent 24 years with the company. He can be contacted at firstname.lastname@example.org .|
MHI’s John Nofsinger: Technology, fundamentals go hand-in-hand
As the world of material handling expands and its importance grows on the plant floor, John Nofsinger is seeing his organization’s influence grow as well. Nofsinger is CEO of Material Handling Industry, an international trade association representing theinterests of the material handling and logistics industry in the U.S.
Before joining MHI, Nofsinger was employed for 20 years by Republic Storage Systems (a Division of Republic Steel Corporation and subsequently of LTV Corporation). A variety of technical and marketing positions led to his last responsibilities as vice president and managing director of the Automated Systems Division.
Nofsinger serves on the Board of Governors and Executive Committee of MHI, Board of Directors of Material Handling Industry of America and Material Handling Education Foundation. He is also managing executive for the Rack Manufacturers Institute and serves on the board of the Charlotte Roundtable of the Council of Logistics Managers.
He will lead his association to NA 2006 in Cleveland’s IX Centre on March 27—29. Nofsinger spoke exclusively with PLANT ENGINEERING editor Bob Vavra about the past, present and future of material handling, and of his expectations for this year’s event:
Q: As you look ahead of NA 06, what are the key issues facing material handling as a discipline and MHIA as an association?
A : As a discipline, the material handling and logistics industry is enjoying its role as the industry that makes the supply chain work. As investment considerations take on a much more strategic dimension, material handling and logistics providers are being required to add value and solutions to outsourcing and off-shoring decisions, worker shortages, supply chain integration as well as events that might disrupt supply chains.
Founded in 1945, MHI, as an Association has evolved through a series of changes — wartime to peacetime, advancements in technology, numerous business cycles, consolidation, globalization and a shift from tactical to strategic infrastructure. In each case, the industry has demonstrated resilience to the challenges presented.
The day-to-day activities of a membership organization like MHI are driven by consensus. Consensus, in turn is fueled by consistency and constancy. One of our larger challenges is to provide valuable programs and platforms through which to maintain the continuing involvement of time-poor executives and to assure relevance of all activities.
Q: While many larger manufacturers have embraced material handling and inventory management, some smaller operations may still be looking to balance the upfront costs against potential savings. Talk about how material handling saves money for manufacturing.
A : An executive of a leading electrical equipment manufacturer once referred to material handling as continuous flow occasionally interrupted be a value add operation. That said, the efficiency and effectiveness through which goods flow offers significant value, including:
Time/Labor Savings: Both direct and indirect labor are conserved by efficient and well organized work flows and stations
Space Savings: Fully utilizing building cube maximizes return on assets and can avoid costly expansion or transportation costs
Safety: Proper application of material handling equipment and systems can significantly reduce worker fatigue and minimize the occasion of immediate of cumulative injury
Inventory Reduction: Moving goods and information concurrently and connecting elements of the supply chain can lead to lower inventories at finished stages as well as postponement strategies — all allowing the freeing of important working capital for more productive application
Security: Well-ordered and secured expendables and high-value inventories can reduce shrinkage
Rapid Response: Material handling when thought of as a system vs. as activities, allows companies to respond quickly to shifts in market demand and differentiate themselves resulting in improved market shares.
Q: RFID has been the hot topic for a few years now. Is it still the ultimate solution? Where are we in terms of implementation, and how long do you see it taking until we’re at full adoption?
A : RFID has been around for quite some time — actually since the end of World War II. While advances had occurred through the 1900s, it wasn’t until a critical mass of early adopters changed the landscape. Over the past five or six years, the imperatives of companies like Wal-Mart, Gillette, Spencer and Marks, Target, the U.S. Department of Defense (to name a few) have created a flurry of developmental activity that have driven price points and technology availability to be more attractive at the unit-load, and occasionally carton level. The promise of tags on every item in a carton is still felt to be some years down the road. All of this said, RFID is as near to a “silver bullet” technology as most have seen in the past 20—30 years.
Q: What are you especially excited about as you look ahead to NA 2006?
A : For me, trade shows are therapeutic. There is energy and enthusiasm seldom seen during the normal course of activities. They are about “new” — new products, technologies, providers, corporate structures. They allow us to renew and develop important relationships. They are a laboratory that satisfies a long list of business objectives that cannot be done more effectively in any other venue. I can’t wait for NA 2006.