Used to connect driving equipment to driven equipment, couplings are a critical machine component that can sometimes be overlooked when it comes to selection and maintenance. These critical components come in many different shapes and sizes, so it can be confusing trying to navigate what couplings are used for, what they do, and which couplings are best for your machinery.
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We have put together the essential list of everything you need to know about each different type of Coupling, including their features and benefits and in what machinery you will often find them:
Flexible Couplings come in many different variations, but all of these have 1 thing in common &#; its elastomeric properties. These properties typically come in the form of an elastomer or elastic type element that is often made of a rubber or urethane compound. This element absorbs impact, dampens vibrations, and cushions shock loads, making it ideal for applications with high shaft misalignment. Depending on your application, the type of flexible coupling you use differs and can provide additional benefits to your machine.
Jaw couplings are made up of 3 parts, two metallic hubs and an elastomeric spacing element (also known as a spider) this means depending on your application, these couplings offer different levels of dampening protection. In addition, these versatile couplings have a fail-safe design, meaning that even if the spider fails, the coupling can continue to operate without its dampening properties until the spider is replaced. They also do not require lubrication during operation, making them a low maintenance option that helps to reduce machine downtime for your business.
Tyre couplings have two metal hubs with a tyre shaped spacer positioned in between &#; this enables the coupling to transmit torque. The tyre also gives this coupling outstanding vibration dampening capabilities and excellent resistance to shock loads, helping to extend the life of the connected equipment in a wide range of industries.
Gear couplings are a great option for universal use and are often suitable for most applications as they can transmit high torque while working at low speeds.
Curved tooth gear couplings are known for their easy of assembly and their maintenance free operation making them ideal for inhospitable environments. These couplings also have a good resistance to chemicals, meaning they offer a low cost, reliable service for industries such as: Minerals & Mills, Pulp & Paper, Petrochemical, Oil & Gas, Machine Tools and Railway.
Claw couplings can be a great option for machinery where either angular, parallel or axial shaft misalignment is an issue. These couplings generally have a short overall length, meaning they are suitable for applications where space on the shaft is limited. Similar to other flexible couplings, a claw couplings elastic element dampens vibration and cushions shock loads, helping to extend the life of surrounding equipment in a range of industries.
Another fail-safe coupling option is pin and bush couplings. These couplings also have fantastic vibration and shock load dampening properties, all while accommodating up to 0.5° of misalignment. Also, their fail-safe design make them ideal for use in safety equipment such as fire pumps and elevator drives.
When your application has high levels of vibration and large shock loads, grid couplings are the ideal solution. Due to the grid that runs between the two slotted hubs of the coupling, these components have a high torsional flexibility. This helps to reduce vibration, which aids in prolonging the life of the surrounding drive equipment.
Back-lash free couplings are often found in applications where precision is key. Back-lash occurs because of the spacing of components within an application, this backlash often accumulates in the couplings as the joining piece for components. Using a back-lash free coupling minimises the effects of backlash within the system, helping to increase accuracy levels. These couplings come in the form of:
Rigid Couplings are often found in lower speed or lower torque applications and are used where a solid connection is required between two shafts of the same size. They tend to be high strength couplings that are used to maintain alignment between shafts. These couplings are often considered the most basic, but most cost-effective coupling.
There are three different variations of rigid couplings, each provide different features and benefits for applications. These variations include:
Torsionally rigid couplings are generally suitable for most industrial applications. This category includes disc and composite couplings. These can feature zero back-lash, low-restoring forces, and electrically insulated designs.
These Couplings can fall into either flexible or rigid depending on the requirements of your application.
Flywheel couplings, also known as flange couplings, is a torsionally rigid coupling that provide maintenance free operation that&#;s free from resonance. This makes them ideal for applications such as cranes, harvesting machines and floor conveyors.
Chain couplings have flanges that are linked with duplex roller chains, meaning they are a high strength coupling option. Not only this, but these couplings can transmit high levels of torque AND accommodate up to 2° of misalignment, helping to protect surrounding equipment from damage.
Where vibration in machinery is high, you may find a fluid coupling. This is because these couplings have an excellent level of vibration dampening and provide a controlled soft start up. Commonly found in applications such as conveyors, mills, shredders, crushers and agitators, fluid couplings can withstand the harsh conditions found in the environments these applications are often found.
This is just a basic overview of the different types of couplings that Acorn can provide, alongside custom solutions, we can help you to find the exact coupling your machine needs. Our specialised Couplings & Drives department are on hand to answer all of your coupling questions and queries, so get in touch with them today.
Rigid couplings are used in applications where misalignment is not a factor, flexibility is not required, and where the coupling is not required to absorb load shocks or torque changes. Rigid couplings connect shafts using bolted flanges, keyed sleeves or ribbed clamps bolted together over the shaft ends with keyways. They are used primarily for vertical drive systems. Lubrication is not required, but larger couplings, or those running at high speeds, may require balancing to reduce vibration.
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Flexible couplings also connect two rotating shafts, but are designed to dampen vibration, absorb some shock loading, and provide some axial movement or end float of the shafts, as well as compensate for minor misalignment. The three categories of flexible couplings include: mechanically flexible, material flexible and metallic grid couplings, that provide a combination of mechanical and material flexibility.
Special purpose couplings include mechanically flexible U-joints, constant velocity joints, magnetic couplings, eddy current, fluid couplings and the Schmidt or offset coupling designed to handle large parallel shaft offsets of up to 18 inches and 1,000,000 in-lbs of torque.
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Does the maintenance group recognize where mechanically flexible couplings should be applied?
Logic: Gear and chain couplings provide a flexible connection by permitting coupling components to move or slide over each other. Some minor misalignment is provided by the clearances between gear teeth and chain, and sprocket teeth respectively, however, shafts should be aligned to less than 0.002 inches to ensure long coupling life. Approximately 75 per cent of gear and chain coupling failures are caused by misalignment or improper or insufficient lubrication. Some newer types of gear and chain couplings contain nylon gear sleeves or nylon chains and these types do not require lubrication.
Are the applications of the material flexible coupling types recognized?
Logic: These couplings incorporate elements that accommodate a certain amount of bending or flexing. The flexing materials that provide the connection between the coupling drive and driven components include laminated discs, bellows, diaphragms or elastomeric materials that may include rubber or plastics, such as neoprene and urethane. Generally speaking, these coupling types require little maintenance other than alignment, and their service life is limited by the fatigue limit of the flexing material itself.
Are the applications of combination mechanical/material flexible couplings recognized?
Logic: These include the popular grid coupling; a compact unit capable of transmitting high torque at speeds up to 6,000 rpm. The construction of the coupling consists of two flanged hubs, each with grooved slots cut axially on the outer edges of flanges that are connected by using a serpentine spring grid that contacts the grooved slots. The flexibility of this grid provides torsional resilience that offers some misalignment and end float, dampens vibration, and may reduce peak or shock loads by up to 25 per cent. Grid type couplings require lubrication using good quality coupling grease in the areas of the grooved slots and serpentine spring.
Does the maintenance group understand why couplings fail?
Logic: Couplings fail for several reasons, but the primary causes are improper selection for the particular application, excessive misalignment, improper, inadequate, or insufficient lubrication, harsh environmental or operating conditions, and excessive speeds or loads. A common application is variable high torque machine trains operating at moderate speeds that require a serpentine spring and grid coupling. Vertical pump shafts would use flanged or split rigid couplings, while transmission of high torque at all speeds would require a chain or gear coupling.
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Does the maintenance group understand the coupling alignment factor?
Logic: There is a perception that flexible couplings can accommodate a great deal of parallel offset and/or angular shaft misalignment. This is untrue. Depending upon the coupling type, flexible couplings can only accommodate from ¼ to about 2 ½ degrees of misalignment and high speed, high load drive applications must be aligned to close tolerances. Symptoms of misalignment include, but are not limited to, coupling noise, powdered rubber particles, or leaking lubricant directly below the coupling (depending on coupling type), premature or frequent bearing or seal failure at one or both machines, &#;soft foot&#; condition at foot bolts, shimmering of oil on base plates or near the foot bolts, high operating temperatures and/or vibration at or near the coupling.
Does the maintenance group understand coupling lubrication application?
Logic: The coupling types that require lubrication are gear, chain, combination mechanical, and material flexible grid couplings. Lubricants used for gear and grid couplings include ISO 460 compounded oils for low speeds and centrifugal G forces up to applications, ISO 100 rust and oxidation inhibited anti-wear oils for high speeds with centrifugal G forces of over 8,000, and low temperatures of -20°C (-4°F) applications. NLGI Grades 1 and 2 coupling greases may be used for gear, chain and grid type couplings.
Centrifugal Force &#;G&#; may be calculated from the equation:
G = 14.2 X 10 &#; 6 dN2
Where d = pitch diameter of the coupling in inches
And N = revolutions per minute.
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Oil or grease can be used at coupling speeds of to rpm. Where coupling speeds that exceed rpm, oil is recommended. These units contain access plugs for re-lubrication that must be properly torqued after re-lubrication to ensure safety. Re-lubrication and inspection intervals should be carried out at least annually or more frequently for high temperature applications. The amount of lubricant is important. Gear couplings may lock up and lose flexibility if they are overfilled. (Any increase in vibration of the coupling after re-lubrication suggests overfilling).
If a coupling requires disassembly for re-lubrication, ensure that the two flange halves are marked for correct reassembly use specified coupling flange bolts properly torqued, and install new seals or gaskets if the old seals and gaskets have any damage. Before reassembly of grid type couplings, inspect the serpentine spring for damage, wear or fatigue cracks, and inspect the grid grooves for excessive wear or evidence of fatigue. On chain and gear type couplings, inspect the chain, sprocket and gear teeth for looseness wear and evidence of fatigue.
Does the maintenance group understand the operational factors?
Logic: Operating conditions play a large role in the long life and reliability of flexible couplings. Operating temperatures will affect not only the lubricant, but can seriously shorten the life of the materials used in elastomeric couplings. Generally, seals and coupling components cannot withstand temperatures much above 104°C (220°F) and group I mineral base oil begins to oxidize at 71°C (160°F). These guidelines should be understood and applied by machinery operators, and as a result, when high operating temperatures and/or where zero &#;backlash&#; is required, flexible disc couplings are recommended over elastomeric types.
In addition, solvents, caustic process fluids and water washing practices may be harmful to some elastomeric couplings and disc, grid or gear couplings may be required. Wherever high loads and speeds are required, disc and gear couplings may be the best choice. Finally, if high temperatures are being experienced, remember that an angular misalignment of as little as 0.001 inches can increase adjacent bearing temperatures by as much as 18°C (65°F). Coupling operating conditions and environment, lubrication and alignment are all related. MRO
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L. (Tex) Leugner, the author of Practical Handbook of Machinery Lubrication, is a 15-year veteran of the Royal Canadian Electrical Mechanical Engineers, where he served as a technical specialist. He was the founder and operations manager of Maintenance Technology International Inc. for 30 years. Tex holds an STLE lubricant specialist certification and is a millwright and heavy-duty mechanic. He can be reached at .