Troubleshooting Power Transmission Couplings

Size: px

Start display at page:

Download "Troubleshooting Power Transmission Couplings"

Gordon Blankenship
6 years ago
Views:

1 Troubleshooting Power Transmission Couplings Introduction Power transmission couplings are used to connect two shafts that turn in the same direction on the same centerline. There are three principle types of couplings; rigid, flexible and special purpose. Rigid couplings are used in applications where misalignment is not a factor, where 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. Rigid couplings 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. 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. There are three fundamental categories of flexible couplings; mechanically flexible, such as gear and chain couplings, material flexible, such as disc, spring, diaphragm, elastomeric and bellows and combination, such as metallic grid couplings, that provide a combination of mechanical and material flexibility. Special purpose couplings include such devices as mechanically flexible Ujoints and constant velocity joints used for automobile applications, magnetic couplings, such as magnet to magnet and eddy current couplings and fluid couplings, such as liquid, silicone and shot filled types. Magnet and fluid couplings provide no contact between drive and driven elements, offer low maintenance and both are capable of absorbing shock loads. 1

A unique special purpose type, the Schmidt or offset coupling is designed to handle large parallel shaft offsets of up to 18 inches and 1,000,000 in-lbs of torque. (SEE FIGURE 1.) FIGURE 1.

2 A unique special purpose type, the Schmidt or offset coupling is designed to handle large parallel shaft offsets of up to 18 inches and 1,000,000 in-lbs of torque. (SEE FIGURE 1.) FIGURE 1. The Schmidt or offset coupling. There are over 80 styles of flexible couplings used today in approximately 90 percent of all industrial applications. The most common are briefly described below under their respective categories. 1. Mechanically flexible couplings, such as 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 inches to ensure long coupling life. These coupling types require lubrication using recommended coupling grease or EP oil. As might be expected, approximately 75% 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 respectively and these types do not require lubrication. (SEE FIGURES 2. & 3.) 2

Material flexible couplings, as the name implies, provide flexibility by incorporating elements that accommodate a

3 FIGURE 2. FIGURE 3. Chain coupling steel. Chain coupling nylon. 2. Material flexible couplings, as the name implies, provide flexibility by incorporating 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, diaphrams and elastomeric materials that may include rubber or plastics, such as neoprene and urethane. 3

4 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. (SEE FIGURES 4. & 5.) FIGURE 4. Laminated disc coupling undergoing laser alignment. 4

5 FIGURE 5. Typical flexible coupling with elastomeric material which might be natural rubber, urethane or neoprene. 3. Combination mechanical/material flexible couplings include the very popular grid coupling, which is 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 specially designed grooved slots cut axially on the outer edges of the flanges. The flanges are connected by using a serpentine spring grid that fits the grooved slots. The flexibility of this grid provides torsional resilience, can provide some misalignment and end float, dampen vibration and may reduce peak or shock loads by up to 25%. Grid type couplings require lubrication using good quality coupling grease in the areas of the grooved slots and serpentine spring. (SEE FIGURES 6 & 7.) 5

6 FIGURE 6. Grid coupling. (The serpentine spring has been removed so that it does not interfere with the alignment procedure). FIGURE 7. The wear areas of the grid coupling are the grooved slots and the serpentine spring. Why Couplings Fail Couplings fail for several reasons, but the primary causes are improper selection for the particular application, excessive misalignment, improper, inadequate, or 1

7 insufficient lubrication, harsh environmental or operating conditions and excessive speeds or loads. 1. The application factor. Following is a general guide for coupling application and a corresponding selection recommendation. (SEE TABLE 1.) 2

8 3 TABLE 1.

FIGURE 8. Some manufacturers like Falk, provide a coupling selector guide that carries the service factor for various equipment types and provides corresponding coupling recommendations. 2.

9 FIGURE 8. Some manufacturers like Falk, provide a coupling selector guide that carries the service factor for various equipment types and provides corresponding coupling recommendations. 2. The alignment factor. 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 much closer tolerances. (SEE FIGURE 9.) 4

10 FIGURE 9. Types of Misalignment Symptoms of misalignment include; Noise at the coupling, Powdered rubber particles or leaking lubricant directly below the coupling (depending upon coupling type), Process fluid and/or oil leaks at the drive, driven (or both) shafts, Premature shaft, shaft keyway, or key fatigue or breakage, Premature or frequent bearing or seal failure at one or both machines, Soft Foot condition at foot bolts, Broken or constantly loosening of foot bolts at one or both machines, Shimmering of oil on base plates, or near the foot bolts, High operating temperatures at or near the coupling, 5

11 High vibration conditions, usually at both machines, Cracked or broken foundation, particularly at or near the foot bolts, Continuing or intermittent leaks at pipe joints caused by pipe strain, High energy consumption, No compensation for thermal growth at either drive or driven machine during initial alignment procedure, Settling of machine or foundation after installation. The following chart lists acceptable misalignment tolerances based on machine speed. (SEE FIGURE 10.) TABLE 2. Allowable Misalignment Tolerances Machine Speed RPM Parallel Offset Misalignment (in MILS) Angular Misalignment (in MILS) < >

12 FIGURE 10. Gear coupling with spacer undergoing laser alignment. The coupling connects an electric motor driving a hammer mill. Fluid couplings are referred to as special purpose, however they are also rigid and cannot accommodate misalignment. (SEE FIGURE 11.) 1

13 FIGURE 11. Fluid coupling undergoing a laser alignment procedure. Alignment can also be determined initially with a dial indicator. To determine parallel offset, mount the indicator on one hub with the point touching the outside diameter (OD) of the other flange. Rotate the hub on which the indicator is mounted while holding the other flange stationary. The parallel misalignment can be determined by obtaining the minimum and maximum readings and dividing by two. (Rotating the flange under the indicator point will not produce a measure of parallel offset). To determine angular misalignment, move the dial indicator point so that it is touching the face of the other hub. Again rotate the hub on which the indicator is mounted while holding the other stationary. 2

14 Again note the minimum and maximum readings. Each inch difference between the two readings for each inch the indicator point is located from the centerline of the shaft indicates an angular misalignment of about one (1) degree. Another issue that may affect alignment is end float or axial shaft movement. There is a tendency for shafts to be pulled closer to their own machine as soon as they are started. As shafts tend to separate at startup, axial end float tries to pull the coupling apart and allowable end float should not exceed ¼ inch. 1. The lubrication factor. There are three flexible coupling types that require lubrication. These are gear and chain mechanically flexible couplings and combination mechanical and material flexible grid couplings. The lubricants used for gear and grid couplings may include; ISO 460 compounded oils with tackiness additives for low speeds and centrifugal G forces of up to 8000 or ISO 100 rust and oxidation inhibited anti-wear oils for high speeds, centrifugal G forces of over 8000 and low temperatures of -20 C (-4 F). NLGI Grades 1 and 2 coupling greases containing high viscosity oil fortified with rust and oxidation inhibitors, extreme pressure additives and tackiness agents to prevent separation at high speeds and high centrifugal G forces, may be recommended for gear, chain and grid type couplings. Note: 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. 3

15 Generally, oil or grease can be used at coupling speeds of 3600 to 6000 rpm. Oil is recommended where coupling speeds exceed 6000 rpm. Gear and grid style couplings incorporate seals or gaskets to contain the lubricant and prevent the entry of foreign material. These units contain access plugs for relubrication which must be properly torqued after re-lubrication to ensure that centrifugal force does not loosen the plugs and cause a serious safety hazard. Re-lubrication and inspection intervals should be considered as preventive maintenance tasks and carried out at least annually, (or more frequently, if conditions such as high temperatures call for shorter intervals). The amount of lubricant used is also important. Gear couplings, if overfilled with lubricant, may lockup and the coupling will no longer maintain its flexibility. (Any increase in vibration of the coupling after re-lubrication suggests excessive lubricant). If a coupling is disassembled for re-lubrication, ensure that the two flange halves are marked for correct reassembly and be certain to use specified coupling flange bolts properly torqued. Install new seals or gaskets if the old seals and gaskets have any damage, nicks or are no longer pliable. Before reassembly of grid type couplings, carefully 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. (SEE FIGURE 12.) 4

16 FIGURE 12. Courtesy American Axle & Manufacturing. Coupling shows failed serpentine spring and grease residue that had undergone oxidation. The old lubricant removed from the coupling should also be inspected for metal particles indicating wear and darkening in colour suggesting oxidation. Finally, after re-lubrication and re-installation, ensure that safety guards or mesh screens are reinstalled and secured. 2. The operational and environmental factors. Operating conditions and environmental considerations 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 back lash is required, flexible disc couplings are recommended over elastomeric types. 5

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.

17 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 inches can increase adjacent bearing temperatures by as much as 18 C (65 F). The lesson is that coupling operating conditions and environment, lubrication and alignment are all related. (SEE TABLE 3.) TABLE 3. 6

18 7

19 8

20 These troubleshooting guidelines are general in nature and cover only those very common flexible couplings described in the guide. For uniquely designed or 9

21 special purpose couplings, contact the appropriate manufacturer for selection, maintenance and troubleshooting recommendations. Conclusion Troubleshooting coupling problems is quite simple, so long as the troubleshooter understands coupling design and application limitations and remembers that reliable machine trains are dependent upon the couplings that connect drive to driven equipment. R e f e r e n c e s Condition Monitoring Standards, Volume 1, IDCON Inc. 2001, PP Industrial Machinery Repair, R. Smith and K. Mobley, 2003, Butterworth- Heinemann, PP Power Transmission Handbook, 4 th Edition, Power Transmission Distributors Association, 2006, PP Coupling Standards, American Gear Manufacturers Association, AGMA 9000, 9001, 9002, 9003, 9004, 9008, The Practical Handbook of Machinery Lubrication, 3 rd Edition, L. Leugner, PP ,

lea) shows a compression type. These couplings are used for

lea) shows a compression type. These couplings are used for Mechanical Equipment - Course 230.1 SHAFT COUPLINGS Couplings Couplings are used to join two shafts provide some means of transmitting power source to a driven member. There are two tiona of couplings,