Courtesy of Kop-Flex/Emerson Power Transmission Corp. JRM16.pdf
Pump Couplings By carefully selecting, installing and maintaining couplings, you can save substantial dollars in reduce reduced d mainte aintenance co costs sts and downt ntime ime..
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variety of flexible couplings are used on pumps. Yet, only a small number of specifications and requirements exist for these coupling types. The American Petroleum Industry (API) addresses a few of these requirements in one of its specifications, API 610, Centrifugal Pumps for General Refinery Service (Seventh Edition). This specification states that couplings must have an alignment diameter concentric to the bore within 0.003 TIR” to help with alignment, and that couplings must meet balance requirements of AGMA 515, Class 8. There are not many other industry standards for pump applications that specify additional requireMore ments for couplings. importantly, these specifications and requirements do not explain how couplings work or help in the selection process. FUNCTIONS OF FLEXIBL FL EXIBLE E COUPLINGS
Flexible couplings join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. The three basic functions of a flexible coupling are to (Figure 1): 1. Transmit power 2. Accommodate misalignment 3.
Compensate for end movement
In general, the power lost by a flexible coupling is small, although
MARCH1994
Mechanical element coupling: grid type, horizontal split cover
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PUMPS AND SYSTEMS MAGAZINE
Mechanical element coupling: gear type, crowned teeth
donut, block, jaw, urethane tire, corded tire, shear donut, disc and diaphragm. GEAR
COUPLINGS
Gear type couplings consist of two hubs with external teeth that engage internal teeth on a two- or one-piece sleeve. The teeth may be straight or curved (crowned). For an application requiring over l/4 degree, curved teeth may be better. These couplings obtain their flexibility because of the looseness (backlash) between the mating teeth. Gear couplings are used for medium and large pump applications, and are probably the most power dense type available. They require periodic lubrication, every l-2 years, depending on duty and type of lubrication. If properly maintained (good lubrication and reasonable alignment), these couplings have a service life of 3-5 years, and in many cases 5-10 years. Some gear couplings have sleeves that are made of plastic (nylon, or high molecular plastic) and do not require lubrication. These have much lower torque capacities than the allsteel couplings and are used mainly on small pumps.
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Mechanical element coupling; gear type, straight teeth
GRID COUPLINGS
Grid type couplings are similar to gear couplings. Composed of all metal, they have some degree of resilience. These couplings can dampen vibration and reduce peak or shock loads by 10-30%. They have two hubs with serrations (grooves) rather than teeth. The grooves are connected by a steel grid. The covers, which keep the lubrication in, are either vertically or horizontally split. These couplings do not transmit as much power (per the same outside diameter) as gear couplings but are usually less costly. Grid couplings are used for medium and small pump applications. These couplings were originally designed for torsional dampings (softness). They are often misapplied in pumps because of their low cost, and will fail immediately with loss of packed lubricants. COMPRESSION
DONUT
COUPLINGS
Compression donut couplings have a precompressed elastomeric donut. Screws force the donut to a smaller diameter. All legs of the donut are in compression before the load is applied. Low torsional and lat-
COUPLINGS
Block couplings use rubber in compression. The rubber blocks are installed in cavities formed by internal sleeve blades, external hub blades and two end plates. This type is unique among couplings due to its “fail-safe” feature. If the elastomeric fails, the coupling may run for sometime on the metal blades. These couplings can provide up to l/2 degree of misalignment and parallel off set capabilities of 1/64-1/4 inch. Large pumps use these type couplings to reduce vibratory torques or because it is torsionally soft and is used to tune a system. Sometimes medium and small pumps will use these couplings because if properly aligned, they require no maintenance (except for replacement of blocks every 3-5 years). JAW COUPLINGS
Jaw couplings have their elastomers in compression. The flex element can be one piece or split to facilitate replacement. They also have
MARCH1994
flexibility, When the coupling fails, usually only the elastomeric element is replaced.
CORDED TIRE COUPllNGS
URETHANE TIRE COUPLINGS Common on small pumps, urethane tire couplings have their elastomeric elements in shear. The tires are split to enable easy assembly without removing hubs. Urethane tire couplings offer a high degree of
Corded tire couplings also have their elastomeric elements in shear. They use a reinforced element (similar to belted auto tires). Because of the reinforcement in the element, the torque capacity is greater per outside diameter than the urethane tire coupling. Most small and some medium pum p applications use these
M e t a l li c e l e m e n t c o u p l i n g ; d i s c t y p e , continuous disc
SHEAR DONUT COUPLiNGS
(UNCLAMPED) Another type common on small pumps, shear donut couplings have their elastomeric elements in shear. They provide low torsional stiffness and low reactionary forces. Oversizing can lead to premature failure by wearing the engaging teeth rather than a material failure from flexing.
Elastomeric element coupling; type, compression donut
MARCH1994
BLOCK
MAGAZINE
a “fail-safe” feature. Flex elements are made of many types of elastomeric materials, such as, rubber and urethane. The properties (that is, hardness, resilience, etc.) can be varied to suit required loads. These couplings are used primarily to accommodate misalignment and transmit power. Small and medium pumps employ these couplings. Be careful, since sometimes these couplings are used to absorb energy and dampen loads, particularly when equipment is engine driven. Also be careful because they are torsionally soft and in the past were used to tune a system out of a torsional critical mode.
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stiffness is achieved with this design. Medium and some small pumps incorporate these couplmgs. era1
compression
can and has been optimized for its characteristics. These couplings are composed of all metal, and do not require lubrication. The discs are usually continuous but can be individual links. Most disc couplings use multiple thin discs rather than one thick disc/link because the stresses from misalignment are proportional to Nts versus T^3. These couplings are used in medium pump applications. If properly installed and misalignment is kept under 1/4 degree, this type will provide a long life with little maintenance. If the misalignment increases beyond 1/2 degree during operation, the flexible element will probably fail in fatigue.
OISC COUPllNGS
DlAPHRAGM COUPLlNGS
Disc couplings transmit torque by a simple tensile force between alternating driving and driven bolts on a common bolt circle. Misalignment is accomplished from the flexibility that comes from the length of material between the bolts. Disc couplings have been around for years but with the use of Finite Element Analysis, this type
Diaphragm couplings transmit torque between the inside and outside diameters of the flex element. The diaphragm can be constant or of variable thickness, and singular or multiple. The diaphragm shapes are usually modified to increase flexibility (such as, tapered, convoluted, and cut out profiles). These couplings can usually handle up to l/2 degree of misalignment. Large, critical pump applications rely on these couplings. How To Select Couplings When selecting a coupling how do you know which one is best? The chart in Table 1 shows there is no PUMPS
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flexibility, When the coupling fails, usually only the elastomeric element is replaced.
a “fail-safe” feature. Flex elements are made of many types of elastomeric materials, such as, rubber and urethane. The properties (that is, hardness, resilience, etc.) can be varied to suit required loads. These couplings are used primarily to accommodate misalignment and transmit power. Small and medium pumps employ these couplings. Be careful, since sometimes these couplings are used to absorb energy and dampen loads, particularly when equipment is engine driven. Also be careful because they are torsionally soft and in the past were used to tune a system out of a torsional critical mode.
CORDED TIRE COUPllNGS
URETHANE TIRE COUPLINGS Common on small pumps, urethane tire couplings have their elastomeric elements in shear. The tires are split to enable easy assembly without removing hubs. Urethane tire couplings offer a high degree of
Corded tire couplings also have their elastomeric elements in shear. They use a reinforced element (similar to belted auto tires). Because of the reinforcement in the element, the torque capacity is greater per outside diameter than the urethane tire coupling. Most small and some medium pum p applications use these
M e t a l li c e l e m e n t c o u p l i n g ; d i s c t y p e , continuous disc
SHEAR DONUT COUPLiNGS
(UNCLAMPED) Another type common on small pumps, shear donut couplings have their elastomeric elements in shear. They provide low torsional stiffness and low reactionary forces. Oversizing can lead to premature failure by wearing the engaging teeth rather than a material failure from flexing.
Elastomeric element coupling; type, compression donut
62
compression
can and has been optimized for its characteristics. These couplings are composed of all metal, and do not require lubrication. The discs are usually continuous but can be individual links. Most disc couplings use multiple thin discs rather than one thick disc/link because the stresses from misalignment are proportional to Nts versus T^3. These couplings are used in medium pump applications. If properly installed and misalignment is kept under 1/4 degree, this type will provide a long life with little maintenance. If the misalignment increases beyond 1/2 degree during operation, the flexible element will probably fail in fatigue.
OISC COUPllNGS
DlAPHRAGM COUPLlNGS
Disc couplings transmit torque by a simple tensile force between alternating driving and driven bolts on a common bolt circle. Misalignment is accomplished from the flexibility that comes from the length of material between the bolts. Disc couplings have been around for years but with the use of Finite Element Analysis, this type
Diaphragm couplings transmit torque between the inside and outside diameters of the flex element. The diaphragm can be constant or of variable thickness, and singular or multiple. The diaphragm shapes are usually modified to increase flexibility (such as, tapered, convoluted, and cut out profiles). These couplings can usually handle up to l/2 degree of misalignment. Large, critical pump applications rely on these couplings. How To Select Couplings When selecting a coupling how do you know which one is best? The chart in Table 1 shows there is no PUMPS
MARCH1994
AND
SYSTEMS
MAGAZINE
Metallic element coupling: diaphragm type, multiple 6. Stiffness requirements 7.
Bearing load restrictions
8.
Potential excitation or critical frequencies
If you change the coupling, make sure that all maintenance personnel understand the requirements. For example, changing from a gear coupling to a disc coupling will require different instructions for setting a motor on its magnetic center. Alignment procedures also differ.
2. The keys are too tight. Grind them until they fall in. 3.
You lost a bolt. Take one from the maintenance department’s bolt can-any one will do.
4.
These bolts are snug enough.
5. The pump or motor is unbalanced. Let’s rebalance and take it all out of the coupling. 6.
Part of the coupling looks bad. Replace the bad part with another part that you took out last
Metallic element coupling: diaphragm type, multiple 6. Stiffness requirements 7.
Bearing load restrictions
8.
Potential excitation or critical frequencies
If you change the coupling, make sure that all maintenance personnel understand the requirements. For example, changing from a gear coupling to a disc coupling will require different instructions for setting a motor on its magnetic center. Alignment procedures also differ. Review all coupling changes to ensure success. INSTALLATION
CONSIDERATIONS
Most coupling failures occur relatively soon after installation or after a coupling has been reassembled on the equipment. Usually failures occur because the installer didn’t read and follow the manufacturers instructions. To achieve success, the installer needs to read and follow those instructions! Things not to do, or a sure way to increase failures: 1.
The coupling doesn’t fit on the shaft. Get out the torch and heat the “H” out of the hub, or open up the bore until the coupling slides on.
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2. The keys are too tight. Grind them until they fall in. 3.
You lost a bolt. Take one from the maintenance department’s bolt can-any one will do.
4.
These bolts are snug enough.
5. The pump or motor is unbalanced. Let’s rebalance and take it all out of the coupling. 6.
Part of the coupling looks bad. Replace the bad part with another part that you took out last year.
7.
Put the same grease in the coupling, like you do in your bearings.
8. Don’t check alignment. The pump has been running fine for 2 years. 9.
You request a coupling to fit on your 3” shaft and with one l/2” keyway. You supply no other information.
10. You buy another coupling type because it costs 30% less than the current one.
Metallic element coupling: diaphragm type, multiple 6. Stiffness requirements 7.
Bearing load restrictions
8.
Potential excitation or critical frequencies
If you change the coupling, make sure that all maintenance personnel understand the requirements. For example, changing from a gear coupling to a disc coupling will require different instructions for setting a motor on its magnetic center. Alignment procedures also differ. Review all coupling changes to ensure success. INSTALLATION
CONSIDERATIONS
Most coupling failures occur relatively soon after installation or after a coupling has been reassembled on the equipment. Usually failures occur because the installer didn’t read and follow the manufacturers instructions. To achieve success, the installer needs to read and follow those instructions! Things not to do, or a sure way to increase failures: 1.
The coupling doesn’t fit on the shaft. Get out the torch and heat the “H” out of the hub, or open up the bore until the coupling slides on.
PUMPS
AND
SYSTEMS MAGAZINE
2. The keys are too tight. Grind them until they fall in. 3.
You lost a bolt. Take one from the maintenance department’s bolt can-any one will do.
4.
These bolts are snug enough.
5. The pump or motor is unbalanced. Let’s rebalance and take it all out of the coupling. 6.
Part of the coupling looks bad. Replace the bad part with another part that you took out last year.
7.
Put the same grease in the coupling, like you do in your bearings.
8. Don’t check alignment. The pump has been running fine for 2 years. 9.
You request a coupling to fit on your 3” shaft and with one l/2” keyway. You supply no other information.
10. You buy another coupling type because it costs 30% less than the current one.
