·        Dry couplings as the name implies operate dry – do not require any type of lubrication.

·        Dry couplings have no radial clearances resulting in a lower potential unbalance.

·        Dry couplings require no maintenance or inspection for a minimum of five years and are designed for infinite life. Coupling vendors recommend that gear couplings be inspected, cleaned and regreased every 12-18 months.

·        Dry couplings eliminate “lock-up” and spool cracking typical in gear type couplings due to lack of lubrication.

·        Dry couplings reduce hub to shaft fretting.

·        Dry couplings exert significantly lower forces on connected equipment. The sliding force exerted by a gear coupling is a function of torque. The deforming force of a disc or diaphragm coupling is spring force and independent of torque. The force dry couplings exert on equipment bearings is roughly 40% of the gear in bending and 10% axially. Companies will sometimes include machine-bearing life when calculating the return on investment.
 

Turbo compressor manufacturers generally prefer to use disc couplings between compressors and from gear to compressor since many of today’s machines require the use of a reduced moment coupling. Disc couplings have an advantage over diaphragm type in the reduced moment design because they can accept a larger shaft. Disc couplings are also lighter in weight and impose lower forces on the connected equipment for the same given torque due the multiplicity of the design.

Turbine manufacturers often prefer diaphragm couplings.  It is common for turbine rotors to have a flanged shaft designed to bolt directly to a diaphragm or rigid coupling. In addition, diaphragm couplings can operate under high axial and angular deflection, which is often a major concern for large steam and gas turbines.  Diaphragm couplings are sometimes preferred over disc for synchronous motor applications where cyclic torsional vibration is present during start-up.   

Yes, some applications such as synchronous motors and reciprocating compressors require the use of a resilient coupling due to potentially harmful torsional excitations during transient and continuous operation. Both O.E.M.s and users for these applications requiring torsional damping often favor the Kop-Flex Max-C coupling.

Many industrial frame gas turbine generator set applications do not require flexible couplings but need to have an engineered rigid coupling or spool to connect the machines. Kop-Flex is also a prime supplier of this type of coupling to turbine O.E.M.s worldwide.

These type couplings are designed to last forever - the API 671 requirement is 5 years - as long as their operating torque and misalignment capacities are not exceeded. However, over the life of a coupling many things can change to affect the coupling life. Equipment foundation settling and unaccounted for pipe strain would alter the misalignment loading. Machine upsets and occurrences such as compressor slugs affect the torque loads and life. Upgrading of equipment without adequate checks happens occasionally. Unexpectedly high vibratory loading can affect the life.

To get maximum life, select the coupling with the maximum practical misalignment and torque rating for the application - being mindful, of course, that usually low weight and only certain ranges of torsional stiffness are allowed due to rotor dynamics considerations.

After the initial installation, check the coupling as often as possible during planned and unplanned outages, looking for cracks or anything unusual in the diaphragms or discs. Follow the manufacturers' inspection recommendations.  

Roughly 70% of today’s turbo compressor trains require reduced moment couplings. Centrifugal compressor rotors have become much lighter and turn faster than older machines. In many cases, the coupling has become an appreciable percentage of the rotor mass system thereby affecting the rotor dynamics of the train.

The coupling’s half weight and CG location or overhung moment affects the lateral critical speeds of the train. In other words, a coupling that is too heavy or hangs off the end of a sensitive compressor or pump rotor shaft can move the lateral critical speed of the train into the operating speed range. This can create unacceptable vibration levels that will shorten the life of the equipment.

To solve this problem, coupling manufacturers have designed a coupling where the hub is inverted, passing through the inside diameter of the flex-element. This moves the half coupling weight and CG location up the shaft, closer to the machine bearings, reducing the overhung moment and therefore moving the lateral critical speed away from the operating speed range. Kop-Flex High Performance Disc coupling types RM and RZ, and also diaphragm RM type are the reduced moment designs.

A.

Component Balance:  The first method of balance is the component balance wherein each component is individually balanced by weight removal so that the residual unbalance left in each part does not exceed a given value. This method of balance is generally used for motor speeds of 1800- 3600 rpm.

Assembly Check Balance:  The second method is called the assembly check balance. After the component balance is performed, the coupling is assembled and spun on a horizontal balance machine and the unbalance recorded. No correction is made to the coupling. The assembled coupling unbalance must not exceed a given value. Assembly check balancing is widely accepted by both the O.E.M. and user as the preferred method of balancing as it allows for the interchangeability of component parts in the field.

Assembly Balance:  The third method of balance is the assembly balance, often used for very sensitive high-speed machines. Corrections are made to the assembled coupling on the horizontal balance machine. Generally, the coupling assembly is balance to the lowest residual unbalance possible. The coupling is match-marked and must be assembled in the same way every time. Because the assembly balance corrects for overall unbalance of the assembly, it may prohibit subsequent field replacement of components.  

Tapered Hydraulic:   Turbo-compressor O.E.M.s generally prefer the tapered hydraulic hub where the torque is transmitted through 100% friction. Hubs are designed to have between 0.002inches (0.051 mm) and 0.003inches (0.076 mm) per inch or mm of interference. They are mounted and removed by hydraulic dilation.

Tapered Key:  Older machines transmit torque through a tapered keyed hub. Single and double-keyed hubs transmit the torque through both the key(s) and hub to shaft interference and are generally mounted using heat. Precision class keys that comply with the AGMA specification are required for high performance couplings.

Flange:  Integral flanges are common on both steam and gas turbines. In this configuration the turbine rotor is flanged and bolts to the coupling flange. Diaphragm couplings and rigids are designed to bolt directly to these flanged shafts.

Spline (gear teeth):  There are a few O.E.M.s that transmit torque through a splined connection. Often proprietary, these interference fit splines are considered by some to be the best method of torque transmission.

Most coupling vendors do not supply the hydraulic mounting equipment as this falls under the machinery O.E. M. scope of supply. End users often request the ancillary equipment from the coupling supplier, particularly when retrofitting. Kop-Flex recognizes the need for this type of equipment and will supply the hydraulic dilation and mounting tooling in a simple, easy to use package that can be shipped with the coupling. Kop-Flex also supplies plug and ring gages when requested; it is recommended the best plug gage be matched to the master ring gage used to machine the rotor shaft.

Coupling vendors do not supply coupling guards as they fall under the machinery O.E. M. scope of supply. Guards are often a subject of discussion and technical papers since the coupling’s windage is a source of heat and turbulence in the enclosure and must be considered by the coupling designer. Kop-Flex has software that can calculate windage for the coupling/guard design and has a technical paper on this subject.