DISC COUPLINGS

TODAY’S HIGH-SPEED TURBOMACHINERY APPLICATIONS DEMAND THE LATEST DISC COUPLING DESIGNS

The Kop-Flex High Performance Disc Coupling [/caption]

By Julian Del Campo

With the advent of new materials and state-of-the-art manufacturing techniques and analysis tools, flexible couplings for industrial applications have advanced since their invention in the early 20th century. Their primary functions, however, have remained the same — they transmit torque between equipment and accommodate different types of misalignment while reducing the reactionary forces on the equipment.

Some couplings also dampen damaging peak torques and torsional vibrations, provide overload protection to stop the transmission of damaging loads, prevent shorting between equipment by adding electrical insulation, and provide live torque and power readings (capable of measuring shaft horsepower with less than 1% error). Flexible element couplings can be separated into two different types: Mechanically flexible and flexible element couplings.

Mechanically flexible couplings generally use clearance-fit parts (i.e., backlash between gear teeth) to allow relative movement between the metallic components. They require lubrication.

Flexible element couplings accommodate misalignment between shafts through the elastic deformation of components (discs or diaphragms) designed to cope with this kind of loading. They require no lubrication. Given their power density, gear couplings remain the preferred style when high torque capacity is required. As a mechanically flexible design, they transmit load through teeth on the hub and sleeve to compensate for misalignment.

The metal-on-metal contact is subject to wear, and while the rate can be mitigated through proper coupling sizing, case hardening and optimal lubrication intervals, they are designed with a finite service life in mind.

As both equipment speeds and the required time between maintenance intervals have increased, so did the prevalence of disc couplings on new equipment trains. Given their dynamic stability and maintenance-free operation, their popularity has increased for low-to-high horsepower drives. Disc couplings are the most common flexible element coupling due to their performance, high misalignment capacity, compact design, and cost.  While there are many different disc couplings designs available on the market, they all operate under the same principals.

Torque is transmitted circumferentially from driving to driven bolt, resulting in tension forces in the discs. Misalignment is accommodated by the bending of the disc so no lubrication is required. The discs are typically made of high-strength stainless steel and are assembled in unitized packs for easy maintenance.

The stacking of multiple discs versus a single thick disc of similar material increases the flexibility and reduces the reaction forces on the equipment, while providing a large cross section for torque transmission. The typical failure mode involves the unravelling of discs one at a time. Increased imbalance will cause vibration response at the 1X frequency to trend upward over time. With accurate vibration monitoring equipment in place, the equipment typically can be shut down safety before catastrophic failure of the disc pack occurs.

As a disc coupling operates, minor movement between the discs occur as the discs accommodate misalignment.  This movement is not of great concern for applications below 3,600 RPM, but for higher-speed applications, PTFE based low-friction coating can be used on high-performance couplings to mitigate fretting corrosion.

To maximize the life of the coupling and the connected equipment, some manufacturers use a scalloped disc design.  By removing unnecessary material from the disc, a scalloped design allows for a more flexible disc, radially and axially. Verifying both models with Finite Element Analysis (FEA), a scalloped disc reduced reaction loads on connected equipment up to 25% in some cases. This increases the flexibility of the disc pack with no loss of torque capacity.  Optimization of the stress distribution through the disc is achieved, since the cross section of material at the bolt hole and the center of the link are equivalent.

For a given diameter, the torque rating and misalignment capacity of a disc coupling can be considered a function of the number of disc pack bolts used in its design. A 3-bolt disc has long links between bolt holes thereby increasing the effective bend length, which decreases stiffness and increases misalignment capacity. A 5-bolt disc pack design increases torque capacity since the bolts evenly distribute the torque, but the shorter link length increases the angular stiffness and decreases the misalignment capacity. As a general rule, 3-bolt designs provide high misalignment capacity, but low torque capacity, while 5-bolt designs provide high torque capacity, but low misalignment capability. A 4-bolt design balances both torque and misalignment and is the most common configuration, operating successfully on a wide variety of applications from pumps to centrifugal compressors. Although relatively simple in theory, coupling design continues to evolve as increasingly accurate modeling and testing technology becomes availabl.

Author: Julian Del Campo is Senior Service Engineer at Regal Beloit America, Inc. – Kop-Flex. Kop-Flex is a coupling business unit of Power Transmission Solutions, a power transmission supplier in North America.  For information visit us at www.RegalPTS.com/Turbomachinery or email Julian.delcampo@regalbeloit.com