Selecting the Right Coupling for Pump Applications
Misalignment occurs when the centre lines of the coupled shafts do not coincide. With pumps driven by motors, couplings are expected to limit the axial float of the equipment to prevent internal friction. Design engineers typically base their coupling selection on experience, price and preference toward coupling manufacturers. Ease of maintenance, replacement of the flexible mid-element, and the lubrication schedule are additional key factors to take into consideration during the selection process. Time spent to determine how well a coupling will interact with a pump system should be a function of the cost of the pump - specifically, how large is the pump and will downtime adversely affect plant operations.
With small pumps, less than 100hp, a plant's operations will probably not be affected by downtime. Couplings with flexible elements that are accessible for inspection and replacement are preferred. It is common to use Oldham or Oldham universal couplings - which give a high degree of flexibility - on small pumps where the mid is made of urethane and easy to replace without removing the hubs. Medium pumps - more than 100hp and less than 1,000hp - are usually not critical to plant operations.
The couplings chosen for medium pumps, and often for small pumps too, are sometimes used to absorb energy and dampen loads, especially when the equipment is engine driven. Jaw couplings are primarily used for torsional dampening in these pump applications. In general, the jaw coupling does not accommodate as much angular and radial misalignment as the Oldham-type couplings, however, it does offer a high peak torque rating, low cost, and large axial shaft play accommodation.
OEP Couplings' type SC jaw coupling is machined from bar on precision CNC machining centres with live tooling. Large pumps - more than 1,000hp - are typically critical to a plant's operation and maintenance and downtime costs are high. Couplings using special alloys are designed and manufactured specifically for these size applications. Block couplings are an optimal choice where very high cycle peaks at start-up occur as they reduce the loads on the equipment. Very few manufacturers still offer the block coupling as they have been replaced by the easier to produce, conventional Oldham coupling.
One of the block coupling's advantages in this type of application lies in its simple design; the grooves lie in the hub and the square mid ridges in these wide grooves. The mid section of this type of coupling is therefore large enough to contain an internal grease reservoir. The block coupling does not have a sacrificial mid section that serves as a mechanical fuse under overload, but other than this it offers all the advantages of a standard Oldham coupling.
These advantages include large radial and axial misalignment, low reactive forces, homokinetic transmission, zero backlash and high torsional stiffness. OEP Couplings' type BC block coupling is a precision CNC-machined unit produced with tight tolerances, optimal surface finishes, the latest surface treatments, and a geometry that maximises peak torque. As the larger pumps' potential failure's effect on plant operations could be catastrophic, the information gathered to determine the correct coupling must include hp and torque, speed, type of equipment connected and shaft size and attachment, including keyway options.
In addition to the parameters listed above, other secondary considerations include environmental conditions, temperature and corrosive atmosphere, space limitations, history of previous problems with couplings, expected misalignment conditions, balance requirements, stiffness requirements, bearing load restrictions and potential frequencies. Finally, each coupling manufacturer has specific guidelines that should be followed for maintenance schedules, which include periodic inspections and lubrication.
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