Couplings are used to join two shafts to transmit rotation and power from one shaft to the other. A typical example is the coupling between an electric motor and water pump as shown on the right. In an ideal world components can be installed in perfect alignment. Rigid couplings are generally used in applications with precise alignment.
In the real world, however, alignment is seldom perfect. Misalignments can be one of several fundamental types, including lateral, axial, angular or skewed/angled. The greater the misalignment the less efficient the motor is in generating speed and torque. Misalignment also contributes to premature wear, broken shafts, bearing failure and excessive vibration.
Flexible couplings are used extensively to compensate for misalignment between components. These couplings range from simple designs (e.g. an elastic element between two shafts) to intricate couplings such as constant velocity (CV) joints. The following metal-to-metal flexible couplings are commonly used in industrial applications:
GEAR COUPLINGS compensate for misalignment via the clearance between gear teeth. Shaft-mounted external gear teeth on both shafts mate with internal gear teeth on a housing that contains a lubricant. Another design mesh external teeth on one shaft with internal teeth mounted on the other shaft.
CHAIN COUPLINGS operate similarly to gear couplings. Sprockets on each shaft end are connected by a roller chain. The clearance between the components, as well as the clearance in mating the chain to the sprockets, compensate for the misalignment. Loading is similar to that of geared couplings.
GRID COUPLINGS use a corrugated steel grid that bends to compensate for loading induced by misalignment. Grooved discs attached to the ends of each shaft house the grid, which transmits torque between them. Low amplitude sliding motion develops between the grid and grooves as the grid deforms under load, widening in some locations and narrowing in others over each revolution.
Unless specifically mentioned by the manufacturer, metal-to-metal couplings are generally grease lubricated. Coupling components are protected primarily by an oil film (which is released from the grease) and seeps into the loading zone to lubricate the metal contact surfaces. Greases formulated with high-viscosity base oils, anti-scuff additives and metal-wetting agents are recommended to overcome the boundary lubrication conditions that often exist in flexible couplings. High oil viscosity also reduces leakage rates.
Centrifugal forces in flexible couplings can be severe, becoming even more extreme as the rotational speed and the diameter of the coupling is increased. Even moderately sized couplings can generate centrifugal forces thousands of times greater than gravity (commonly referred to as G-force). Centrifugal forces have a centrifuge effect on the grease inside flexible couplings. If a general purpose grease (in which the thickener is of higher density than the oil) is used, the thickener and the oil may separate – similar to a cream separator that splits the heavier milk from the lighter cream. One problem with separation of the oil and thickener is that the oil will tend to leak out of the coupling. A much greater problem, however, is that the thickener which is separated out, is moved by centrifugal force to the outer part of the coupling and against the torque transmission elements (e.g. the gear teeth in a geared flexible coupling). The thickener coats the transmission elements and keeps the oil component of the grease from lubricating them.
Blue Chip HSC 350 High Speed Coupling Grease is specifically formulated to resist separation of the oil, even under the high centrifugal forces encountered in couplings. This ensures reliable coupling lubrication over extended periods, even during high speed operation. For more information about HSC 350 High Speed Coupling Grease and the complete range of Blue Chip greases, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.