Choose the right spider when looking for flexible couplings
Jaw couplings are widely used for continuous-duty electric motor-driven machinery, pumps and gearboxes. In this design, the thick, stubby protrusions, called jaws, of the driving and driven hubs are separated by blocks of elastomeric material, usually molded into a single asterisk-shaped element called a "spider."
Selecting the right type of spider is important because it is a key element in determining the jaw coupling’s torque rating, and also affects the coupling’s response to vibration, temperature, chemicals, misalignment, high RPM, space limitations and ease of installation.
In standard jaw couplings, the spider is said to be "in compression", because the driving jaws push the driven jaws, using the legs of the elastomeric spider as cushions between them. This contrasts with "in shear" couplings, in which the driving hub pulls the driven hub through an elastomeric element suspended between them, where it can absorb some of the torque force by twisting.
When elastomeric coupling elements break down, it’s often due to cyclic loading when hysteresis (internal heat build-up in the elastomer) exceeds the material’s limits. Some elastomerics are also more vulnerable to high ambient temperatures and some types of oil, chemical or atmospheric exposure. For this reason, elastomeric couplings offer a selection of element materials to suit specific operating conditions:
NBR (nitrile butadiene rubber). Sometimes called Buna N, NBR is the most economical and widely used standard coupling element material. It resembles natural rubber in its resilience and elasticity, resistance to oil, hydraulic fluid and most chemicals and has operating temperature ranges from -40ºC to +100ºC. With hardness of 80 Shore A, NBR provides the best damping capability among elastomeric elements.
Urethane. Urethane has 1.5 times the torque capacity of NBR with very good chemical and oil resistance, but less damping capability (90 Shore A hardness) and narrower operating temperature range of -39ºC to +71ºC. Urethane spiders are good choices when the application calls for greater torque in a confined space, or for resistance to atmospheric effects such as ozone, sunlight and hydrolysis in tropical conditions.
Hytrel7. Hytrel7 is designed for high operating temperatures of -51ºC to +121ºC, with excellent resistance to oils and chemicals and can carry three times the torque of standard NBR. It also provides resistance to ozone, sunlight, and hydrolysis in tropical conditions. With hardness of 55 Shore D, however, it cuts angular misalignment ratings in half, and damping capacity is low.
Bronze. It’s not exactly elastomeric, therefore these rigid, porous, oil-impregnated metal inserts are used only for slow speed (250 RPM maximum) applications requiring high torque or high temperature resistance. Bronze spiders can withstand virtually all chemicals and temperatures from -40ºC to +232ºC, but their rigidity offers zero damping capacity.
In addition to the available variety of materials, four basic mechanical designs allow further choices to suit specific applications:
- The standard solid-centre spider is the most commonly used design in general power transmission applications where the BE dimension (distance between ends of driving and driven shafts) affords a suitable gap and will remain fairly constant.
- The open-centre type (OCT) design is often used where equipment positioning results in a BE dimension smaller than the thickness of the solid-centre spider. Because this design has no full-diameter support, however, it has lower maximum-speed ratings.
- A snap-wrap is a flat-strip, open-end design that connects the spider legs around the perimeter of the coupling rather than at the centre, allowing easy removal or installation without disturbing the alignment of either coupling hub. Wrapped around the jaws, this type of spider is held in place by a ring or collar. Like the OCT spider, it also accomodates minimal BE applications, but when retained by a collar attached to one hub, it affords the same RPM rating as the standard jaw coupling.
- Load cushions are small, separate blocks that may easily be installed and removed radially, which can be very helpful for maintenance in heavy-duty applications. They are typically available in NBR and Hytrel materials, but only for certain models of coupling; load cushions must be held in place by a collar.
Standard jaw couplings typically tolerate angular shaft misalignment up to 1E, but two recent variations broaden their capabilities. The "curved jaw" design, so named because jaws and spider legs have both radial and axial curvature (crowning), offers more torsional softness at lower torque levels than the standard jaw coupling, and extends angular misalignment capability up to 1.5E.
Another development allows standard driving and driven hubs to be backed away from each other so their jaws rotate in separate planes, connected by a wider, wrap-around spider that transmits torque in-shear. This jaw in-shear design provides torsionally softer performance, permits angular misalignment up to 2E, and allows greater axial float.
Mark McCullough is the director, marketing and application engineering for Lovejoy, Inc.