Rubber and Polyurethane Spiders are an Essential Component of Jaw Couplings
A jaw coupling is a power transmission coupling that can also be used in motion control applications, for example, a motor. It connects two shafts while minimizing vibrations and compensating for any parallel, angular, and axial misalignment.
Jaw couplings are made up of three parts: two metal hubs sandwiched around an elastomer insert commonly called a “spider.” The three parts press-fit together, and a metal jaw from each hub meshes with the lobes of the spider. Torque is transmitted through the elastomer spider in compression.
The spider fails first
Spiders are designed to be fail-safe, which means that if the elastomer wears out, the hubs’ teeth will mate and the coupling will continue to transmit torque without damaging the equipment. Failure can be attributed to many causes which include misalignment, hub failure, extreme temperature, vibration, over-torque, and exposure to chemicals.
Spiders come in various materials with rubber heading the list
Material for the spider is chosen to accommodate a variety of factors: its ability to control vibration, handle misalignment, and function within a certain temperature range. The speed of machinery and chemical conditions are also important considerations. There are four materials from which to choose, rubber is the most popular:
Nitrile butadiene rubber (NBR)
Buna N, as it’s often called, is the most economical and widely used spider material. It has much in common with natural rubber with its resilience and elasticity. It is also resistant to oil, hydraulic fluid, and most chemicals.
Rubber offers the best vibration damping quality, along with excellent compression set resistance and elongation properties. Its operating temperature ranges from 240 to 212 degrees Fahrenheit.
Urethane has more torque capacity than NBR along with good chemical and oil resistance. It does not control vibration as well, however, and has a narrower operating range of 230 to 160 degrees Fahrenheit. It is also up to twice as costly as NBR.
Made for high operating temperatures (260 to 250 F), Hytrel is resistant to oils and chemicals while carrying three times the torque as NBR. But it has low angular misalignment ratings and damping capacity at three times the cost of rubber.
These oil-impregnated metal spiders are used in applications where slow speeds, high torque, and higher temperature resistance are required. They have a small amount of misalignment capability and no damping capacity. They are at least three times more expensive than spiders made from NBR.
Rubber spiders come in a variety of designs
Along with the type of materials, there are also four basic designs for the coupling spiders:
By far the most commonly used design especially when the distance between the ends of the driving and driven shafts is large enough to accommodate the spider thickness.
Open-center type (OCT) spider
These spiders work best when the shaft ends must be positioned closer together. Since by such a thin segment of material joins the spider’s legs, they have limited support and speeds are limited to 1,750 rpm for NBR spiders.
With this style the spider legs are connected around the perimeter of the coupling, making it possible to install them without disturbing the hub alignment. One downside: They cost twice as much as the first two.
Load cushions are held in place by a collar and are easy to remove and install. They are only available for specific collar models.