Plastics are a natural

Sept. 1, 2006
90% of the time, elastomer-insert or spider couplings are employed to connect drive motors and pump units to systems. Their puncture resistance and vibration

90% of the time, elastomer-insert or spider couplings are employed to connect drive motors and pump units to systems. Their puncture resistance and vibration damping make them suitable for even severe conditions. Several industries benefit from their dynamics and rigidity under impact. Usually spider-coupling hubs are made of grey cast iron or die-cast aluminum. In fact, this metal/elastomer type has been used for over thirty years, often for the economic advantage of large-quantity production. But development engineers are eliminating the disadvantages of these metal/elastomer couplings by replacing them with thermoplastic/elastomer units.

There are three distinct plastic classifications: Duroplastics, elastomers, and thermoplastics. So why are some plastic hubs made from the latter? The major reason is that the material can be injection molded, a technique that produces accurate part geometry quickly; even complex parts can be manufactured without needing finishing in this way. Granules are melted in a screw and squeezed into a preheated mold under high pressure.

In operation, engineered thermoplastics survive temperatures to 150° C. The benefit here is that these plastics are far less expensive than high-temperature varieties, and yet have similar mechanical properties. Fortunately, this is a realistic temperature parameter; normal coupling applications rarely come close to that value.

Plastics for molding come in many granules and grades, but hub strength and tenacity are generally highest with semi-crystalline structures. The molecular chains are loosely arranged in a specific order, so the effect of the secondary valences is enhanced — magnifying that of the primary valences. (A valence is the combining capacity of an atom or radical, determined by the number of electrons that it loses, adds, or shares when it reacts with other atoms.) In other words, the molecules in semi-crystalline structures are oriented to complement the chemical bond strengths within the molecules themselves — for improved physical and chemical properties.

Reinforcement with glass can stiffen thermoplastic hubs, boosting coupling strength and elastic modulus for dimensional integrity under loading. The ideal ratio of glass to plastic varies with component size, shape, and intended use; too much glass can make materials brittle, and (much like an object of pure glass) subject to fracture under impact stress. But again, when the proper amount is added, glass extends material life.

Plastic solution

Used in food and dairy industries, semiconductor manufacturing, water treatment and textile plants, aggregate manufacturing, compressor manufacturing, and biotechnology, spider couplings operate under a variety of conditions. However, the requirements placed on the couplings share similarities.

  • Often, total coupling weight must be as low as possible, so its moment of inertia does not affect the dynamics of the mechanical system. All-plastic couplings reduce weight by almost 90% as compared to conventional types.

  • Often, noise emission must be kept to a minimum. Noise is damped by plastic couplings, as sound waves are transmitted far less readily than by metal, if at all. Plastic hubs possess a far lower natural frequency than those of commonly used metals. For this reason, they act as vibration sinks for the audible resonance common in motor and gear drives.

  • Torque impacts occur if the rotational direction or speed is changed. So, couplings must be strong to avoid breaking jaws. Since the fibers in plastic couplings are cross-linked, they're much more resistant to impact than metal their counterparts. In short, impacting plastic is like hitting a web, whereas impacting cast metal couplings is like hitting a rigid frame: Plastic's cross-linked geometry effectively distributes loads across a wide base of material. In cast materials, on the other hand, stress is relatively concentrated around the location of impact.

  • Often couplings must maintain their dimensions under torque. The dimensions of plastic hubs remain the same under load, just as conventional couplings do. The majority of plastics are softer than metals (with weaker elasticity moduli) so a common perception is that the dimensional integrity of plastic components must be weaker than that of metal. In fact, the internal structure of engineered plastic supports itself from many angles — to better resist dimensional changes under stress.

  • In dynamic applications, hubs must be balanced. Injection molding balances even standard parts. Uniform material density throughout the hub, and several symmetrical planes across the coupling's axis are two factors largely responsible for the balance of molded couplings. Even the largest size smoothly rotates at speeds in excess of 6,000 rpm — significantly faster than the typical 1,750-rpm requirement of most elastomer jaw couplings.

With or without backlash

Elastomer inserts, also called spiders, must not only transmit torque, but also compensate for lateral, angular, and axial misalignments between linked shafts. For this reason elastomer inserts come in three different shore hardness categories.

The first distinct field of application is servo drive technology. It requires precise transmission of the torque and position, which also requires that offset between the input and output sides of the coupling be kept to a minimum. Zero-backlash of the vibration-damping element is achieved by preload between the insert and the jaw geometry.

The second field of application for elastomer couplings is pump and compressor technology. Here, in addition to torque transmission, a high degree of offset compensation is required. Offsets are caused by the structural situation in industrial pump plants, for example. For this purpose, some spiders are specifically designed to exhibit backlash. Geometric changes can increase surface-area contact between each jaw and the insert, so the abrasion and wear of the traditional spider are reduced.

It is very challenging to compensate for large shaft misalignments and maintain high rotational positioning accuracy. The type of coupling flexture, and in the case of jaw couplings, the material of the insert, affect which of these two performance objectives a coupling is most suitable. Fortunately, in most cases where a large shaft misalignment exists, precision transmission of velocity and torque are not requirements, particularly since the majority of drive applications are unidirectional. In the case of spiders which allow for backlash and high misalignment, a clearance, sometimes measuring to several mm, exists between the hub jaw and spider. This translates into a very small delay, only fraction of a second, between the movement of the input and output hubs of the coupling at startup

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For consistency

Analysis with computer simulation optimizes batch parameters during molding. This prevents unacceptably inconsistent coupling hub density from one jaw to another, which can result in imbalances during rotation. Where too much material is concentrated in a single jaw, the hub moment of inertia is increased slightly. Fortunately, this is unlikely to be of great concern, as these couplings exhibit very low inertia anyway. But it's another story when one jaw is not dense enough: Couplings can be weakened and can fracture under the application of rated torque. Also, in cases where the coupling's physical jaw dimensions are inconsistent internal misalignment load can affect the elastomer insert and cause premature wear, rough running, and even jaw fracture.

Temperature during formation

Molded plastic couplings must not only withstand impact stress common in elastomer coupling applications, but also exhibit thermal stability. During their manufacture, the mandrels used to shape bore and keyway dimensions must form hubs for the proper shape after cooling — not during formation in the mold.

Thermally unstable materials tend to cool into one set of dimensions under certain atmospheric conditions, and another set in other temperature and humidity levels. Because a consistent shaft-hub fit across all common conditions is a requirement, poor thermal stability can be a limiting factor.

On coupling hubs, suitable angles, locations, and cross-point radii of individual crosspieces maximize torsional rigidity. Symmetrical structures keep the residual imbalance very low. Therefore, couplings with molded hubs can be used for rotational speeds up to 5,000 rpm without post-molding balancing.

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