The function of a coupling is to connect two rotating shafts, for the transfer of rotary motion and torque. For a coupling to work at its optimum efficiency, it must match all required conditions, including performance, environmental, use and service factors. If all of these factors have been taken into consideration when selecting a coupling the coupling should have no failure issues over its lifetime. However, if just one of these factors is not met, a coupling can prematurely fail, causing anything from a small inconvenience, to a significant financial loss, and even the potential of personal injury.
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This article will help you to identify the main reasons why couplings fail and provide you with useful information and advice to help you minimise the risk.
Couplings are often selected extremely late in the application design process, without meeting the complex requirements of the system. By considering couplings early on in the design process, each criteria can be considered individually, ensuring that the coupling chosen is suitable for the functions required.
Several criteria must be considered when deciding on a type of coupling, including the type of application, torque, misalignment, stiffness, inertia, RPM, shaft mounting, environmental factors, space limitations, service factors, and cost. Each criteria must be individually considered to ensure that the coupling will be suitable for the application and not result in premature failure. This process of evaluation must also be repeated for any change in conditions throughout the application&#;s lifecycle.
An essential consideration when selecting a coupling is the misalignment conditions of the application. This may be angular, parallel or axial, or a combination of more than one misalignment (complex misalignment). Flexible couplings should be considered in these conditions, although the type will depend on the type of misalignment present. For example, an oldham coupling is suitable for large amounts of parallel misalignment, but cannot tolerate a high level of angular or axial misalignment, whilst a single beam coupling can withstand large amounts of angular and axial misalignment, but not parallel.
Even flexible couplings which are designed for use on misaligned shafts have their limits. A common point of failure is the under-estimation of the degree of misalignment, creating loads that surpass the coupling specifications. This causes the coupling to wear at an accelerated rate, and has the potential to cause other components, such as bearings, to also fail prematurely. Where misalignment exists beyond the manufacturer specifications for the coupling, this should first be rectified with shaft realignment before installing the coupling.
The torque of an application is frequently under-estimated. The maximum instantaneous torque for the application needs to be considered, in addition to the steady state torque. Flexible couplings have different static torque ratings depending on the design type. For example, a double disc coupling will typically offer a 15-20% higher static torque rating than an identically sized Oldham coupling with an acetal disc.
Windup is also known as torsional compliance or torsional rigidity, and is present in all couplings. It is the rotational deflection between the driver and the load, similar to winding up a spring. The most significant problem with this is maintaining accuracy of location due to a difference in angular displacement from one end of the coupling to the other.
Backlash is the loss of motion momentarily in a coupling. For example, when torsion is applied in one direction, the coupling bends and compresses under that stress. When the direction of torsion is changed, backlash is experience within the coupling. Any amount of backlash in a motion control application could be detrimental to the application, potentially causing lack of accuracy in positioning, and difficulty in tuning the system. Zero backlash couplings should be considered in these scenarios.
Dampening refers to the minimisation of shock and vibration and is particularly important in motion control and power transmission applications to reduce the waste of energy and the unnecessary stress on system components. Shock dampening helps to reduce the effects of impulse loads, minimising shock to the motor and other sensitive equipment. The potential for premature coupling failure can be accelerated when the selection of coupling type does not fully take into consideration the dampening levels required.
Inertia refers to the coupling&#;s resistance to change in angular velocity, and governs the tendency of the coupling to remain at a constant speed in response to applied external forces (eg torque). Too much coupling inertia in an application can seriously degrade the performance of the entire system by introducing resonance and adding to the natural frequency of the system. A low inertia coupling can allow the system to be tuned to a higher performance level, and is a very good choice for high precision applications.
Failure to consider the coupling&#;s maximum safe operating speed during the design stage can quickly result in failure, sometimes with tragic consequences. A balanced coupling is essential in high speed applications. Any degree of misalignment can also affect the coupling&#;s safe operating speed.
Electrical isolation is the separation of two mechanical components to prevent electrical current transfer, whilst still allowing mechanical energy transfer. Oldham and jaw couplings can be electrically isolating when non-metallic or polymer inserts are used, and other types of coupling can also be manufactured in electrically isolating materials.
A fuse coupling disallows further energy transfer upon failure, whereas a fail-safe coupling is designed to continue working, even after failure. For example, a jaw coupling would be considered fail-safe, as even if the spider fails, the jaws of the two hubs interlock, allowing continued power transmission. Both have their uses, but it is important to establish which type is required for the application during the design stage.
Those clever chaps at Ruland have put together a 5 question quiz to test your coupling failure knowledge. All it takes is two minutes to complete, so why not give it a go. Have fun and remember, failure is a building block to success!
Information courtesy of Ruland Couplings and Shaft Collars.
For help and advice on choosing the right coupling for your application, contact our Couplings & Drives expert.
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For our discussion here, we'll discuss two types of "Tire Couplings". In today's world, the second type (Bonded Urethane Type) is the most
common and the one A.R.&E. will recommend, should your application be practical for the "Tire" type coupling solution.
Corded
tire types
This design came about in the late 's as a solution for dealing with transient torque peaks and shock loads in diesel-driven pumps. Named for their resemblance to
an auto tire, this design consists of two flanged hubs equipped with clamping plates, which grip the coupling's hollow, ring-shaped element, by its inner rims. Furthering the
similarity, tire coupling elements usually are rubber derivative elastomers with layers of cord, such as nylon, vulcanized into the tire shape. The coupling transmits torque
through the friction of the clamp applied to the inner rims of the tire and a shearing of the element. Slippage of the coupling may be expected to occur at about four times
the rated torque.
The two significant limitations to the corded tire type coupling are speed and space constraints. As speed increases, the coupling exerts axial
forces on the shafts due to the centrifugal forces working on the elastomer. And the geometry of the tire itself makes for a large outside diameter for its torque capability.
A design variation includes an inverted tire coupling in which the tire element arcs inward toward the axis, thus overcoming the centrifugal forces at speed. This affords
10-30% higher RPM service, depending on its size.
The corded tire coupling is torsionally soft and can dampen vibration. High radial softness accommodates angular misalignment up to 4° and parallel offset up to 1/8". Rare among elastomeric couplings is its capability to allow a certain amount of axial shaft movement. These properties give corded tire designs a wide variety of applications including those driven by internal combustion engines. This coupling is offered in spacer designs as well as with hubs which can accept bushings.
Bonded Urethane Tire
This design was first marketed in the 's and has found success primarily in the process pump industry because of several features that the corded tire lacks.
The design utilizes a urethane material that is bonded to two half-circle metal rings, referred to as "shoes", which are then bolted to the two hubs. Torque is transmitted from the
hubs through the shoes/bonded joint and then the shear-plane of the split urethane tire.
The design offers advantages such as radial removal of the element halves,
high angular misalignment capability (4°), and shock load cushioning. In its standard close coupled configuration, it can span greater BSE lengths than most in-compression
couplings, and it also has the large opening in the center of the tire to allow complete flexibility in positioning shaft ends. The outside diameter (OD) of this design is
also smaller than the Corded Tire type for similar shaft and torque capacities.
Spacer couplings are achieved by using the same shaft hubs and simply extending the
lengths of the steel shoes onto which the elastomer is bonded. Hubs can also be reversed in their mounting orientation to further add to the BSE permutations possible.
Bushings are also commonly used on this style of coupling. A heavy duty elastomer option (25% more torque) is available, but it reduces the misalignment capacity by 50%.
It has proven to be ideal in applications such as pumps, screw compressors, blowers, mixers, crushers, and general power transmission drives.
Limitations of the urethane tire type include the large number of fasteners required for installation and removal of the elastomer, and the fatigue of the element and the bond between steel and elastomer under torsional vibrations.And here's a resource for you... it doesn't have to do with just this, but instead is a general knowledge base of couplings. It is an internet source sponsored and created by "Lovejoy, Inc.", one of the leading manufacturers of couplings and known for their "Jaw Coupling". Lovejoy became a household name and the coupling of choice a long time ago, and they're still a major force in the industry. Click on this link to be taken to their resource website called... "Coupling Answers.com" . It's a great site and you'll learn a lot by spending some time there.
Call us to discuss whether a "Tire Coupling" is the answer to your problem, or to discuss your new application.
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