Shaft Coupling
Shaft coupling is a mechanical device that is used to connect two shafts of two different equipment for transmit the power and torque. The pump shaft coupling is used with machinery to connect driver and driven equipment and compensate small amounts of misalignment, power and torque transfer.
There are many types of shaft coupling used to connect the shaft, depending upon its features and it is applications. Generally, two types of shaft coupling are used, flexible coupling and rigid coupling. The rigid coupling allows the same amount of same parallel misalignment between both machinery.
Some special types of coupling is also available in the market which is allowed to disconnect both machinery to each other if any abnormality found during the operations or more torque required to operate or if both machinery exceed the set limit of the design operating condition save from any catastrophic failure.
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American Petroleum Institute (API)
API has a give detailed specification about Special-Purpose Couplings use in Refinery Service and specification should be follow as per API 671 guideline. But in that specification did not cover general purpose couplings.
API 610 Centrifugal Pumps for General Refinery Service 7th Edition in This specification coupling's hub diameter must have a concentric to the bore within 0.003 TIR to aid alignment.
And all coupling couplings balancing requirements specification should be followed as per AGMA 515 class 8, but new update also available AGMA standard for balancing specification is AGMA- which supersedes AGMA 515.
In the above mentioned standard did not explain how couplings work or how to select best coupling for your application. So general question coming your mind, how do we know.
Which one is use for general purpose and which one is special purpose applications?
The Simple answer is that API 610 will be used for general purpose couplings and API 671 use for special purpose couplings.
General Purpose Couplings are categories depending upon driver and driven equipment capacity if equipment is operated Under 100 hp is considered as small. If it is operated Between 100 and hp is considered medium and if operated above hp is considered critical.
Above hp operating equipment is considered as SPECIAL PURPOSE COUPLING. Flexible couplings can be compensating following types of misalignment.
Parallel offset
The axes of two machine shafts coupling the hub are parallel but not the center of both shafts are not in the same straight line.
Angular offset
The axes of two machine shafts intersect at the center point of the coupling, but total center hub position in some amount in angle position.
Combined angular and offset
The axes of both machine shafts hub are not in straight line center and not in parallel as well.
Type of shaft coupling are divided on bases of purpose of use
As described in the above section.
SPECIAL PURPOSE COUPLINGS
This type of couplings is completely covered in API 671. But this has also categorized in to two categories lubricated and non-lubricated. There is some old rotating equipment that has still used lubricated couplings. BUT Most of new equipment or upgrade equipment operate with non-lubricated couplings arrangement. Generally, the types are gear coupling (mechanical element) that require oil or grease lubrication and metallic element types that require no lubrication.
There are mainly two types of non-lubricated couplings is that the diaphragm type and the disc type. For some large equipment, elastomeric element type are used, all will be cover in API 671.
It is divided into two-part, detailed descriptions that is below
If you are looking for more details, kindly visit CNG.
This type of coupling used to transmit the power and torque from one machine shaft to another machine server with allow a little misalignment to each other or in both shaft it will also compensate very little angular misalignment approx. 1.5 degree with compensate little vibrations as well.
What is rigid type coupling?
Rigid coupling
As a now a days rigid coupling more used. It is also categorized in two part one is metallic type and other is type Elastomeric.
In Metallic type, the parts are used in little loose fit and in many parts and membranes used in it. Elastomeric parts are used with flexible material like rubber, plastics to transmit the power and torque between metallic hubs.
Benefits of metallic type coupling
Application of elastomeric type coupling
Accurate figure is not available in public domain because it's varies material to material and that's misalignment compensate by flexible type decided by equipment vendor but the thumb rule is that angular misalignment compensate is approx. 21.5 degree.
What is factor effect for select a best coupling?
There are many factors for select good coupling these are following. Operating is speed required, Ambient temperature, weight of the coupling, moment of inertia, torsional stiffness, required outside diameter and inside diameter of the coupling, shaft deflection due to loads, torsional deflection of couplings, backless compensate, misalignment capacity between the shaft balance and unbalance of hubs and coupling as well
Now a days in the market there are many types of rigid shaft coupling available. Some of these are mentioned below
Sleeve coupling muff coupling
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.
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.