Shaft Couplings

View A Video on Shaft Couplings - A Quick Introduction

Shaft couplings are utilized by manufacturers to connect rotary equipment such as power transmissions, driveshafts, line shafts, generators, wheels, pumps and turbines in a variety of automotive, oil and gas, aerospace, water/waste treatment and construction industries. Rigid shaft couplings connect well-aligned shafts linearly. Fluid couplings transmit energy through oil by a pump and turbine construction. Flexible couplings such as bellows couplings or Oldham couplings connect shafts and equipment which require misalignment flexibility or non-linear alignment. Gear couplings transfer rotary motion at a 90 degree angle, while universal joints work similarly, connecting shafts with a joint which allows a full range of motion. Motor couplings like flywheel couplings and torque limiters are types of flexible couplings that are found in transmissions and motors of many different vehicles. Jaw couplings offer zero backlash and provide the best vibration dampening.

Shafts aid in positioning equipment and supplying it with an axis of rotation; engineers mount rotary equipment such as wheels upon rigid or linear shafts, using shaft couplings to connect the shafts of rotating equipment in order to allow some axial and angular misalignment while maintaining torque power. In certain industrial applications, shafts remain aligned with one another continuously, in which case rigid shaft couplings remain an effective choice. A commonly used rigid shaft is the driveshaft, which transfers power from a car, truck or motorcycle's transmission to its wheels.  Universal joints, which are also found in driveshafts in automobiles, allow the shaft to rotate and bend while still providing torque. They have central hubs where the shafts are joined to a third element.  The arrangement allows the joint to bend to a higher degree than in flexible shaft couplings. Gear couplings are used in similar applications to connect shafts while simultaneously providing torque. Gear couplings are identical in design to sprockets, using a gearhead in the same way sprockets move chains. Shafts and equipment that require a wider range of motion use universal joints or gear couplings to accommodate more than 5 degrees of misalignment which is typically the maximum angle at which flexible couplings still perform well.

During operation, some types of shafts tend to shift, causing misalignment. In applications in which shaft misalignment is common, manufacturers use flexible shaft couplings that are able to maintain proper and effective function even if the shafts are not in perfect alignment. Flexible shaft couplings provide efficient accommodation for moderate shaft misalignment which occurs when the shafts' axes of rotation become skewed. Movement, bumps or vibration cause shaft movement, which results in parallel, angular or skewed shaft misalignment. Parallel misalignment occurs when shafts' axes are parallel to one another, but do not intersect with one another; angular misalignment occurs when the axes of shafts intersect with one another at an angle, and skewed misalignment occurs when axial and angular misalignment occur in conjunction with one another. Flexible shaft couplings provide the means by which rotating equipment can continue to function during moderate displacement measuring up to 5° of misalignment. In addition to transmitting torque and accommodating shaft misalignment, flexible shaft couplings perform other functions. Flexible shaft couplings accommodate axial displacement, also called end float, which occurs when the shafts move along the axis of rotation, either toward or away from one another. Flexible shaft couplings also provide shock absorption and lessen the intensity of vibrations, a process known as damping.

Shaft coupling manufacturers fabricate couplings for a broad range of applications, often specializing in certain flexible applications, automotive applications, linear applications or industry-specific equipment applications. Correct flexible coupling requires careful analysis of the industrial application and environment in which the coupling will be used, as well as the performance of individual couplings. Flexible shaft couplings are categorized according to their means of flexibility and their application into three main categories: mechanical flexible shaft couplings, elastomeric flexible shaft couplings, and metallic membrane flexible shaft couplings. Not all coupling manufacturers fabricate all types of flexible couplings; some specialize instead in gear couplings, bellows couplings and other industry-specific types. Many coupling manufacturers also manufacture various types of shafts, such as driveshafts, line shafts and bearing supported stub shafts, offering custom fabrication for unusual coupling applications. The most common methods of construction for flexible couplings include electroforming, chemical deposition, mechanical forming and welding.  Electroformed couplings are made by adding layers of metal on a mandrel until the desired thickness is reached and the mandrel is melted away to leave behind the bellows. Chemical deposition is a similar method except that the materials are added by electro-deposition. Mechanical forming includes roll-forming or extrusion. Welded couplings are made by welding a series of rings or washers on both their inside and outside.

Images Provided by R + W America L.P.

Images Provided by Ringfeder Corporation

Shaft Coupling Types

• Bellows couplings are connectors for shafts and equipment that require flexibility and non-linear alignment.
• Compression couplings offer several advantages, which include efficient torque transmission, high overload tolerance and high torsional stiffness. Compression couplings include jaw couplings, pin and bush couplings and donut couplings.
  
• Diaphragm couplings permit angular misalignment, but only a small degree of axial motion. A sub-type of metallic membrane coupling.
  
• Disc couplings accommodate angular misalignment, but do not accommodate parallel misalignment or axial motion. A sub-type of metallic membrane coupling.
  
• Driveshafts transmit torque from engines to moving parts.
• Elastomeric couplings contain a resilient element, such as rubber or plastic. Elastomeric couplings possess many advantages, including a high damping ability, high shock absorption, a high degree of misalignment accommodation, and do not require lubrication; however, elastomeric couplings are larger than metallic couplings, and possess sensitivity to ultraviolet light, chemicals and high temperatures.
  
• Fail-safe couplings are designed to continue operating for a certain length of time after the application of torque has ceased.
  
• Flexible couplings transmit power while compensating for any misalignment.
• Flexible shaft couplings accommodate moderate shaft misalignment while transferring power.
• Fluid couplings are hydrodynamic devices that transmit rotation between shafts by acceleration and deceleration of hydraulic fluid.
• Flywheel couplings connect two shafts together while absorbing vibration and shock and providing a steady dampening effect.
• Gear couplings offer efficient torque transmission in a compact mechanism, and permit a high degree of axial movement. However, gear couplings require piloting, and may malfunction at very high or very low torques.
  
• Grid couplings offer adequate vibration damping and shock absorption. However, significant axial movement may compromise coupling resilience.
• Jaw couplings are a type of compression coupling that control motion by transmitting torque and dampening vibrations that could cause damage to other system components.
• Mechanical flexible couplings obtain flexibility through loosely fitted parts that move past one another. Mechanical flexible couplings maintain high torsional stiffness and accommodate a high degree of angular misalignment; however, mechanical couplings require lubrication and maintenance, and may not permit adequate axial movement.
  
• Metallic flexible couplings do not contain a rubber or a plastic element. Metallic flexible couplings offer high torsional stiffness and great resistance to chemicals, ultraviolet radiation and high temperatures.
  
• Metallic membrane or metallic element couplings obtain flexibility through the bending of a membrane within the coupling. Metallic membrane couplings require low maintenance and no lubrication; however, these couplings may be more expensive than mechanical flexible couplings.
• Motor couplings are flexible devices that provide distance changes along a lateral axis between a motor shaft and a device.
• Oldham couplings are constant velocity couplings used for connecting two parallel shafts that are close together and run at the same speed.
• Shear couplings provide a high degree of misalignment accommodation, produce low reactionary loads and offer high torsional softness. Shear couplings include tire, sleeve and molded-element couplings.
• Torque limiters are devices that protect equipment from damage by mechanical overload.
• Universal joints are hinges that enable rigid rods to bend.

Shaft Coupling Terms

Angular Misalignment - The condition in which the axes of shafts intersect with one another at an angle; angular misalignment is calculated by measuring the angle at the intersection of the connected axes, expressed in degrees.
 
Axis of Rotation - An imaginary line in the center of an object around which the object rotates.
 
Axial - Characterized by movement along the axis of rotation.
 
Axial Displacement - Condition in which shaft's move axially; also called end float.
 
Axial Freedom - The amount of movement along the shaft axes that a coupling can permit.
 
Axial Stretch - The change in a shafts length, whether an increase or decrease in length, upon the application of a load.
 
Backlash - The extent of shaft movement.  
 
Bending Flexibility - Measurement indicating the comparison of flexibility between shafts.
 
Bending Stiffness - The shaft's resistance to the application of torque.
 
Bore - The hole on the shaft onto which the coupling is mounted.
 
Burst Pressure - The pressure at which a device fails and loses ability to retain fluid.
 
Damping - The reduction of vibration between shafts; this reduction is caused by an elastomer in the coupling.
 
Field Repairable - In such cases the entire coupling does not have to be replaced and only certain components are repaired on site of the coupling.
 
Horsepower (hp) - Measurement unit indicating the time rate of work a piece of equipment produces. With regard to mechanical power, horsepower equals the movement of 33,000 pounds one foot per minute or the movement of 550 pounds one foot per second. Horsepower equals 746 watts of electrical power.
 
Hysteresis - A delay in the response of an object to forces, especially magnetic forces, acting upon the object; often observed in elastic and magnetic objects.
 
Inherent Balance - Balance of the coupling found in the original design of the coupling itself. This also can be a factor of the materials used in construction of the coupling, as certain stocks of metal are better for equilibrium.
 
Keyway - Rectangular opening in the coupling bore in which a key may be inserted to lock couplings and shaft parts into place.
 
Parallel Misalignment - The condition in which shafts axes are parallel, but do not intersect with one another; also called parallel offset or radial misalignment.
 
Piloting - The process of guaranteeing that shafts and couplings maintain the same axis of rotation.
 
Reactionary Load - The coupling's exertion of force upon shafts during parallel misalignment, which causes the shafts to bend.
 
RPM (Revolutions Per Minute) - Measurement of operating speed indicating the number of full rotations a shaft completes in one minute.
 
Shear Pin - A protective pin used in some couplings to prevent cut off.
 
Shrouded Bolt - An optional bolt found on some flexible couplings, which are used during high speed applications. Installed with a socket wrench.
 
Thermal Expansion - Lengthening of shafting caused by change in environmental temperature.
 
Torque - The measurement of the extent to which a force applied to an object causes the object to rotate.
 
Torsional Softness - The low resistance of a shaft to twisting motion, opposite of torsional stiffness.
 
Torsional Stiffness - The measurement of a shaft's resistance to twisting during operation. High torsionsal stiffness indicates minimal shaft twisting; low torsional stiffness indicates little resistance to twisting.
 
Torsional Vibration - The change in a rotational system's torque.