Coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. In a more general context, a coupling can also be a mechanical device that serves to connect the ends of adjacent parts or objects. Couplings do not normally allow disconnection of shafts during operation, however there are torque-limiting couplings which can slip or disconnect when some torque limit is exceeded. Selection, installation and maintenance of couplings can lead to reduced maintenance time and maintenance cost.
Uses
Shaft couplings are used in machinery for several purposes. A primary function is to transfer power from one end to another end.Other common uses:
- To alter the vibration characteristics of rotating units
- To connect the driving and the driven part
- To introduce protection
- To reduce the transmission of shock loads from one shaft to another
- To slip when overload occurs
Types
Beam
A beam coupling, also known as helical coupling, is a flexible coupling for transmitting torque between two shafts while allowing for angular misalignment, parallel offset and even axial motion, of one shaft relative to the other. This design utilizes a single piece of material and becomes flexible by removal of material along a spiral path resulting in a curved flexible beam of helical shape. Since it is made from a single piece of material, the beam style coupling does not exhibit the backlash found in some multi-piece couplings. Another advantage of being an all machined coupling is the possibility to incorporate features into the final product while still keeping the single piece's integrity.Changes to the lead of the helical beam provide changes to misalignment capabilities as well as other performance characteristics such as torque capacity and torsional stiffness. It is even possible to have multiple starts within the same helix.
The material used to manufacture the beam coupling also affects its performance and suitability for specific applications such as food, medical and aerospace. Materials are typically aluminum alloy and stainless steel, but they can also be made in acetal, maraging steel and titanium. The most common applications are attaching rotary encoders to shafts and motion control for robotics.
Beam couplings can be known by various names depending upon industry. These names include flexible coupling, flexible beam coupling, flexible shaft coupling, flexure, helical coupling, and shaft coupling.
The primary benefit to using a flexible beam coupling to join two rotating shafts is to reducing vibration and reaction loads which in turn will reduce overall wear and tear on machinery and prolong equipment life.
Bush pin flange
Bush pin flange coupling is used for slightly imperfect alignment of the two shafts.This is modified form of the protected type flange coupling. This type of coupling has pins and it works with coupling bolts. The rubber or leather bushes are used over the pins. The coupling has two halves dissimilar in construction. The pins are rigidly fastened by nuts to one of the flange and kept loose on the other flange. This coupling is used to connect shafts which have a small parallel misalignment, angular misalignment or axial misalignment. In this coupling the rubber bushing absorbs shocks and vibration during its operations. This type of coupling is mostly used to couple electric motors and machines.
Constant velocity
There are various types of constant-velocity couplings: Rzeppa joint, Double cardan joint, and Thompson coupling.Clamp or split-muff
In this coupling, the muff or sleeve is made into two halves parts of the cast iron and they are joined by means of mild steel studs or bolts. The advantages of this coupling is that assembling or disassembling of the coupling is possible without changing the position of the shaft. This coupling is used for heavy power transmission at moderate speed.Diaphragm
Diaphragm couplings transmit torque from the outside diameter of a flexible plate to the inside diameter, across the spool or spacer piece, and then from inside to outside diameter. The deforming of a plate or series of plates from I.D. to O.D accomplishes the misalignment.Disc
Disc couplings transmit torque from a driving to a driven bolt tangentially on a common bolt circle. Torque is transmitted between the bolts through a series of thin, stainless steel discs assembled in a pack. Misalignment is accomplished by deforming of the material between the bolts.Elastic
An elastic coupling transmits torque or other load by means of an elastic component. One example is the coupling used to join a windsurfing rig to the sailboard. In windsurfing terminology it is usually called a "universal joint", but modern designs are usually based on a strong flexible material, and better technically described as an elastic coupling. They can be tendon or hourglass-shaped, and are constructed of a strong and durable elastic material. In this application, the coupling does not transmit torque, but instead transmits sail-power to the board, creating thrust.Flexible
Flexible couplings are usually used to transmit torque from one shaft to another when the two shafts are slightly misaligned. They can accommodate varying degrees of misalignment up to 1.5° and some parallel misalignment. They can also be used for vibration damping or noise reduction. In rotating shaft applications a flexible coupling can protect the driving and driven shaft components from the harmful effects of conditions such as misaligned shafts, vibration, shock loads, and thermal expansion of the shafts or other components.At first, flexible couplings separate into two essential groups, metallic and elastomeric.
Metallic types utilize freely fitted parts that roll or slide against one another or, on the other hand, non-moving parts that bend to take up misalignment.
Elastomeric types, then again, gain flexibility from resilient, non-moving, elastic or plastic elements transmitting torque between metallic hubs.
Fluid
Gear
A gear coupling is a mechanical device for transmitting torque between two shafts that are not collinear. It consists of a flexible joint fixed to each shaft. The two joints are connected by a third shaft, called the spindle.Each joint consists of a 1:1 gear ratio internal–external gear pair. The tooth flanks and outer diameter of the external gear are crowned to allow for angular displacement between the two gears. Mechanically, the gears are equivalent to rotating splines with modified profiles. They are called gears because of the relatively large size of the teeth.
Gear couplings and universal joints are used in similar applications. Gear couplings have higher torque densities than universal joints designed to fit a given space while universal joints induce lower vibrations. The limit on torque density in universal joints is due to the limited cross sections of the cross and yoke. The gear teeth in a gear coupling have high backlash to allow for angular misalignment. The excess backlash can contribute to vibration.
Gear couplings are generally limited to angular misalignments, i.e., the angle of the spindle relative to the axes of the connected shafts, of 4°–5°. Universal joints are capable of higher misalignments.
Single joint gear couplings are also used to connect two nominally coaxial shafts. In this application the device is called a gear-type flexible, or flexible coupling. The single joint allows for minor misalignments such as installation errors and changes in shaft alignment due to operating conditions. These types of gear couplings are generally limited to angular misalignments of °–°.
Geislinger
Giubo
Grid
A grid coupling is composed of two shaft hubs, a metallic grid spring, and a split cover kit. Torque is transmitted between the two coupling shaft hubs through the metallic grid spring element.Like metallic gear and disc couplings, grid couplings have a high torque density. A benefit of grid couplings, over either gear or disc couplings, is the ability their grid coupling spring elements have to absorb and spread peak load impact energy over time. This reduces the magnitude of peak loads and offers some vibration dampening capability. A negative of the grid coupling design is that it generally is very limited in its ability to accommodate the misalignment.
Highly flexible
Highly flexible couplings are installed when resonance or torsional vibration might be an issue, since they are designed to eliminate torsional vibration problems and to balance out shock impacts.They are used in installations where the systems require a high level of torsional flexibility and misalignment capacity. This type of coupling provides an effective damping of torsional vibrations, and high displacement capacity, which protects the drive. The design of the highly flexible elastic couplings makes assembly easier. These couplings also compensate shaft displacements and the torque is transmitted in shear. Depending on the size and stiffness of the coupling, the flexible part may be single- or multi-row.