Forged Fittings - Couplings

The primary purpose in most cases is power and torque transmission from a driving shaft to a driven shaft — for example, a coupling connecting a motor to a pump or a compressor.

Shock and vibration absorption

A shaft coupling can smooth out any shocks or vibrations from the driving element to the driving element. This feature reduces the wear on the components and increases the service life of the setup.

Accommodate any misalignment

Misalignments between shafts can result from initial mounting errors or may develop over time due to other reasons. Most couplings can accommodate some degree of misalignment (axial, angular, and parallel) between shafts.

Interrupt heat flow

A shaft coupling can also interrupt the flow of heat between the connected shafts. If the prime mover tends to heat up during operation, the machinery on the drive side is protected from being exposed to this heat.

Overload protection

Special couplings known as Overload Safety Mechanical Coupling are designed with the intention of overload protection. On sensing an overload condition, these torque-limiting couplings sever the connection between the two shafts. They either slip or disconnect to protect sensitive machine

Couplings come in a host of different shapes and sizes. Some of them work great for generic applications, while some others are custom-designed for really specific scenarios.

In order to make an informed decision, one should be aware of the capabilities and differences between different types of couplings. This section describes the following types of couplings:

• Rigid coupling

Engineers prefer rigid couplings when precise alignment is required. They permit little to no relative movement between the shafts. 

This type of shaft coupling can restrict any undesired shaft movement. Examples of this type of shaft coupling include a sleeve, compression, and flange couplings.

In vertical applications, such as vertical pumps, rigid couplings are used to connect two shafts and function as a single shaft.

High-torque applications, such as large turbines, also use them to transmit torque. Because they cannot use flexible couplings, most turbines now use rigid couplings between turbine cylinders, ensuring that the shaft acts as a continuous rotor. High-torque applications, such as large turbines, also use them to transmit torque. Because they cannot use flexible couplings, most turbines now use rigid couplings between turbine cylinders, ensuring that the shaft acts as a continuous rotor.

• Flexible coupling

An individual shaft coupling allowing some degree of relative motion between its constituent shafts as well as vibration isolation is known as a flexible coupling. The machines would not need a flexible coupling if shafts were aligned perfectly all the time and they did not move or vibrate during operation.

Unfortunately, this is not how machines operate in reality, so designers have to deal with all of the above issues in machine design. As a result of the flaws and dynamics that are part of almost every system, the flexible coupling can reduce the amount of wear and tear on machines.

There are many different types of flexible couplings, but gear couplings, universal joints, and Oldham couplings are some popular examples.

•  Sleeve or muff coupling

It consists of a cast-iron sleeve (hollow cylinder) or muff with an internal diameter equal to the external diameter of the shafts being connected, making it the simplest rigid coupling. By restricting relative motion between shafts and sleeves, a gib head key prevents slippage.

During assembly, some sleeve couplings and shafts have threaded holes that match up to prevent shafts from moving axially. Power is transferred from one shaft to the other through the sleeve, the keyway, and the key. This shaft coupling is intended for light to medium torques.

As long as all the parts are designed with torque values in mind, the sleeve coupling is a sturdy choice.

• Split muff coupling

To facilitate the assembly or disassembly of a sleeve coupling, the sleeve can be divided into two parts. By doing this, the technician no longer needs to move the shafts.

A split muff coupling or a compression coupling is made up of two halves held together by studs or bolts.

These couplings transmit power through the key, similar to sleeve couplings. Split muff couplings are used in heavy-duty applications.

• Flange coupling

Each shaft is connected with a flange coupling. By using set screws or a tapered key, you can prevent the flange hub from sliding backward and exposing the shaft interfaces. The flanges are connected with studs or bolts and on the shaft by keys.

There is one flange with a protruding ring, while the other has a recess to accommodate it. This design keeps the shafts aligned without undue stress being placed on them.

In addition to power transmission, flange couplings are used in pressurized fluid systems. There are three major types of flange couplings:

• • Unprotected type flange coupling
  • Protected type flange coupling
  • Marine-type flange coupling
• Gear coupling

Gear couplings are very similar to flange couplings, but they are flexible and can be used for shafts that are not parallel. Gear couplings accommodate angular misalignments of about 2 degrees and parallel misalignments of 0.25…0.5 mm. 

Two hubs (with external gear teeth), two flange sleeves (with internal gear teeth), seals (O-rings and a gasket), and fasteners are included in the gear coupling setup.

A gear coupling transmits power between its two ends through its internal and external gears.

• For optimal performance, gear couplings need periodic lubrication (grease) since they are capable of high-torque transmission.

A universal joint can accommodate small angular misalignments while providing high torque transmission capacity when two shafts aren’t parallel.

Two hinges are connected through a cross-shaft to form a universal joint. The cross-shaft maintains the orientation of the two hinges, as well as transmits power. The universal joint is not a constant velocity coupling, i.e., the driving and driven shafts rotate at different speeds.

The most common application for universal joints is in car gearboxes and differentials.

• Oldham Coupling

Oldham couplings are used for lateral shaft misalignment when two shafts are parallel but not collinear.

In addition to two flanges that attach to the shaft, there is a center disc. Each face of the center disc has a lug. Each lug is actually a rectangular projection that fits into a groove cut out in the flanges perpendicular to one another.

By attaching the flanges to the shaft through keys, power is transmitted from the driving shaft to the key to the flange to the center disc and then to the driven shaft through the second flange.

Whenever there is a parallel offset between two shafts, Oldham couplings are ideal. When power needs to be transferred between shafts at different elevations, such parallel misalignment can result. As the shafts move, the center disc adjusts for lateral variation by moving back and forth.

• Diaphragm coupling

A diaphragm coupling is an all-around shaft coupling. It can accommodate parallel misalignment, high angular misalignment, and axial misalignment. It can also transmit torque at high speeds without lubricating.

A diaphragm coupling is available in several styles and sizes. It consists of two diaphragms connected by an intermediate member. Through bolts on both sides, the drive flanges on the shafts are connected to the intermediate member by the diaphragm.

As a result of their high-speed function, diaphragm couplings have gained much use in other rotating equipment as well. They were originally designed for helicopter drive shafts. Today, turbines, compressors, generators, aircraft, and more are some examples of applications.

• Jaw coupling

Jaw couplings are material flexing couplings that are used in general low-power transmission and motion control applications. Like diaphragm couplings, jaw couplings do not require lubrication.

In this coupling, two hubs with intermeshing jaws fit inside an elastomeric spider. The spider is usually made from copper alloys, polyurethane, Hyrtel, or NBR.

As a result of the spider’s elastic nature, it is suitable for transmitting shock loads. It can also dampen vibrations and reactionary forces.

Jaw couplings are used in applications such as compressors, blowers, mixers, and pumps.

• Beam coupling

Among the best low-power transmission couplings, beam couplings are machined couplings with high flexibility in terms of parallel, axial and angular misalignment.

With beam-style couplings, the helical cuts are positioned in a cylindrical structure. These cuts can be modified in terms of their lead and number of starts to offer varying degrees of misalignment capability. Since beam coupling is a single-piece structure, engineers are able to make these changes without compromising its integrity. Thus, beam coupling is also known as helical coupling.

Beam couplings are actually flexible beams that can be used in single-beam or multi-beam configurations. Multi-beam couplings can handle greater parallel misalignment than single-beam couplings.

Beam couplings are used in robotics and servo motors to avoid torsional windup in low-load applications.

• Fluid coupling

Fluid couplings transmit torque from one shaft to another using hydraulic fluid.

The shaft coupling consists of an impeller connected to the driving shaft and a runner connected to the driven shaft. The whole setup is enclosed in a housing.

During rotation of the drive shaft, the impeller accelerates the fluid, which then comes into contact with the runner blades and transfers mechanical energy to them. 

In automobile transmissions, marine propulsion, locomotives, and some industrial applications with cyclic loads, fluid couplings are used.

The following parameters should be considered when choosing

Whenever used correctly, shaft couplings provide incredible advantages to motion control and power transmission systems.

Designers must take many factors into account to make the right choice. Understanding these factors will reduce coupling failures and improve system performance.

• Torque levels
• Alignment limits
• Rotational speeds
• Lubrication constraints