Choosing the Right Mechanical Coupling

A coupling (or transmission joint) is a mechanical system for transmitting power and torque from a driving shaft to a rotating driven shaft.

It is rare for coupled shafts to be perfectly aligned. For this reason, certain couplings (such as flexible couplings and universal joints) are also used to compensate for misalignments between shafts. The type of misalignment will be a determining factor in the choice of coupling.

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  • How to choose a mechanical coupling

    Mechanical couplings can be divided into four main categories:

    • Flexible couplings
    • Torsionally-rigid flexible couplings
    • Rigid couplings
    • Hydrodynamic couplings

    Lovejoy couplings

    There is a wide variety of couplings available on the market. Sometimes, it’s easy to know which one to choose. For example, if there is no significant misalignment, a rigid coupling will be the cheapest and most reliable solution. If torsional damping is the main concern, then a jaw coupling is the best choice.

    Couplings have several functions and advantages:

    • Transmit movement from the drive side to the driven side of a rotary system
    • Quickly connect two components of a system (for example, a motor and a pump) and easily separate them for maintenance purposes
    • Compensate for axial, radial, or angular misalignment
    • Add more flexibility to a system
    • Absorb vibrations and mechanical shocks
    • Protect equipment from torque overloads

    The choice of coupling should be based on the following criteria:

    • Type of misalignment between the two axes (radial, angular, or axial)
    • Maximum torque to be transmitted (generally, in the technical characteristics of the coupling, the manufacturer indicates the maximum torque allowed)
    • Characteristics and operating conditions of the drive side and the driven side of the system (frequency of starts, shocks and vibrations, operating temperature, etc.)
    • Maximum rotation speed
    • Coupling dimensions
    • Diameter of input and output shafts
    • Cost/performance ratio

    The service life of the coupling in relation to its replacement cost is an important selection criterion. In certain cases, for example, if downtimes are particularly costly or it is difficult to dismantle the components to carry out maintenance on the coupling, it is advisable to opt for a coupling with a longer service life that requires little or no maintenance.

    System performance versus cost must also be considered. For example, if the motor or driven components are expensive or fragile, you should opt for a more expensive coupling that exerts softer reactive forces.

  • How to choose a flexible coupling

    Flender tire coupling

    Flexible couplings consist of rigid parts and an elastic connection. This elastic element makes them torsionally flexible. As such, they can compensate for certain misalignments but are mainly intended to absorb shocks and vibrations in the transmission resulting from variations in torque (from a pulsating torque, for example).

    There are various types of flexible couplings, such as elastomer couplings, jaw couplings, tire couplings, grid couplings, etc.

    They have various applications in conveyor belts, mixers, pumps, etc.

    Choosing a coupling of this type depends on several factors, such as the space available for mounting it, the durability required, and the transmission capacity.

    The operating temperature is an equally important aspect. The product’s technical characteristics usually indicate the temperature range in which the coupling can operate.

    As with any mechanical assembly, it is difficult to achieve perfect alignment between the parts. As such, there are misalignments between the drive shaft and the driven shaft that can complicate the transmission of movement. These misalignments can be axial, radial, or angular. Misalignment can lead to fatigue or wear of the coupling. Therefore, when selecting a coupling, you must consider the speed of rotation, the maximum permissible misalignment, and the torque the assembly can withstand. This information is provided by the manufacturer for each product.

    When transmitting high powers, ensure that the torque transmitted is less than the maximum torque indicated in the product’s technical characteristics, leaving a margin proportional to the expected misalignment. The rotational speed of flexible couplings is limited, so they are not suitable for shafts with high rotational speeds, especially if there is significant misalignment.

    Concerning mechanical shocks and vibrations, these couplings can generate vibrations if they start to show backlash due to wear. Fixing these assemblies with a flange helps prevent marks from being produced on the shafts, offering greater resistance to sudden reversals and vibrations.

  • How to choose a torsionally-rigid coupling

    Miki Pulley torsionally-rigid coupling

    Torsionally rigid couplings are suitable for compensating for misalignment and transmitting high torques.

    This category includes Oldham couplings, disc couplings, membrane couplings, chain couplings, gear couplings, and bellows couplings. Torque is transmitted through rigid welded elements. For this reason, these couplings cannot absorb torque variations. Torsional rigidity, usually expressed in Nm/rad, indicates the resistance of a coupling to torsion.

  • How to choose a rigid coupling

    OEP Couplings rigid coupling

    Rigid couplings are, as their name suggests, permanent rigid couplings (or joints) connecting two shafts or shaft/hub. Unlike flexible couplings, they do not allow misalignment.

    They ensure that machines can be driven without any unwanted movement between the shafts. They have a very low capacity to absorb misalignment, making them ideal for transmissions with high torques and no misalignment between the shafts. They are also suitable for systems with low rotational speeds.

    Rigid couplings are mounted precisely between the two shafts and are easy to assemble, disassemble, and adjust. They are unable to accommodate misalignment. If misalignment does occur, due to assembly misalignment or thermal expansion during operation, the use of rigid couplings can result in significant reactive forces on the bearings. However, if these misalignments are very small and the bearings are robust, rigid couplings will be the best choice both for their low cost and their durability.

  • How to choose a hydrodynamic coupling

    Flender hydrodynamic coupling

    With hydrodynamic couplings, torque is transmitted through a fluid pumped against a turbine, causing it to rotate.

    Hydrodynamic couplings have few components. For this reason, they are extremely resistant to adverse environmental conditions. These couplings are used in aggressive environments, and in applications that require the gradual acceleration of large masses (such as conveyor belts, mills, crushers, grinders, mixers, and motor vehicles).

    Hydrodynamic couplings offer the following advantages:

    • Slow and gentle starting of machines
    • Motor at no load operation, acceleration of large masses without oversizing the motor
    • Electrical installation protection
    • Protection of the various pieces of equipment that make up the system
    • Protection of the kinematic chain
    • Power transmission without wear
    • Vibration absorption
    • Resistance to adverse environmental conditions

    The disadvantage of this type of coupling is its weight: a hydrodynamic coupling is generally heavier than a conventional mechanical coupling.

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