First things first! You choose a coupling to connect two shafts together at their ends to transmit power. The primary purpose of coupling is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both.
Normally, couplings do not allow shafts to disconnect during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded. Hence, you should carefully select, install and maintain couplings; as this could give you substantial savings in the form of reduced maintenance costs and downtime.
A gear coupling is a mechanical device to join two rotating shafts (that are not collinear) for efficient transmission of mechanical power. These are crucial because although you may align the shafts (for example, between the gearbox and the mixing mill roll) at the time of installation, it is likely that during the operation there may be disruption to this alignment.
Disruptions could occur due to setting of foundation, thermal expansion, shaft deflection, wearing out of other parts, improper maintenance and many more reasons. At these times, where the misalignment occurring during the operation is unavoidable, a gear coupling compensate or minimize the effect of misalignment thus providing a good solution to prevent your rubber machinery, bearings, seals and shafts from being subjected to the additional forces.
Gear couplings are power intensive and considered as the king of the coupling types. You would see the gear couplings being adopted for wide range of application in drive technologies. Hence, we love love gear coupling (and I think, you would too!).
Gear Couplings and Universal Joints
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.
Structure of a Gear Coupling
Gear couplings are torsionally rigid and consist of hubs with multi crowned teeth at flank, tip and chamfering on teeth, gaskets, sleeve and O-rings. The hubs attach to the machinery shafts, and sleeves span the gap from one hub to the next. They are available in two designs – completely flexible and flexible/rigid.
The hub is the heart of any gear coupling. Hence manufacturers take great care to impart a superior design and mostly manufacture the hub on CNC machine for precision. Together with the tooth profile of the hub, these aspects enable the gear coupling to function well under all operating conditions with increased reliability and long life.
The tooth flanks and outer diameter of the external gear are crowned to allow for angular displacement between the two gears. The multi crowned teeth reduce the alignment adjustment and improve the load carrying capacity of the teeth. The backlash between the teeth is minimal due to multi-crown tooth design.
A completely flexible coupling comprises two hubs with an external gear and two outer sleeves with an internal gear. It’s a universal coupling for all sorts of applications and accommodates all possible misalignment (angular, radial and combined) as well as large axial moments.
A flexible/rigid coupling comprises one flexible geared half and one rigid half. It does not accommodate parallel displacement of shafts but does accommodate angular misalignment.
Information to order Gear Coupling
When you are buying rubber machinery, your machinery supplier or OEM would take care of this. However, you need to understand that there is a check list of information that needs to be provided to a gear coupling supplier that includes,
- Specify quantity required and delivery expectations.
- Specify shaft or bore sizes and key-way dimensions. You need to give exact dimensions with tolerances.
- Specify load – HP and/or torque at a specific RPM. State the normal and maximum conditions of use.
- Specify Speed of operation – Minimum, Normal and Maximum.
- Specify Application. Most Rubber & Tyre Machinery (Mixers, Kneaders, Mills, Calenders, Extruders, etc) conforms to a service factor classification where heavy shock conditions or frequent reversing peak loads that do not exceed 150 per cent average horse power is observed. Uneven load is usually present during operation. Service factor ranges from 1.25 to 1.50.
- Specify Coupling Series, Type and Size.
- Specify Space limitations – envelope dimensions, shaft extensions and shaft spacing.
- Specify unusual misalignment conditions if any.
- Specify Modifications – tapered bores, special keys, hub cut off, counter bores or others
- Specify unusual operating conditions ambient temperatures and atmospheres, if any.
Maintenance of Gear Coupling
Gear Coupling maintenance is mostly simple – requiring a regularly scheduled inspection of each coupling. Your schedule should consist of:
- Performing visual inspections – checking for signs of wear or fatigue, and cleaning couplings regularly.
- Checking and changing lubricant Each coupling half is provided with grease nipple/plug on the housing from where grease can be filled directly. The coupling is supplied with some quality of grease to facilitate assembly. It is recommended to fill the required quality and quantity of grease before putting the coupling in operation. This maintenance is required annually for most couplings and more frequently for couplings in adverse environments or in demanding operating conditions.
- Documenting the maintenance performed on each coupling, along with the date.
Here’s a 1:17 min video of dismantling of a gear coupling so you could appreciate the components that make-up this coupling much better.
Failure of Gear Coupling
Caution! Even with proper maintenance, your gear couplings can fail. The other causes of failure include:
- Improper installation
- Poor coupling selection
- Operation beyond design capabilities
The only way to improve your coupling life is to understand what caused the failure and to correct it prior to installing a new coupling. You could check for some external signs that indicate potential coupling failure like,
- Abnormal noise, such as screeching, squealing or chattering
- Excessive vibration or wobble
- Failed seals indicated by lubricant leakage or contamination
Summarizing, gear coupling is a mechanical device to join two rotating shafts for efficient transmission of mechanical power, are power intensive and is the king amongst the coupling types. They offer the highest load capacity and stiffness, yet easy to maintain. These are some of our reasons to love this gear coupling on the rubber machinery.
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