An Introduction to Couplings
A coupling is a mechanical device used to transmit power or torque from one shaft to another. Their other uses include connecting separately-manufactured components, providing flexibility while transmitting power, providing overload protection, and reducing transmission shock loads from one shaft to another. While there are many types of couplings, we will discuss the six most common: muff/sleeve couplings, split muff couplings, flange couplings, bush pin flexible couplings, gear couplings, and fluid couplings.
Muff/sleeve couplings are simply a hollow cylinder or pipe. They are manufactured based on the diameter of the shaft so that it will fit into the sleeve. The driving and driven shafts are then inserted into the sleeve and held in place through the two threaded holes in the sleeve and shaft. These couplings are easy to manufacture due to the low number of parts. They are used in light- to medium-duty applications where shafts do not require alignment.
In the split muff coupling, the sleeve is split into two parts rather than being a single component. They are semi-cylindrical and fit over the shaft. Similar to muff/sleeve couplings, they feature threaded holes allowing the shafts to be joined with steel bolts or studs. The defining feature of these couplings is their ability to be assembled and disassembled without changing the position of the shaft. They are used in medium- to heavy-duty loads at moderate speeds.
Flange couplings are similar to sleeve couplings, but feature flanges on both sides of the two sleeves. Both flanges have threaded holes which are used to join with nuts and bolts. To prevent slipping, there is also a key section on the hub and shafts. A tapered key is used in these couplings to ensure the hub doesn’t loosen up or move from the shaft. Flange couplings are used for medium & heavy-duty applications.
Bush pin flexible couplings are, in many ways, an upgraded version of the flange coupling. The main difference between the two is the bush pin flexible couplings’ use of rubber bushings. These bushings are designed to allow studs or bolts to perfectly fit inside the provided holes. The primary advantage of these couplings is that they can be used in slightly misaligned shafts as the rubber bushings provide added flexibility in addition to shock and vibration absorption. Couplings of this type are ideal for use in applications where there is angular, parallel, or axial misalignment. They are most often found in medium-duty applications in electric motors and machines.
Another modified version of the flange coupling is the gear coupling. In gear couplings, the flange and hub are different parts instead of a single component like in flange couplings. Hubs are externally splined but are thick enough to be considered teeth. They also have internal teeth and a gear ratio of 1:1. Gear couplings are ideal for use in heavy-duty applications where the requirement of torque transmission is high.
Finally, the fluid coupling is a coupling consisting of two parts: a pump and a turbine. Both of these feature internal blades mounted at a certain angle. The pump is mounted on the end of the driving shaft while the turbine is on the driven shaft. Fluid couplings operate thusly: the fluid enters into the pump through its center, causing the drive shaft to rotate and push the fluid out. The design of the coupling then diverts the fluid into the turbine to rotate the blades. When a reactor is included between the pump and turbine, the device becomes a torque converter. Fluid couplings are most commonly used in marine and industrial applications to control the start-up of power transmissions.
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