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Coupling refers to a device that connects two shafts or shafts and rotating parts, rotates together in the process of transmitting motion and power, and does not disengage under normal circumstances. Sometimes it is also used as a safety device to prevent the connected parts from being subjected to excessive load, and plays the role of overload protection. Under normal circumstances, we design and select the coupling according to the principles of easy disassembly, light weight, small size, easy disassembly and installation, and the location as close to the bearing as possible. Couplings are generally divided into rigid couplings and flexible couplings (called elastic couplings when using elastic elements). Rigid couplings are used in places where the two shafts are strictly aligned and there is no relative offset during work. The flexible coupling is more used where the two shafts have relative displacement.
Rigid couplings are divided into flange couplings, radial key flange couplings, sleeve couplings, jacket couplings and parallel shaft couplings Flange couplings: use bolts to connect the two halves The flange of the shaft coupling realizes the coupling of the two shafts. Radial key flange coupling: the coupling of the two halves of the coupling is connected by radial keys and ordinary bolts. Sleeve Coupling: using a common sleeve Couplings that connect two shafts in a certain way Couplings: Couplings that are clamped in some way to achieve two-shaft coupling using two axially split collets. Parallel shaft couplings: use Coupling in which the intermediate disc passes through the pin to realize the connection of two parallel shafts.
Rigid couplings consist of very rigid parts. The elastic coupling contains elastic elements, which not only have the functions of buffering and vibration reduction, but also compensate for the displacement between the axes of the two shafts. The difference between flexible coupling and rigid coupling is as follows:
1. Different characteristics
An flexible coupling is a coupling that can compensate for the axial displacement and radial displacement between the connecting shafts, while a rigid coupling is a coupling that cannot connect the axial displacement and radial displacement between the shafts.
2. The structure is different
Compared with the flexible coupling, the rigid coupling has a relatively simple structure, and the elastic coupling has a relatively complex structure, which is more suitable for the situation where the concentricity of the two shafts is not good, especially for the situation with torsional vibration, because its The elastic structure can have the effect of damping vibration. Rigid coupling is relatively suitable for two shafts with good concentricity, and the price is relatively cheap.
3. Different assembly requirements
The shaft concentricity error of the two shafts of the rigid coupling needs to be within 0.05mm, and a more accurate coaxiality error value is required, while the coaxiality error of the elastic coupling for the two shafts can be within 0.1mm. The distance between the drive shaft and the pump end should be kept 5~10mm.
Larger loads can be carried with a pin coupling. A hole is drilled through the hub and the shaft. Having the pin through the entire assembly doubles the shear area, since there are two shaft/hub interfaces. This is crucial in designs where the pin is to purposely fail in an overload or shock-load situation. A pin with a reduced diameter, which causes it to fail in order to protect more critical parts, is called a shear pin. The ability to carry a load can be calculated by taking the force divided by the area of the pin, on both interfaces, in shear.
A problem with pin couplings can be its tolerance fit&#;the potential of the pin being too tight or too loose in the hole. A pin that&#;s too loose can fall out and create backlash. If a pin is too tight, installation and removal can become difficult. A tapered pin can fix some of the problems, but often a spring pin is used. A spring pin is where the diameter is slightly smaller than the pin so that it forms a press fit into the assembly.
Another problem with all pin-type couplings is they require a hole that adds a stress concentration. To reduce the effects of a stress concentration, the pin can be laid parallel to the shaft and nest as a key between the interface of the shaft and hub.
For more information, please visit Flexible Coupling Types.
Keys
A pin key will increase the cross-section in shear, making it able to handle larger loads. Woodruff keys are semicircular keys that are recommended (rather than pins) if the application has light loads and requires ease of assembly and disassembly. Another common key is a square or rectangle key.
Keys are simple ways to join the shaft to hubs and are relatively easy to design, considering their basic shear-force and compressive-force calculations. More complex designs often turn to spline couplings (depending on the design, they might also be called gear, sheave, or sprocket), because of their ability to handle great loads and reduce backlash. These couplings can greatly increase cost, though.
Splines
Spline couplings make a strong assembly between the shaft and hub. An SAE spline&#;s torque capacity is 1,000 psi bearing stress on the side of the spline. This information yields the equation:
T = Nrh
where T= torque; N = the number of splines; r = radius; and h = depth of the spline.
A spline coupling is not designed to break to preserve components downstream from high or shock loads. Splines are essentially a set of axial keys, but have many benefits over keys. A spline transfers torque more uniformly, reducing force on the hub. Splines are precisely machined to resist movement, while keys will wear over time and can compress, causing movement and backlash in the assembly. Also, in the case of involute splines, the major diameter fit allows for accurate concentricity, and this form tends to center the shaft in the hub.
Keyless
Eliminating the key can make things easier for assembly and maintenance. Taper and screw, shrink and press fit, and clamp are some common types of keyless couplings.
Taper and screw is where the shaft and hub can be tapered, and is held in place by threading on the end of the shaft with a nut and washer to mate the components. Torque, load, and shock are limited with this coupling. Due to the machining involved it can be costly, too. Furthermore, if the nut is not tightened properly, it may not transfer torque, or it induces stress or fatigue in the shaft that causes it to break.
Taper and screw couplings are good for concentricity, and a key can be added to the taper and screw design to increase torque transmission. This type of coupling can be beneficial for applications with low loads and where the hub needs to be removed for maintenance.
Shrink and press fit are difficult to take on and off, but can form a tight keyless bond between the shaft and hub. These would not be good for applications that require the hub to be removed for maintenance. This is similar to the taper and screw, as they both rely on friction forces to hold the assembly tight. The more surface area and greater the force (without inducing a stress concentration), the more load the coupling will be able to handle.
Torque is important for many, if not all, industrial applications, but for keyless couplings it&#;s critical. In terms of clamp couplings, the fasteners that clamp the shaft and hub together must be torqued properly for two reasons: To gain the friction or crush necessary to transmit energy from the shaft to the hub; and not to be over-torqued, inducing stresses onto the coupling or other components.
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