and abutted against each other which are enveloped by muff or sleeve. A gib head sunk keys hold the two shafts and sleeve together (this is the simplest type of the coupling) It is made from the cast iron and very simple to design and manufacture. It consists of a hollow pipe whose inner diameter is same as diameter of the shafts. The hollow pipe is fitted over a two or more ends of the shafts with the help of the taper sunk key. A key and sleeve are useful to transmit power from one shaft to another shaft. Tapered shaft lock A tapered lock is a form of keyless shaft locking device that does not require any material to be removed from the shaft. The basic idea is similar to a clamp coupling but the moment of rotation is closer to the center of the shaft. An alternative coupling device to the traditional parallel key, the tapered lock removes the possibility of play due to worn keyways. It is more robust than using a key because maintenance only requires one tool and the self-centering balanced rotation means it lasts longer than a keyed joint would, but the downside is that it costs more. Twin spring A flexible coupling made from two counter-wound springs with a ball bearing in the center, which allows torque transfer from input to output shaft. Requires no lubrication to consistently run as it has no internal components. Universal joint Maintenance and failure Coupling maintenance requires a regularly scheduled inspection of each coupling. It consists of: Performing visual inspections Checking for signs of wear or fatigue Cleaning couplings regularly Checking and changing lubricant regularly if the coupling is lubricated. 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. Even with proper maintenance, however, couplings can fail. Underlying reasons for failure, other than maintenance, include: Improper installation Poor coupling selection Operation beyond design capabilities. External signs that indicate potential coupling failure include: Abnormal noise, such as screeching, squealing or chattering Excessive vibration or wobble Failed seals indicated by lubricant leakage or contamination. Balance Couplings are normally balanced at the factory prior to being shipped, but they occasionally go out of balance in operation. Balancing can be difficult and expensive, and is normally done only