JNTU B.Tech II MINE MECHANISATION-I Previous Question Paper (November/December - 2020 - R16)

A hydraulic coupling is a hydrodynamic device used to transmit rotating mechanical power from a prime mover (such as an electric motor or diesel engine) to a driven load (such as a conveyor or pump) using hydraulic fluid as the medium. It provides a smooth, shock-free start and protects the drive system from overload.

The device consists of three primary components enclosed in a housing: an impeller (pump) connected to the input shaft, a runner (turbine) connected to the output shaft, and the working fluid (usually oil). When the prime mover rotates the impeller, the oil is accelerated outward by centrifugal force. This kinetic energy is transferred to the runner, where the oil strikes the blades and causes the runner to rotate, thereby transmitting torque to the load. Because there is no mechanical connection between the input and output shafts, the coupling allows for a degree of slip, which is essential for soft starting.

Key features of the hydraulic coupling include:

• Soft Start: It allows the motor to reach its operating speed under no-load or light-load conditions before gradually accelerating the driven machinery, reducing peak starting currents.
• Overload Protection: If the driven equipment jams, the coupling allows the impeller to continue rotating while the runner stalls, preventing the motor from burning out.
• Vibration Damping: The fluid medium acts as a buffer, absorbing torsional vibrations and shocks from the load, which extends the life of the motor and gearbox.

A typical sketch of a hydraulic coupling features a circular casing containing two facing vaned wheels. The impeller is mounted on the input shaft, and the runner is mounted on the output shaft. The space between them forms a toroidal vortex chamber. As the impeller spins, oil flows in a circular path from the impeller to the runner and back, creating a continuous power transmission loop. The efficiency of the coupling is defined by the speed ratio between the runner and the impeller, with a small amount of slip (typically 2-4%) being necessary to maintain torque transmission.

A jaw clutch is a mechanical device used to transmit torque between two rotating shafts by engaging interlocking teeth or jaws. It is primarily used in applications where the shafts are required to rotate at the same speed or where positive engagement is necessary without slippage.

The clutch consists of two main components: a fixed jaw member keyed to the driving shaft and a sliding jaw member mounted on the driven shaft. The sliding member is typically moved axially along a splined shaft using a shifting lever or fork to engage or disengage the teeth. Because the jaws have a square or trapezoidal profile, they provide a positive drive, meaning there is no friction-based slip.

Key characteristics include:

• Positive Engagement: It ensures a rigid connection, making it suitable for low-speed, high-torque applications.
• Engagement Limitation: Engagement is generally only possible when the shafts are stationary or rotating at very low, synchronized speeds, as the impact of the jaws at high speeds can cause mechanical failure or damage.
• Design Variations: Jaws can be square-cut for bidirectional drive or spiral/ramp-cut for unidirectional drive, where the clutch automatically disengages if the torque exceeds a certain limit or if the direction of rotation reverses.

Sketch Description: Imagine two circular discs facing each other. The left disc (driving) has several protruding rectangular teeth (jaws) spaced evenly around its face. The right disc (driven) has corresponding slots or recesses that match the shape of the teeth. A shifting collar is shown on the driven shaft, which allows the right disc to slide axially to lock into the left disc, creating a solid mechanical coupling.