scoop coupling

Prepared by

Jishnu V

Engineer, BHEL ISG

1. WORKING PRINCIPLE OF A SCOOP COUPLING Scoop coupling consists of two power transmitting

elements called Impeller and Runner, both having a large number of straight radial vanes

In a conveyor system the prime mover (motor) drives the impeller. The runner is connected to the driven machine or load (which is pulley via gearbox)

In running condition, the working circuit of the scoop coupling is filled with the oil and the impeller acts as a centrifugal pump, creating oil flow radiallyoutward, which crosses the gap to the runner, which acts as a turbine

1. WORKING PRINCIPLE OF A SCOOP COUPLING The oil flow gives up power as it return to the impeller

again and thus the cycle is repeated

The input side of the scoop coupling connected to the motor is known as the pump side and the output side connected to the load is known as the turbine side

Hence while running the power is transmitted from the motor to driven machine through the rotating forced vortex of oil

As the fluid flow from impeller to runner there is slip between these two parts.

1. WORKING PRINCIPLE OF A SCOOP COUPLING Mathematically Slip (S)= ωT –ωP

Where ωT is the angular velocity of runner and ωP is the angular velocity of impeller.

Efficiency η= 1-S

Due to the slip there is an energy loss which is converted to the form of heat energy resulting in heating of oil, which requires external cooling system for oil

The oil pump (gear pump) continuously supplies oil to the coupling.

1. WORKING PRINCIPLE OF A SCOOP COUPLING The output speed of coupling can be changed by varying the slip,

which depends upon the oil quantity maintained in the working circuit by changing the position of the scoop tube.

2.0 SPEED CONTROL OF A SCOOP COUPLING

If the scoop tube is moved into the rotating ring of oil, it progressively removes the oil from the chamber and working circuit / blocks the path of fluid flow and discharges it into the sump.

Inversely, moving the scoop tube from the oil ring allows retaining more oil in the working circuit

The scoop tube position can be changed from 0% to 100% to regulate the output speed of the coupling.

In 0% position, the scoop tube is at FULL IN position and does not allow oil level to build up in the coupling.


At this position of scoop tube (0%), there is maximum slip and minimum torque transmission.

In 100% position, the scoop tube is at FULL OUT position at which maximum oil is in the working circuit.

This (FULL OUT) gives maximum torque transmission at minimum slip

For intermediate position of the scoop tube, intermediate value of slip and torque transmission is obtained.


A) No drive: (output shaft is stationary or scoop is in full in position) Tip of the scoop tube is extended to a radius slightly more than that of the working circuit scoop trims all the oil from rotating members and returns it to the sump for recirculation, the output shaft is stationary.

B) Intermediate speed: Scoop tip is withdrawn to an intermediate radius. The working circuit and scoop casing are partially filled to the level determined by scoop tip. Output shaft and driven machine are running at reduced speed

3.0 OIL CIRCUIT

3.0 OIL CIRCUIT

The oil pump delivers oil from the sump (oil tank) to the oil cooler (radiator type or shell and tube heat exchanger type) through oil filter.

From oil cooler oil goes into working circuit of the Scoop Coupling. A small lube line is taken out from the main oil line that provides oil to bearing.

After transmitting power, the scoop tube collects oil from working circuit and delivers back to the oil tank. Also, some splashed oil in the coupling is returned to the oil tank.

3.0 OIL CIRCUIT

The pressure switch and temperature switch give an electrical trip signal to the control panel in case of oil flow failure and temperature rise respectively.

The manually operated butterfly valve (bypass valve) allows either oil to flow into the working circuit or totally bypasses back to the oil tank.

When the fluid coupling is at rest, the oil is well below the openings in the box through which the shafts pass

When the scoop coupling starts operating, the oil pump raises the oil from the sump and delivers it through the heat exchanger

3.0 OIL CIRCUIT

The oil flows through the scoop housing into the working circuit via drilled ports of the impeller. When the coupling is running, the oil is escaping from the working chamber into the scoop chamber via drilled ports. The tip of the adjustable scoop, sliding radiallyin the scoop housing, has its open tip projecting into the scoop chamber. The movement of this scoop is controlled externally and the radial position of the tip determines the depth of oil permitted to remain in the scoop chamber, and hence in the working circuit.

3.0 OIL CIRCUIT

The oil trimmed out of the scoop chamber by the scoop tube returned to the sump for re-circulation.

The speed out output shaft is determined by the volume of oil in the working circuit and is controlled by the depth of ring of oil in the scoop chamber and regulated by the position of tip of scoop tube

Full in of scoop Depth of oil ring decreases Less oil in working circuit

Retracting of scoop Depth of oil ring increases More oil in working circuit

3.0 OIL CIRCUIT

4.0 MATERIAL OF CONSTRUCTION

scoop coupling

Engineering