Question

With reference to lifeboats, describe with the aid of sketches:

(a) The handbrake used for lowering and state how it may be tested

(b) The centrifugal brake and state the method of testing

(c) (i) A roller ratchet and state the method of testings

(ii) Explain why the ratchet is incorporated into the system.

Answer 1

Part (a)

The illustrated handbrake mechanism utilizes a dead man's handle to control a brake band engaging a drum attached to the lifeboat lowering system. In the normal resting position (as shown in the sketch), the weight of the dead man's handle keeps the brake band taut, preventing the lifeboat from lowering. To lower the lifeboat, the operator must actively lift and hold the dead man's handle, releasing the brake band. The lifeboat then descends under its own weight.

Testing of Handbrake:

Static Test: A load of 1.5 times the maximum working load of the lifeboat is applied. The lifeboat is then raised 50-100 mm above its resting position and the brake engaged. The brake must securely hold the lifeboat in this position.

Dynamic Test: A load of 1.1 times the maximum working load is applied to the lifeboat. The lifeboat is lowered at maximum speed, and the brake is then applied. The total distance travelled by the lifeboat after brake application must not exceed 1 meter.

Part (b)

The centrifugal brake, illustrated below, regulates the speed of the lifeboat's descent, limiting it to a maximum of 36 m/min. It operates using the principle of centrifugal force. As the lifeboat is lowered, a rotating drum (connected to the lowering mechanism) spins. Attached to this drum are flyweights carrying brake pads. The centrifugal force generated by the spinning flyweights pushes the brake pads against a stationary drum, applying braking force. The braking force is proportional to the square of the rotational speed.

Adjusting the braking force involves altering the mass of the flyweights. Holes can be drilled into the flyweights to reduce mass (reducing braking force) or filled with plugs to increase mass (increasing braking force).

Testing of the Centrifugal Brake involves a dynamic test only: A proof load of at least 1.1 times the maximum working load of the lifeboat is applied. The lifeboat is lowered. The lowering speed must not exceed 36 m/min throughout the descent.

Part (c)

A roller ratchet in the lifeboat hoisting mechanism is a unidirectional clutch. It functions as a safety device, preventing the outer gear of the hoisting mechanism from rotating in the reverse direction should the electric power supply fail. This feature avoids the uncontrolled lowering of the lifeboat. Specifically, when the hoisting shaft attempts to rotate counter-clockwise (due to gravity or other forces after power loss), the pawl within the ratchet mechanism is pressed against its casing due to friction. This frictional engagement prevents the ratchet, and consequently the wire drum and lifeboat, from rotating backwards.

Testing of Roller Ratchet:

Operate the lifeboat hoisting mechanism using the motor.
Disconnect the electric supply while hoisting the lifeboat.
Observe if the roller ratchet successfully prevents any reverse movement of the lifeboat or unwinding of the drum.
Verify that the ratchet holds the lifeboat securely at the position where power was lost.

Part (d)

Purpose of Ratchet in the System

Prevent Reverse Motion: Ensure the lifeboat does not lower back unintentionally when hoisting operations are interrupted due to power failure.
Enhance Safety: Act as a fail-safe mechanism to secure the lifeboat in its position, preventing sudden descent that could result in injury or damage.
Unidirectional Control: Function as a unidirectional clutch, allowing only controlled motion during hoisting while locking movement in the opposite direction.

✓ Verified Model Answer (Text Solution)

Purpose of Ratchet in the System