52.  State feature and classification of friction clutch?

Features of Friction clutch

- Structure is simple, weight and dimensions are small.

- Energy loss is small, transmission efficiency is high.

- Transition stage is short (unstable working period).

Classification

According to form of friction surface, the friction clutch used on the ships can be classified as follows:

- Disk friction clutch.

- Cone friction clutch.

- Air-actuated friction clutch.

53.  Describe structure and operational principle of dingler disk friction clutch?

1-      Actuating shaft

2-      Receiving shaft

3-      Actuating friction disk

4-      Receiving friction disk

5-      Brake lining (friction material)

In a disk friction clutch, the torque is transmitted by friction between surfaces of a disk couple maintained in contact by an axial thrust force.

The single-disk friction clutch consists of two-friction disks (3)- actuating disk and (4)- receiving disk, installed on the shafts (1) and (2). The actuating shaft (1) is connected to the crankshaft of the main engine. The receiving shaft (2) is connected to the propeller shaft.

When the main engine is working, the actuating shaft and the friction disk (3) are turned. Due to axial thrust force Q, the friction disk (4) is pressed into the friction disk (3). On the surfaces of the friction disk couple, a friction force is generated: T = mQ (m- friction coefficient). Therefore, torque of the main engine is transmitted from the actuating shaft (1) to the receiving shaft (2) and it turns the propeller shaft.

When the axial thrust force Q is stopped, the friction disk couple is released, and revolution of the receiving shaft will decrease and stop.

The working process of the friction clutch can be classified by three periods:

- Loading period: This is unstable working state. The friction force on the friction disk surface increases from 0 to maximum and revolution of the receiving shaft also gradually increases. In this period, revolution of the receiving shaft is always smaller in comparison with the actuating shaft because of relative slip of the friction disks.

- Stable working period: Revolution of the actuating shaft and the receiving shaft is the same; transmission torque is equal to friction moment of the friction clutch. When operating, a little power is lost so torque on the receiving shaft is little smaller in comparison with the actuating shaft. The friction clutch must be cooled to decrease temperature of the friction disk surfaces.

- Unloading period: This is also unstable working state of the friction clutch. Revolution of the receiving shaft is little by little decreased and then stopped.

54.  State the working process of the friction clutch?

The working process of the friction clutch can be classified by three periods:

- Loading period: This is unstable working state. The friction force on the friction disk surface increases from 0 to maximum and revolution of the receiving shaft also gradually increases. In this period, revolution of the receiving shaft is always smaller in comparison with the actuating shaft because of relative slip of the friction disks.

- Stable working period: Revolution of the actuating shaft and the receiving shaft is the same; transmission torque is equal to friction moment of the friction clutch. When operating, a little power is lost so torque on the receiving shaft is little smaller in comparison with the actuating shaft. The friction clutch must be cooled to decrease temperature of the friction disk surfaces.

- Unloading period: This is also unstable working state of the friction clutch. Revolution of the receiving shaft is little by little decreased and then stopped.

55.  State requirements and classify the material for friction clutch?

Material for manufacturing the friction clutch has to meet the following requirements:

- Abrasion resistance ability is high.

- Friction coefficient is high and not depends on temperature.

- Friction surfaces must not be stuck to each other at high temperature.

- Longevity must be high.

- It will not be deformed due to heat.

- Heat conductibility is high.

Main materials: Cast iron and steel (most commons), brass, asbestos, leather, and wood.

56.  Describe the structure and operational principle of cone friction clutch?

The cone friction clutch consists of friction cones (3) and (4), which are installed on shafts (1) and (2). The actuating shaft (1) is connected to the crankshaft; the receiving shaft is connected to the propeller shaft. The friction cone (3) is fixed on the shaft (1), the friction cone (4) can be freely slid along the shaft (2).

The cone friction clutch and the disk friction clutch have the same principle. In case of the cone friction clutch, average radius of the friction cone can be determined by the shaft diameter: Rtb = (3 ¸ 5)d.

When designing, the angle a of the friction cone surfaces must be suitably calculated so that the friction cone surfaces can be released when stopping the axial thrust force Q.

Features of the cone friction clutch

- In comparison with the same diameter disk friction clutch, the axial thrust force of the cone friction clutch is only equal to 2/3 when the same transmission torque.

- Dimensions of the friction cones are large so the cone friction clutch is unwieldy.

- In comparison with the multiple-disk friction clutch, the relative slip of the cone friction clutch is higher so it is less stable.

- It is complicated to manufacture.

- It must be accurately balanced when it has a large dimension and high speed.

58.  State about advantages, disadvantages and notice of air actuated friction clutch?

Advantages:

- When working, it can center itself.

- Transmission torque of the air-actuated friction clutch depends on pressure of the compressed air. When air pressure increased, the relative slip reduces then revolution of the receiving shaft will increase. However, centrifugal force also increases therefore transmission torque a little decrease. So transmission torque is not proportional with the air pressure. Suitable pressure is about 6 ¸ 10 (KG/cm2).

- Noise and vibration can be decreased

- It is complex in the structure.

Disadvantages: it is complex in the structure

Notices

- Suitable working range of temperature is about (- 30 ¸ 50)oC.

- It must be accurately balanced especially when working at high revolution.

- It must be replaced when friction surfaces worn excessively.

59.  Describe the structure and operational principle of a simple gear box?

A simple reduction gear consists of the sun gear (1), which is fixed on the crankshaft. It works as a driving gear. The internal gearing pinion (2) is fixed (fixed center gear), the planet pinion (3) has a mobile shaft and the connecting rod (planet carrier) (4) is connected to the propeller shaft.

In this gear-transmission, the planet pinion (3) takes part in two movements at the same time: It turns around the planet carrier (4) and moves relatively to the sun gear (1) and the internal gearing pinion (2). Transmission ratio of the reduction gear: i = n1/n2

Where:

- n1 revolution of the main engine.

- n2 revolution of the propeller.

60.  State concept critical revolution and establish the formula of it?

Shaft line on the ship can be considered as a continuous beam with many supports. At a certain range of revolution, the shaft vibrates strongly and this phenomenon is called "jumping up" of the shaft. Cause of this phenomenon is the shaft moving in clearance of the bearings. If the "jumping up" occurs in a long time, the bearing and shaft line will be damaged. The revolution at which cause "jumping up" is called the critical revolution.