Introduction to Power Transmission

Power has got a wide field in any type of engineering. It is available in various forms and can be had through natural or artificial resources. We are concerned with electrical and mechanical forms of energy for the purpose of Machine Shop.

As it is described above, energy is supplied in its mechanical and electrical forms but the latter is mostly used in modern machine shops. The sources of mechanical energy are chiefly internal and external combustion engines e.g. petrol engines and diesel engines. The sources of electrical powers are hydro-electric projects and various thermoelectric plants. The form of electrical energy is preferred due to its being readily available at various places.

To convert the electrical energy into mechanical energy it has to pass through an electrical machine named as electrical motor. We are to study how the power is transmitted from its readily available source to the required point.

Definition          

The technique of conveying the power from the point of source to the point where it is required is known as power transmission.

Elements of Power Transmission

Power is transmitted with the help of various elements. Important elements are mentioned as:

(a) Shafts

(b) Pulley

(c) Sprocket wheels

(d) Belts

(e) Ropes

(f) Chains

(g) Gears

Transmission of Power from Main Shaft to Machine

The elements described above are arranged in such a manner so as to transmit the power from one shaft to another. Generally the driver and driven shafts are co-axial to each other. There are methods of transmitting power from electric motor to machine. These are explained below.

Group or Common Drive

This system consists of a large electric motor which drives the overhead shaft called the main-shaft or line-shaft. The main-shaft drives the countershafts which further drive the main spindles of various machines. The main disadvantage of this system is that the line-shaft has to be driven even if a single machine is to be used. The only advantage of the system is that it is cheap and the overall cost per horse power is low compared to an individual drive system. This system consists of belts, ropes, pulleys, chains, etc. and is used for transmitting power for long distances.

Individual Drive

In this system every machine carries its own electric motor and starter. The motor is either connected to the machine spindle directly or through belts. Machines containing individual drives are also called self contained units. The individual drive system has largely replaced the group drive system in most modem workshops, because in the former each and every machine can be operated independently. This results in appreciable saving in the recurring consumption of electricity. Workshops using individual drive machines have a better get up due to the elimination of the large line overhead shaft and long crossing belts. While selecting the suitability of a process, many factors, such as initial investment, overall economy of the plant, and convenience in control and operation need to be considered.

Coupling of Shaft

Shafts used in power transmission may be cold rolled, cold drawn, forged or turned out of mild steel stock. They are supplied in standard lengths. To increase the length of shafts pieces are joined together by couplings. The two principal types of couplings used for joining shafts are rigid coupling and flexible or non-rigid coupling.