Power Transmission

POWER TRANSMISSION A transmission system is a system that transmits mechanical power from a prime mover—such as an engine or electric motor—to some form of useful output device, normally rotary in form, and generally at a reduced rate of angular speed but at a higher motive torque. Transmission systems are classified based on a) Distance between driving system and driven system b) Speed c) Power

POWER TRANSMISSION SYSTEMS

Belt drives

Rope drives

Chain drives

Gear drives

Belt Drives - Employed whenever power or rotary motion is to be transmitted between two parallel shafts. Low cost. Driven pulley

Driving pulley

Portion of belt having less tension – slack side Portion of belt with higher tension - tight side Their difference in tension causes rotation of driven pulley For clockwise rotation-lower side of belt-tight side,upper side will be slack side and vice versa:

OPEN BELT DRIVE Employed when two parallel shafts have to rotate in the same direction

CROSSED BELT DRIVE Employed when two parallel shafts have to rotate in opposite direction. Shafts are placed at a maximum distance from each other and operated at low speeds to avoid excessive wear.

LENGTH OF BELT OPEN SYSTEM

L =3.14*(r1+r2) + (r1-r2)^2/X+2*X Where L= length of pulley r1= radius of larger pulley r2= radius of smaller pulley X= distance between centres of two pulleys

Crossed system

L = 3.14*(r1+r2) + (r1+r2)^2/X + 2X

Where L= length of pulley r1= radius of larger pulley r2= radius of smaller pulley X= distance between centres of two pulleys

VELOCITY RATIO OF BELT DRIVES It is the ratio of the speed of the driven pulley to the speed of the driving pulley. Diameter d2,

Diameter d1,

speed N2

speed N1

V.R = N2/N1 or d1/d2

EFFECT OF THICKNESS OF BELT ON V.R When thickness of belt drive is considered V.R = N2/N1 or d1+t/d2+T

POWER TRANSMITTED BY A BELT DRIVE POWER= (T1-T2)*V/(60*1000) KW

Where V is velocity of belt in m/min T1 and T2 are tensions on tight and slack sides respectively.

Advantages of belt drive are: They are simple. They are economical. Parallel shafts are not required.. Noise and vibration are damped out. Machinery life is prolonged because load fluctuations are cushioned (shock-absorbed). They are lubrication-free. They require only low maintenance. They are highly efficient (90–98%, usually 95%).. They are very economical when shafts are separated by large distances.. Disadvantages include: The angular-velocity ratio is not necessarily constant .Heat buildup occurs. Speed is limited to usually 35 meters per second. Power transmission is limited to 500 horsepower. Operating temperatures are usually restricted to –35 to 85°C. Some adjustment of center distance or use of an idler pulley is necessary for wear and stretch compensation. A means of disassembly must be provided to install endless belts

CHAIN DRIVES Used for small centre distances( generally 3 m) Can transmit high power ( upto 100 KW) Velocity ratio can be as high 8:1 they are positive non slip drives Used in bicycles,motor cycles,rolling mills,agricultral machinaries,machine tools etc.

Chiefly consist of two parts- a chain and two sprockets Sprockets are teethed wheels over which an endless chain is fitted. Two types of chains used are roller chains and silent chains. Roller chains-commonly used chain Silent chain-inverted tooth chain extensively used for smooth and noiseless operations at high velocities

Velocity ratio V.R = N2/N1 = T1/T2

Where T1 and T2 are number of teeth on driving and driven sprockets respectively

Disadvantages 1)Perfect alignment between shafts required 2) Requires more lubrication 3) High initial cost 4)More velocity fluctuations 5)Cannot be used for operations having periodic reversals in intervals