Dept of Mechanical Engg. CHAPTER -2 POWER TRANSMISSION By Vasanthkumar.ch M tech (Robotics) Dept of Mechanical Engg.

Power transmission (rotational power) Belt drives Chain drives Gear drives

BELT DRIVE A belt drive is a method of transferring rotary motion between two shafts(attached with pulleys). It is a looped strip of flexible material, used to mechanically link two or more rotating shafts. Belt drives may be used as a source of motion, to efficiently transmit power, or to track relative movement. Generally belt drives are friction drives.

Simple belt drive system

Application Where the rotational speeds are different Distance between the shafts is high Motion or power that needs to be transmitted to more number of applications

Types of belts Flat belt V belt Round belt Timing belt

Pictorial view of belts Flat belt

Round belt

V belt

Timing or grooved belt

Pictorial view

Pictorial view

Materials used for belt drives Oak tanned leather – fairly stiff Chrome leather belt – for oil and steam environment Fabric belts - (Canvas or cotton duck) Rubber belts – layers of fabric impregnated with rubber or vulcanized rubber

Types of Belt drives Open belt drive Crossed belt drive

Open belt drive

Crossed belt drive

Terminology Length of the belt Velocity ratio Slip Tight side and slack side Angle of contact Ratio between the belt tensions Power transmitted

Ratios b/w belt tension and Power R= T1/T2 =eµθ T1 = Tension in the tight side T2 = Tension on the slack side µ= coefficient of friction θ= angle of contact Power transmitted P = (T1-T2 )V V= velocity of the belt V= πd1 * N/60 m/s

Merits of belt drive mechanism They are simple. They are economical. Parallel shafts are not required. Overload and jam protection are provided. 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%). Some misalignment is tolerable. They are very economical when shafts are separated by large distances.

Demerits of belt drive Non compact Constant velocity cannot be obtained Slippage Not applicable for heavy loads

Real time applications Lathe , drilling and sewing machines Compression systems Automobiles Water systems Power generation units Air conditioning systems

Example An engine running at 300 rpm drives a line shaft by means of belt drive. The engine pulley is 600mm in diameter and the pulley on the shaft is 400mm in diameter. Determine the speed of the line shaft.(assume no slip).

Example Following details of the cross and open belt drive Diameter of the driver= 300mm Diameter of the follower = 600mm Center distance of the drive is =3mts Speed of the drive is = 500rpm Angle of contact = 195.6 ° Determine the length of the belt required for both of the drive systems

Example For the example in the previous slide the tension on the tight side is 1.3KN and the coefficient of friction between the pulley is 0.25 find the power capacity of the drive.

CHAIN DRIVES

Animated view

Applications Where slippage needs to be reduced to a considerable amount Initial torque developed is more Continuous drive systems Smaller center distance Agro machinery , automobiles,cranes

Gear systems Machine elements that transmit the motion and power between the rotating shafts by means of successively engaging teeth . Compact than the other drive systems More accurate power transmission Less slippage or little backlash

Common forms of gear configuration Gears for connecting parallel shafts Gears for connecting intersecting shafts Gears for connecting neither parallel nor intersecting shafts

Gear types for connecting parallel shafts Spur gears (internal and external) Parallel helical gears Rack and pinion arrangement

Spur gear

Helical gear

Herring bone gear

Rack and pinion

Gears for connecting interesting shafts Straight bevel gear

Neither interesting nor parallel

Nomenclature

Terminologies Pitch circle Tooth space Addendum circle Backlash Root circle Circular pitch Addendum Diametral pitch Dedundum Module Clearance Face of the tooth Flank of tooth Tooth space Tooth thickness or circular thickness

Velocity ratio of the Gear Drive Angular speeds of two gears ω1 = 2π N1 ω2 = 2π N2. Peripheral velocity of the Driver gear Vp = ω1 d1 /N1 = π d1N1 = ω2d2 /N2 = π d2N2 Velocity ratio = n= ω1/ ω2=N1/N2

Gear trains A combination of two or more gears for transmission of energy. Size /number of teeth makes a speed change(reduction or increment). Preferred when large speed change is required in a compact space.

Types of gear trains Simple gear train Compound gear train Planetary gear train

Simple gear train

Compound gear train

Velocity ratio of Gear trains ω1 = angular velocity of Gear 1 ω2 = angular velocity of Gear 2 ω3 = angular velocity of Gear 3 N1= speed of gear 1 N2 = speed of gear 2 N3 = speed of gear 3 T1= teeth of gear 1 T2= teeth of gear 2 T3= teeth of gear 3 ω1/ ω2= N1/N2 = T2/T1; ω2/ ω3 = N2/N3 =T3/T2 ω1/ ω3= ω1/ ω2 * ω2/ ω3 = N1/N2* N2/N3 = T2/T1* T3/T2

Planetary gear train Also termed a epicyclical gear

Gears in real time application Automobiles Machine systems Pumping systems Machine tools Timing and related equipments

Merits and Demerits Merits: Compactness Greater speed amplification and reduction of speeds Less slip Direct contact with the driver

Merits and Demerits Demerits Wear and tear Required coolant for reduction of heat developed Back lash Noise and vibration