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The power is transmitted from one shaft to the another shaft by a belt, chain and gear drives. The belts and ropes are used where distance between the two shafts is large. The chains are used for intermediate distances. The gears are used for shorter distance between the shafts. Gear drive is a positive drive because there is no slip. Belts(or ropes) transmit power because of friction between the belt (or rope) and the pulley. Due to slip and creep in belts, this drive is not a positive drive. Thus it finds limited applications.
Belt drive consists of
(i) Driver and driven pulleys
(ii) Motor and machine shafts
(iii) Two keys
(iv) One belt
TYPES OF BELT DRIVE
Open Belt Drive
Cross Belt Drive
Compound Belt Drive (3 shafts and two belts, four shafts and three belts)
Quarter turn belt drive
Definition of a belt
Belt is in the form of a loop. It connects mechanically two shafts for transmitting power smoothly. A belt drive consists of shafts, pulleys and a belt.
Materials Of Belts
Standard Thicknesses of belts
5, 6.5, 8, 10 and 12 mm are the standard thicknesses.
Standard widths of flat belts
25, 32, 40, 50, 63, 71, 80, 90, 100, 112,
125, 140, 160, 180, 200, 224, 250, 280,
315, 400, 450, 500, 560 and 600 mm
Types Of Flat Belts
Based on Orientation
Based on Load or duty or power to be transmitted
DUTY or LOAD
7.5 to 15 kW
> 15 kW
Important terminology for belts
(i) Speed ratio
Neglecting the slip and thickness of the belt.
Speed ratio= diameter of bigger pulley/diameter of smaller pulley
Hence Speed ratio = N1/N2
Where N1 is the motor or engine speed OR higher speed
N2 is the machine speed or lower speed
(ii) Center distance
(iii) Peripheral velocity
LAW OF BELTING
The center line of belt as it approaches the pulley must coincide with the central plane of that pulley. Otherwise belt will fly away from the pulley.
TYPES OF PULLEYS USED WITH BELTS
There are two types of pulleys used with belts.
(I) Flat pulleys——–Used with flat belts.
(ii) Grooved pulleys—–Used with V-belts. Grooved pulleys are called sheaves.
Belt Drive and its Types
A belt drive consists of two shafts, two pulleys and a belt. One of these shaft is a motor shaft on which electric motor is mounted. On the other shaft the machine is mounted to which power is transmitted by the belt drive. Belt drive is used to transmit power from the motor to the machine shaft. It can be from the engine to the machine shaft. This can also be from turbine to generator.
Motor speed and Pulleys sizes
Normally speed of the motor is high because high speed motors are more efficient. Therefore, motor shaft is the driving shaft and the machine shaft is the driven shaft. Thus, in most of the cases the machine has Lesser RPM than that of the motor. Hence, pulley on motor will be smaller pulley and the pulley on the machine shaft will be a larger pulley. Belts do not transmit with 100 % efficiency because of SLIP and stretching of the belt. There are two types of belt drive, namely flat belt drive and V-belt drive. The flat belt drive is of further two types, Open and cross belt drives.
Open Belt Drive
Both drive and driven shafts run in the same direction in case of an open belt drive. For smooth power transmission, belt on one side is more tight than the other side. In a horizontal drive, tightened side is always kept in the lower side of two pulleys. The sag of the upper side slightly increases the angle of contact of the belt on the two pulleys. More angle of contact means more power transmission.
Cross Belt Drive
Rope Belt Drive
Advantages of a belt drive
Belt drive is a simple drive.
It is cheap.
No lubrication required.
Have high efficiency.
Requires less maintenance.
Some misalignment is adjustable without loss in efficiency.
Durable and has long life.
Can be used for parallel and non parallel shafts.
Easy to reduce vibrations and noise.
Disadvantages of a belt drive
Velocity ratio is not truly constant because of slip and stretching.
Heating occurs due to friction. It disturbs its perfect working at higher temperatures.
There is a speed limit of 35 m/s.
Requires center distance adjustment due to stretching.
PRACTICAL APPLICATIONS OF A FLAT BELT DRIVE
(ii) Water pumps
(iv) Saw mills
(v) Electrical generators
PRACTICAL APPLICATIONS OF A V- BELT DRIVE
(i) Stone crushers
(ii) Machine tools
(iii) Paper industry
(iv) Textile industry
(v) High power mills
(vi) Refrigeration and air conditioning machinery
ANALYSIS OF FLAT BELT DRIVE
OPEN FLAT BELT DRIVE
When centrifugal tension is neglected
T1 / T2 = eµθ
T1 Tight side tension in Newton (N)
T2 Slack side tension in Newton (N)
µ = Coefficient of friction between the pulley and belt materials
θ = Angle of contact=angle of lap on the SMALLER PULLEY
Initial tension = Ti = (T1 + T2)/2
Power transmitted = (T1 –T2) v Watts (W)
v is the linear velocity of the belt in meters
Hence v =πD N/60
Where N is RPM and D is the diameter of the pulley in meters
WHEN CENTRIFUGAL TENSION IS CONSIDERED
Tc = m v2
Where m is mass of belt PER UNIT LENGTH, kg/m
Initial tension = Ti = ((T1 + T2)/2 + Tc)
Firstly Ttight = T1 + Tc
Secondly Tslack = T2 + Tc
Thirdly Ttight / Tslack = eµθ
Power transmitted = (Ttight — Tslack) v Watt
CONDITION FOR MAXIMUM POWER WITH CENTRIFUGAL TENSION (Tc)
Firstly Tmax = 3 m v2
Secondly T1 = (2/3) Tmax
Thirdly T2 = (1/3) Tmax
V = (Tmax/3 m) 0.5
Max Power = (2/3) Tmax (1 — eµθ)( (Tmax/3 m)0.5) Watts