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Power Transmission Through Belts
INTRODUCTIONA belt is a loop of flexible material used to mechanically link two or more rotating shafts, most often parallel.
Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement.
Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel.
ABSTRACT We are trying to understand the working of belt drives, their different components, the forces involved and how are they transferred, to create a device of our own, innovating the current belt drive system and developing our own system based on concepts of belt drive.
Why Used ? Transfer power (torque) from one location to another.
From driver: motor, peddles, engine, windmill, turbine to driven: conveyor belt, back wheels, generator rock crusher, dryer.
Used to span large distances or need flexible x-mission elements. Gear drives have a higher torque capability but not flexible or cheap.
Often used as torque increaser (speed reducer)
Advantages of belt driveThey 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.
Disadvantages of belt driveThe angular-velocity ratio is not necessarily constant or equal to the ratio of pulley diameters, ,because of belt slip and stretch.
Heat buildup occurs. Speed is limited to usually 7000 feet per minute (35 meters per second). POperating temperatures are usually restricted to –31 to 185°F (–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.
Type of Belt Drive
Type of belt drive
FLAT BELT DRIVEThey comprise of a simple flexible belt running between
the pulleys.
This type of belts are limited to transmission of less energy because of their tendency to slip.
They might also have problems due to wrong alignment of belts.
Picture Source : Google Images
Round Belt DriveThey are just like ropes stretched between two pulleys.
They are very strong and are used in factories to transmit huge amount of power.
They are used to transfer power across large distances
Picture Source : Google Images
V-BELTSThey have a trapezoidal “V” shape and track in a mating groove in the pulley.
The “V” shape of the belt solves the slipping and alignment problems.
They provide the best combination of Traction, Speed and Movement.
They need narrow but deeper grooves on the pulley for their functioning.
They can be used for transmission of large amount of power due to their strength.
They tend to wedge into grooves as the load increases.
TIMING BELTS
These belts have tooth to have better grip on the pulley.
They runs on a matching toothed pulley. They have no slippage.
They can be used to transfer high amount of energy.
Due to their non-slipping mechanism they are used in Engines of vehicles and in turbochargers.
They are also used to transfer motion and timing purposes.
Picture Source : Google Images
ARRANGEMENT OF BELT DRIVE
1 •Open Belt Drive
2 •Closed Belt Drive
Open Belt DriveIn this type of arrangement the belt is put up on both the pulleys straightly without any twist or turns.
This makes the driven pulley rotate in the same direction as that of the driver pulley.
This method is good for delivering more power as the belts rupture very less when compared to other methods of arrangement.
Picture Source : Google Images
Closed Belt DriveIn this type of arrangement the belt is given a turn as it runs between the pulleys.
The driver pulley and the driven pulley rotates in the opposite direction while in this type of arrangement.
It cannot be used for delivering much power as the belt rubs against itself and gets ruptured very fast in the process.
Length of the Belt It is always desirable to know the length of Belt drive. It will be required to know the length of belt required. The actual length is slightly shorter than the theoretic values.
The length can be determined by the geometric considerations
OPEN BELT DRIVE
Nomenclature dL - Diameter of the larger pulley
dS – Diameter of the smaller pulley
αL- Angle of wrap of the larger pulley
αS – Angle of wrap of the smaller pulley
C- Center distance between the two pulleys
Basic Formulae
αL = 180ο + 2β
αS = 180ο - 2β
Where angle β is,
β
L0 = Length of open belt
R ++2C
This formulae may be verified by simple geometry.
CLOSED BELT DRIVE Basic Formulae
aL=aS=180o + 2β
Where β is
β
Lc = Length of close belt R ++2C
Tension ratio𝑇2−𝑚𝑉 2
(𝑇 1−𝑚𝑉 2 )=𝑒𝜇𝛼
If we neglect the mass of pulleys then
𝑇2
(𝑇 1 )=𝑒𝜇𝛼
Power Transmitted by Belt Drive Let T1 be the tension on the tight side in ‘N’
T2 be the tension on the slack side in ‘N’, and
V be the speed of the belt in m/sec.
The effective turning (driving) force at the circumference of the driven pulley or follower is the difference between the two tensions (i.e. T1 – T2).
P=( T1 – T2)V Watt
Relationship between Wheel Speed Let
and be the tension in the slack side and tight side of the belt, respectively.
and be the corresponding speeds of driving and driven pulley, respectively.
And be the radius of driving and driven pulley, respectively.
V be the velocity of the belt in m/s passing over the driver.
and be the respective diameter.
Referencewww.wikipidea.org
www.nptel.ac.in
www.srmuniv.ac
www.engineeringtoolbox.com
Engineering Mechanics – Irving H. Shames
Bhandari, V.B., “Design of Machine Elements”, Tata McGraw-Hill Publishing Company Ltd., 1994.
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