1. Design of Machine elements.
Belt Drives.
Prepared by :Ruchik Bhayani : 150050119009
Kaushik Bhuva : 150050119010
Guided by : Ankur sir.
2. INTRODUCTIONINTRODUCTION
1.1.A belt is a looped strip of flexible material, used to mechanically linkA belt is a looped strip of flexible material, used to mechanically link
two or more rotating shafts.two or more rotating shafts.
2.2.They may be used as a source of motion, to efficiently transmitThey may be used as a source of motion, to efficiently transmit
power, or to track relative movement. Belts are looped over pulleys.power, or to track relative movement. Belts are looped over pulleys.
3.3.In a two pulley system, the belt can either drive the pulleys in theIn a two pulley system, the belt can either drive the pulleys in the
same direction, or the belt may be crossed, so that the direction ofsame direction, or the belt may be crossed, so that the direction of
the shafts is opposite.the shafts is opposite.
3. Advantages of belt drive
•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.
4. • The angular-velocity ratio is not necessarily constant or equal toThe angular-velocity ratio is not necessarily constant or equal to
the ratio of pulley diameters, because of belt slip and stretch.the ratio of pulley diameters, because of belt slip and stretch.
• Heat buildup occurs. Speed is limited to usually 7000 feet perHeat buildup occurs. Speed is limited to usually 7000 feet per
minute (35 meters per second). Power transmission is limitedminute (35 meters per second). Power transmission is limited
to 370 kilowatts (500 horsepower).to 370 kilowatts (500 horsepower).
• Operating temperatures are usually restricted to –31 to 185°FOperating temperatures are usually restricted to –31 to 185°F
(–35 to 85°C).(–35 to 85°C).
• Some adjustment of center distance or use of an idler pulley isSome adjustment of center distance or use of an idler pulley is
necessary for wear and stretch compensation.necessary for wear and stretch compensation.
• A means of disassembly must be provided to install endlessA means of disassembly must be provided to install endless
belts.belts.
Disadvantages of belt drive
16. Velocity ratio of belt
it is the ratio between the velocities of the driver
and the follower or driven.
d1 = Diameter of the driver,
d2 = Diameter of the follower
N1 = Speed of the driver in r.p.m., and
N2 = Speed of the follower in r.p.m.
N2 / N1 = d1 + t / d2 + t
17. Velocity of belt drive considring slip
the motion of belts and shafts assuming a firm frictional grip between the
belts and the shafts. But sometimes, the frictional grip becomes insufficient.
This may cause some forward motion of the driver without carrying the belt
with it. This may also cause some forward motion of the belt without
carrying the driven pulley with it. This is called slip of the belt and is
generally expressed as a percentage.
The result of the belt slipping is to reduce the velocity ratio of the system. As
the slipping of the belt is a common phenomenon, thus the belt should never
be used where a definite velocity ratio is of importance (as in the case of
hour, minute and second arms in a watch).
Let s1 % = Slip between the driver and the belt, and
s2 % = Slip between the belt and the follower.
If thickness of belt is considred
N2/N1= d1+t/d2+t(1-s/100)
18. GEOMETRICAL RELATIONSHIPS FOR BELT DRIVES
Flat belts are used in open and crossed configurations.
The figures below show the geometry of open flat belt and crossed flat belt drives.
The open belt drive is used with shafts arranged parallel and rotating in the same
direction. The driver pulls the belt from one side and delivers it to the other side.
Thus the tension in the upper side will be more and on the lower side will be less.
The crossed belt drive is used with shafts arranged parallel and rotating in the
opposite direction. The driver pulls the belt from one side and delivers it to the other
side. Thus the tension in the upper side will be less and on the lower side will be
more.
20. In a flat belt pulley, the rim surface is given a convex shape by increasing
the thickness of a rim at the center. This increased thickness is called
crown and the process is known as crowning of pulley.
OBJECTIVES OF CROWNING OF PULLEY
i)In flat belt drive, if the two shaft are not exactly parallel, there is
tendency of belt to come off from the pulley in running condition. The
crowning prevents the coming off of the belt from the pulley.
ii)The crowning helps to keep the belt near the mid plane of the pulley in
running conditions.
CROWNING OF PULLEY
21. Creep In Belt
This motion of the belt relative to drive and driven pulley due to
unequal stretching of two sides of the drive is known as creep.
The effect of creep back on the driving pulley is to slow down the
speed of the belt with respect to driving pulley and the effect of
creep forward on the driven pulley is to slow down the speed of
the driven pulley with respect to the belt.
It is important to note that, the creep which is due to the elastic
property of the belt is totally different than the conventional slip,
which is due to insufficient frictional grip between the belt and
pulley.
22. Method of belt tensioning
In order to transmit the power, the belt must be provided with
the sufficient initial tension. The power transmitting capacity of
the belt drive also depends upon the initial tension.
Some of the method use to adjust the belt tensions are as follows:-
1. Manual adjustment
2. Pivoted overhung motor
3. Weighted idler pulley
