Power is transmitted from source
to the place where it is required in many ways. Principle types are explained
below.
(a) Belt drive
(b) Chain drive
(c) Rope drive
(d) Gear drive
Belt Drive
When power is to be transmitted
from one shaft to another shaft belt drive is more commonly used. The power is
transmitted form one pulley to the other by the frictional resistance between
the belt and the pulley. In practice, however, certain amount of slip takes
place in this method of drive and little creep too develops. Hence the drive is
not positive due to the slippage of belt.
Since belts are flexible they are
not only used to connect parallel shafts but also shafts located in more than
one plane may be connected by twisting the belt.
Two pulleys of either the same
size or of different size may be connected, as to give an open drive or crossed
drive. In the open belt drive the direction of
rotation of both the pulleys will be the same, whereas in the cross belt drive,
the direction of the driven pulley or wheel is reversed, and the arc of contact
between the belt and the pulley is increased.
When two pulleys are coupled by a
belt, pulley mounted on the shaft of an electric motor is called the driver, and
the pulley to which it is belted is called the driven. If both the driver and
driven is of the same size or diameter, both will rotate at the same speed or
revolutions per minute. If the driver pulley is twice the size of the driven
(DN) pulley then the smaller will rotate at twice the RPM of the larger (DR).
The general rule therefore is smaller the pulley the faster it rotates, and
larger the pulley the slower it rotates.
Considering the open belt drive
when the driver begins to rotate, the tension in the lower portion of the belt
is increased and therefore that portion of the belt stretches. At the same time
there is a decrease in tension to the top side. The lower side is called the
tight side (T1) and the top side is called the slack side (T2). The tensions of
both the sides are unequal. This difference in tension coupled with force of
friction makes the belt drive possible.
The centre distance between the
pulleys should not be less than three times the larger diameter pulley and ratio
of pulley diameter should not normally exceed six to one. If condition is such
that this ratio has to be exceeded, a Jockey pulley should be fitted to
increase the arc of contact on the smaller pulley.
Material of Belts: The belts are made out of the following
materials:
(a) Leather. Generally leather belts are of single thickness, but two
or more belts may be cemented or stitched together. The thickness of the single
belts differs from 7/32” to ¼” and the double belts from 11/32” to 3/8”. Leather belt should be kept separate and free from cracks by applying
some form of dressing. Tallow is the best for this purpose. If belt slips, to
increase the friction between the belt and pulley, powdered resin may be
applied.
(b) Cotton and Canvas. These belts are woven and folded longitudinally
to form the plies which are stuck together with cement or rubber solution.
These belts are made in various thicknesses. The strength of these belts is
about the same as that of leather belts. Greasy dressing should never be used
to woven belts.
(c) Rubber. These are useful in damp places and out of doors. These are
not very reliable in comparison with the cotton and canvas belts because the
presence of moisture causes the plies to come off.
Joining of Belts: The following are the methods of
joining the belts:
(a) Lacing
(b) Cementing (by means of glue
or paste)
(c) Fastening
Types of Belts Fasteners:
(a) Bats. Bats belt fasteners are
used for medium size belts.
(b) Jackson. Jackson belt
fasteners are used when driving heavy drive on commercial purpose.
(c) Clipper. Clipper used for
belts of small width.
Types of Belts as per Shape:
(a) Flat Belt. These are usually made out of leather, canvas, cotton and rubber. These belts are
used to connect shafts up to 8–10 m apart with speed as high as 22 m/s.
(b) V’ belt. These are made of
rubber impregnated fabric with angle of ‘V’ between 280 to 400. These are used
to connect shafts up to 4 m apart.
Types of Pulleys. The following are the various types of pulleys:
(a) Split Pulley. This is used on main shaft from where the drive is
taken to the respective machines. In order to mount these pulleys anywhere on
main shaft easily, these are made in two half and rigidly bolted together.
These pulleys are made out of cast iron or sometimes out of wood also for lightness.
(b) Fast and Loose Pulleys. These pulleys are fitted on the counter
shaft and are made out of cast iron. The fast or tight pulley is keyed to the
shaft where as the loose pulley which is fitted with a bush in its bore is free
to rotate on the shaft. The bush fitted in the bore of the loose pulley if worn
out may be replaced.
(c) Flat Pulley. These pulleys may be of single flat type or may be of
cone pulley type in which three or four different diameters are made in order
to obtain the change in speeds. These pulleys are not actually flat.
They are tapered slightly towards
the centre from either side so that the diameter of the pulley is a little
larger at the centre. This is called “Crown”. Some pulleys may have convex
shape periphery. The reason, why the flat pulley is made slightly larger in the
centre is that a flat belt will always climb towards the highest part of the
pulley. So if the centre is slightly higher, the belt will run and stay in the
centre of the pulley, which is most desirable condition.
d) “V” pulleys. Grooved pulleys for V–belts are generally made
entirely of cast iron, but occasionally the arms are made of wrought iron. They
are mainly used for the transmission of large powers over great distances. The
effect of the groove is to increase the frictional grip of the belts on the
pulley and thus reduces the tendency to slip. The grooves on the pulleys are
V-shaped. The angle between the two faces being from 280 to 400. The belt rests
on the two sides and not on the bottom of the groove.
(e) Guide pulley (Jockey pulley). Guide pulleys are mainly used to
adjust the tension
of the belt
and also for
right angle drive. A single endless belt may connect the shafts which
are not parallel and which may not intersect, if guide pulleys are used. The
belt will run equally well in both direction, and the guide pulleys do not
affect the velocity ratio between the driver and driven pulleys. Guide pulleys
are also used when the shafts to be connected are very close. By properly
placing guide pulleys it is possible to make the belt run equally in either
direction. It gives more tension to the belt.
Belt Creep. The word creep indicates the lateral shifting movement
of the belt. If the driver and driven pulleys are not parallel, the belt is
likely to move or creep towards the lower side of the pulley. This action of
the belt coming out of pulley is called the creep. It might also occur if the
surface of the pulley is worn out considerably.
Hints on Belt Drive
(a) A comfortable working system
for a single belt is about 50 lbs per inch of breadth and for double belt 80lbs
per every inch.
(b) A belt running with a soft
flesh side makes the pulley wear less.
(c) When a pulley has an
appearance of polished silver then belt may slip. To avoid this, apply belt
grease.
(d) The driving side of the belt
should be kept moist (sticking).
(e) When a belt gets saturated
with the oil, apply chalk powder to absorb oil.
(f) Maximum speed in belt drive
should not exceed 4000 ft/min.
Rope Drive
The ropes are often used instead
of belt for transmitting power especially when the shafts are at long distance
apart. Cotton ropes are based an account of the flexibility but manila and hemp
ropes are also used as they are less flexible than cotton ropes.
The use of ropes necessitates
grooves on pulleys to increase frictional grip on rope and thus reduce the
possibility of rope slipping. When large amount of power is transmitted to a
greater distance wire ropes are used. The bottom of groove in this case
consists of wood, composite wood and leather etc. The wire ropes make contact
only at the sides of the groove and not at the bottom.
Chain Drive
There is no slip and driving
efficiency is high. The loss of power in a well designed chain drive does not
exceed 2%. It occupies little space and gives efficient service for the drives
where the centre distance of two shafts is too small for belt drive and too
large for gear drive. There are several types of chains used for transmitting
purpose.
(a) The inverted tooth type for
high speed
(b) Bush or roller type for low and
medium speed.
The chief drawback of the use of
chain is stretching of links, which takes place after sometimes due to wear of
pins and rollers.
Gear Drive
Gear drive is used when power is
to be transmitted positively (i.e. without any slip taking place in the drive)
and a constant velocity ratio between two shafts is to be maintained. The speed
ratio between two shafts connected by means of two gears (spur) depends upon
the number of teeth cut in both the gears. Normally smaller gear turns the
bigger. Suppose a 15 teeth gear is driving a gear of 60 teeth. The smaller gear has to rotate 4 times to
cause the bigger to rotate once. Therefore RPM of shaft will be inversely
proportional to the number of teeth in the gear.
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