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Belt drives
A belt is a flexible power transmission element that seats tightly on a set of pulleys or sheaves.When the belt is used for speed reduction, the typical case, the smaller sheave is mounted on the high-speed shaft, such as the shaft of an electric motor. The larger sheave is mounted on the driven machine. The belt is designed to ride around the two sheaves without slipping.
Law of Belting
The law of belting states that the center line of the belt when it approaches the pulley must lie in the mid-plane of that pulley which should be perpendicular to the axis of the pulley. Otherwise the belt will run off the pulley.
Centrifugal effect on belts
In operation as the belt passes over the pulley the centrifugal effect due to its self-weight tends to lift the belt from the pulley surface. This reduces the normal reaction and hence the frictional resistance. The centrifugal force produces an additional tension in the belt.
Belt Creep
A change in belt tension due to friction forces between the belt and pulley will cause the belt to elongate or contract and move relative to the surface of the pulley. This motion is called elastic creep.
Slip
When the belt moves forward without taking the belt with it is known as slip of a belt. Slip is type of loss occurring in the belt drive. Slip occurs when the frictional force between the belt and pulley is not sufficient due to which there is weaker grip between the belt and pulley.
Tight side and slack side
When the belt is transmitting power, friction causes the belt to grip the driving sheave, increasing the tension in one side, called the "tight side," of the drive. The tensile force in the belt exerts a tangential force on the driven sheave, and thus a torque is applied to the driven shaft. The opposite side of the belt is still under tension, but at a smaller value. Thus, it is called the "slack side."
Types of drive belt
Open Belt Drive
Quarter turn Belt
DriveCrossed Belt Drive
Advantages of Belt Drives
1. Belts permit flexibility ranging from high horsepower drives to slow speed and high speed drives.
2. Belt drives are less expensive than chain drives for low horsepower and low ratio applications.
3. Belts require no lubrication.
4. Single belt drives will accept more misalignment than chain drives.
5. Flat belts are best for extremely high speed drives.
6. Belt drives cushion shock loads and load fluctuations.
7. Belts will slip under overload conditions, preventing mechanical damage to shafts, keys, and other machine parts.
Disadvantages of Belt Drives
1. Belts cannot be used where exact timing or speed is required because slippage does occur (only timing belts can be used).
2. Belts are easily damaged by oil, grease, abrasives, some chemicals, and heat.
3. Belts can be noisy; also loose or worn belts can be a major cause of machinery vibration.
Idler Pulleys
A properly designed V-belt drive does not require an idler to deliver fully rated horsepower if proper belt tension and area of contact are maintained. Idlers put an additional bending stress on the belt, which reduces belt life. Also, the smaller the idler pulley, greater reduction in belt life.
The best location for an inside idler is on the slack side of the drive.
Belt rating
Flat-belts are made of different sizes such as 3 ply, 4 ply and V - belts are made of different grades such as A, B, C, D and E grade belts. Belt rating is defined as the power transmitting capacity of unit size flat belt or a particular grade single V belt.
Factors should be considered during the selection of a belt drive
1. Amount of power to be transmitted
2. Peripheral and angular speeds
3. Speed ratio
4. Efficiency
5. Centre distance between staffs
6. Space available
7. Working environment
Flat belts
The flat belt is the simplest type, often made from leather or rubber-coated fabric. The sheave surface is also flat and smooth, and the driving force is therefore limited by the pure friction between the belt and the sheave. Some designers prefer flat belts for delicate machinery because the belt will slip if the torque tends to rise to a level high enough to damage the machine.
The ply of belt
Flat belts are made of thin strips and laminated one over the other in order to get thick belt. These thin strips or sheets are called as plies of belt. Usually flat belts are made of 11 ply, 4 ply, 5 ply, 6 ply and 8 ply belt etc. And 4 ply belt is thicker than 3 ply belt and so-on.
Types of joints employed for joining flat-belts
1. Cemented joints
2. Laced joints
3. Crest joints
4. Hinged joints
Life of belts
The life of a belt is a function of the centre distance between the driver and driven shafts. The shorter the belt, the more often it will be subjected to additional bending stresses while running around the pulleys at a given speed, and quicker it will be destroyed due to fatigue. Hence, a longer belt will last more than a shorter belt
Wipping
If the centre distance between two pulleys is too long then the belt begins to vibrate in a direction perpendicular to the direction of motion of belt. This phenomenon is called as wipping. Wipping can be avoided by using idlers pulleys.
V belts
A widely used type of belt, particularly in industrial drives and vehicular applications, is the V-belt drive. The V-shape causes the belt to wedge tightly into the groove, increasing friction and allowing high torques to be transmitted before slipping occurs. Most belts have high-strength cords positioned at the pitch diameter of the belt cross section to increase the tensile strength of the belt. The cords, made from natural fibres, synthetic strands, or steel, are embedded in a firm rubber compound to provide the flexibility needed to allow the belt to pass around the sheave.
V Belt Cross Sections
The cross-sectional dimensions of V belts have been standardized by manufacturers,with each section designated by a letter of the alphabet (A, B, C, D, E) for sizes in inch dimensions.
V-belt drive being preferred to flat belt drive
V-belt drive is preferred to the flat belt drive due to the following advantages
1. Power transmitted is more due to wedging action in the grooved pulley.
2. Higher velocity ratio (up to 10 ) can be obtained.
3. V-belt drive is more compact, quiet and shock absorbing.
4. The drive positive because the slip is negligible
Design of chain drives
A chain drive is a flexible mechanical drive which may be considered to be intermediate between belt drive and gear drive in that it has features common to both, Chains are suitable for long as well as short centre distance drives and give a more compact, drive than is possible with belts. Chain drives are similar to belt drives in which the chains, are operated between toothed wheels called as sprockets.
Applications of chain drives
Chain drives are employed in transportation machineries like motor-cycle, bi-cycles, automobiles and technological machineries, like agricultural machines, crushes etc.
Advantages of chain drives
1. Are having more power transmitting capacity.
2. Have higher efficiency and compact size.
3. Exert -less load on shafts since no initial tension is applied on the sprocket shafts.
4. Require easy maintenance
Drawbacks of chain drives
1. The design of chain drive is more complicated.
2. The operation is noisy and production cost is high.
3. They require more accurate assembly bf shafts than for belts.
Chordal action in chain drives
Chordal action results in a pulsating and jerk motion of a chain.In order to reduce the variation in chain speed, the number of teeth on the sprocket should beincreased.
What is a silent chain
Inverted tooth chains are called silent chains because of their relatively quiet operation.Silent Drives are preferred for high-power, high-speed, and smooth operation.
Types of ropes
1. Fibre ropes
2. Wire ropes
Wire ropes are superior to fibre ropes
1. Wire ropes are stronger, more durable than fibre ropes.
2. Wire ropes can withstand' shock loads.
3. Their 'efficiency in high.
4. They can be operated for Very long centre distance even up to 1000 m.
Wire-ropes classification
Based on nup4ber of strands and wires
1. 6x7
2. 6 x 19
3. 6 x 37
4. 8 x 19
Based on the direction of wire with respect to strands in twisting
1. Cross lay ropes.
2. Parallel-lay ropes.
3. Compound lay ropes.
Wire-ropes designated
A wire-rope is designated by the number of strands and the number of wires in each strand. For example, a wire rope having six strands, and each strand containing nineteen wires can be denoted as 6 x 19 rope.
Various stresses induced in wire ropes
1. Direct tensile load due to load and self-weight of the rope.
2. Bending stress when the rope winds round the drum.
3. Stresses due to changes in starting and stopping etc.
Gear
It is a machine part with gear teeth. When two gears run together, the one with the larger number of teeth is called the gear.
Pinion
It is a machine part with gear teeth. When two gears run together, the one with the smaller number of teeth is called the pinion.
Gear drives superior to belt drives and chain drives
1. The gear drives possess high load carrying capacity, high compact layout.
2. They can transmit power from very small values to several kilowatts.
Materials for making gears
1. Ferrous metals such as carbon steels, alloy steels of nickel, chromium and vanadium.
2. Cast-iron of different grades.
3. Non-ferrous metals such as brass, bronze, etc.
4. Non-metals like phenolic resins nylon, Bakelite etc.
Among them steel with proper heat treatment is extensively, employed in many of the engineering applications.
Possible gears-failures
1. Tooth breakage
2. Pitting of tooth surface.
3. Abrasive- wears
4. Seizing of teeth
Correct gearing (or) state the law of gearing
The law of gearing states that for obtaining a constant velocity ratio, at any instant of teeth the common normal at each of contact should always pass through a pitch point,situated on the line joining the centres of rotation of the pair of mating gears.
Gear ratio (i)
It is the ratio of number of teeth of larger gear to that of smaller gear. At is also defined as the ratio of high speed to the low speed in a gear drive. Usually, the gear ratio should always be greater than 1.
Factors influence backlash
The factors like errors in tooth thickness, pitch, tooth spacing, mounting misalignment, etc. influence the backlash.
Stresses are induced in gear tooth
1. Surface compressive stress
2. Bending stress
Pinion is harder than gear
Because the teeth of pinion undergo more number of cycles than those of gear and hence quicker wear.
Methods of manufacturing a gear
1. Gear milling
2. Gear generating (Gear hobbing, Gear shaping)
3. Gear molding (Injection molding, Die casting, Investment casting)
Advantages of helical gears
1. Transmit more power
2. Provide smooth and soundless operation
3. Can be used for high speed and high velocity ratio processes
Helix angle
Helix angle -is the angle between the axis of the gear and the through tooth face. For helical gear, teeth are cut at an inclined axis, specified as helix angle and its value ranges from 80◦ to 25◦ the case of spur gear, tooth-are cut parallel to the axis, the spur gear is zero.
Herringbone gears
A herring bone gear is made of two single helical gears attached other hence called as double helical gear in which the teeth of be set in the opposite direction to the teeth of another gear arrangement. Sometimes, a single cylindrical block is ova-ployod for making herringbone gears.
Design of bevel gears
Bevel gear is the type of gear for which the teeth are cut on conical surface in contrast with spur and helical gears for which the teeth are cut on cylindrical surfaces. The structure of bevel gear is similar to and uniformly truncated frustum of a cone. When the power is to be transmitted in an angular, direction, i.e., between the shafts whose axes intersecting at an angle, bevel gears are employed. Bevel gears are classified in two ways based on the shape of teeth.
1. Straight bevel gears.
2. Spiral bevel gears
Crown gear
A crown gear is a type of bevel gear whose shaft angle is 90 degree and angle of pinion is not equal to the pitch angle of gear.
Mitre gear
Mitre gear is the special type of crown gear in-which the shaft angles is90◦ and the pitch angles of pinion and gear are equal and each angle to 45◦.
Design of worm gears
Worm gear, drive find wide applications like milling machine indexing head, table fan, and steering rod of automobile and so on.When we require to transmit power between nonparallel and non-intersecting shafts and very high velocity ratio, of about 100, worm gears, can be employed. Also worm-gears provide self-locking facility.
Merits of worm gears
1. Used for very high velocity ratio of about 100
2. Smooth and noiseless operation.
3. Self-locking facility is available.
Demerits of worm gears
1. Low efficiency.
2. More heat will be produced and hence this drive can be operated inside an oil reservoir or extra cooling fan is required in order to dissipate the heat from the drive.
3. Low power transmission.
Purpose of gear-box housing
Gear-box -housing or casing is used as container inside which, the gears, shafts, bearings and other components are "mounted.' Also it prevents the entry of dust inside the housing and reduces noise of operation. That is, the housing Safe-guard the inner components.
Function of spacers in a gear-box
Spacers are sleeve like components, which are mounted, in shafts in-between gears and bearings or one gear and another gear in order to maintain the distance between them so as to avoid interruption between them.
Speed diagram or structural diagram of gear box
Speed diagram or structural diagram is the graphical representation different speeds of output shaft, motor shaft and intermediate shafts.
Purpose we are using gear-box
Since the gear-box is provided with number of gears of different size arranged is different forms, we can get number of output speeds by operated motor/engine at single speed.
Ray-diagram of gear
The ray-diagram or kinematic arrangement of a gear box indicates arrangement of various gears in various shafts of the gear box in order to obtain the different output speeds from the single speed of the motor.
Step ratio
Step ratio is the ratio of one speed of the shaft to its previous lower speed since the spindle speeds are arranged in geometric progression, the ratios adjacent speeds (i.e., step ratios) are constant
Preferred Numbers
Preferred numbers are the conventionally rounded off values derived from geometricseries. There are five basic series denoted as R 5, R 10, R 20, R 40 and R 80 series
Sliding mesh gear box
Sliding mesh gearbox is the oldest and simplest form of gear box. In order to mesh gears on the splined main shaft with appropriate gears on the layshaft for obtaining different speeds, they are moved to the right or left. It derives its name from the fact that the meshing of the gears takes place by sliding of gears on each other.
Constant mesh gear box
In constant mesh gear box all the gears are always in mesh and the engagement between the gears which are freely rotating on the transmission main shaft and the transmission main shaft is effected by moving the dog clutches.
Types of brakes
1. Radial brakes (Band brakes, block brakes, and internal expanding rim)
2. Axial brakes (Cone brakes and disc brakes)
Types of brake linings
1. Organic linings
2. Semi-metallic linings
3. Metallic linings
Self-locking brake
When the frictional force is sufficient enough to apply the brake with no external force, then the brake is said to be self-locking brake.
Self-energizing brake
When the moment of applied force and the moment of the frictional force are in the same direction, then frictional force helps in applying the brake. This type of brake is known as a self- energizing brake.
Positive clutch
Which transmits power from driving shaft to the driven shaft by jaws or teeth is called positive clutch. No slipping is there.
Power is transmitted by clutches
In clutches, power transmission is achieved through
1. Interlocking
2. Friction
3. Wedging
Cone clutches better than disc clutches
Since the cone discs are having large frictional areas and they can transmit a larger torque than disc clutches with, the same oil diameter and actuating force and hence cone clutches are preferred over disk clutches. But usually cone clutches are mainly used in low peripheral applications
Need for generated heat dissipated in clutch operation
In order to save the friction plates and materials from melting by the heat produced during operation, the generated heat should be dissipated.
Theories applied for the design of friction clutches.
1. Uniform Pressure theory
2. Uniform wear theory
Difference between clutches and brakes
The clutch used to engage the driving and driven members and keep them moving (i.e., rotating) together, whereas brakes are employed to stop a moving member or reduce its speed.
Clutches are usually designed on the basic of uniform wear
In clutches, the value of normal pressure, axial load for the given clutch is limited by the rate of wear that can be tolerated in the brake linings. Moreover, theassumption of uniform wear rate gives a lower calculated clutch capacity than theassumption of uniform pressure. Hence clutches are usually designed on the basis of uniform wear.
Cam
Cam is a rotating mechanical member used for transmitting desired motion to a follower by direct contact.
Angle of ascend and descend
The angle of rotation of the cam from the position when the follower begins to rise till it reaches its highest points is angle of ascend.
The angle through which the cam rotates during the time the follower returns to the initial position is angle of descend.
Dwell
The zero displacement or the absence of motion of the follower during the motion of the cam is called dwell.
Angle of dwell
It is the angle through which the cam rotates while the follower remains stationary at the highest or the lowest.
Angle of action
The total angle moved by the cam during its rotation between the beginning of rise and the end of return of the follower
Radial or disc cams
In radial cams the follower reciprocates or oscillates in a direction perpendicular to the cam axis. The cams are all radial rams. In actual practice, radial cams are widely used due to their simplicity and compactness.