Belts

BELT DRIVE
A belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. They may be used as a source of motion, to efficiently transmit power, or to track relative movement. Belts are looped over pulleys. In 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 of the shafts is opposite. As a source of motion, a conveyor belt is one application where the belt is adapted to continually carry a load between two points.

Belts are the cheapest utility for power transmission between shafts that may not be parallel. Power transmission is achieved by specially designed belts and pulleys. The demands on a belt drive transmission system are large and this has led to many variations on the theme. They run smoothly and with little noise, and cushion motor and bearings against load changes, albeit with less strength than gears or chains. However, improvements in belt engineering allow use of belts in systems that only formerly allowed chains or gears.

Flat belts

Fig 1.Belts on a Yanmar 2GM20 marine diesel engine.
Fig.1 shows the application of a flat belt drive where power is transmitted from motor to engine flywheel. Flat belts were used early in line shafting to transmit power in factories. It is a simple system of power transmission that was well suited to its day. It delivered high power for high speeds (500 hp for 10,000 ft/min), in cases of wide belts and large pulleys. These drives are bulky, requiring high tension leading to high loads, so vee belts have mainly replaced the flat-belts (except when high speed is needed over power.

Fig 2. Flat belt
Flat belt pulleys shown in fig.2 need to be carefully aligned to prevent the belt from slipping off. Because flat belts tend to slip towards the higher side of the pulley, pulleys were made with a slightly convex or "crowned" surface (rather than flat) to keep the belts centered. The flat belt also tends to slip on the pulley face when heavy loads are applied. Many proprietary dressings were available that could be applied to the belts to increase friction, and so power transmission. Grip was better if the belt was assembled with the hair (i.e. outer) side of the leather against the pulley although belts were also often given a half-twist before joining the ends (forming a Möbius strip), so that wear was evenly distributed on both sides of the belt (DB). Belts were joined by lacing the ends together with leather thonging, or later by patent steel comb fasteners. A good modern use for a flat belt is with smaller pulleys and large central distances. They can connect inside and outside pulleys, and can come in both endless and jointed construction.

 

 

Round belts

Fig 3. Round belt
Round belts  shown in fig.3 are a circular cross section belt designed to run in a pulley with a circular (or near circular) groove. They are for use in low torque situations and may be purchased in various lengths or cut to length and joined, either by a staple, gluing or welding (in the case of polyurethane). Early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great effect.

Vee belts

Fig 4. Vee belt
Vee belts  shown in fig.4 (also known as V-belt or wedge rope) solved the slippage and alignment problem. It is now the basic belt for power transmission. They provide the best combination of traction, speed of movement, load of the bearings, and long service life. The V-belt was developed in 1917 by John Gates of the Gates Rubber Company. They are generally endless, and their general cross-section shape is trapezoidal. The "V" shape of the belt tracks in a mating groove in the pulley (or sheave), with the result that the belt cannot slip off. The belt also tends to wedge into the groove as the load increases — the greater the load, the greater the wedging action — improving torque transmission and making the vee belt an effective solution, needing less width and tension than flat belts. V-belts trump flat belts with their small center distances and high reduction ratios. The preferred center distance is larger than the largest pulley diameter, but less than three times the sum of both pulleys. Optimal speed range is 1000-7000 ft/min. V-belts need larger pulleys for their larger thickness than flat belts. They can be supplied at various fixed lengths or as a segmented section, where the segments are linked (spliced) to form a belt of the required length. For high-power requirements, two or more vee belts can be joined side-by-side in an arrangement called a multi-V, running on matching multi-groove sheaves. The strength of these belts is obtained by reinforcements with fibers like steel, polyester or aramid (e.g. Twaron). This is known as a multiple-belt drive. When an endless belt does not fit the need, jointed and link vee-belts may be employed. Alas, they are weaker and only speed up to 4000 ft/min. A link v-belt is a number of rubberized fabric links held together by metal fasteners. They are length adjustable by dissasembling and removing links when needed.

Fig.5 Open belt drive.

Fig.6 Cross belt drive.
The most common belt-pulley arrangement, by far, is the open belt drive (Fig. 5). Here both shafts are parallel and rotate in the same direction. The cross-belt drive of Fig. 6 shows parallel shafts rotating in opposite directions. Timing and standard V-belts are not suitable for cross-belt drives because the pulleys contact both the inside and outside belt surfaces.
Velocity ratio :
N2 / N1 =D1 / D2
­ N1 = Speed of driver pulley in rpm
­                N2 = Speed of driven pulley in rpm
­                D1 = Diameter of driver pulley in mm
­                D2 = Diameter of driven pulley in mm
Length of belt
Length of the belt = L =2C+ П / 2 (D1+ D2) + (D2 –  D1)2 / 4C  ( For open belt)
Length of the belt = L =2C+ П / 2 (D1+ D2) + (D1+ D2)2 / 4C  ( For cross belt)
C - Center Distance between the pulleys
Angle of Contact :
Angle of contact is the angle subtended by the belt overlapon pulley, The angle of contact for smaller pulley will be (180-2a) whereas for bigger pulley, it is  (180+2a)
Ratio between belt tensions
T1 / T2 = e mq       
T1 and T2 are the tensions in the tight and slack side
m is the coefficient of friction between belt and pulley surface
q is the angle of contact of the belt with the pulley in radians
Power transmitted by belt drive
Power transmitted, P = (T1 - T2) V    (Watt)
V is the velocity of the belt = П D1 N1 /  60 (m/sec)
Industrial belts are usually reinforced rubber or leather, the rubber type being predominant. Non reinforced types, other than leather, are limited to light-duty applications.
Belts probably fail by fatigue more often than by abrasion. The fatigue is caused by the cyclic stress applied to the belt as it bends around the pulleys. Belt failure is accelerated when the following conditions are present: high belt tension; excessive slippage; adverse environmental conditions; and momentary overloads caused by shock, vibration, or belt slapping.
Slip of the belt
            The power transmission in belt drive is caused by friction between belt and pulleys. However, some relative movement will always exist at driver-belt interface and belt-driven pulley interface due to ineffective friction. This phenomenon is called as slip the belt. Slip is expressed in percentage. Due to slip, the belt speed will be less than the peripheral speed of the driving wheel and slightly more than peripheral speed of the driven wheel.
Tight and slack side
            In clockwise rotation of the driver, the driver pulls belt from lower side and delivers it to the upper side. Thus the tension in the lower side belt will be more than that of the upper side belt. Hence the lower side is called as tight side and upper side is called as slack side.

 

 Film belts
Though often grouped with flat belts, they are actually a different kind. They consist of a very thin belt (0.5-15 millimeters or 100-4000 microns) strip of plastic and occasionally rubber. They are generally intended for low-power (10 hp or 7 kW), high-speed uses, allowing high efficiency (up to 98%) and long life. These are seen in business machines, tape recorders, and other light-duty operations.

Timing Belts

Fig.7 Timing belt
Timing belts, shown in fig.7 (also known as Toothed, Notch or Cog)  are positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tensioned, they have no slippage, run at constant speed, and are often used to transfer direct motion for indexing or timing purposes (hence their name). They are often used in lieu of chains or gears, so there is less noise and a lubrication bath is not necessary. Camshafts of automobiles, miniature timing systems, and stepper motors often utilize these belts. Timing belts need the least tension of all belts, and are among the most efficient. They can bear up to 200 hp (150 kW) at speeds of 16,000 ft/min.
Timing belts with a helical offset tooth design are available. The helical offset tooth design forms a chevron pattern and causes the teeth to engage progressively. The chevron pattern design is self-aligning. The chevron pattern design does not make the noise that some timing belts make at idiosyncratic speeds, and is more efficient at transferring power (up to 98%).
Disadvantages include high cost, need for toothed pulleys, less protection from overload and jam, no clutch action.

Standards for use

The open belt drive has parallel shafts rotating in the same direction, whereas the cross-belt drive also bears parallel shafts but rotate in opposite direction. The former is far more common, and the latter not appropriate for timing and standard V-belts, because the pulleys contact both the both inner and outer belt surfaces. Nonparallel shafts can be connected if the belt's center line is aligned with the center plane of the pulley. Industrial belts are usually reinforced rubber but sometimes leather types, non-leather non-reinforced belts, can only be used in light applications.

Selection criteria

Belt drives are built under the following required conditions: speeds of and power transmitted between drive and driven unit; suitable distance between shafts; and appropriate operating conditions. The equation for power is:
power (kW) = (torque in newton-meters) × (rpm) × (2π radians)/(60 sec × 1000 W)

Belt wear

Fatigue, more so than abrasion, is the culprit for most belt problems. This wear is caused by stress from rolling around the pulleys. High belt tension; excessive slippage; adverse environmental conditions; and belt overloads caused by shock, vibration, or belt slapping all contribute to belt fatigue.

Specifications

To fully specify a belt, the material, length, and cross-section size and shape are required. Timing belts, in addition, require that the size of the teeth be given. The length of the belt is the sum of the central length of the system on both sides, half the circumference of both pulleys, and the square of the sum (if crossed) or the difference (if open) of the radii. Thus, when dividing by the central distance, it can be visualized as the central distance times the height that gives the same squared value of the radius difference on, of course, both sides. When adding to the length of either side, the length of the belt increases, in a similar manner to the Pythagorean theorem. One important concept to remember is that as D1 gets closer to D2 there is less of a distance (and therefore less addition of length) until its approaches zero.
On the other hand, in a crossed belt drive the sum rather than the difference of radii is the basis for computation for length. So the wider the small drive increases, the belt length is higher. Otherwise it is similar.
Advantages of belt drive are:

Disadvantages include:

There are four general types of belts: flat belts, V-belts, film belts, and timing belts. Each has its own special characteristics, limitations, advantages, and special-purpose variations for different applications.
Flat belts, in the form of leather belting, served as the basic belt drive from the beginning of the Industrial Revolution. They can transmit large amounts of power at high speeds. Flat belts find their widest application where high-speed motion, rather than power, is the main concern. Flat belts are very useful where large center distances and small pulleys are involved. They can engage pulleys on both inside and outside surfaces, and both endless and jointed construction are available.
V-belts are the basic power-transmission belt, providing the best combination of traction, operating speed, bearing load, and service life. The belts are typically endless, with a trapezoidal cross section which runs in a pulley with a V-shaped groove. The wedging action of the belt in the pulley groove allows V-belts to transmit higher torque at less width and tension than flat belts. V-belts are far superior to flat belts at small center distances and high reduction ratios. V-belts require larger pulleys than flat belts because of their greater thickness. Several individual belts running on the same pulley in separate grooves are often used when the power to be transmitted exceeds that of a single belt. These are called multiple-belt drives.
Film belts are often classified as a variety of flat belt, but actually they are a separate type. Consisting of a very thin strip of material, usually plastic but sometimes rubber, their widest application is in business machines, tape recorders, and other light-duty service.
Timing belts have evenly spaced teeth on their bottom side which mesh with grooves cut on the periphery of the pulleys to produce a positive, no-slip, constant-speed drive. They are often used to replace chains or gears, reducing noise and avoiding the lubrication bath or oiling system requirement. They have also found widespread application in miniature timing applications. Timing belts, known also as synchronous or cogged belts, require the least tension of all belt drives and are among the most efficient.

 

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Chain drive

Fig.8 Roller chain and sprocket
Fig.8 shows the application of a chain drive. Chain drive is a way of transmitting mechanical power from one place to another. It is often used to convey power to the wheels of a vehicle, particularly bicycles and motorcycles. It is also used in a wide variety of machines besides vehicles.
Most often, the power is conveyed by a roller chain, known as the drive chain, passing over a sprocket gear, with the teeth of the gear meshing with the holes in the links of the chain. The gear is turned, and this pulls the chain putting mechanical force into the system. Another type of drive chain is the Morse chain, invented by the Morse Chain Company of Ithaca, New York, USA. This has inverted teeth.
Sometimes the power is output by simply rotating the chain, which can be used to lift or drag objects. In other situations, a second gear is placed and the power is recovered by attaching shafts or hubs to this gear. Though drive chains are often simple oval loops, they can also go around corners by placing more than two gears along the chain; gears that do not put power into the system or transmit it out are generally known as idler-wheels. By varying the diameter of the input and output gears with respect to each other, the gear ratio can be altered, so that, for example, the pedals of a bicycle can spin all the way around more than once for every rotation of the gear that drives the wheels.

 

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Drive chains are similar to drive belts in many ways, and which device is used is subject to several design tradeoffs. Drive chains are most often made of metal, while belts are often rubber, plastic, or other substances. This makes drive chains heavier, so more of the work put into the system goes into moving a chain versus moving a belt.[ On the other hand, well-made chains are often stronger than belts. Also, drive belts can often slip (unless they have teeth) which means that the output side may not rotate at a precise speed, and some work gets lost to the friction of the belt against its rollers.
Teeth on toothed drive belts generally wear faster than links on chains, but wear on rubber or plastic belts and their teeth is often easier to observe; you can often tell a belt is wearing out and about to break more easily than a chain. Chains often last longer.
Chains are often narrower than belts, and this can make it easier to shift them to larger or smaller gears in order to vary the gear ratio. Multi-speed bicycles with derailleurs make use of this. Also, the more positive meshing of a chain can make it easier to build gears that can increase or shrink in diameter, again altering the gear ratio.
Both can be used to move objects by attaching pockets, buckets, or frames to them; chains are often used to move things vertically by holding them in frames, as in industrial toasters, while belts are good at moving things horizontally in the form of conveyor belts. It is not unusual for the systems to be used in combination; for example the rollers that drive conveyor belts are themselves often driven by drive chains.
Drive shafts are another common method used to move mechanical power around that is sometimes evaluated in comparison to chain drive; in particular shaft drive versus chain drive is a key design decision for most motorcycles. Drive shafts tend to be even tougher and more reliable than chain drive, but weigh even more (robbing more power), and impart rotational torque.

Use in vehicles

Bicycles

Chain drive was the main feature which differentiated the safety bicycle introduced in 1885, with its two equal-sized wheels, from the direct-drive penny-farthing or "high wheeler" type of bicycle. The popularity of the chain-driven safety bicycle brought about the demise of the penny-farthing, and is still a basic feature of bicycle design today.

Automobiles

Transmitting power to the wheels

Fig.9 Side chain gear
Fig.9 shows athe application of a chain drive.Chain drive was a popular power transmission system from the earliest days of the automobile. It gained prominence as an alternative to the Système Panhard with its rigid Hotchkiss driveshaft and universal joints.
A chain drive system uses one or more roller chain to transmit power from a differential to the rear axle. This system allowed for a great deal of vertical axle movement (for example, over bumps), and was simpler to design and build than a rigid driveshaft in a workable suspension. Also, it had less unsprung weight at the rear wheels than the Hotchkiss drive, which would have had the weight of the driveshaft to carry as well, which in turn meant that the tires would last longer.

Inside motors

Internal combustion engines often use chain drive to power the timing chain used to drive overhead camshaft valvetrains. This is an area in which chain drives frequently compete directly with belt drive systems, and an excellent example of some of the differences and similarities between the two approaches. For this application, chains last longer, but are often harder to replace. Being heavier, the chain robs more power, but is also less likely to fail. The camshaft of a four stroke engine must rotate at half crankshaft speed, so some form of reduction gearing is needed and a direct drive from the crankshaft isn't possible. Alternatives to chain drives include gear trains, bevel gear and shaft drives, or toothed flexible belt drives.
Chain drive versus belt drive or use of a driveshaft is a fundamental design decision in motorcycle design; nearly all motorcycles use one of these three designs. See Motorcycle construction for more details.
PROBLEMS

Solution:
N1 = 200 rpm
D1=750 rpm
D2=450 rpm
N2 = N1 * (D1/D2)
= 200(750/450)
= 333.34 rpm.

Diameter of the driver                 :           200 mm
Diameter of the follower             :           400 mm
Center distance of the drive         :           2m
Speed of the drive                        :           400 rpm
Angle of contact                          :           197.3 °
Determine the length of the belt required.
Solution:
D1 = 200 mm
D2= 400 mm
C =  2m
N1 = 400 rpm
Length of the belt = L =2C+ П / 2 (D1+ D2) + (D1+ D2) / 4C
= (2x2) + П / 2 (0.2+ 0.4) + (0.2+ 0.4) / 4x2
= 4.99 m

T1 = 1.3 kN
q = 197.3 x (П /180) radians
V = П D1 N1 /  60
= (П x 0.2) (400/60)
= 4.2 m/s
T1 / T2 = e mq
1.2 x 103 / T2 = e (0.25 x 197.3 x П /180 )
T2 = 506.33 N
Power transmitted, P = (T1 - T2) V
(1200-506.33)x4.2
= 2.9 kW  
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