V Belts

The v-belts are made of rubber with fabric cords to transmit power and its covered with a protective layer. The cords transmit the force from the driver to the driven pulley, thereby transmit the power. The number of cords are increased based on the force requirements. The rubber layer transmits the force in cord to the side layers.            

        

V-belts are widely used in industry and automobiles because of its power transmitting capacity. the wedge shape of the belt increases the area of contact with the pulley there by increasing more friction which makes it ti carry more power without slip.

V-belt cross sections

There are different sizes of v belt cross sections named as A,B,C,D,E based on the width of belts. These are standard sizes manufactured by the belt manufacturers.

V-belts are preferred over flat belts

V-belts are preferred over flat belts the reasons follows,

  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
  5. They are rugged–they will give years of trouble-free performance when given just reasonable attention even under adverse conditions.
  6. They are clean–require no lubrication.
  7. They are efficient–performing with an average of 94-98% efficiency.
  8. They are smooth starting and running. 
  9. They cover extremely wide horsepower ranges.
  10. They permit a wide range of driven speeds, using standard electric motors.
  11. They dampen vibration between driving and driven machines.
  12. They are quiet.
  13. They act as a “safety fuse” in the power drive because they refuse to transmit a severe overload of power, except for a very brief time.
  14. V-belts and sheaves wear gradually–making preventive corrective maintenance simple and easy.

V Belt Selection – Using PSG Design Data Book

  Step:1
Belt cross section  Select standard v-belt cross section from PSG 7.58 based on motor power(kW)

 
 Step:1
Pulley diameters  Calculate the diameters of the smaller and larger pulley using the relation

  – Diameter of larger pulley (mm)
  – Speed of the larger pulley (rpm)
  – Diameter of small pulley (mm)
  – Speed of the small pulley (rpm)
  – velocity ratio
 Step:3
Center distance  Calculate center distance value based on velocity ratio/speed ratio from PSG 7.61 


 C – Center distance (mm)
 
 Step:4
Nominal pitch length  Calculate nominal pitch length using formula from PSG 7.61
 
 
 – Diameter of larger pulley (mm)
  – Diameter of small pulley (mm)
 C – Center distance (mm) 
 Step:5
Maximum power capacity of belt  Calculate the power capacity of belt using formula in PSG 7.62 for the selected belt cross section.

Calculate the velocity of belt / belt speed using the formula
 

 

  – Diameter of small pulley (mm)
  – Speed of the small pulley (rpm)
 – Velocity of belt of speed of belt (m/s)
 Power capacity – (kW)
 Step:6
Number of Belts  Calculate the number of belts required using the formula in PSG 7.70
 
For,
   – Refer PSG 7.69
   – Refer PSG 7.58, 7.59 and 7.60
   – Refer PSG 7.68
 Step:7
Actual center distance  Calculate the center distance using the formula in PSG 7.61 
 

 C
 – Center distance (mm)

WHAT IS BELTING?

V BELT

A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, 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

POWER TRANSMISSION

Belts are the cheapest utility for power transmission between shafts that may not be axially aligned. 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.

Power transmitted between a belt and a pulley is expressed as: P = (T1 ? T2)v frac{T_1}{T_2} = e^{mualpha}

where T1 and T2 are tensions in the tight side and slack side of the belt respectively, ? is the coefficient of friction, and ? is the angle subtended by contact surface at the centre of the pulley

PROS AND CONS

Belt drive, moreover, is simple, inexpensive, and does not require axially aligned shafts. It helps protect the machinery from overload and jam, and damps and isolates noise and vibration. Load fluctuations are shock-absorbed (cushioned). They need no lubrication and minimal maintenance. They have high efficiency (90-98%, usually 95%), high tolerance for misalignment, and are inexpensive if the shafts are far apart. Clutch action is activated by releasing belt tension. Different speeds can be obtained by step or tapered pulleys.

The angular-velocity ratio may not be constant or equal to that of the pulley diameters, due to slip and stretch. However, this problem has been largely solved by the use of toothed belts. Temperatures ranges from ?31 °F (?35 °C) to 185 °F(85 °C). Adjustment of center distance or addition of an idler pulley is crucial to compensate for wear and stretch

FLAT BELTS

Flat belts were used early in line shafting to transmit power in factories.They were also used in countless farming, mining, and logging applications, such as bucksaws, sawmills, threshers, silo blowers, conveyors for filling corn cribs or haylofts, balers, water pumps (for wells, mines, or swampy farm fields), and electrical generators. The flat belt is a simple system of power transmission that was well suited for 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. The Industrial Revolution soon demanded more from the system, and flat belt pulleys needed to be carefully aligned to prevent the belt from slipping off. Because flat belts tend to climb 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. Flat belts also tend to slip on the pulley face when heavy loads are applied and 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 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.

VEE BELTS

Vee belts (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 V-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 or Kevlar). This is known as a multiple-V-belt drive (or sometimes a “classical V-belt drive”). When an endless belt does not fit the need, jointed and link V-belts may be employed. However they are weaker and only usable at speeds 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 disassembling and removing links when needed.

RIBBED BELT

A ribbed belt is a power transmission belt featuring lengthwise grooves. It operates from contact between the ribs of the belt and the grooves in the pulley. Its single-piece structure it reported to offer an even distribution of tension across the width of the pulley where the belt is in contact, a power range up to 600 kW, a high speed ratio, serpentine drives (possibility to drive off the back of the belt), long life, stability and homogeneity of the drive tension, and reduced vibration. The ribbed belt may be fitted on various applications : compressors, fitness bikes, agricultural machinery, food mixers, washing machines, lawn mowers, etc

MULTI-GROOVE BELTS

A multi-groove or poly groove belt is made up of usually 5 or 6 “V” shapes along side each other. This gives a thinner belt for the same drive surface, thus is more flexible, although often wider. The added flexibility offers an improved efficiency, as less energy is wasted in the internal friction of continually bending the belt. In practice this gain of efficiency is overshadowed by the reduced heating effect on the belt, as a cooler-running belt lasts longer in service.

A further advantage of the poly groove belt, and the reason they have become so popular, stems from the ability to be run over pulleys on the ungrooved back of the belt. Although this is sometimes done with vee belts and a single idler pulley for tension, a poly groove belt may be wrapped around a pulley on its back tightly enough to change its direction, or even to provide a light driving force.

Any vee belt’s ability to drive pulleys depends on wrapping the belt around a sufficient angle of the pulley to provide grip. Where a single-vee belt is limited to a simple convex shape, it can adequately wrap at most three or possibly four pulleys, so can drive at most three accessories. Where more must be driven, such as for modern cars with power steering and air conditioning, multiple belts are required. As the poly groove belt can be bent into concave paths by external idlers, it can wrap any number of driven pulleys, limited only by the power capacity of the belt.

This ability to bend the belt at the designer’s whim allows it to take a complex or “serpentine” path. This can assist the design of a compact engine layout, where the accessories are mounted more closely to the engine block and without the need to provide movable tensioning adjustments. The entire belt may be tensioned by a single idler pulley

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 micrometres) 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, printers, tape recorders, and other light-duty operations

TIMING BELTS

Timing belts, (also known as toothed, notch, cog, or synchronous belts) are apositive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tension-ed, 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 a relatively high purchase cost, the need for specially fabricated toothed pulleys, less protection from overloading and jamming, and the lack of clutch action