Chapter 10 - Belt Drives Flashcards
What are the merits of belt drives?
- offer a good wide range of shaft centres (the distance between the axes of two parallel shafts) with versatility of sheave or pulley diameters
How is power transmitted from pulley to belt?
- friction drives
- transmit power by means of a belt pressed tightly onto a pulley
- the power of the prime mover is transmitted to the pulley by the belt
How is area of contact determined on a belt?
- depends on:
— pulley diameter
— centre-to-centre distance
— take-up devices used
What types of materials are required for belts?
- cotton
- leather
- rubber
- nylon or other synthetics
How is the coefficient of friction determined between belt and pulley?
- the frictional grip varies with the coefficient of friction between the belt and pulley
- it is determined by the belt and pulley materials and their surface structures
Define installed tension and effective tension
- installed tension
— the tension on the belt when the drive is not running
— for flat and v-belts, this is expressed as percent elongation or percent tension - effective tension
— the tension needed on the belt to transmit power without slipping
— when the drive is running, the pull on the belt increases the tension and stretch on the tight side as it overcomes the resistance of the load
— the slack side had no tension increase because it simply returns the belt to the driven pulley
How much force is required to deflect the following new belt:
D size V-belt correctly installed on sheaves, 40” centre-to-centre?
- 26 pounds for new belt
- 22 pounds after run-in
How much deflection should the mentioned belt have?
D size V-belt correctly installed on sheaves, 40” centre-to-centre?
- 40/64 = 0.625” = 1-1/8”
What is the approximate length of belt required for the following drive:
1 sheave 16”
1 sheave 8”
Centre-to-centre distance is 50”
- belt length = (D+d/2)Pi + 2C
D = large diameter pulley d = small diameter pulley C = shaft centres Pi = 3.1416
What are the causes of slip and how is slip controlled?
- slip is caused by the surface characteristics of the belt and pulley
- severe slippage burns the belt
- it also polished the belt and pulley surfaces, reducing friction grip between the belt and pulley (pulley should look smooth and rather dull, NOT shiny)
- Causes: — the driving pulley is too small — the load is too great — the belt is running too loose — the belt is not being properly cleaned and tensioned
- Controls:
— using smaller pulley ratios (3:1 or 4:1)
— aligning the pulleys correctly - ensuring adequate take-up
Describe creep
- a physical characteristic of a belt affecting power transmission
- caused by the elasticity of the belt
1 - belt on the slack side is delivered to the pulley slower than the rate at which it comes off the tight side
2 - the belt creeps ahead on the pulley and he belt surface runs slower than the belt as a whole
What are the purposes of flat belts?
- used in conveyors and drip belts, where the belt itself carries the load
- in a flat belt, the drive members (power plies) carry the forces to transmit power
- they may be embedded in substances such as rubber and encased in covers to provide protection and traction
- if the top side is unrecognizable, it will be marked
- if the drive direction is important, it will be marked
- flat belts may be manufactured as:
— endless (continuous loop)
— reel-stock (open-ended), must be fastened together
Briefly describe the following belt materials:
- leather
- have single or multiple layers
- May be combined with other materials such as cords and polymers
- leather belts are being replaced by rubber or synthetic belts
Briefly describe the following belt materials:
- rubber
- made from fabric or cord impregnated with natural or synthetic rubber compounds
- provide various degrees of:
— strength
— stretch
— pulley grip
— protection against abrasion, oil, or moisture
Briefly describe the following belt materials:
- fabric
- made from cotton or synthetic fabrics, with or without rubber impregnation
- made in layers and are 3-12 ply, depending on their width
- direction of the weave varies, but alternate layers have their fibres running in different directions for added strength
- used for moderate loads and speeds
Briefly describe the following belt materials:
- cord
- have drive members made from twisted cotton or synthetic cords embedded in rubber
- the rubber has a fabric cover to protect against wear
- heavy-duty belts, used for high-speed, small-pulley, shock-load applications
- compass belting (folded or rolled edges) contains a single row of cords
Briefly describe the following belt materials:
- steel-cable
- similar in construction to cord belts
- high-capacity and lower stretch than cord flat belts
Briefly describe the following belt materials:
- nylon and other synthetics
- flexible to permit use on small-diameter pulleys
- nylon member is sandwiched between the outside cover and the pulley-side cover
Briefly describe the following methods of joining belts:
- vulcanized splices
- use heat and pressure to create a bond
- these splices are the strongest and most efficient belt joints
Briefly describe the following methods of joining belts:
- chemical splices or synthetic ply belts
- use adhesives
- the type of belt determines the adhesive to be used
- old and new belts may be chemically spliced together
- a belt containing more than one synthetic may need two different chemicals
Briefly describe the following methods of joining belts:
- mechanical flat-belt fasteners
- it is important to cut the ends properly so that the ends align properly before being fastened
- various types of fasteners are used:
— wire lacing
— steel hinges
— plate fasteners
What are the advantages of v-belts
- wedging action permits lower arc of contact on small pulley and large speed ratio
- shorter centre distances can be used to achieve a compact drive
- they absorb shocks to cushion motors and bearings against load fluctuations
- vibration and noise levels are low
- maintenance and replacements are quick and easy
- power transmission efficiency can be as high as 93% after run-in
Describe the construction of V-belts
- v-belts can have a variety of sizes and cross sections, each with its own particular function
- each has four sections:
— the top section of the belt is the extension section. It is rubber and stretches as the belt wraps around the sheave
— the bottom section is the compression section. It compresses when wedged into, and shaped around, the sheave
— the neutral section of the belt neither compresses nor stretches. The driving members in this area give the belt it’s tensile strength
— the cover section protects the inner parts of the belt from wear. The cover is the only part in direct contact with the sheave.
What are the applications of the following V-belts:
- single-layer compass construction
- used mainly on short-centre drives with small diameter pulleys
- each strand is separated by, and bonded in, rubber.
What are the applications of the following V-belts:
- multi-layered cord construction
- used for long, heavy drives
- each strand is separated by, and bonded in, rubber
- this type has two main bands of blocks or cords
- these v-belts are designed to flex in only one direction and should not be used with reverse bending
What are the applications of the following V-belts:
- grommet-style construction
- the layer or layers of small cords can be replaced by two grommets or ropes to carry the load
- this style has both high load capacity and great flexibility
How are fabric ply belts constructed?
- several plies of fabric can be bonded together to make a driving section
- like cord belts, they can be supplied with one of the following:
— a single band on the pitch line
— one band on the pitch line and another near the bottom of the belt
What are the reasons for concave side-wall V-belts, notched V-belts, and joined V-belts?
- concave side walls have more contact on sheaves when loaded
- notches give belts increased flexibility and heat-dissipation capabilities
- they are normally used in small-sheave, short-centre, and high-speed drives
Are V-belts made to close tolerances when compared to roller chains?
- v-belts are not manufactured with the same close tolerances as gears or roller chains
- v-belts are made to an industry standard of section sizes and angles so that belts from different companies can be used on the same sheaves
What are the five sizes of standard V-belts?
- A, B, C, D, E
- also called standard cross section
- May have either straight or concave side walls
What is the purpose of high-capacity V-belts?
- also called wedge, hi-torque or narrow v-belts
- sizes designated as: 3V, 5V and 8V
- they can transmit up to three times the horsepower of standard v-belts in the same drive space, due to sheave groove design and increased contact area
What size belt is a high-capacity 5V300?
— 5/8” width and 30” pitch-line length
How are light-duty V-belts codes?
- they have cross sections the same as conventional v-belts but coded 2L, 3L, 4L and 5L
- these codes indicate width increments of 1/8”
What are the purposes of double-angle and wide V-belts?
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What are the advantages and disadvantages of positive-drive belts compared to gears and chains?
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How are the pitch circle and the pitch line related in a positive-drive belt?
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What is the meaning of the code 400 H 100 with regard to a positive-drive belt?
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Why are positive drive-pulleys usually flanged?
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What are the purposes of double positive-drive belts and linked V-belts?
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Briefly describe poly V-belts
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What is the length of the following belt?
700L6
Briefly describe power band
V-belts
How does a millwright ensure that matched belts are the same length?
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Calculate the rim and belt speed of the following:
Diameter= 11” RPM = 1750
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What is the preferred location of an outside idler and what size is recommended?
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What is the preferred location of an inside idler and what size is recommended?
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What is used to install and disassemble tapered bushings?
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How are the size (diameter) and width of pulleys for a flat belt determined?
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What are the purposes of crowned pulleys?
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What are the applications for flanged flat belt pulleys?
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Where is the pitch line located on a V-belt?
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How should a belt fit the sheave?
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What is the advantage of combination-groove sheaves compared to regular sheaves?
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Briefly describe a V-flat belt drive
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Describe the two general types of variable-speed belt drives
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When should speed adjustments be made?
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What problems may be encountered when adjusting variable-speed drives?
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What factors ensure maximum belt life?
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How is alignment achieved?
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How are rubber belts stored?
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How and when should belts be inspected?
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What causes the following problems:
- slips and squeals?
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What causes the following problems:
- runs crooked?
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What causes the following problems:
- cracked outside ply?
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What causes the following problems:
- cracked inside ply?
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