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Bread is a staple food prepared from a dough of flour (usually wheat) and water, usually by baking. Throughout recorded history and around the world, it has been an important part of many cultures’ diets. It is one of the oldest human-made foods, having been of significance since the dawn of agriculture, and plays an essential role in both religious rituals and secular culture.

Bread may be leavened by naturally occurring microbes (e.g. sourdough), chemicals (e.g. baking soda), industrially produced yeast, or high-pressure aeration, which creates the gas bubbles that fluff up bread. Bread may also be unleavened. In many countries, mass-produced bread often contains additives to improve flavor, texture, color, shelf life, nutrition, and ease of production.

Etymology

The Old English word for bread was hlaf (hlaifs in Gothic: modern English loaf) which appears to be the oldest Teutonic name.^[1] Old High German hleib^[2] and modern German Laib derive from this Proto-Germanic word, which was borrowed into some Slavic (Czech: chléb, Polish: bochen chleba, Russian: khleb) and Finnic (Finnish: leipä, Estonian: leib) languages as well. The Middle and Modern English word bread appears in other Germanic languages, such as West Frisian: brea, Dutch: brood, German: Brot, Swedish: bröd, and Norwegian and Danish: brød; it may be related to brew or perhaps to break, originally meaning “broken piece”, “morsel”.^{[3][better source needed]}

History

Main article: History of bread

Bread is one of the oldest prepared foods. Evidence from 30,000 years ago in Europe and Australia revealed starch residue on rocks used for pounding plants.^[4][5] It is possible that during this time, starch extract from the roots of plants, such as cattails and ferns, was spread on a flat rock, placed over a fire and cooked into a primitive form of flatbread. The oldest evidence of bread-making has been found in a 14,500-year-old Natufian site in Jordan’s northeastern desert.^[6][7] Around 10,000 BC, with the dawn of the Neolithic age and the spread of agriculture, grains became the mainstay of making bread. Yeast spores are ubiquitous, including on the surface of cereal grains, so any dough left to rest leavens naturally.^[8]

An early leavened bread was baked as early as 6000 BC in southern Mesopotamia, cradle of the Sumerian civilization, who may have passed on the knowledge to the Egyptians around 3000 BC. The Egyptians refined the process and started adding yeast to the flour. The Sumerians were already using ash to supplement the dough as it was baked.^[9]

There were multiple sources of leavening available for early bread. Airborne yeasts could be harnessed by leaving uncooked dough exposed to air for some time before cooking. Pliny the Elder reported that the Gauls and Iberians used the foam skimmed from beer, called barm, to produce “a lighter kind of bread than other peoples” such as barm cake. Parts of the ancient world that drank wine instead of beer used a paste composed of grape juice and flour that was allowed to begin fermenting, or wheat bran steeped in wine, as a source for yeast. The most common source of leavening was to retain a piece of dough from the previous day to use as a form of sourdough starter, as Pliny also reported.^[10][11]

The ancient Egyptians, Greeks, and Romans all considered the degree of refinement in the bakery arts as a sign of civilization.^[9]

The Chorleywood bread process was developed in 1961; it uses the intense mechanical working of dough to dramatically reduce the fermentation period and the time taken to produce a loaf. The process, whose high-energy mixing allows for the use of grain with a lower protein content, is now widely used around the world in large factories. As a result, bread can be produced very quickly and at low costs to the manufacturer and the consumer. However, there has been some criticism of the effect on nutritional value.^[12][13][14]

Types

Main article: List of breads

Brown bread (left) and whole grain bread

Dark sprouted bread

Ruisreikäleipä, a flat rye flour loaf with a hole

Bread is the staple food of the Middle East, Central Asia, North Africa, Europe, and in European-derived cultures such as those in the Americas, Australia, and Southern Africa. This is in contrast to parts of South and East Asia, where rice or noodles are the staple. Bread is usually made from a wheat–flour dough that is cultured with yeast, allowed to rise, and baked in an oven. Carbon dioxide and ethanol vapors produced during yeast fermentation result in bread’s air pockets.^[15] Owing to its high levels of gluten (which give the dough sponginess and elasticity), common or bread wheat is the most common grain used for the preparation of bread, which makes the largest single contribution to the world’s food supply of any food.^[16]

Bread is also made from the flour of other wheat species (including spelt, emmer, einkorn and kamut).^[17] Non-wheat cereals including rye, barley, maize (corn), oats, sorghum, millet and rice have been used to make bread, but, with the exception of rye, usually in combination with wheat flour as they have less gluten.^[18]

Gluten-free breads are made using flours from a variety of ingredients such as almonds, rice, sorghum, corn, legumes such as beans, and tubers such as cassava. Since these foods lack gluten, dough made from them may not hold its shape as the loaves rise, and their crumb may be dense with little aeration. Additives such as xanthan gum, guar gum, hydroxypropyl methylcellulose (HPMC), corn starch, or eggs are used to compensate for the lack of gluten.^{[19][20][21][22]}

• Sangak, an Iranian flatbread
• Strucia — a type of European sweet bread

Properties

Physical-chemical composition

In wheat, phenolic compounds are mainly found in hulls in the form of insoluble bound ferulic acid, where it is relevant to wheat resistance to fungal diseases.^[23]

Rye bread contains phenolic acids and ferulic acid dehydrodimers.^[24]

Three natural phenolic glucosides, secoisolariciresinol diglucoside, p-coumaric acid glucoside and ferulic acid glucoside, can be found in commercial breads containing flaxseed.^[25]

Glutenin and gliadin are functional proteins found in wheat bread that contribute to the structure of bread. Glutenin forms interconnected gluten networks within bread through interchain disulfide bonds.^[26] Gliadin binds weakly to the gluten network established by glutenin via intrachain disulfide bonds.^[26] Structurally, bread can be defined as an elastic-plastic foam (same as styrofoam). The glutenin protein contributes to its elastic nature, as it is able to regain its initial shape after deformation. The gliadin protein contributes to its plastic nature, because it demonstrates non-reversible structural change after a certain amount of applied force. Because air pockets within this gluten network result from carbon dioxide production during leavening, bread can be defined as a foam, or a gas-in-solid solution.^[27]

Acrylamide, like in other starchy foods that have been heated higher than 120 °C (248 °F), has been found in recent years to occur in bread. Acrylamide is neurotoxic, has adverse effects on male reproduction and developmental toxicity and is carcinogenic. A study has found that more than 99 percent of the acrylamide in bread is found in the crust.^[28]

A study by the University of Hohenheim found that industrially produced bread typically has a high proportion of FODMAP carbohydrates due to a short rising time (often only one hour). The high proportion of FODMAP carbohydrates in such bread then causes flatulence. This is particularly problematic in intestinal diseases such as irritable bowel syndrome. While in traditional bread making the dough rises for several hours, industrial breads rise for a much shorter time, usually only one hour. However, a sufficiently long rising time is important to break down the indigestible FODMAP carbohydrates. Some flours (for example, spelt, emmer and einkorn) contain fewer FODMAPs, but the difference between grain types is relatively small (between 1 and 2 percent by weight). Instead, 90% of the FODMAPs that cause discomfort can be broken down during a rising time of four hours. In the study, whole-grain yeast doughs were examined after different rising times; the highest level of FODMAPs was present after one hour in each case and decreased thereafter. The study thus shows that it is essentially the baking technique and not the type of grain that determines whether a bread is well tolerated or not. A better tolerance of bread made from original cereals can therefore not be explained by the original cereal itself, but rather by the fact that traditional, artisanal baking techniques are generally used when baking original cereals, which include a long dough process. The study also showed that a long rising time also breaks down undesirable phytates more effectively, flavors develop better, and the finished bread contains more biologically accessible trace elements.^[29][30]

Culinary uses

Bread can be served at many temperatures; once baked, it can subsequently be toasted. It is most commonly eaten with the hands, either by itself or as a carrier for other foods. Bread can be spread with butter, dipped into liquids such as gravy, olive oil, or soup;^[31] it can be topped with various sweet and savory spreads, or used to make sandwiches containing meats, cheeses, vegetables, and condiments.^[32]

Bread is used as an ingredient in other culinary preparations, such as the use of breadcrumbs to provide crunchy crusts or thicken sauces; toasted cubes of bread, called croutons, are used as a salad topping; seasoned bread is used as stuffing inside roasted turkey; sweet or savoury bread puddings are made with bread and various liquids; egg and milk-soaked bread is fried as French toast; and bread is used as a binding agent in sausages, meatballs and other ground meat products.^[33]

Nutritional significance

Bread is a good source of carbohydrates and micronutrients such as magnesium, iron, selenium, and B vitamins. Whole grain bread is a good source of dietary fiber and all breads are a common source of protein in the diet, though not a rich one.^[34][35]

Crust and crumb

The mass of bread consists of two primary components: the crust and crumb.^[36]

Bread crust is formed from surface dough during the cooking process. It is hardened and browned through the Maillard reaction using the sugars and amino acids due to the intense heat at the bread surface. The crust of most breads is harder, and more complexly and intensely flavored, than the rest. Old wives’ tales suggest that eating the bread crust makes a person’s hair curlier.^[37] Additionally, the crust is rumored to be healthier than the remainder of the bread. Some studies have shown that this is true as the crust has more dietary fiber and antioxidants such as pronyl–lysine.^[38]

Bread crumb is the internal porous material consisting of bubbles with elastic walls. As the bread ages (becomes stale), the crumb becomes more firm.^[36]

Preparation

Doughs are usually baked, but in some cuisines breads are steamed (e.g., mantou), fried (e.g., puri), or baked on an unoiled frying pan (e.g., tortillas). It may be leavened or unleavened (e.g. matzo). Salt, fat and leavening agents such as yeast and baking soda are common ingredients, though bread may contain other ingredients, such as milk, egg, sugar, spice, fruit (such as raisins), vegetables (such as onion), nuts (such as walnut) or seeds (such as poppy).^[39]

Methods of processing dough into bread include the straight dough process, the sourdough process, the Chorleywood bread process and the sponge and dough process.

Formulation

Professional bread recipes are stated using the baker’s percentage notation. The amount of flour is denoted to be 100%, and the other ingredients are expressed as a percentage of that amount by weight. Measurement by weight is more accurate and consistent than measurement by volume, particularly for dry ingredients. The proportion of water to flour is the most important measurement in a bread recipe, as it affects texture and crumb the most. Hard wheat flours absorb about 62% water, while softer wheat flours absorb about 56%.^[40] Common table breads made from these doughs result in a finely textured, light bread. Most artisan bread formulas contain anywhere from 60 to 75% water. In yeast breads, the higher water percentages result in more CO₂ bubbles and a coarser bread crumb.

Dough recipes commonly call for 500 grams (about 1.1 pounds) of flour, which yields a single loaf of bread or two baguettes.

Calcium propionate is commonly added by commercial bakeries to retard the growth of molds.^{[citation needed]}

Flour

Main article: Flour

Flour is grain ground into a powder. Flour provides the primary structure, starch and protein to the final baked bread. The protein content of the flour is the best indicator of the quality of the bread dough and the finished bread. While bread can be made from all-purpose wheat flour, a specialty bread flour, containing more protein (12–14%), is recommended for high-quality bread. If one uses a flour with a lower protein content (9–11%) to produce bread, a shorter mixing time is required to develop gluten strength properly. An extended mixing time leads to oxidization of the dough, which gives the finished product a whiter crumb, instead of the cream color preferred by most artisan bakers.^[41]

Wheat flour, in addition to its starch, contains three water-soluble protein groups (albumin, globulin, and proteoses) and two water-insoluble protein groups (glutenin and gliadin). When flour is mixed with water, the water-soluble proteins dissolve, leaving the glutenin and gliadin to form the structure of the resulting bread. When relatively dry dough is worked by kneading, or wet dough is allowed to rise for a long time (see no-knead bread), the glutenin forms strands of long, thin, chainlike molecules, while the shorter gliadin forms bridges between the strands of glutenin. The resulting networks of strands produced by these two proteins are known as gluten. Gluten development improves if the dough is allowed to autolyse.^[42]

Fortification

Processing of flours usually involves removal of the outer layers, which contain important nutrients. Such flours, and bread made from them, may be fortified by adding nutrients. Fortification with added calcium, iron, thiamine (Vitamin B1) and niacin (Vitamin B3) is a legal requirement in the UK (wholemeal flours, from which the nutrients have not been stripped, are exempt).^[43] The unregulated term “wheatmeal” is used to describe flour containing some but not all of the outer covering and central part of the wheat grain.^[44]

Liquids

Water, or some other liquid, is used to form the flour into a paste or dough. The weight or ratio of liquid required varies between recipes, but a ratio of three parts liquid to five parts flour is common for yeast breads.^[45] Recipes that use steam as the primary leavening method may have a liquid content in excess of one part liquid to one part flour. Instead of water, recipes may use liquids such as milk or other dairy products (including buttermilk or yogurt), fruit juice, or eggs. These contribute additional sweeteners, fats, or leavening components, as well as water.^[46]

Fats or shortenings

Fats, such as butter, vegetable oils, lard, or that contained in eggs, affect the development of gluten in breads by coating and lubricating the individual strands of protein. They also help to hold the structure together. If too much fat is included in a bread dough, the lubrication effect causes the protein structures to divide. A fat content of approximately 3% by weight is the concentration that produces the greatest leavening action.^[47] In addition to their effects on leavening, fats also serve to tenderize breads and preserve freshness.

Bread improvers

Main article: Bread improver

Bread improvers and dough conditioners are often used in producing commercial breads to reduce the time needed for rising and to improve texture and volume and to give antistaling effects. The substances used may be oxidising agents to strengthen the dough or reducing agents to develop gluten and reduce mixing time, emulsifiers to strengthen the dough or to provide other properties such as making slicing easier, or enzymes to increase gas production.^[48]

Salt

Salt (sodium chloride) is very often added to enhance flavor and restrict yeast activity. It also affects the crumb and the overall texture by stabilizing and strengthening^[49] the gluten. Some artisan bakers forego early addition of salt to the dough, whether wholemeal or refined, and wait until after a 20-minute rest to allow the dough to autolyse.^[50]

Mixtures of salts are sometimes employed, such as employing potassium chloride to reduce the sodium level, and monosodium glutamate to give flavor (umami).

Leavening

See also: Unleavened bread

Leavening is the process of adding gas to a dough before or during baking to produce a lighter, more easily chewed bread. Most bread eaten in the West is leavened.^[51]

Chemicals

A simple technique for leavening bread is the use of gas-producing chemicals. There are two common methods. The first is to use baking powder or a self-raising flour that includes baking powder. The second is to include an acidic ingredient such as buttermilk and add baking soda; the reaction of the acid with the soda produces gas.^[51] Chemically leavened breads are called quick breads and soda breads. This method is commonly used to make muffins, pancakes, American-style biscuits, and quick breads such as banana bread.

Yeast

Main article: Baker’s yeast

Many breads are leavened by yeast. The yeast most commonly used for leavening bread is Saccharomyces cerevisiae, the same species used for brewing alcoholic beverages. This yeast ferments some of the sugars producing carbon dioxide. Commercial bakers often leaven their dough with commercially produced baker’s yeast. Baker’s yeast has the advantage of producing uniform, quick, and reliable results, because it is obtained from a pure culture.^[51] Many artisan bakers produce their own yeast with a growth culture. If kept in the right conditions, it provides leavening for many years.^[52]

The baker’s yeast and sourdough methods follow the same pattern. Water is mixed with flour, salt and the leavening agent. Other additions (spices, herbs, fats, seeds, fruit, etc.) are not needed to bake bread, but are often used. The mixed dough is then allowed to rise one or more times (a longer rising time results in more flavor, so bakers often “punch down” the dough and let it rise again), loaves are formed, and (after an optional final rising time) the bread is baked in an oven.^[51]

Many breads are made from a “straight dough“, which means that all of the ingredients are combined in one step, and the dough is baked after the rising time;^[51] others are made from a “pre-ferment” in which the leavening agent is combined with some of the flour and water a day or so ahead of baking and allowed to ferment overnight. On the day of baking, the rest of the ingredients are added, and the process continues as with straight dough. This produces a more flavorful bread with better texture. Many bakers see the starter method as a compromise between the reliable results of baker’s yeast and the flavor and complexity of a longer fermentation. It also allows the baker to use only a minimal amount of baker’s yeast, which was scarce and expensive when it first became available. Most yeasted pre-ferments fall into one of three categories: “poolish” or “pouliche”, a loose-textured mixture composed of roughly equal amounts of flour and water (by weight); “biga“, a stiff mixture with a higher proportion of flour; and “pâte fermentée”, which is a portion of dough reserved from a previous batch.^[53][54]

• Before first rising
• After first rising
• After proofing, ready to bake

Sourdough

Main article: Sourdough

Sourdough is a type of bread produced by a long fermentation of dough using naturally occurring yeasts and lactobacilli. It usually has a mildly sour taste because of the lactic acid produced during anaerobic fermentation by the lactobacilli. Longer fermented sourdoughs can also contain acetic acid, the main non-water component of vinegar.^[55][56][57]

Sourdough breads are made with a sourdough starter. The starter cultivates yeast and lactobacilli in a mixture of flour and water, making use of the microorganisms already present on flour; it does not need any added yeast. A starter may be maintained indefinitely by regular additions of flour and water. Some bakers have starters many generations old, which are said to have a special taste or texture.^[55] At one time, all yeast-leavened breads were sourdoughs. Recently there has been a revival of sourdough bread in artisan bakeries.^[58]

Traditionally, peasant families throughout Europe baked on a fixed schedule, perhaps once a week. The starter was saved from the previous week’s dough. The starter was mixed with the new ingredients, the dough was left to rise, and then a piece of it was saved to be the starter for next week’s bread.^[51]

Steam

The rapid expansion of steam produced during baking leavens the bread, which is as simple as it is unpredictable. Steam-leavening is unpredictable since the steam is not produced until the bread is baked. Steam leavening happens regardless of the raising agents (baking soda, yeast, baking powder, sour dough, beaten egg white) included in the mix. The leavening agent either contains air bubbles or generates carbon dioxide. The heat vaporises the water from the inner surface of the bubbles within the dough. The steam expands and makes the bread rise. This is the main factor in the rising of bread once it has been put in the oven.^[59] CO₂ generation, on its own, is too small to account for the rise. Heat kills bacteria or yeast at an early stage, so the CO₂ generation is stopped.

Bacteria

Salt-rising bread does not use yeast. Instead, it is leavened by Clostridium perfringens, one of the most common sources of food-borne illness.^[60][61]

Aeration

Aerated bread is leavened by carbon dioxide being forced into dough under pressure. From the mid-19th to mid-20th centuries, bread made this way was somewhat popular in the United Kingdom, made by the Aerated Bread Company and sold in its high-street tearooms. The company was founded in 1862, and ceased independent operations in 1955.^[62]

The Pressure-Vacuum mixer was later developed by the Flour Milling and Baking Research Association for the Chorleywood bread process. It manipulates the gas bubble size and optionally the composition of gases in the dough via the gas applied to the headspace.^[63]

Cultural significance

Main article: Bread in culture

Bread has a significance beyond mere nutrition in many cultures because of its history and contemporary importance. Bread is also significant in Christianity as one of the elements (alongside wine) of the Eucharist,^[64] and in other religions including Paganism.^[65]

In many cultures, bread is a metaphor for basic necessities and living conditions in general. For example, a “bread-winner” is a household’s main economic contributor and has little to do with actual bread-provision. This is also seen in the phrase “putting bread on the table”. The Roman poet Juvenal satirized superficial politicians and the public as caring only for “panem et circenses” (bread and circuses).^[66] In Russia in 1917, the Bolsheviks promised “peace, land, and bread.”^[67][68] The term “breadbasket” denotes an agriculturally productive region. In parts of Northern, Central, Southern and Eastern Europe bread and salt is offered as a welcome to guests.^[69] In India, life’s basic necessities are often referred to as “roti, kapra aur makan” (bread, clothes, and house).^[70]

Words for bread, including “dough” and “bread” itself, are used in English-speaking countries as synonyms for money.^[1] A remarkable or revolutionary innovation may be called the best thing since “sliced bread“.^[71] The expression “to break bread with someone” means “to share a meal with someone”.^[72] The English word “lord” comes from the Anglo-Saxon hlāfweard, meaning “bread keeper.”^[73]

Bread is sometimes referred to as “the staff of life”, although this term can refer to other staple foods in different cultures: the Oxford English Dictionary defines it as “bread (or similar staple food)”.^[74][75] This is sometimes thought to be a biblical reference, but the nearest wording is in Leviticus 26 “when I have broken the staff of your bread”.^[76] The term has been adopted in the names of bakery firms.^[77]

Fictional breads

Lembas bread: a fictional bread from The Lord of the Rings. It was given to Frodo by Galadriel and kept him alive through his journey.^{[citation needed]}

Bread of the two elders: a magical type of bread from Hungarian Folk Tales (from the Ördög és a kenyér story). It was able to talk and ward off the Ördög.^[78]

Fraud

Bread has been subject to food fraud and adulteration with fillers. In medieval times, sand was used as a filler.^[79]

The Russo-Ukrainian War has made sourcing wheat flour more challenging and raised concerns of bread flour fraud.^[

A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently or to track relative movement. Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel.

In a two pulley system, the belt can either drive the pulleys normally in one direction (the same if on parallel shafts), or the belt may be crossed, so that the direction of the driven shaft is reversed (the opposite direction to the driver if on parallel shafts). The belt drive can also be used to change the speed of rotation, either up or down, by using different sized pulleys.

As a source of motion, a conveyor belt is one application where the belt is adapted to carry a load continuously between two points.

History

[edit]

The mechanical belt drive, using a pulley machine, was first mentioned in the text of the Dictionary of Local Expressions by the Han Dynasty philosopher, poet, and politician Yang Xiong (53–18 BC) in 15 BC, used for a quilling machine that wound silk fibres onto bobbins for weavers’ shuttles.^[1] The belt drive is an essential component of the invention of the spinning wheel.^[2][3] The belt drive was not only used in textile technologies, it was also applied to hydraulic-powered bellows dated from the 1st century AD.^[2]

Power transmission

[edit]

Belts are the cheapest utility for power transmission between shafts that may not be axially aligned. Power transmission is achieved by purposely designed belts and pulleys. The variety of power transmission needs that can be met by a belt-drive transmission system are numerous, and this has led to many variations on the theme. Belt drives run smoothly and with little noise, and provide shock absorption for motors, loads, and bearings when the force and power needed changes. A drawback to belt drives is that they transmit less power than gears or chain drives. However, improvements in belt engineering allow use of belts in systems that formerly only allowed chain drives or gears.

Power transmitted between a belt and a pulley is expressed as the product of difference of tension and belt velocity:

P=(T1−T2)v,

where T1 and T2 are tensions in the tight side and slack side of the belt respectively. They are related as

T1T2=eμα,

where μ is the coefficient of friction, and α is the angle (in radians) subtended by contact surface at the centre of the pulley.

Power transmission loss form

[edit]

Belt type	Power loss[citation needed]
Cyclothane-A 83A	10% (8–14%)
Cyclothane-B 85A High Tension	20% (17–22%)
Cyclothane-A 88A HEHT	24% (18–25%)
Cyclothane-A 88A/90A matte green/blue	11% (8–16%)
Cyclothane-A 90A Super Red	15% (9–15%)
Cyclothane-A 92A	7.5% (7–12%)
Cyclothane-A 70A	15% (12–18%)
Cyclothane-E 85A	12.5% (10–14%)
Hytrel 92A	7% (5–8%)
Cyclothane 90ASD Anti-Static	9% (8-10%)
Twisted 83A belts (coiled like a spring)	18% (15–28%)
Flat belts width dependent use tension calculator	(1/2–10%)
All-polyester reinforced belts	1% (1/2–2%)

Pros and cons

[edit]

Belt drives are simple, inexpensive, and do not require axially aligned shafts. They help protect machinery from overload and jam, and damp and isolate 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 of relatively low cost if the shafts are far apart. Clutch action can be achieved by shifting the belt to a free turning pulley or by releasing belt tension. Different speeds can be obtained by stepped or tapered pulleys.

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

Flat belts

[edit]

Flat belts were widely used in the 19th and early 20th centuries in line shafting to transmit power in factories.^[4] 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. Flat belts are still used today, although not nearly as much as in the line-shaft era. The flat belt is a simple system of power transmission that was well suited for its day. It can deliver high power at high speeds (373 kW at 51 m/s; 115 mph), in cases of wide belts and large pulleys. Wide-belt-large-pulley drives are bulky, consuming much space while requiring high tension, leading to high loads, and are poorly suited to close-centers applications. V-belts have mainly replaced flat belts for short-distance power transmission; and longer-distance power transmission is typically no longer done with belts at all. For example, factory machines now tend to have individual electric motors.

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 allow the belt to self-center as it runs. Flat belts also tend to slip on the pulley face when heavy loads are applied, and many proprietary belt dressings were available that could be applied to the belts to increase friction, and so power transmission.

Flat belts were traditionally made of leather or fabric. Early flour mills in Ukraine had leather belt drives. After World War I, there was such a shortage of shoe leather that people cut up the belt drives to make shoes. Selling shoes was more profitable than selling flour for a time.^[when?] Flour milling soon came to a standstill and bread prices rose, contributing to famine conditions.^[5] Leather drive belts were put to another use during the Rhodesian Bush War (1964–1979): To protect riders of cars and busses from land mines, layers of leather belt drives were placed on the floors of vehicles in danger zones. Today most belt drives are made of rubber or synthetic polymers. Grip of leather belts is often better if they are assembled with the hair side (outer side) of the leather against the pulley, although some belts are instead given a half-twist before joining the ends (forming a Möbius strip), so that wear can be evenly distributed on both sides of the belt. Belts ends are joined by lacing the ends together with leather thonging (the oldest of the methods),^[6][7] steel comb fasteners and/or lacing,^[8] or by gluing or welding (in the case of polyurethane or polyester). Flat belts were traditionally jointed, and still usually are, but they can also be made with endless construction.

Rope drives

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In the mid 19th century, British millwrights discovered that multi-grooved pulleys connected by ropes outperformed flat pulleys connected by leather belts. Wire ropes were occasionally used, but cotton, hemp, manila hemp and flax rope saw the widest use. Typically, the rope connecting two pulleys with multiple V-grooves was spliced into a single loop that traveled along a helical path before being returned to its starting position by an idler pulley that also served to maintain the tension on the rope. Sometimes, a single rope was used to transfer power from one multiple-groove drive pulley to several single- or multiple-groove driven pulleys in this way.

In general, as with flat belts, rope drives were used for connections from stationary engines to the jack shafts and line shafts of mills, and sometimes from line shafts to driven machinery. Unlike leather belts, however, rope drives were sometimes used to transmit power over relatively long distances. Over long distances, intermediate sheaves were used to support the “flying rope”, and in the late 19th century, this was considered quite efficient.^[9][10][11]

Round belts

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Round belts are a circular cross section belt designed to run in a pulley with a 60 degree V-groove. Round grooves are only suitable for idler pulleys that guide the belt, or when (soft) O-ring type belts are used. The V-groove transmits torque through a wedging action, thus increasing friction. Nevertheless, round belts are for use in relatively low torque situations only and may be purchased in various lengths or cut to length and joined, either by a staple, a metallic connector (in the case of hollow plastic), 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.

Spring belts

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Spring belts are similar to rope or round belts but consist of a long steel helical spring. They are commonly found on toy or small model engines, typically steam engines driving other toys or models or providing a transmission between the crankshaft and other parts of a vehicle. The main advantage over rubber or other elastic belts is that they last much longer under poorly controlled operating conditions. The distance between the pulleys is also less critical. Their main disadvantage is that slippage is more likely due to the lower coefficient of friction. The ends of a spring belt can be joined either by bending the last turn of the helix at each end by 90 degrees to form hooks, or by reducing the diameter of the last few turns at one end so that it “screws” into the other end.

V belts

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V belts (also style V-belts, vee belts, or, less commonly, 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. They are generally endless, and their general cross-section shape is roughly trapezoidal (hence the name “V”). 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 1,000–7,000 ft/min (300–2,130 m/min). V-belts need larger pulleys for their thicker cross-section than flat belts.

For high-power requirements, two or more V-belts can be joined side-by-side in an arrangement called a multi-V, running on matching multi-groove sheaves. This is known as a multiple-V-belt drive (or sometimes a “classical V-belt drive”).

V-belts may be homogeneously rubber or polymer throughout, or there may be fibers embedded in the rubber or polymer for strength and reinforcement. The fibers may be of textile materials such as cotton, polyamide (such as nylon) or polyester or, for greatest strength, of steel or aramid (such as Technora, Twaron or Kevlar).

When an endless belt does not fit the need, jointed and link V-belts may be employed. Most models offer the same power and speed ratings as equivalently-sized endless belts and do not require special pulleys to operate. A link v-belt is a number of polyurethane/polyester composite links held together, either by themselves, such as Fenner Drives’ PowerTwist, or Nu-T-Link (with metal studs). These provide easy installation and superior environmental resistance compared to rubber belts and are length-adjustable by disassembling and removing links when needed.

History of V-belts

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Trade journal coverage of V-belts in automobiles from 1916 mentioned leather as the belt material,^[12] and mentioned that the V angle was not yet well standardized.^[13] The endless rubber V-belt was developed in 1917 by Charles C. Gates of the Gates Rubber Company.^{[14][non-primary source needed]} Multiple-V-belt drive was first arranged a few years later by Walter Geist of the Allis-Chalmers corporation, who was inspired to replace the single rope of multi-groove-sheave rope drives with multiple V-belts running parallel. Geist filed for a patent in 1925, and Allis-Chalmers began marketing the drive under the “Texrope” brand; the patent was granted in 1928 (U.S. patent 1,662,511). The “Texrope” brand still exists, although it has changed ownership and no longer refers to multiple-V-belt drive alone.^[15]

Multi-groove belts

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A multi-groove, V-ribbed, or polygroove belt^{[16][full citation needed]} is made up of usually between 3 and 24 V-shaped sections alongside each other. This gives a thinner belt for the same drive surface, thus it 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 causes a reduced heating effect on the belt, and a cooler-running belt lasts longer in service. Belts are commercially available in several sizes, with usually a ‘P’ (sometimes omitted) and a single letter identifying the pitch between grooves. The ‘PK’ section with a pitch of 3.56 mm is commonly used for automotive applications.^[17]

A further advantage of the polygroove belt that makes them popular is that they can run over pulleys on the ungrooved back of the belt. Though this is sometimes done with V-belts with a single idler pulley for tensioning, a polygroove belt may be wrapped around a pulley on its back tightly enough to change its direction, or even to provide a light driving force.^[18]

Any V-belt’s ability to drive pulleys depends on wrapping the belt around a sufficient angle of the pulley to provide grip. Where a single-V-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 polygroove 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.^[18]

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.

The nomenclature used for belt sizes varies by region and trade. An automotive belt with the number “740K6” or “6K740” indicates a belt 74 inches (190 cm) in length, 6 ribs wide, with a rib pitch of ⁹⁄₆₄ of an inch (3.6 mm) (a standard thickness for a K series automotive belt would be 4.5mm). A metric equivalent would be usually indicated by “6PK1880” whereby 6 refers to the number of ribs, PK refers to the metric PK thickness and pitch standard, and 1880 is the length of the belt in millimeters.^[19]

Ribbed belt

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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 is 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.

Film belts

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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 (less than 10 watts), 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

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Timing belts (also known as toothed, notch, cog, or synchronous belts) are a 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 instead 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 (4,900 m/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 and does not make the noise that some timing belts make at certain speeds, and is more efficient at transferring power (up to 98%).

The advantages of timing belts include clean operation, energy efficiency, low maintenance, low noise, non slip performance, versatile load and speed capabilities.

Disadvantages include a relatively high purchase cost, the need for specially fabricated toothed pulleys, less protection from overloading, jamming, and vibration due to their continuous tension cords, the lack of clutch action (only possible with friction-drive belts), and the fixed lengths, which do not allow length adjustment (unlike link V-belts or chains).

Specialty belts

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Belts normally transmit power on the tension side of the loop. However, designs for continuously variable transmissions exist that use belts that are a series of solid metal blocks, linked together as in a chain, transmitting power on the compression side of the loop.

Rolling roads

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Belts used for rolling roads for wind tunnels can be capable of 250 km/h (160 mph).^[20]

Standards for use

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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 unless there is a twist between each pulley so that the pulleys only contact the same belt surface. 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.

The pitch line is the line between the inner and outer surfaces that is neither subject to tension (like the outer surface) nor compression (like the inner). It is midway through the surfaces in film and flat belts and dependent on cross-sectional shape and size in timing and V-belts. Standard reference pitch diameter can be estimated by taking average of gear teeth tips diameter and gear teeth base diameter. The angular speed is inversely proportional to size, so the larger the one wheel, the less angular velocity, and vice versa. Actual pulley speeds tend to be 0.5–1% less than generally calculated because of belt slip and stretch. In timing belts, the inverse ratio teeth of the belt contributes to the exact measurement. The speed of the belt is:

Speed = Circumference based on pitch diameter × angular speed in rpm

International use standards

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Standards include:

• ISO 9563: This standard specifies requirements and test methods for endless power transmission V-belts and V-ribbed belts.
• ISO 4184: This standard specifies the dimensions of classical and narrow V-belts for general use.
• ISO 9981: This standard deals with the dimensions of rubber synchronous belt drives.
• ISO 9982: This standard covers the dimensions of polyurethane synchronous belt drives.
• DIN 22101: This standard covers the design principles for belt conveyors used in bulk material handling, including safety requirements and testing methods.
• ASME B29.1: This standard specifies the dimensions, tolerances, and quality requirements for roller chain drives, which include belts and sprockets.
• ANSI/RMA IP-20 is a standard developed by the American National Standards Institute (ANSI) and the Rubber Manufacturers Association (RMA) that focuses on elastomeric belts used in industrial applications. This standard covers important aspects such as dimensions and tolerances, ensuring that the belts perform reliably and efficiently in various industrial settings.
• SAE J1459 is a standard developed by the Society of Automotive Engineers (SAE) that focuses on automotive V-belts and V-ribbed belts. These belts are used in various automotive applications, such as power transmission between the engine and different accessories, including the alternator, power steering pump, air conditioning compressor, and water pump. The standard specifies test procedures, performance requirements, and dimensions to ensure the belts are reliable, durable, and suitable for automotive use.
• ASTM D378 is a standard developed by the American Society for Testing and Materials (ASTM), which focuses on the testing of conveyor belts used in various industries for specific applications. Conveyor belts are essential for material handling and transportation in industries such as mining, construction, agriculture, and manufacturing. ASTM D378 covers the testing methods to evaluate conveyor belts for performance characteristics, such as fire resistance and oil resistance, ensuring that they meet safety and operational requirements.^[21]

Selection criteria

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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 [N·m]) × (rotational speed [rev/min]) × (2π radians) / (60 s × 1000 W).

Factors of power adjustment include speed ratio; shaft distance (long or short); type of drive unit (electric motor, internal combustion engine); service environment (oily, wet, dusty); driven unit loads (jerky, shock, reversed); and pulley-belt arrangement (open, crossed, turned). These are found in engineering handbooks and manufacturer’s literature. When corrected, the power is compared to rated powers of the standard belt cross-sections at particular belt speeds to find a number of arrays that perform best. Now the pulley diameters are chosen. It is generally either large diameters or large cross-section that are chosen, since, as stated earlier, larger belts transmit this same power at low belt speeds as smaller belts do at high speeds. To keep the driving part at its smallest, minimal-diameter pulleys are desired. Minimum pulley diameters are limited by the elongation of the belt’s outer fibers as the belt wraps around the pulleys. Small pulleys increase this elongation, greatly reducing belt life. Minimal pulley diameters are often listed with each cross-section and speed, or listed separately by belt cross-section. After the cheapest diameters and belt section are chosen, the belt length is computed. If endless belts are used, the desired shaft spacing may need adjusting to accommodate standard-length belts. It is often more economical to use two or more juxtaposed V-belts, rather than one larger belt.

In large speed ratios or small central distances, the angle of contact between the belt and pulley may be less than 180°. If this is the case, the drive power must be further increased, according to manufacturer’s tables, and the selection process repeated. This is because power capacities are based on the standard of a 180° contact angle. Smaller contact angles mean less area for the belt to obtain traction, and thus the belt carries less power.

Belt friction

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Main article: Belt friction

Belt drives depend on friction to operate, but excessive friction wastes energy and rapidly wears the belt. Factors that affect belt friction include belt tension, contact angle, and the materials used to make the belt and pulleys.

Belt tension

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Power transmission is a function of belt tension. However, also increasing with tension is stress (load) on the belt and bearings. The ideal belt is that of the lowest tension that does not slip in high loads. Belt tensions should also be adjusted to belt type, size, speed, and pulley diameters. Belt tension is determined by measuring the force to deflect the belt a given distance per inch (or mm) of pulley. Timing belts need only adequate tension to keep the belt in contact with the pulley.

Belt wear

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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.

Belt vibration

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Vibration signatures are widely used for studying belt drive malfunctions. Some of the common malfunctions or faults include the effects of belt tension, speed, sheave eccentricity and misalignment conditions. The effect of sheave Eccentricity on vibration signatures of the belt drive is quite significant. Although, vibration magnitude is not necessarily increased by this it will create strong amplitude modulation. When the top section of a belt is in resonance, the vibrations of the machine is increased. However, an increase in the machine vibration is not significant when only the bottom section of the belt is in resonance. The vibration spectrum has the tendency to move to higher frequencies as the tension force of the belt is increased.

Belt dressing

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Belt slippage can be addressed in several ways. Belt replacement is an obvious solution, and eventually the mandatory one (because no belt lasts forever). Often, though, before the replacement option is executed, retensioning (via pulley centerline adjustment) or dressing (with any of various coatings) may be successful to extend the belt’s lifespan and postpone replacement. Belt dressings are typically liquids that are poured, brushed, dripped, or sprayed onto the belt surface and allowed to spread around; they are meant to recondition the belt’s driving surfaces and increase friction between the belt and the pulleys. Some belt dressings are dark and sticky, resembling tar or syrup; some are thin and clear, resembling mineral spirits. Some are sold to the public in aerosol cans at auto parts stores; others are sold in drums only to industrial users.

Specifications

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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^{[further explanation needed]} there is less of a distance (and therefore less addition of length) as it 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.

V-belt profiles

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Metric v-belt profiles (note pulley angles are reduced for small radius pulleys):

Classic profile	Width	Height	Angle*	Remarks
Z	10 mm	6 mm	40°
A	13 mm	9 mm	40°	12.7 mm = 0.5 inch width, 38° pulley angle imperial belts
B	17 mm	11 mm	40°	16.5 mm = 21/32 inch width, 38° angle imperial belts
C	22 mm	14 mm	40°	22.2 mm = 7/8 inch width, 38° angle imperial belts
D	32 mm	19 mm	40°	31.75 mm = 1.25 inch width, 38° angle imperial belts
E	38 mm	25 mm	40°	38.1 mm = 1.5 inch width, 38° angle imperial belts
Narrow-profile	Width	Height	Angle*	Remarks
SPZ	10 mm	8 mm	34°
SPA	13 mm	10 mm	—
SPB	17 mm	12 mm	—
SPC	22 mm	18 mm	—
High-performance narrow-profile	Width	Height	Angle*	Remarks
XPZ	10 mm	8 mm	—
XPA	13 mm	10 mm	—
XPB	17 mm	13 mm	—
XPC	22 mm	18 mm-	—

* Common pulley design is to have a higher angle of the first part of the opening, above the so-called “pitch line”.

E.g. the pitch line for SPZ could be 8.5 mm from the bottom of the “V”. In other words, 0–8.5 mm is 35° and 45° from 8.5 and above.