The wheel and axle is a simple machine.
The traditional form as recognised in 19th century textbooks is as shown in the image. This also shows the most widely recognised application, ie lifting water from a well. The form consists of a wheel that turns an axle and in turn a rope converts the rotational motion to linear motion for the purpose of lifting.
By considering the machine as a torque multiplier, ie the output is a torque, items such as gears and screwdrivers can fall within this category.
The doorknobs is an example of the same form as the water well, the mechanism uses the axle as a pinion to withdraw the latch.
The simple Chain Fall is another example. Here the user pulls on the wheel using the input chain, so the input motion is actually linear.
The Screwdrivers is an examples of the rotational form. The diameter of the handle gives a mechanical advantage (compared to a screwdriver with no handle!)
Gears are examples of the rotational form.
Sunday, July 13, 2008
Mechanical Advantage
The beam shown is in static equilibrium around the fulcrum. This is due to the moment created by vector force "A" counterclockwise (moment A*a) being in equilibrium with the moment created by vector force "B" clockwise (moment B*b). The relatively low vector force "B" is translated in a relatively high vector force "A". The force is thus increased in the ratio of the forces A : B, which is equal to the ratio of the distances to the fulcrum b : a. This ratio is called the mechanical advantage. This idealised situation does not take into account friction. For more explanation, see also lever.
Wheel and axle notion (e.g. Screwdrivers, doorknobs): A wheel is essentially a lever with one arm the distance between the axle and the outer point of the wheel, and the other the radius of the axle. Typically this is a fairly large difference, leading to a proportionately large mechanical advantage. This allows even simple wheels with wooden axles running in wooden blocks to still turn freely, because their friction is overwhelmed by the rotational force of the wheel multiplied by the mechanical advantage.
Pulley: Pulleys change the direction of a tension force on a flexible material, e.g. a rope or cable. In addition, pulleys can be "added together" to create mechanical advantage, by having the flexible material looped over several pulleys in turn. More loops and pulleys increases the mechanical advantage.
Wheel and axle notion (e.g. Screwdrivers, doorknobs): A wheel is essentially a lever with one arm the distance between the axle and the outer point of the wheel, and the other the radius of the axle. Typically this is a fairly large difference, leading to a proportionately large mechanical advantage. This allows even simple wheels with wooden axles running in wooden blocks to still turn freely, because their friction is overwhelmed by the rotational force of the wheel multiplied by the mechanical advantage.
Pulley: Pulleys change the direction of a tension force on a flexible material, e.g. a rope or cable. In addition, pulleys can be "added together" to create mechanical advantage, by having the flexible material looped over several pulleys in turn. More loops and pulleys increases the mechanical advantage.
transmissions
Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, in support of pumping, milling, and hoisting.
Most modern gearboxes either reduce an unsuitable high speed and low torque of the prime mover output shaft to a more stable lower speed with higher torque, or do the opposite and provide a mechanical advantage (i.e increase in torque) to allow higher forces to be generated. Some of the simplest gearboxes merely change the physical direction in which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed
Most modern gearboxes either reduce an unsuitable high speed and low torque of the prime mover output shaft to a more stable lower speed with higher torque, or do the opposite and provide a mechanical advantage (i.e increase in torque) to allow higher forces to be generated. Some of the simplest gearboxes merely change the physical direction in which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed
Environmental effects
Trucks contribute to air, noise, and water pollution in a similar fashion to automobiles. In the case of air pollution emissions, trucks may actually emit lower emissions than autos on a per pound of vehicle mass basis, although the absolute level on a vehicle mile traveled basis is higher and diesel soot is especially problematic for health.[2] With respect to noise pollution trucks emit considerably higher sound levels at all speeds compared to typical automobiles; this contrast is particularly strong in the case of heavy duty trucks.[3]
Concerns have been raised about the effect of trucking on the environment, particularly as part of the debate on global warming. In the period from 1990 to 2003, carbon dioxide emissions from transportation sources increased by 20%, despite improvements in vehicle fuel efficiency.[4]
In 2005, Transportation accounted for 27% of U.S. greenhouse gas emission, increasing faster than any other sector.[5]
Between 1985 and 2004, in the U.S., energy consumption in freight transportation grew nearly 53%, while the number of ton-miles carried increased only 43%.[6] "Modal shifts account for a nearly a 23% increase in energy consumption over this period. Much of this shift is due to a greater fraction of freight ton-miles being carried via truck and air, as compared to water, rail, and pipelines."
According to a 1995 U.S. Government estimate, the energy cost of carrying a ton of freight a distance of one mile averages 514 Btu for water, 337 Btu for rail, 3,100 for trucks and nearly 20,000 for air transport.[7] and many environment organizations favor laws and incentives to encourage the switch from road to rail, especially in Europe.
Concerns have been raised about the effect of trucking on the environment, particularly as part of the debate on global warming. In the period from 1990 to 2003, carbon dioxide emissions from transportation sources increased by 20%, despite improvements in vehicle fuel efficiency.[4]
In 2005, Transportation accounted for 27% of U.S. greenhouse gas emission, increasing faster than any other sector.[5]
Between 1985 and 2004, in the U.S., energy consumption in freight transportation grew nearly 53%, while the number of ton-miles carried increased only 43%.[6] "Modal shifts account for a nearly a 23% increase in energy consumption over this period. Much of this shift is due to a greater fraction of freight ton-miles being carried via truck and air, as compared to water, rail, and pipelines."
According to a 1995 U.S. Government estimate, the energy cost of carrying a ton of freight a distance of one mile averages 514 Btu for water, 337 Btu for rail, 3,100 for trucks and nearly 20,000 for air transport.[7] and many environment organizations favor laws and incentives to encourage the switch from road to rail, especially in Europe.
Chassis
The chassis or frame of a truck is commonly constructed mainly of two beams, and several crossmembers. A truck chassis consists of two parallel straight C-shaped beams, or in some cases stepped or tapered beams, these held together by crossmembers. In most instances, gussets help attach the crossmembers to the beams. The "C-shape" of the beams has a middle vertical and longer side, and a short horizontal flange at each end; the length of the beams is variable. The chassis is usually made of steel, but can be made (whole or in part) of aluminium for a lighter weight. The integrity of the chemical composition (carbon, molybdenum, etc.) and structure of the beams is of uttermost importance to its strength, and to help prevent cracking or breaking of beams, and to help maintain rigidity and flexibility of the frame, welding, drilling and other types of modifications should not be performed by unlicenced persons. The chassis is the main structure of the truck, and the other parts attach to it. A tow bar may be found attached at one or both ends.
Drivetrain
Small trucks use the same type of transmissions as almost all cars which have either an automatic transmission or a manual transmission with synchronisers. Bigger trucks often use manual transmissions without synchronisers which have less bulk and weight although synchromesh transmissions are used in larger trucks as well. Transmissions without synchronisers known as "crash boxes" require double clutching for each shift, (which can lead to repetitive motion injuries), or a technique known colloquially as "floating," a method of changing gears which doesn't use the clutch, except for starts and stops, due to the physical effort of double clutching especially with non power assisted clutches, faster shifts, and less clutch wear. Double clutching allows the driver to control the engine and transmission revolutions to synchronize, so that a smooth shift can be made e.g. when upshifting, accelerator pedal is released and the clutch pedal is depressed while the gear lever is moved in to neutral, clutch pedal is then released and quickly pushed down again while the gear lever is moved to the next highest gear. Finally, the clutch pedal is released and accelerator pedal pushed down to obtain required engine rpms. Although this is a relatively fast movement perhaps a second or so while transmission is in neutral it allows the engine speed to drop and synchronize engine and transmission revolutions relative to the road speed.
Engine
Trucks can use all sorts of engines. Small trucks such as SUVs or pickups, and even light medium-duty trucks in North America and Russia will use gasoline engines. Most heavier trucks use four stroke turbo intercooler diesel engines, although there are alternatives. Huge off-highway trucks use locomotive-type engines such as a V12 Detroit Diesel two stroke engine.
North American manufactured highway trucks almost always use an engine built by a third party, such as CAT, Cummins, or Detroit Diesel. The only exceptions to this are Volvo and its subsidiary Mack Trucks, which are available with own engines. Freightliner, Sterling Trucks and Western Star, subsidaries of DaimlerChrysler, are available with Mercedes-Benz and Detroit Diesel engines. Trucks and buses built by the Navistar International can also contain International engines. The Swedish truckmaker Scania claims they stay away from the U.S.-market because of this third party tradition.
In the European union all truck engines must comply with Euro 4 regulations, the regulations will become more restrictive in 2008 with the introduction of Euro 5.
Formerly in Britain some lorries were coal-fired steam-powered: for more information see Traction engine.
North American manufactured highway trucks almost always use an engine built by a third party, such as CAT, Cummins, or Detroit Diesel. The only exceptions to this are Volvo and its subsidiary Mack Trucks, which are available with own engines. Freightliner, Sterling Trucks and Western Star, subsidaries of DaimlerChrysler, are available with Mercedes-Benz and Detroit Diesel engines. Trucks and buses built by the Navistar International can also contain International engines. The Swedish truckmaker Scania claims they stay away from the U.S.-market because of this third party tradition.
In the European union all truck engines must comply with Euro 4 regulations, the regulations will become more restrictive in 2008 with the introduction of Euro 5.
Formerly in Britain some lorries were coal-fired steam-powered: for more information see Traction engine.
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