Axle in the context of Leading wheel

A wheel is a rotating component (typically circular in shape) that is intended to turn on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be moved easily facilitating movement or transportation while supporting a load, or performing labor in machines. Wheels are also used for other purposes, such as a ship's wheel, steering wheel, potter's wheel, and flywheel.

Common examples can be found in transport applications. A wheel reduces friction by facilitating motion by rolling together with the use of axles. In order for a wheel to rotate, a moment must be applied to the wheel about its axis, either by gravity or by the application of another external force or torque.

The wheel and axle is a simple machine, consisting of a wheel attached to a smaller axle so that these two parts rotate together, in which a force is transferred from one to the other. While the wheel and axle exist separately from each other, they are only a simple machine when used in conjunction. The wheel and axle can be viewed as a version of the lever, with a drive force applied tangentially to the perimeter of the wheel, and a load force applied to the axle supported in a bearing, which serves as a fulcrum.

A pulley is a wheel on an axle or shaft enabling a taut cable or belt passing over the wheel to move and change direction, or transfer power between itself and a shaft.

A pulley may have a groove or grooves between flanges around its circumference to locate the cable or belt. The drive element of a pulley system can be a rope, cable, belt, or chain.

A tire (North American English) or tyre (Commonwealth English) is a ring-shaped component that surrounds a wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide traction on the surface over which the wheel travels. Most tires, such as those for automobiles and bicycles, are pneumatically inflated structures, providing a flexible cushion that absorbs shock as the tire rolls over rough features on the surface. Tires provide a footprint, called a contact patch, designed to match the vehicle's weight and the bearing on the surface that it rolls over by exerting a pressure that will avoid deforming the surface.

The materials of modern pneumatic tires are synthetic rubber, natural rubber, fabric, and wire, along with carbon black and other chemical compounds. They consist of a tread and a body. The tread provides traction while the body provides containment for a quantity of compressed air. Before rubber was developed, tires were metal bands fitted around wooden wheels to hold the wheel together under load and to prevent wear and tear. Early rubber tires were solid (not pneumatic). Pneumatic tires are used on many vehicles, including cars, bicycles, motorcycles, buses, trucks, heavy equipment, and aircraft. Metal tires are used on locomotives and railcars, and solid rubber (or other polymers) tires are also used in various non-automotive applications, such as casters, carts, lawnmowers, and wheelbarrows.

A portable engine is an engine, either a steam engine or an internal combustion engine, that sits in one place while operating (providing power to machinery), but (unlike a stationary engine) is portable and thus can be easily moved from one work site to another. Mounted on wheels or skids, it is either towed to the work site or moves there via self-propulsion.

Portable engines were in common use in industrialised countries from the early 19th through early 20th centuries, during an era when mechanical power transmission was widespread. Before that, most power generation and transmission were by animal, water, wind, or human; after that, a combination of electrification (including rural electrification) and modern vehicles and equipment (such as tractors, trucks, cars, engine-generators, and machines with their engines built in) displaced most use of portable engines. In developing countries today, portable engines still have some use (typically in the form of modern small engines mounted on boards), although the technologies mentioned above increasingly limit their demand there as well. In industrialised countries they are no longer used for commercial purposes, but preserved examples can often be seen at steam fairs driving appropriate equipment for demonstration purposes.

A gear or gearwheel, also called a toothed wheel, is a rotating machine part typically used to transmit rotational motion or torque by means of a series of "teeth" that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or cavities machined on the part, or separate pegs inserted into it. In the latter case, the gear is usually called a cogwheel. A cog may be one of those pegs or the whole gear. Two or more meshing gears are called a gear train.

The smaller member of a pair of meshing gears is often called pinion. Most commonly, gears and gear trains can be used to trade torque for rotational speed between two axles or other rotating parts or to change the axis of rotation or to invert the sense of rotation. A gear may also be used to transmit linear force or linear motion to a rack, a straight bar with a row of compatible teeth.

A toll road, also known as a tollway, or (mainly in the US) a turnpike, pike or expressway, is a public or private road for which a fee (or toll) is assessed for passage. It is a form of road pricing typically implemented to help recoup the costs of road construction and maintenance.

Toll roads have existed in some form since antiquity, with tolls levied on passing travelers on foot, wagon, or horseback; a practice that continued with the automobile, and many modern tollways charge fees for motor vehicles exclusively. The amount of the toll usually varies by vehicle type, weight, or number of axles, with freight trucks often charged higher rates than cars.

The pedal is the part of a bicycle that the rider pushes with their foot to propel the vehicle. It provides the connection between the cyclist's foot or shoe and the crank allowing the leg to turn the bottom bracket spindle and propel the bicycle's wheels. A pedal usually consists of a spindle that threads into the end of the crank, and a body on which the foot rest is attached, that is free to rotate on bearings with respect to the spindle.

Pedals were initially attached to cranks connecting directly to the driven (usually front) wheel. The safety bicycle, as it is known today, came into being when the pedals were attached to a crank driving a sprocket that transmitted power to the driven wheel by means of a roller chain.

A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface.

The term originally referred to portions of a log that had been riven (split lengthwise) into four or six sections. The radial members of a wagon wheel were made by carving a spoke (from a log) into their finished shape. A spokeshave is a tool originally developed for this purpose. Eventually, the term spoke was more commonly applied to the finished product of the wheelwright's work than to the materials they used.

A train wheel or rail wheel is a type of wheel specially designed for use on railway tracks. The wheel acts as a rolling component, typically press fitted on to an axle and mounted directly on a railway carriage or locomotive, or indirectly on a bogie (CwthE) or truck (NAmE). The powered wheels under the locomotive are called driving wheels. Wheels are initially cast or forged and then heat-treated to have a specific hardness. New wheels are machined using a lathe to a standardised shape, called a profile. All wheel profiles are regularly checked to ensure proper interaction between the wheel and the rail. Incorrectly profiled wheels and worn wheels can increase rolling resistance, reduce energy efficiency and may even cause a derailment. The International Union of Railways has defined a standard wheel diameter of 920 mm (36 in), although smaller sizes are used in some rapid transit railway systems and on ro-ro carriages.

A wheelset is a pair of railroad vehicle wheels mounted rigidly on an axle. Wheelsets are often mounted in a bogie ("truck" in North America) – a pivoted frame assembly holding at least two wheelsets – at each end of the vehicle. Most modern freight cars and passenger cars have bogies each with two wheelsets, but three wheelsets (or more) are used in bogies of freight cars that carry heavy loads, and three-wheelset bogies are under some passenger cars. Four-wheeled goods wagons that were once near-universal in Europe and Great Britain and their colonies have only two wheelsets; in recent decades such vehicles have become less common as trainloads have become heavier.

A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the part of the shaft in contact with the bearing slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.

Plain bearings, in general, are the least expensive type of bearing. They are also compact and lightweight, and they have a high load-carrying capacity.

Machine element or hardware refers to an elementary component of a machine. These elements consist of three basic types:

1. structural components such as frame members, bearings, axles, splines, fasteners, seals, and lubricants,
2. mechanisms that control movement in various ways such as gear trains, belt or chain drives, linkages, cam and follower systems, including brakes and clutches, and
3. control components such as buttons, switches, indicators, sensors, actuators and computer controllers.

While generally not considered to be a machine element, the shape, texture and color of covers are an important part of a machine that provide a styling and operational interface between the mechanical components of a machine and its users.

A rotaxane (from Latin rota 'wheel' and axis 'axle') is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation). The two components of a rotaxane are kinetically trapped since the ends of the dumbbell (often called stoppers) are larger than the internal diameter of the ring and prevent dissociation (unthreading) of the components since this would require significant distortion of the covalent bonds.

Much of the research concerning rotaxanes and other mechanically interlocked molecular architectures, such as catenanes, has been focused on their efficient synthesis or their utilization as artificial molecular machines. However, examples of rotaxane substructure have been found in naturally occurring peptides, including: cystine knot peptides, cyclotides or lasso-peptides such as microcin J25.

A bogie (/ˈboʊɡi/ BOH-ghee) (or truck in North American English) comprises two or more wheelsets (two wheels on an axle), in a frame, attached under a vehicle by a pivot. Bogies take various forms in various modes of transport. A bogie may remain normally attached (as on many railroad cars and semi-trailers) or be quickly detachable (as for a dolly in a road train or in railway bogie exchange). It may include suspension components within it (as most rail and trucking bogies do), or be solid and in turn be suspended (as are most bogies of tracked vehicles). It may be mounted on a swivel, as traditionally on a railway carriage or locomotive, additionally jointed and sprung (as in the landing gear of an airliner), or held in place by other means (centreless bogies).

Although bogie is the preferred spelling and first-listed variant in various dictionaries, bogey and bogy are also used.

A single-decker bus or single-deck bus, is a bus that has a single deck for passengers. Normally the use of the term single-decker refers to a standard two-axled rigid bus, in direct contrast to the use of the term double-decker bus, which is essentially a bus with two passenger decks and a staircase. These types of single-deckers may feature one or more doors, and varying internal combustion engine positions. The majority of single-deckers have a length of up to 12 m (39 ft 4 in), although some exceptions of longer buses exist. They also typically weigh between 11 and 14 t (12 and 15 short tons).

In regions where double-deckers are not common, the term single-decker may lack common usage, as in one sense, all other main types of bus have a single deck. Also, the term may become synonymous with the name transit bus or related terms, which can correctly be applied to double-deckers too.

An articulated bus, also referred to as a slinky bus, bendy bus, tandem bus, vestibule bus, stretch bus, or an accordion bus, is an articulated vehicle, typically a motor bus or trolleybus, used in public transportation. It is usually a single-decker, and comprises two or more rigid sections linked by a pivoting joint (articulation) enclosed by protective bellows inside and outside, and a cover plate on the floor. This allows a longer legal length than rigid-bodied buses, and hence a higher passenger capacity (94–120), while still allowing the bus to maneuver adequately.

Due to their high passenger capacity, articulated buses are often used as part of bus rapid transit schemes, and can include a mechanical guidance system and electric bus or trolleybus.Articulated buses are typically 18 m (59 ft) long, in contrast to standard rigid buses at 11 to 14 m (36 to 46 ft) long. The common arrangement of an articulated bus is to have a forward section with two axles leading a rear section with a single axle, with the driving axle mounted on either the front or the rear section. Some articulated buses have a steering arrangement on the rearmost axle that turns slightly in opposition to the front steering axle, allowing the vehicle to negotiate tighter turns, similar to hook-and-ladder fire trucks operating in city environments. A less common variant of the articulated bus is the bi-articulated bus, where the vehicle has two trailer sections rather than one. Such vehicles have a capacity of around 200 people, and a length of about 25 m (82 ft); as such, they are used almost exclusively on high-capacity, high-frequency arterial routes and on bus rapid transit services.