Dashpot

A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring (which acts to resist displacement). The process and instrumentation diagram (P&ID) symbol for a dashpot is . Simplified diagram of linear dashpot Types Two common types of dashpots exist - linear and rotary. Linear dashpots are generally specified by stroke (amount of linear displacement) and damping coefficient (force per velocity). Rotary dashpots will have damping coefficients in torque per angular velocity. A less common type of dashpot is an eddy current damper, which uses a large magnet inside a tube constructed of a non-magnetic but conducting material (such as aluminium or copper). Like a common viscous damper, the eddy current damper produces a resistive force proportional to velocity. Dashpots frequently use a one-way mechanical bypass to permit fast unrestricted motion in one direction and slow motion using the dashpot in the opposite direction. This permits, for example, a door to be opened quickly without added resistance, but then to close slowly using the dashpot. For hydraulic dashpots this unrestricted motion is accomplished using a one-way check-valve that allows fluid to bypass the dashpot fluid constriction. Non-hydraulic dashpots may use a ratcheting gear to permit free motion in one direction. Applications Dashpot in a Zenith-Strombergcarburetor A dashpot is a common component in a door closer to prevent it from slamming shut. A spring applies force to close the door, and the dashpot forces fluid to flow through an orifice between reservoirs (the orifice is often adjustable), which slows the motion of the door. Consumer electronics often use dashpots where it is undesirable for a media access door or control panel to suddenly pop open when the door latch is released. The dashpot provides a steady, gentle motion until the access door has fully opened. Dashpots are commonly used in dampers and shock absorbers. The hydraulic cylinder in an automobile shock absorber is a dashpot. They are also used on carburetors, where the return of the throttle lever is cushioned just before the throttle fully closes, then is allowed to fully close slowly to reduce emissions. The British SU carburettor's main piston carries a stepped needle. This needle is held in the fuel flow orifice. The manifold vacuum causes this piston to rise allowing more fuel into the airflow. The SU's dashpot has a fixed hydraulic piston, damping the main piston as it moves upward. A valve in the piston disables the damping as the...

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Coil spring

A coil spring, also known as a helical spring, is a mechanical device which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces. They are made of an elastic material formed into the shape of a helixwhich returns to its natural length when unloaded. Under tension or compression, the material (wire) of a coil spring undergoes torsion. The spring characteristics therefore depend on the shear modulus, not Young's Modulus. A coil spring may also be used as a torsion spring: in this case the spring as a whole is subjected to torsion about its helical axis. The material of the spring is thereby subjected to a bending moment, either reducing or increasing the helical radius. In this mode, it is the Young's Modulus of the material that determines the spring characteristics. Metal coil springs are made by winding a wire around a shaped former - a cylinder is used to form cylindrical coil springs. Coil springs for vehicles are typically made of hardened steel. A machine called an auto-coiler takes spring wire that has been heated so it can easily be shaped. It is then fed onto a lathe that has a metal rod with the desired coil spring size. The machine takes the wire and guides it onto the spinning rod as well as pushing it across the rod to form multiple coils. The spring is then ejected from the machine and an operator will put it in oil to cool off. The spring is then tempered to lose the brittleness from being cooled. The coil size and strength can be controlled by the lathe rod size and material used. Different alloys are used to get certain characteristic’s out of the spring, such as stiffness, dampening and strength  A compression coil spring A tension coil spring A selection of conical coil springs Oxy-cut spring showing deformation due to loss of tempering in adjacent turn Spring Rate Spring rate is the measurement of how much a coil spring can hold until it compresses 1 inch. The spring rate is normally specified by the manufacture. If a Spring has a rate of 100 then the spring would compress 1 inch with 100lbs of load.  Variants Volute spring suspension on an M4 Sherman tank Types of coil spring are: Tension/extension coil springs, designed to resist stretching. They usually have a hook or eye form at each end for attachment. Compression coil springs, designed to resist being compressed. A typical use for compression coil springs is in car suspension systems. Volute springs are...

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Crank (mechanism)

A crank is an arm attached at a right angle to a rotating shaft by which reciprocating motion is imparted to or received from the shaft. It is used to convert circular motion into reciprocating motion, or vice versa. The arm may be a bent portion of the shaft, or a separate arm or disk attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod (conrod). The end of the rod attached to the crank moves in a circular motion, while the other end is usually constrained to move in a linear sliding motion. The term often refers to a human-powered crank which is used to manually turn an axle, as in a bicycle crankset or a brace and bit drill. In this case a person's arm or leg serves as the connecting rod, applying reciprocating force to the crank. There is usually a bar perpendicular to the other end of the arm, often with a freely rotatable handle or pedal attached. Hand crank of a winch on a sailing boat. A compound crank Examples Hand crank on a pencil sharpener Animation of a multi-cylinder engine Familiar examples include: Hand-powered cranks Mechanical pencil sharpener Fishing reel and other reels for cables, wires, ropes, etc. Manually operated car window The carpenter's brace is a compound crank. The crank set that drives a handcycle through its handles. Hand wiches. Foot-powered cranks The crankset that drives a bicycle via the pedals. Treadle sewing machine Engines Almost all reciprocating engines use cranks (with connecting rods) to transform the back-and-forth motion of the pistons into rotary motion. The cranks are incorporated into a crankshaft. History East Asia Han China Tibetan operating a quern(1938). The upright handle of such rotary handmills, set at a distance from the centre of rotation, works as a crank. The earliest hand-operated cranks appeared in China during the Han Dynasty (202 BC-220 AD), as Han era glazed-earthenware tomb models portray, and was used thereafter in China for silk-reeling and hemp-spinning, for the agricultural winnowing fan, in the water-powered flour-sifter, for hydraulic-powered metallurgic bellows, and in the well windlass. In order to create a handle by means of a wheel to easily rotate their grain winnowers, the Chinese invented the crank handle and applied the centrifugal fan principle in the 2nd century BC. The crank handle was used in well-windlasses, querns, mills, and many silk making machines. The rotary winnowing fan greatly increased the efficiency of separating grain from husks and stalks. Harvesting grain by the use of rotary winnowing fan would not reach the Western World until the eighteenth century, where harvested grain was initially thrown up in the air by shovels or winnowing baskets. However, the potential of the crank...

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Stroke (engine)

In the context of an Internal combustion engine, the term stroke has the following related meanings: A phase of the engine's cycle (eg compression stroke, exhaust stroke), during which the piston travels from top to bottom or vice-versa. The type of power cycle used by a piston engine (eg two-stroke engine, four-stroke engine). "Stroke length", the distance travelled by the piston in each cycle. The stroke length- along with bore diameter- determines the engine's displacement. Phases in the power cycle] The phases/strokes of a four-stroke engine. 1: intake 2: compression 3: power 4: exhaust Commonly-used engine phases/strokes (ie those used in a four-stroke engine) are described below. Other types of engines can have very different phases. Induction/Intake stroke The induction stroke is the first phase in a four-stroke internal combustion engine cycle. It involves the downward movement of the piston, creating a partial vacuum that draws a fuel/air mixture (or air alone, in the case of a direct injection engine) into the combustion chamber. The mixture enters the cylinder through an intake valve at the top of the cylinder. Compression stroke The compression stroke is the second of four stages in an otto cycle or diesel cycle internal combustion engine. In this stage, the fuel/air mixture (or air alone, in the case of a direct injection engine) is compressed to the top of the cylinder by the piston. This is the result of the piston moving upwards, reducing the volume of the chamber. Towards the end of this phase, the mixture is ignited — by a spark plug for petrol engines or by self-ignition for diesel engines. Combustion/Power/Expansion stroke The combustion stroke is the third phase, where the ignited air/fuel mixture expands and pushes the piston downwards. The force created by this expansion is what creates an engine's power. Exhaust stroke The exhaust stroke is the final stage in a four stroke internal combustion engine cycle. In this stage, the piston moves upwards, squeezing out the gasses that were created during the combustion stroke. The gasses exit the cylinder through an exhaust valve at the top of the cylinder. At the end of this phase, the exhaust valve closes and the intake valve opens, which then closes to allow a fresh air/fuel mixture into the cylinder so the process can repeat itself. Types of power cycles The thermodynamic cycle used by a piston engine is often described by the number of strokes to complete a cycle. The most common designs of for engines are two-stroke and four-stroke. Less common designs include five-stroke engines, six-stroke engines and two-and-four stroke engines. Two-stroke engine Two-stroke engines complete a power cycle every two strokes, which means a power cycle is completed with every crankshaft revolution. Two-stroke engines are commonly used in (typically large) marine engines, outdoor power tools (e.g....

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Diesel cycle

The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated during the compression of air in the combustion chamber, into which fuel is then injected. This is in contrast to igniting the fuel-air mixture with a spark plug as in the Otto cycle (four-stroke/petrol) engine. Diesel engines are used in aircraft, automobiles, power generation, diesel-electric locomotives, and both surface ships and submarines. The Diesel cycle is assumed to have constant pressure during the initial part of the combustion phase ( to  in the diagram, below). This is an idealized mathematical model: real physical diesels do have an increase in pressure during this period, but it is less pronounced than in the Otto cycle. In contrast, the idealized Otto cycle of a gasoline engine approximates a constant volume process during that phase. Idealized Diesel cycle p-V Diagram for the ideal Diesel cycle. The cycle follows the numbers 1-4 in clockwise direction. The image on the left shows a p-V diagram for the ideal Diesel cycle; where  is pressure and V the volume or  the specific volume if the process is placed on a unit mass basis. The ideal Diesel cycle follows the following four distinct processes: Process 1 to 2 is isentropic compression of the fluid (blue) Process 2 to 3 is reversible constant pressure heating (red) Process 3 to 4 is isentropic expansion (yellow) Process 4 to 1 is reversible constant volume cooling (green) The Diesel engine is a heat engine: it converts heat into work. During the bottom isentropic processes (blue), energy is transferred into the system in the form of work , but by definition (isentropic) no energy is transferred into or out of the system in the form of heat. During the constant pressure (red, isobaric) process, energy enters the system as heat . During the top isentropic processes (yellow), energy is transferred out of the system in the form of , but by definition (isentropic) no energy is transferred into or out of the system in the form of heat. During the constant volume (green, isochoric) process, some of energy flows out of the system as heat through the right depressurizing process . The work that leaves the system is equal to the work that enters the system plus the difference between the heat added to the system and the heat that leaves the system; in other words, net gain of work is equal to the difference between the heat added to the system and the heat that leaves the system. Work in () is done by the piston compressing the air (system) Heat in () is done by the combustion of the fuel Work out ()...

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Otto cycle

An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine.It is the thermodynamic cycle most commonly found in automobile engines. The Otto cycle is a description of what happens to a mass of gas as it is subjected to changes of pressure, temperature, volume, addition of heat, and removal of heat. The mass of gas that is subjected to those changes is called the system. The system, in this case, is defined to be the fluid (gas) within the cylinder. By describing the changes that take place within the system, it will also describe in inverse, the system's effect on the environment. In the case of the Otto cycle, the effect will be to produce enough net work from the system so as to propel an automobile and its occupants in the environment. The Otto cycle is constructed from: Top and bottom of the loop: a pair of quasi-parallel and isentropic processes (frictionless, adiabatic reversible). Left and right sides of the loop: a pair of parallel isochoric processes (constant volume). The isentropic process of compression or expansion implies that there will be no inefficiency (loss of mechanical energy), and there be no transfer of heat into or out of the system during that process. Hence the cylinder, and piston are assumed impermeable to heat during that time. Work is performed on the system during the lower isentropic compression process. Heat flows into the Otto cycle through the left pressurizing process and some of it flows back out through the right depressurizing process. The summation of the work added to the system plus the heat added minus the heat removed yields the net mechanical work generated by the system. Thermodynamics The classical Carnot heat engine Branches Laws Systems System properties Material properties Equations Potentials History Culture Scientists Book:Thermodynamics v t e Pressure–volume diagram Temperature-Entropy diagram The idealized diagrams of a four-stroke Otto cycle Both diagrams: the  intake (A)  stroke is performed by an isobaric expansion, followed by an adiabatic compression (B)  stroke. Through the combustion of fuel, heat is added in a constant volume (isochoric process) process, followed by an adiabatic expansion process power (C) stroke. The cycle is closed by the  exhaust (D)  stroke, characterized by isochoric cooling and isentropic compression processes. Processes The processes are described by: Process 0–1 a mass of air is drawn into piston/cylinder arrangement at constant pressure. Process 1–2 is an adiabatic (isentropic) compression of the charge as the piston moves from bottom dead centre (BDC) to top dead centre (TDC). Process 2–3 is a constant-volume heat transfer to the working gas from an external source while the piston is at top...

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