DIFFERENT KINDS OF MECHANISMS

CRANK

n a mechanical linkage or mechanism, a link that can turn about a center of rotation. The crank's center of rotation is in the pivot, usually the axis of a crankshaft, that connects the crank to an adjacent link. A crank is arranged for complete rotation (360°) about its center; however, it may only oscillate or have intermittent motion. A bell crank is frequently used to change direction of motion in a linkage (see illustration)

Cranks (a) for changing radius of rotation, and (b) for changing direction of translation.

BELL CRANK

A bell crank is a type of crank that changes motion through an angle. The angle can be any angle from 0 to 360 degrees, although 90 degrees and 180 degrees are common. The name comes from its first use, changing the vertical pull on a rope to a horizontal pull on the striker of a bell used for calling staff in large houses or commercial establishments.

A typical 90 degree bell crank consists of an "L" shaped crank pivoted where the two arms of the L meet. Moving rods (or cables or ropes) are attached to the ends of the L arms. When one is pulled, the L rotates around the pivot point, pulling on the other arm.

A typical 180 degree bell crank consists of a straight bar pivoted in the center. When one arm is pulled or pushed, the bar rotates around the pivot point, pulling or pushing on the other arm.

Changing the length of the arms changes the mechanical advantage of the system. Many applications do not change the direction of motion, but instead to amplify a force "in line", which a bell cranks, can do in a limited space. There is a tradeoff between range of motion, linearity of motion, and size. The greater the angle traversed by the crank, the more non-linear the motion becomes (the more the motion ratio changes).

Bell cranks are often used in aircraft control systems to connect the pilot's controls to the control surfaces. For example: on light aircraft, the rudder often has a bell crank whose pivot point in the rudder hinge. A cable connects the pilot's rudder pedal to one side of the bell crank. When the pilot pushes on the rudder pedal, the rudder rotates on it's hinge. The opposite rudder pedal is connected to the other end of the bell crank to rotate the rudder in the opposite direction.

Bell cranks are also seen in automotive applications, as part of the linkage connecting the throttle pedal to the carburetor, and connecting the brake pedal to the master brake cylinder.

Slider-crank mechanism

A four-bar linkage with output crank and ground member of infinite length. A slider crank is most widely used to convert reciprocating to rotary motion (as in an engine) or to convert rotary to reciprocating motion (as in pumps), but it has numerous other applications. Positions at which slider motion reverses are called dead centers. When crank and connecting rod are extended in a straight line and the slider is at its maximum distance from the axis of the crankshaft, the position is top dead center (TDC); when the slider is at its minimum distance from the axis of the crankshaft, the position is bottom dead center (BDC).

Principal parts of slider-crank mechanism

Bevel Gear

Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone.

The bevel gear is used to change the axis of rotational motion. By using gears of differing numbers of teeth the speed of rotation can also be changed.

Spur gear

Spur gears are the most common type of gears. They have straight teeth, and are mounted on parallel shafts. Sometimes, many spur gears are used at once to create very large gear reductions.

CAM A cam is a rotating or sliding

piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice versa. It is often a part of a rotating wheel (e.g. an eccentric wheel) or shaft (e.g. a cylinder with an irregular shape) that strikes a lever at one or more points on its circular path. The cam can be a simple tooth, as is used to deliver pulses of power to a steam hammer, for example, or an eccentric disc or other shape that produces a smooth reciprocating (back and forth) motion in the follower which is a lever making contact with the cam. Gear ratios out of this gear set.

Classification of cams. (a) Translating. (b) Disk. (c) Positive motion. (d) Cylindrical. (e) With yoke follower. (f) With flat-face follower

Universal joint

A universal joint, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft.

Roller chain

Roller chain or bush roller chain is the type of chain most commonly used for transmission of mechanical power on bicycles, motorcycles, and in industrial and agricultural machinery. It is a simple, reliable, and efficient means of power transmission.

GENEVA DRIVE

The Geneva drive or Maltese cross is a mechanism that translates a continuous rotation into an intermittent rotary motion. It is an intermittent gear where the drive wheel has a pin that reaches into a slot of the driven wheel and thereby advances it by one step. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position between steps.

In the most common arrangement, the driven wheel has four slots and thus advances for each rotation of the drive wheel by one step of 90°. If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.

INTERNAL GENEVA DRIVE

Besides the external Geneva drive, there is also an internal Geneva drive. The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses. The axis of the drive wheel of the internal Geneva drive can have a bearing only on one side. The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180° in an external Geneva drive and always greater than 180° in an internal one, where the switch time is therefore greater than the time the driven wheel stands still.

PISTON

This mechanism is used to convert between rotary motion and reciprocating motion, it works either way. Notice how the speed of the piston changes. The piston starts from one end, and increases its speed. It reaches maximum speed in the middle of its travel then gradually slows down until it reaches the end of its travel.

LINKAGE

A mechanical linkage is a series of rigid links connected with joints to form a closed chain, or a series of closed chains. Each link has two or more joints, and the joints have various degrees of freedom to allow motion between the links. A linkage is called a mechanism if two or more links are movable with respect to a fixed link. Mechanical linkages are usually designed to take an input and produce a different output, altering the motion, velocity, acceleration, and applying mechanical advantage.

FOUR BAR LINKAGE

A typical four bar mechanism, as the name denotes, is formed of a kinematic chain of four members connected by revolute joints. This mechanism can have four possible configurations with a different link fixed as frame each time.

Types of four-bar linkages, s = shortest link, l = longest link

The plane four-bar linkage (Fig. 1) consists of four pin-connected links forming a closed loop, in which all pin axes are parallel. The spherical four-bar linkage consists of four pin-connected links forming a closed loop, in which all pin axes intersect at one point. The skew four-bar linkage (Fig. 2) consists of four jointed links forming a closed loop, in which crank 2 and link 4 are pin-connected to ground 1 and the axes of the pins are generally nonparallel and nonintersecting; coupler 3 is connected to crank 2 and link 4 by ball joints.

Fig. 1 Plane four-bar linkage with joints at A, B, C, and D. φ, ψ, and μ are angles defining orientations of joints.

Fig. 2 perpendicular between axes of pin joints at A and D; φ, ψ, and ξ are angles defining orientations of joints.">Skew four-bar linkage with joints at A, B, C, and D. OA = f; ED = g; OE = common perpendicular between axes of pin joints at A and D; φ, ψ, and ξ are angles defining orientations of joints.

Four-bar linkages are most frequently used to convert a uniform continuous rotation (the motion of crank 2) into a no uniform rotation or oscillation (the motion of link 4). In instrument applications the primary function of the linkage is the conversion of motion, while in power applications both motion conversion and power transmission are fundamental.

Each of the above linkages can be proportioned for three types of motion, or linkage types: crank-and-rocker, drag, and double-rocker.

Crank-and-rocker linkages have a motion in which the crank (link 2) is capable of unlimited rotation, while the output link (link 4) oscillates or rocks through a fraction of one turn (usually less than 90°). This is the most common form of the plane and the skew four-bar linkage, and is used in machinery and appliances of all types.

In drag linkages the motions of cranks 2 and 4 are both capable of unlimited rotations. The plane drag linkage has been used for quick-return motions. The most common drag linkage is the spherical drag linkage. One such linkage is the Hooke-type universal joint, or hooke joint. See also Universal joint.

In double-rocker linkages, neither crank 2 nor 4 is capable of complete rotations. Such motions occur in hand tools and mechanical equipment in which only limited rotations are required.

RACK AND PINION

The rack and pinion is used to convert between rotary and linear motion. The rack is the flat, toothed part, the pinion is the gear. Rack and pinion can convert from rotary to linear of from linear to rotary.

RECIPROCATOR

This mechanism converts rotary motion to reciprocating motion in two axis.

QUICK RETURN MECHANISM

A quick return mechanism such as the one seen below is used where there is a need to convert rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and backwards. Many machines have this type of mechanism and in the school workshop the best example is the shaping machine.