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Mechanisms or machine elements make up the basic components of mechanical systems. The primary purpose of a mechanism is to transfer or transform force from one form or direction into another.
The most basic elements of mechanisms were described as "simple machines" by Renaissance scientists and included the following:
Gears and cams were developed as an offshoot of several of these elements and are also important types of mechanisms. The classical concept of decomposing machines into these simple elements still has relevance today, though there are elements that do not fall directly into these categories. Mechanisms and simple machines can be thought of as the building blocks of more complex machines.
Machine elements include components that allow power to be transmitted from one mechanism to another. Elements such as bearings, couplings, clutches, brakes, belts, and chains are examples of components that facilitate movement.
7.1 Cam-Driven Devices
One method of translating rotary motion into linear motion is the use of a cam on a rotating shaft. By offsetting the center of a round or oval disc on the shaft, the cam surface will vary in distance from the shaft center. This can be used to drive a shaft linearly; the linear shaft is often called a follower. Springs are used to keep the end of the follower in contact with the cam as it rotates. FIG. 50 shows a cam and follower arrangement.
Cams are used extensively in line shaft-type applications. They allow stations to be synchronized with a master motor and run at higher speeds than standard asynchronous applications. The downside is that they are more difficult to modify movement profiles as the cams must be machined or replaced.
7.2 Ratchet and Pawl Systems
A ratchet is a mechanism that allows a linear or rotary movement in one direction only. Movement in the opposite direction is prevented by a spring-loaded pawl that engages teeth on the ratchet as it turns. The teeth are angled such that the pawl cannot be forced out of the slots between the ratchet teeth when the ratchet is reversed. FIG. 51 illustrates this arrangement.
Ratchet and pawl systems are used in lifting mechanisms such as jacks and winding mechanisms, and even plastic cable ties. Ratchet gearing may be used to transmit intermittent motion or to simply prevent the reverse movement of the gear.
7.3 Gearing and Gear Reduction
Gearing is used to translate rotary motion from one speed, direction, or force into another. A gear is a mechanism, usually round, that has teeth that engage with another toothed device. The mating interface between the two machine parts is called a spline.
Gears can be combined into "trains," which can change speeds and therefore torque outputs incrementally rather than all at once.
They may have teeth on the outside or inside circumference or a combination of both. Gear tooth profiles are almost always slightly curved in a shape called an involute curve. This curve is based on the diameter of the gear and is important in keeping gear movement and interfacing smooth and consistent.
Gear reduction is the process of converting a high-speed low torque component, such as a servomotor, into a lower-speed higher torque output without creating excessive backlash. This is usually done with a self-contained gearbox, which also may change the shaft or rotation direction.
The simplest type of gear is the cylindrical spur gear. Spur gears only mesh if the gear axles are parallel with each other. There are two types of spur gears: internal and external. External spur gears transmit drive between parallel shafts, making them rotate in opposite directions. This is illustrated in FIG. 52. They work well at moderate speeds but can be noisy at higher speeds. Internal spur gears are spur gear arrangements that transmit motion to shafts rotating in the same direction.
Helical gears have teeth cut at an angle to the axis. As with the spur gear arrangements, they can be arranged in an internal or external manner. Unlike spur gears, however, helical gears can also mesh on nonparallel axes. Whereas spur gears must be parallel but produce thrust only perpendicular to the load, helical gears produce axial thrust when arranged with parallel shafts. They may be arranged with the shafts at an angle to each other, even completely perpendicular- his is known as a crossed helical gear arrangement. FIG. 53 illustrates a set of helical gears at a slight angle to each other.
Helical gears can take more torque than spur gears and are often used in higher-speed applications. The higher torque causes more axial thrust, however, which can be mitigated by using double helical gears. These are the equivalent of two mirrored helical gears stacked on top of each other and are sometimes called herringbone gears.
Because of the more gradual engagement of the teeth, helical gears are quieter than spur gears. The more complex profile makes double helical gears more expensive than helical or spur gears.
Bevel gears are conical gears with the teeth cut at an angle to the shaft. They are designed to connect two shafts on intersecting axes, as shown in FIG. 54. Straight bevel gears have teeth that are cut radially toward the apex of the conical section. Spiral bevel gears have curved oblique teeth that reduce noise and improve the smoothness of meshing between gears, similar to the effect of the helical gear.
Hypoid bevel gears are a combination of spiral bevel gears and worm gears. The axes of these gears do not intersect. The distance between the axes is called the offset. Hypoid bevel gears allow a greater gear ratio than regular bevel gears, making them a good choice for gear reduction in mechanical differentials.
Crown gears have teeth that project perpendicular to the plane of the gear or parallel with the shaft. They are considered to be part of the bevel gear grouping and are sometimes called contrate gears.
They may be meshed with other bevel-type gears or spur gears.
A worm gear is used to transmit motion at a right angle to its shaft.
They have line tooth contact and are often meshed with disk-type gears, sometimes called the wheel or worm wheel. A worm gear resembles a screw and may have one or more toothed tracks running around it, as shown in FIG. 55. Because the pitch of the worm can be made quite small, high ratios of gear reduction can be achieved; however, this is at the cost of efficiency. When a worm and gear combination is used, the worm can always drive the gear; however, the reverse is not always true. If the ratio is high enough, the teeth may lock together because the force exerted by the wheel cannot overcome the friction of the worm gear. This can be an advantage if it is preferable to hold a worm-driven object in position against the force of gravity.
Worm gearing can be divided into two general categories-fine and coarse pitch gearing. The main purpose of fine pitch gearing is to transmit motion rather than power, while the opposite is true of coarse pitch.
A rack is a linear toothed rod or bar that usually engages with a round gear called a pinion gear. This is a common method of converting rotational motion into linear motion and vice versa. As with other gear types, teeth may be cut straight or at an angle to the axis of motion. A rack is often used in gear theory as a gear of infinite radius.
A rack and pinion system is an excellent method of moving a linear axis rapidly over a long distance. The rack is usually the fixed component, and the pinion gear is rotated from the traveling part of the system, which is guided by linear bearings. FIG. 56 shows an industrial rack and pinion.
Planetary or epicyclic gearing is a method of combining gears in such a way that one or more of the gear axes is movable, usually one rotating around another. There are various arrangements that accomplish this using bevel or spur-type gears. Epicyclic gearing is a very compact method of achieving gear reduction and is often used in servo gearboxes. FIG. 57 shows a cutaway of a planetary gearbox.
7.4 Bearings and Pulleys
Bearings allow sliding or rolling contact between two or more parts.
They fall into three general categories based on their purpose: radial bearings that support rotating shafts or journals, thrust bearings that support axial loads on rotating elements, and guide bearings that support and guide moving elements in a straight line. Bearings are also often described by the principle of operation or the direction of the applied load.
Bearings that provide sliding contact are known as plain bearings.
Relative motion between the parts of plain bearings may be of the lubricated type, either a hydrodynamic interface (a wedge or film buildup of lubricant is produced by the bearing surface) or hydrostatic interface (a lubricant is introduced into the mating surfaces under pressure). The motion interface may also be unlubricated with a material such as nylon, brass, or Teflon. Plain bearings are also known as bushings when they operate on a shaft.
Rolling contact-type bearings use rolling elements, such as balls or rollers, in place of lubricants or direct contact. FIG. 58 shows a cutaway of a roller bearing with cylindrical rollers. Roller bearings generally have a much lower friction coefficient than plain bearings and therefore have less energy loss. They also generally hold tighter tolerances and are consequently more precise. Normally, the rolling elements and the races they ride in are hardened to reduce wear.
Roller-type bearings are also generally shielded or sealed to reduce the chance of contaminants entering the bearing races.
The use of linear roller bearings and rails is one of the most common guiding methods for linear movement. They often support ball screw or worm gear-driven axes in motion control applications.
A rail with two bearing blocks is shown in FIG. 59.
An air bearing is a pneumatic device that uses a film of air between surfaces. They are often used in moving heavy loads on a floor surface, similar to a hovercraft or air hockey table. Rotary, spindle, and slide air bearings provide almost no resistance to motion and are very precise. Air bearings may be externally pressurized with a continuous flow or generate a film of air from the relative motion of the two surfaces.
A pulley, sometimes called a sheave, is a wheel or drum mounted on an axle. It generally has a groove or channel between two flanges that carries a belt, chain, or cable. Pulleys can change the direction or speed of a motion, much in the same manner as gearing. Pulleys of different dimensions transfer speed changes in proportion to the diameters or circumferences of the pulleys. For example, if a pair of pulleys has a diameter ratio of 2 to 1, the speed will be increased proportionally.
Pulleys are often used with flexible belts in industrial applications.
It is common to use a steel-reinforced toothed belt in a belt-driven actuator to provide linear motion. In this case, the pulley will also have grooves in the surface parallel to the shaft, as shown in FIG. 60. These are called positive drive belts. Belt and pulley combinations without positive drive are known as friction drives.
The number of degrees that the belt is in contact with the pulley is called the wrap angle. Belt and pulley combinations should be selected such that the wrap angle on the smaller pulley is at least 120° for a friction drive belt. For a toothed belt, the wrap angle can be closer to 90°. Any less than this can cause the belt to skip teeth.
Pulleys can be arranged in combinations that provide mechanical advantage along with the speed reduction. For any given pair of pulleys, it is usually best to not exceed a ratio of 8 to 1, and 6 to 1 is a reasonable maximum. If a greater ratio is required, it is best to use a compound drive of several pulleys. This is illustrated in FIG. 61.
A servomechanism is a combination of mechanical and control hardware that uses feedback to affect the control of a system. The feedback is in the form of an error or difference between the sensed parameter and its desired value. Servomechanisms generally operate on the principle of negative feedback, where the error is subtracted from the output.
A servomechanism is known as a closed-loop system. This has already been described in PID Control (section 2.2.2) and the Servomotor (section 3.6.4) sections of this book; however, it is important to mention that servomechanisms control not only position, speed, and torque, but may control variables such as temperature, pressure, or anything measurable. An example of a servomechanism that does not involve a servomotor is a hydraulic actuator that has its speed and position controlled by a spool valve using feedback from an externally mounted analog position sensor. FIG. 62 illustrates the physical layout of a servo system.
7.6 Ball Screws and Belt-Driven Linear Actuators
A ball screw is a mechanical linear actuator that translates rotary motion to linear motion with little friction. A threaded shaft provides a spiral raceway for ball bearings, which act as a precision screw. The ball assembly acts as the nut, while the threaded shaft is the screw.
They are made to close tolerances and are therefore suitable for use in situations where high precision is necessary.
The pitch of a ball screw determines the potential linear speed of a linear actuator as well as its ability to hold a load against the force of gravity. Higher pitch (more turns per inch) provides more precision and a greater ability to stop a vertical load from turning the screw, but it requires a higher motor speed to move at the same rate as a lower pitch screw. FIG. 63 shows a 12 mm ball screw and nut with a 4 mm screw pitch.
Belt-driven linear actuators use a toothed belt and gears to move a carriage attached to the belt. The linear speed of a belt-driven actuator is typically faster than that of a ball screw but can be less rugged and precise. Belt-driven actuators are more likely to slip with a heavy load; toothed belts can even be damaged if the load is beyond the rating of the actuator as teeth can be stripped off the belt. FIG. 64 shows a cutaway of an actuator showing the belt inside the sealed housing.
7.7 Linkages and Couplings
A linkage is a combination of rigid elements and hinges or joints that constrain element movement. A linkage can be used to multiply or translate force or motion between mechanisms or components.
Examples of linkages are scissor lifts and four bar linkages.
The simplest linkage is the lever. By pinning a point on the length of the lever to a fixed location, the lever pivots around that point, the fulcrum. A point nearer to the fulcrum will rotate in a smaller arc than a point farther from the fulcrum, creating a multiplication of distance or velocity. With this comes a reduction in the force output; the reverse of this is also true, with a large movement driving a smaller one, the force output is multiplied proportionally. This is known as a mechanical advantage.
A four bar linkage is a group of four joints and four bars that allows points on the linkage to move in restrained ways when one or two of the joints are fixed in a location. Depending on the lengths of the bars and whether the joints can rotate through a full circular motion, different arcs and motions can be created, as shown in FIG. 65.
This is a type of mechanism that uses a four bar linkage with two degrees of freedom known as a pantograph. It allows shapes to be duplicated at scaled sizes-a tool especially useful for engraved lettering. Similar arrangements can be used to constrain movement in mechanisms.
A common type of mechanism that uses a toggle linkage is the clamp. A toggle mechanism is a type of four bar linkage that folds and locks at a certain position. Some pneumatically driven clamps perform a rotate-and-lower movement that is partially cam based, but a manual toggle clamp uses a four bar linkage to produce a high clamping force, as shown in FIG. 66. A well-known manufacturer of toggle and other types of clamps is DE-STA-CO.
A coupling connects two shafts or rotating members together. A coupling can be rigid or flexible. A rigid coupling has the advantage of keeping shafts precisely aligned and holding the ends very securely.
Flexible couplings allow for misalignment and tend to dampen vibration. Helical flexible couplings are commonly used to drive a shaft with a servomotor, AC motor, or DC motor in automated systems. Another name for a helical coupling is a beam coupling; an example is shown in FIG. 67.
A Lovejoy or spider coupling has two metal-toothed hubs and an elastomer insert or "spider" to reduce vibrations. The three parts fit together with a press fit. These are also used extensively in servo systems but are not as forgiving to angular misalignment as helical couplings.
Universal joints allow shafts to be driven at an angle, as do gear joints. These are often used in power transmission. For lighter applications, hoses or bellows-type couplings are sometimes used for nonaligned shafts.
7.8 Clutches and Brakes
A clutch allows two rotating elements to be engaged and disengaged with each other. The most common type of clutch is a friction clutch.
There are several different types of friction clutch, including conical, radially expanding, contraction band, and friction disk clutches. It is common for the driving and driven members of friction clutches to be held in contact with each other by spring pressure. They may then be actuated to the open position by pneumatic, hydraulic, electrical, magnetic, or even centrifugal force. Clutch surfaces can be made of a variety of materials, including metals, ceramics, resin-wire fabric, and rubbers with a high durometer.
Brakes are used to rapidly stop a rotating member, also using frictional methods. Typically, brakes use a disk or pad to contact a rotating surface, such as a plate or shaft. In industrial applications, most brakes are composed of semimetallic, organic, or ceramic materials. Like clutches, brakes may be actuated in a number of ways, including pneumatics and hydraulics, electrical, or mechanical methods.
A clutch brake is a device that either engages a clutch to provide motion from a rotating to a nonrotating member or quickly brings the driven member to a stop when the clutch is not engaged. Clutch brakes are used extensively in conveyor applications and often in other servo and motor-driven machinery.