Sections Threaded Fasteners and Power Screws Rivets Springs Sliding Bearings Rolling-Element Bearings Spur Gears Helical, Bevel, and Worm Gears Threaded Fasteners and Power Screws Classes of Fasteners They may be classified into two broad categories: Removable and Permanent. Removable Fasteners are defined as those that can be easily removed with hand tools and without damaging any parts. Ordinary nuts and bolts are typical examples. Permanent Fasteners are those that are installed so as to remain permanently in place; they would not be removed for routine maintenance work. Such as Rivets. Advantages of threaded fasteners: Ease of assembly Ease of disassembly Can be accomplished with relative ease by unskilled workers using minimum of special tooling and in a relatively short time. Simple technology. Easily inspected. Large products can be shipped and assembled at the customer site. Replace worn components or to make adjustments. Types of Threaded Fasteners 1.Screws A screw is an external threaded fastener that is generally assembled into a blind threaded hole. Screws are by far the most common types. screws are intended for screwing into tapped holes. 2. Bolt and nut A bolt is an externally threaded fastener that is inserted through holes in the parts and screwed into a nut on the opposite side. A nut is an internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form. 3. Stud and nut A stud is threaded on both ends and is usually screwed permanently into a tapped hole. Threads on the two ends may or may not be identical. 4. Threaded rod and nutA threaded rod is the least common type. It is an external threaded fastener, but without the usual head possessed by a bolt. It is usually used when a very long threaded member is desired. A washer is a hardware component often used with threaded fasteners to ensure tightness of the mechanical joint. Thread Loosening and Thread Locking The greater the helix angle, the greater the loosening tendency. The greater the initial tightening, the greater the frictional force that must be overcome to initiate loosening. Soft or rough clamping surfaces tend to promote slight plastic flow which decreases the initial tightening tension and thus promotes loosening. Surface treatments and conditions that tend to increase the friction coefficient provide increased resistance to loosening. Slotted and Castle Nuts Are used with a cotter pin or wire that fits in diametrically opposite slots and passes through a drilled hole in the bolt. This provides a positive lock but may require slight over tightening or under tightening in order to align a pair of slots with the bolt hole. Terms of Thread External Thread – a thread on the outside of a cylinder (Bolt, Screw) Internal thread – a thread on the inside of a hole (Nut, threaded hole) Right Handed – Thread advances when turn Clockwise with respect to its mating part Left Handed – Thread advances when turn counter-Clockwise with respect to its mating part? What is Power Screws? Power screws (linear actuators) used to convert rotary motion of either the nut or the screw to relatively slow linear motion of the mating member along the screw axis. The sliding friction between screw and nut threads has been replaced with approximate rolling contact between the balls and the grooves in the screw and nut. There is sliding friction in place of the static friction. Its friction decreases drastically, with efficiencies commonly 90 percent or higher. Ball-Bearing Screw to obtain a great mechanical advantage in lifting weights. (in screw-type jacks). ?to exert large forces, (in presses and tensile machine, home garbage compactors, and C-clamps). ?obtain precise positioning of the axial movement ( micrometer screws or the lead screw of a lathe). Properties of Ball-Bearing Screw ?a brake must be used to hold a load in place. ?it also means that the screw is reversible. (that is the linear motion can be converted to relatively rapid rotary motion). ? Smooth operation without the “slip-stick” action commonly observed in regular power screws). ?The load-carrying capacity of ball-bearing screws is normally greater than that of regular power screws of the same diameter. ? Cleanliness and a thin film of lubricant are important to ball-bearing screws. ?Where dry operation is unavoidable, load capacity should be reduced by as much as 90 percent. Rivets Rivets – metal pin used to make a permanent fastening between two or more materials. An unthreaded, headed pin used to join two parts by passing the pin through holes in the parts and then forming a second head in the pin on the opposite side by hammering, pressing or forging· Helical Extension Springs Springs are elastic members that exert forces, or torques, and absorb energy, which is usually stored and later released. Torsion Bar Spring Spiral springs are used to power clocks and toys. Spiral springs are usually made of thin rectangular wire. Helical Compression Spring Automatic “over-load stop”A broken compression spring may continue to provide a “stop” that holds the end plates apart. Helical Extension Springs Extension springs do not have the automatic “over-load stop” feature. A sufficient static overload can elongate any extension spring to failure. Compression springs are generally preferred to extension springs in safety critical applications. Torsion Springs Torsion springs are of two general types: helical, and spiral. Helical springs are used to counterbalance garage overhead doors and to roll up window shades, the angular deflection can be several complete turns. Long helical springs with many turns usually have a center rod for support. Spiral springs are used to power clocks and toys. Spiral springs are usually made of thin rectangular wire. Bearing A machine or structural part that supports another part. A device that supports, guides, and reduces the friction of motion between fixed and moving machine parts. Something that supports weight. The part of an arch or beam that rests on a support. A machine or structural part that supports another part. A device that supports, guides, and reduces the friction of motion between fixed and moving machine parts. Something that supports weight. The part of an arch or beam that rests on a support. A machine or structural part that supports another part. A device that supports, guides, and reduces the friction of motion between fixed and moving machine parts. Something that supports weight. The part of an arch or beam that rests on a support. Journal Bearings The general principle is that a shaft rotates inside a bearing bore slightly larger than the shaft diameter, and a lubricant is supplied to the annular gap. The portion of the shaft within the bearing is called the At low speeds, the fluid wedge does not fully develop, and partial contact occurs. The coefficient of friction f is about 0.10. This is called Boundary Lubrication. As speed increases, contact occurs less frequently, and f decreases into the range of .004 < f < 0.10. This is Mixed Lubrication. Finally, under hydrodynamic lubrication, no contact occurs, and 0.002 < f < 0.010. Advantages of Journal Bearings ?are well suited for high rotating speeds with impact and momentary overloads. The higher the rotating speed, the more effective the hydrodynamic pumping action. ?do not normally generate noise and may dampen noise from other sources. ?are less expensive than ball or roller bearings for requiring minimal lubrication provisions. Elements of Bearings These parts include: inner rings: connection to the rotating shaft Outer rings: connection to the stationary housing. Rolling elements: such as balls or rollers Cage (retainer or separator): which keeps the ball or rollers evenly spaced and separated. Seals: keep grease in the bearing and dirt and debris out of it. Gears Gears, defined as toothed members transmitting rotary motion from one shaft to another, are among the oldest devices and inventions of man. A gear is a wheel with teeth along its rim. It is used to transmit effort from one shaft to another。 Terms of Spur Gearset The pinion and gear are regarded as two cylinders that are pressed against one another and smoothly roll together. Speed In Gearset As the pinion rotates, the speed of a point on its pitch circle is vp=rpùp. LiKewise, that of gear is vg=rgùg Because the velocity of points in contact are the same, the velocity ratio is: Gear Train Combination of two or more meshed gears. Used to change the rate of rotation, the direction of rotation, and the amount of torque. Planetary Geartrains Planetary geartrains: the centers of certain gears move. And their motion is similar in some respects to the orbit of a planet around a star. The planet gear rotates about its own center, meshes with the sun gear, and orbits as a whole about the center of the sun gear. The link that connects the centers of the two gears is called the carrier. It can rotate as well about one of its ends. When multiple planet gears are present to reduce noise, vibration and force, the carrier is sometimes called the spider. Bevel Gear The teeth shape of bevel gear are like ordinary spur gears, but the tooth surfaces are made up of conical elements. Helical Gear The teeth of helical gears are still formed on a cylinder. However the teeth are not parallel to the gear’s shaft. Instead, the teeth on a helical gear are inclined at an angle so that each tooth wraps on the gear’s cylinder in the shape of a shallow helix. Property of Helical Gear As helical gears rotate, each tooth comes first into engagement on one side, with contact spreading across the tooth as rotation continues. Transmit motion between parallel shafts. Helix angle is the same on each gear, but one gear must have a right-hand helix and the other a left-hand helix. Applications of Helical Gear lLess noise lLess vibration lHigher rotating speeds lGreater torque and power lComplex to analyze and manufacture lSubjected to a thrust load, This can be eliminated by using double helical or herringbone gears. Worm and Worm Gear Worm gears are similar to crossed helical gears. The pinion or worm has a small number of teeth, usually one to four, and since they completely wrap around the pitch cylinder they are called threads. Its mating gear is called a worm gear. For each revolution of the worm, the worm gear advances by just on tooth in its rotation. SIGNIFICANT FIGURE “Significant figures” are all the digits in a measurement that are known with certainty plus a last digit that must be estimated. The last significant digit in a measured quantity is the first digit of uncertainty TRAPPED ZEROES Trapped zeroes are zeroes between two nonzero digits. Trapped zeroes are always significant. 101 s – 3 significant figures 20013 m – 5 significant figures 0.3006 cm – 4 significant figures (the leading zero is not significant)