[Pawl And Ratchet Mechanism Pdf Download
Abdul Soumphonphakdy
A ratchet (occasionally spelled rachet) is a mechanical device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Ratchets are widely used in machinery and tools. The word ratchet is also used informally to refer to a ratcheting socket wrench.
A ratchet consists of a round gear or a linear rack with teeth, and a pivoting, spring-loaded finger called a pawl (or click, in clocks and watches[1][2]) that engages the teeth. The teeth are uniform but are usually asymmetrical, with each tooth having a moderate slope on one edge and a much steeper slope on the other edge.
When the teeth are moving in the unrestricted (i.e. forward) direction, the pawl easily slides up and over the gently sloped edges of the teeth, with a spring forcing it (often with an audible 'click') into the depression between the teeth as it passes the tip of each tooth. When the teeth move in the opposite (backward) direction, however, the pawl will catch against the steeply sloped edge of the first tooth it encounters, thereby locking it against the tooth and preventing any further motion in that direction.
In theoretical statistical physics, the concept of a ratchet, often termed a "Brownian Ratchet," is used to explain the origin of directed motion arising from a combination of time-reversal symmetry breaking and left-right symmetry breaking. When non-thermal forces (e.g. alternating pushes and pulls) are applied to an asymmetric substrate (e.g. an asymmetric gear), directed motion generically appears. This principle is known as the "Ratchet Principle" or "Curie's Principle," after Pierre Curie.[3]
While the ratchets referenced in statistical physics are typically at the molecular or microscopic scales, the concept was inspired by the ratchet and pawl in its introductions by Marian Smoluchowski and Richard Feynman.
Yi Zhang
with
Susan Finger
Stephannie BehrensTable of Contents Chapter 8. Other Mechanisms 8.1 Ratchet Mechanisms A wheel provided with suitably shaped teeth, receiving an intermittentcircular motion from an oscillating or reciprocating member, is calleda ratchet wheel. A simple form of ratchet mechanism is shownin Figure 8-1. Figure 8-1 RatchetA is the ratchet wheel, and B is an oscillatinglever carrying the driving pawl, C. A supplementarypawl at D prevents backward motion of the wheel. When arm B moves counterclockwise, pawl C will force thewheel through a fractional part of a revolution dependent upon themotion of B. When the arm moves back (clockwise), pawl Cwill slide over the points of the teeth while the wheel remains atrest because of fixed pawl D, and will be ready to push thewheel on its forward (counterclockwise) motion as before. The amount of backward motion possible varies with the pitch of theteeth. This motion could be reduced by using small teeth, and theexpedient is sometimes used by placing several pawls side by side onthe same axis, the pawls being of different lengths. The contact surfaces of wheel and pawl should be inclined so that theywill not tend to disengage under pressure. This means that the commonnormal at N should pass between the pawl and the ratchet-wheelcenters. If this common normal should pass outside these limits, thepawl would be forced out of contact under load unless held byfriction. In many ratchet mechanisms the pawl is held against thewheel during motion by the action of a spring. The usual form of the teeth of a ratchet wheel is that shown in theabove Figure, but in feed mechanisms such as used on many machinetools it is necessary to modify the tooth shape for a reversible pawlso that the drive can be in either direction.The following SimDesign example of a ratchet also includes a four bar linkage. If you try this mechanism, you may turn the crank of the link mechanism. The rocker will drive the driving pawl to drive the ratchet wheel. The corresponding SimDesign datafile is:/afs/andrew.cmu.edu/cit/ce/rapidproto/simdesign/ratchet.sim8.2 Overrunning ClutchA special form of a ratchet is theoverrunning clutch. Have you ever thought about what kind ofmechanism drives the rear axle of bicycle? It is a free-wheelmechanism which is an overrunning clutch. Figure 8-2 illustrates asimplified model. As the driver delivers torque to the driven member,the rollers or balls are wedged into the tapered recesses. This iswhat gives the positive drive. Should the driven member attempt todrive the driver in the directions shown, the rollers or balls becomefree and no torque is transmitted. Figure 8-2 Overrunning clutch8.3 Intermittent GearingA pair of rotating members may be designed so that, for continuousrotation of the driver, the follower will alternately roll with thedriver and remain stationary. This type of arrangement is know by thegeneral term intermittent gearing. This type of gearing occursin some counting mechanisms, motion-picture machines, feed mechanisms,as well as others. Figure 8-3 Intermittent gearingThe simplest form of intermittent gearing, as illustrated in Figure 8-3has the same kind of teeth as ordinary gears designed forcontinuous rotation. This example is a pairof 18-tooth gears modified to meet the requirement that the follower advance one-ninth of a turn for each turn of the driver. The intervalof action is the two-pitch angle (indicated on both gears). The singletooth on the driver engages with each space on the follower toproduce the required motion of a one-ninth turn of the follower. Duringthe remainder of a driver turn, the follower is locked againstrotation in the manner shown in the figure. To vary the relative movements of the driver and follower, the meshingteeth can be arranged in various ways to suit requirements. Forexample, the driver may have more than one tooth, and the periods ofrest of the follower may be uniform or may vary considerably. Countingmechanisms are often equipped with gearing of this type. 8.4 The Geneva WheelAn interesting example of intermittentgearing is the Geneva Wheel shown in Figure 8-4. In thiscase the driven wheel, B, makes one fourth of a turn forone turn of the driver, A, the pin, a,working in the slots, b, causing the motion of B.The circular portion of the driver, coming in contact with thecorresponding hollow circular parts of the driven wheel, retains it inposition when the pin or tooth a is out of action. The wheelA is cut away near the pin a as shown, to provideclearance for wheel B in its motion.Figure 8-4 Geneva wheelIf one of the slots is closed, A can only move through part ofthe revolution in either direction before pin a strikes theclosed slot and thus stops the motion. The device in this modifiedform was used in watches, music boxes, etc., to preventoverwinding. From this application it received the name Genevastop. Arranged as a stop, wheel A is secured to the springshaft, and B turns on the axis of the spring barrel. Thenumber of slots or interval units in B depends upon the desirednumber of turns for the spring shaft. An example of this mechanism has been made in SimDesign, as in the following picture.The corresponding SimDesign data file is:/afs/andrew.cmu.edu/cit/ce/rapidproto/simdesign/geneva.sim8.5 The Universal Joint The engine of an automobile is usually located in front part. How doesit connect to the rear axle of the automobile? In this case,universal joints are used to transmit the motion. Figure 8-5 Universal jointThe universal joint as shown in Figure 8-5 is also known in theolder literature as Hooke's coupling. Regardless of how it isconstructed or proportioned, for practical use it has essentially theform shown in Figure 8-6, consisting of two semicircular forks 2and 4, pin-jointed to a right -angle cross 3. Figure 8-6 General form for a universal jointThe driver 2 and the follower 4 make the complete revolution at thesame time, but the velocity ratio is not constant throughout therevolution. The following analysis will show how complete informationas to the relative motions of driver and follower may be obtained forany phase of the motion. 8.5.1 Analysis of a Universal Joint Figure 8-7 Analysis of a universal jointIf the plane of projection is taken perpendicular to the axis of 2,the path of a and b will be a circle AKBL asshown in Figure 8-7. If the angle between the shafts is , the path of c andd will be a circle that is projected as the ellipseACBD, in whichOC = OD = OKcos =OAcos(8-1)If one of the arms of the driver is at A, an arm of thefollower will be at C. If the driver arm moves through theangle to P, thefollower arm will move to Q. OQ will be perpendicularto OP; hence: angle COQ = . But angle COQ is theprojection of the real angle describes by the follower. Qn isthe real component of the motion of the follower in a directionparallel to AB, and line AB is the intersection of theplanes of the driver's and the follower's planes. The true angle described by the follower, whilethe driver describes the angle , can be found by revolvingOQ about AB as an axis into the plane of the circleAKBL. Then OR = the true length of OQ, andROK = = the trueangle that is projected as angle COQ = .Nowtan= Rm/Om andtan= Qn/OnBut Qn = RmHenceThereforetan=costanThe ratio of the angular motion of the follower to that of the driveris found as follower, by differentiating above equation, rememberingthat is constant Eliminating: Similarly, can be eliminated: According to the above equations, when the driver has a uniformangular velocity, the ratio of angular velocities varies betweenextremes of cos and1/cos. Thesevariations in velocity give rise to inertia forces, torques, noise,and vibration which would not be present if the velocity ratio wereconstant. 8.5.2 Double Universal JointBy using a double joint shown on the right in Figure 8-7, the variation of angular motion between driver andfollower can be entirely avoided. This compensating arrangement is toplace an intermediate shaft 3 between the driver and followershafts. The two forks of this intermediate shaft must lie in the sameplane, and the angle between the first shaft and the intermediateshaft must exactly be the same with that between the intermediateshaft and the last shaft. If the first shaft rotates uniformly, theangular motion of the intermediate shaft will vary according to theresult deduced above. This variation is exactly the same as if thelast shaft rotated uniformly, driving the intermediateshaft. Therefore, the variable motion transmitted to the intermediateshaft by the uniform rotation of the first shaft is exactlycompensated for by the motion transmitted from the intermediate to thelast shaft, the uniform motion of either of these shafts will impart,through the intermediate shaft, uniform motion to the other. Universal joints, particularly in pairs, are used in manymachines. One common application is in the drive shaft which connectsthe engine of an automobiles to the axle. Table of Contents Complete Table of Contents1 Physical Principles2 Mechanisms and Simple Machines3 More on Machines and Mechanisms4 Basic Kinematics of Constrained Rigid Bodies5 Planar Linkages6 Cams7 Gears8 Other Mechanisms 8.1 Ratchet Mechanisms 8.2 Overrunning Clutch 8.3 Intermittent Gearing 8.4 The Geneva Wheel 8.5 The Universal Joint 8.5.1 Analysis of a Universal Joint 8.5.2 Double Universal Joint IndexReferences
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