Analysis And Usage Of Cams And Their Followers Engineering Essay

Analysis And Usage Of Cams And Their Followers Engineering Essay A cam follower, also known as a track follower,[1] is a specialized type of roller or needle bearing designed to follow cams. Cam followers come in a vast array of different configurations, however the most defining characteristic is how the cam follower mounts to its mating part; stud style cam followers use a stud while the yoke style has a hole through the middle.[2] The first cam follower was invented and patented in 1937 by Thomas L. Robinson of the McGill Manufacturing Company.[3] It replaced using just a standard bearing and bolt. The new cam followers were easier to use because the stud was already included and they could also handle higher loads.[ / CAM AND MECHANISMS A cam mechanism consists of three elements: the cam, the follower (or follower system), and the frame. The follower is in direct contact with the cam. The cam may be of various shapes. The follower system includes all of the elements to which motion is imparted by the cam. This may be connected directly to the follower, or connected through linkages and gearing. The frame of the machine supports the bearing surfaces for the cam and for the follower. A CAM changes the input motion, which is usually rotary motion (a rotating motion), to a reciprocating motion of the follower. They are found in many machines and toys WHAT IS THE CONCEPT BEHIND CAM? A CAM is a rotating machine element which gives reciprocating or oscillating motion to another element known as follower. The cam and follower has a point or line contact constitute a higher pair or you can say that it is the mechanicl component of a machine that is used to transmit the motion to the another component of the machine called the follower, through a prescribed program by direct contact.The contact between them is maintained by an external force which is generally provided by the spring or sometimes by the weight of the follower itself ,when it is sufficient. Cam is the driver member and the follower is the driven member. The follower is in direct contact with the cam. CAM MECHANISM CONSIST OF THREE MECHANISMS CAM:It may be of many shapes FOLLOWER: It includes all the elements to which motion is imparted by the cam.This may be connected directly by the cam.This may be connected directly to the follower, or connected through linkages and gearing. FRAME: The frame of the machine supports the bearing surfaces for the cam and for the follower. APPLICATIONS OF CAM AND FOLLOWERS Cam and follower are widely used for operating inlet and exhaust valve of I C engine. These are used in wall clock. These are used in feed mechanism of automatic lathe Machine. These are used in paper cutting machine. Used in weaving textile machineries. The cam mechanism is a versatile one. It can be designed to produce almost unlimited types of motioning the follower. It is used to transform a rotary motion into a translating or oscillating motion. On certain occasions, it is also used to transform one translating or oscillating motion into a different translating or oscillating motion. Cams are used in a wide variety of automatic machines and instruments. The certain usuages of cam and followers that includes textile machineries, computers, printing presses, food processing machines, internal combustion engines, and countless other automatic machines, control systems and devices. The cam mechanism is indeed a very important component in modern mechanization. CLASSIFICATION OF CAMS Based on the physical shape Disk or plate cams Working of the disc cam with reciprocating follower. Working of the disc cam with oscillating follower. Cylindrical cam Translating cam CLASSIFICATION OF FOLLOWES (i) Based on surface in contact (a) Knife edge follower (b) Roller follower (c) Flat faced follower (d) Spherical follower (ii) Based on type of motion Oscillating followe Translating follower (iii) Based on line of action Radial (in line) follower (b) Off-set follower Cams can be conveniently classified into two main groups Group a: Cams that impart motion to the follower in a plane in line with the axis of rotation of the cam (as does a cylindrical cam). Group b: Cams that impart motion to the follower in a plane at 90 degrees to the axis of rotation, as with face or edge cams.Most cams fall into this category. TYPES OF CAMS Eccentric cam: A circular cam is often called an eccentric cam because the axis of rotation of the cam is offset from the geometric center of the circular disc. Concentric disc: A concentric disc attached to a rotating shaft would have its axis of rotation coinciding with its geometric center. PROFILE SHAPES OF SOME CAMS: PEAR-SHAPED CAMS: These type cams are often used for controlling valves. For example, they are used on motor car camshafts to operate the engine valves. A follower controlled by a pear-shaped cam remains motionless for about half a revolution of the cam. During the time that the follower is stationary, the cam is in a dwell period. During the other half revolution of the cam, the follower rises and then falls. As the pearshaped cam is symmetrical, the rise motion is the same as the fall motion. Edge cams It must be appreciated that this type of cam, where the follower is in contact with the edge of the cam disc, is only capable of imparting positive motion to its follower in one direction, that is, during the rise portion of the cam movement. During the fall portion of the cam movement the follower must be maintained in contact with the cam either by the mass of the follower and its mechanism or, more usually, by a spring. Both methods have their advantages. Box cams A groove can be milled in the face of cam discs. As the cam rotates, a follower located in the groove has its motion guided by the groove. This type of cam is called a box cam. Cylindrical cams: Cylindrical cams are used when motion has to be transmitted parallel to the axis of rotation of the cam. The cylindrical or barrel cam consists of a rotating cylinder with a helical (screw shaped) groove in its curvedsurface. A follower with a tapered roller end is located in the groove. As the cylinder turns, the follower moves in a straight line parallel to the axis of the rotation barrel cam. This type of cam is often used to guide thread on sewing machines, looms and fabric making machines. CIRCULAR CAMS: These cams are sometimes called eccentric cams. The cam profile is a circle. The center of rotation of the cam is often from the geometric center of the circle. The circular cam produces a smooth form of motion called a simple harmonic motion. These cams are often used to produce motion in pumps. Circular cams are often used to operate steam engine valves. As the cam is symmetrical, the rise and fall motions are the same. HEART SHAPED CAMS: This cam causes the follower to move with a uniform velocity. Heart-shaped cams are essential when the follower motion needs to be uniform or steady as, for example, in the mechanism that winds thread evenly on the bobbin of a sewing machine. A heart-shaped cam can be used for winding wire evenly on the former of a solenoid. UNIFORM ACCELERATION AND RETARDATION CAMS: A cam shaped as shown controls the motion of the follower so that it moves with uniform acceleration and retardation. The follower gains and looses velocity at a constant rate. Uniform acceleration and retardation cams are used to controls the motion of linkages in complex machinery. Types of Cam Followers There are three types of cam followers, and since the type of follower influences the profile of the cam it is worthwhile considering the advantages and disadvantages of each type. The three types are the knife-edge, the roller follower and the flatfoot or mushroom follower. The Knife Edge Follower: This is the simplest type, is not often used due to the rapid rate of wear. When it is adopted, it is usually for reciprocating motion, running in slides and there is considerable side thrust, this being a component of the thrust from the cam. The Roller Follower: This eliminates the problem of rapid wear since the sliding effect is largely replaced by a roller action. Some sliding will still take place due to the varying peripheral speed of the cam profile, due to the changing radius of the point of contact. Note also that the radial position of the contact between the cam and the roller, relative to the follower center, will change according to whether a rise or fall motion is taken place: this fact has to be considered when constructing the cam profile. Again,with the roller follower, considerable side thrusts are present, a disadvantage when dealing with reciprocating motions. This side thrust will be increased when using small rollers. The Flat Foot or Mushroom Follower: This has the advantage that the only side thrust present is that due to the friction between the follower and the cam. The problem of wear is not so great as with the knife-edge follower, since the point of contact between the cam and follower will move across the face of the follower according to the change of shape of the cam. A trick to lessen further the effect of wear is to design the follower to be capable of axial rotation and arrange the axis of the follower to lie to one side of the cam. Thus the contact with the cam will tend to cause rotation of the follower. The cam profile, to work with a flatfoot follower, must be convex at all parts, in order to prevent the corners of the follower digging into the cam profile. The minimum cam radius should be as small as possible to minimize sliding velocity and friction. All three types of cam followers can be mounted in the following ways: 1) In-line with the cam center line, 2) Offset from the cam center line, or 3) Mounted on a swinging radial arm. CAM-VALVE CAM TERMINOLOGY Trace point: A theoretical point on the follower, corresponding to the point of a fictitious knife-edge follower. It is used to generate the pitch curve. In the case of a roller follower, the trace point is at the center of the roller. 2.Pitch curve: The path generated by the trace point at the follower is rotated about a stationary cam. 3. Working curve: The working surface of a cam in contact with the follower. For the knife-edge follower of the plate cam, the pitch curve and the working curves coincide. In a close or grooved cam there is an inner profile and an outer working curve. 4. Pitch circle: A circle from the cam center through the pitch point. The pitch circle radius is used to calculate a cam of minimum size for a given pressure angle. 5. Prime circle (reference circle): The smallest circle from the cam center through the pitch curve. 6. Base circle: The smallest circle from the cam center through the cam profile curve. 7. Stroke or throw: The greatest distance or angle through which the follower moves or rotates. 8. Follower displacement: The position of the follower from a specific zero or rest position (usually its the position when the follower contacts with the base circle of the cam) in relation to time or the rotary angle of the cam. 9. Pressure angle: The angle at any point between the normal to the pitch curve and the instantaneous direction of the follower motion. This angle is important in cam design because it represents the steepness of the cam profile. Some question arises regarding cam and followers: When is a flat faced follower preferred as compared to roller followers and why? Flat faced followers are preferred to roller followers where space is limited for eg: cams with flat followers are used to operate valves of an automobile engine but in case of stationary and oil engines, roller followers are preferred because more space is available. What data is plotted on displacement diagram of cam and follower motion? Ans:Plot of linear displacement i.e. lift or stroke (s) of follower (on Y axis direction) versus angular displacement (ÃŽÂ ¸) of the cam for one rotation (on X axis direction)