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دورة كاملة عن الكامات Introduction to Cams

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    دورة كاملة عن الكامات Introduction to Cams

    Introduction to Cams


    A cam is a mechanical component of a machine that is used to transmit motion to another component, called the follower, through a prescribed motion program by direct contact.
    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.



    Classifying cams
    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.

    The cam, as a means of producing a given type of motion, is simple and reasonable to design, provided the simple principles are understood. Another advantage is that, generally, a cam can easily be changed or modified to allow a change of motion, without interfering with the remainder of the mechanism.



    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.








    • 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 cam
    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 curved surface. 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.


    cylindrical cam animation



    PROFILE SHAPES OF SOME CAMS
    The most common kind of cam is the plate cam. It consists of a narrow plate or disc, which is fixed to a rotating shaft. The plate is shaped so that the follower will produce a pre-determined form of motion. Most cams are designed to have a smooth curved shape so that the motion transmitted to the follower is smooth and without sudden jerks.





    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 pear-shaped cam is symmetrical, the rise motion is the same as the fall motion.









    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.








    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.





    The drawing below shows the three types of cam followers:


    knife-edge follower with cam


    roller follower with cam




    flat foot follower with cam










    Cam design considerations



    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.






    All cam followers wear at different rates depending on the follower shape, size, type etc. the next section shows how to reduce wear in a flat-foot follower.







    Reducing wear in a flat-face cam follower:

    One method of reducing wear on a flat-face follower is to allow the follower to rotate as the cam rotates. This leads to an even amount of wear on the follower.




    Overcoming resistance to cam motion:

    When using a spring anchored to a fixed point, the force exerted by the spring will vary during the camshaft revolution, reaching a maximum when the follower is at the full diameter of the cam and the spring at its maximum extension. The camshaft drive has to overcome this increasing resistance to rotation. During the fall portion of the cam motion, the spring loading (which is decreasing) is tending to push the cam round, assisting rotation. These conditions give rise to unbalanced torque (turning moments) requirements from the camshaft driving motor. Using a double-arm cam lever with two followers, one on each arm and each running on its cam, can eliminate this fluctuating load. The cams must be designed to match each other, the second cam rising when the first cam falls, but care must be taken to provide a small amount of clearance: when one cam is driving, the other follower must be slightly clear of its cam. This is to ensure that the followers do not lock when passing over peaks. This difficulty can be eliminated if two separate arms, with followers on two mating cams, are used, the arms being spring-loaded together. This will give an almost constant spring length during the camshaft revolution.


    Considerations to be taken in
    cam design:

    If the cam follower and its mechanism are made too heavy, then during the early part of the cam rise (that is, the acceleration portion) the force imparted to the follower by the cam will be large. This gives a high torque requirement at this part of the camshaft revolution. Furthermore, once the heavy follower and its mechanism are moving, the momentum will tend to keep it moving at that speed. This means that the cam may not be in control, during the second part of the rise, and the heavy spring loading will be necessary to maintain contact between the cam and the follower at the end of the rise.

    From these considerations, it can be seen that among the main aims to be borne in mind during cam design are:

    High accelerations and declarations are to be avoided to minimize force requirements;
    1. Maximum accelerations should occur during the middle of the stroke;


      The mass of the mechanism to be moved by the cam should be as small as is practicable.





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    Camshaft of an Engine

    The use of cam and follower systems are vital in engines, where they are used to open and close the inlet valve and the exhaust valve to the cylinder head. The diagram shown opposite shows us a typical camshaft that could be found in a lawnmower engine. The cam and follower system is a plate cam and flat follower system, and of course the function of the system is to open and close the valves at the correct time during the four stroke cycle of the engine (this will be dealt with in more depth later). If you examine the image close you will see that the peaks of the cams are offset by approximately 120 degrees. These ensures that the both valves aren't fully open at the same time.

    Camshaft with Plate Cams imparting on a Flat Followers and Valves





    Another variation of a cam and follower system that could be used to open and close the inlet and exhaust valves in an engine
    Camshaft of an Engine incorporating a Rocker Arm
    The diagram shown below is another typical cam and follower system that could be used in an engine. This system incorporates a rocker arm (shown in blue in the image). In this case the motion the cam imparts on the follower is translated to the valve through a push rod amd the rocker arm.
    Camshaft with Plate Cam imparting on a Flat Follower, a Push Rod, a Rocker arm and the Valve.


    Animation of the cam and follower from the mechanism shown above
    What mechanism does the cam and follower system of an engine interaect with inside the engine
    Part 2

    The other mechanism that the cam and follower system interact with in an engine is the actual mechanism that produces the power, i.e. the crankshaft, connection rod and piston mechanism. The cam and follower system in an engine is an integral part of the production of power by the engine

    How the cam and follower system interacts with this mechanism
    Part 3



    In doing this we will
    1. take a brief look at what the history of the combustion engine and the four stroke cycle.
    2. examine what the cam and follower system is required to do.
    3. look at an example of a cam and follower system at work within an internal combustion engine.

    History of the Combustion Engine and the Four Stroke Cycle
    The First Commercial Internal Combustion Engine:
    A person by the name of Etienne Lenoir, in 1860, was the first person to make a successful comercial internal combustion engine. This engine ran on coal gas. However it worked on a cycle that didn't involve compression of the gas before ignition and therefore lacked any great efficiency.

    The Four Stroke Engine
    The four strokes of the four stroke cycle are:
    The Induction Stroke

    Starting with the induction stroke and the piston at top dead centre, t.d.c., as the crankshaft rotates the piston travels down the cylinder. The camshaft has at this stage opened the inlet valve so as the piston travels down the cylinder, the cylinder fills with a mixture of petrol vapour and air.


    The Compression Stroke
    As the crankshaft rotates the piston passes bottom dead centre, b.d.c., and starts to travel up the cylinder, beginning the compression stroke. The rotation of the cam shaft will have closed the inlet valve at this stage so that as the piston moves up the cylinder the gaseous mixture that filled the cylinder during the induction stroke will be compressed.
    Top Dead Centre and Bottom Dead Centre


    The Power Stroke
    At the end of the compression stroke the compressed gaseous mixtrue is ignited by a timed spark that jumps across a gap on a spark plug. This ignites the gaseous mixture and thus increasing the temperature and pressure above the piston head. As a result the piston is driven down the cylinder. This is known as the power stroke
    The Exhaust Stroke
    The final stroke is the exhaust stroke as as the name suggests this is the stroke where the burnt gases are expelled. Again the camshaft will have timed the opening of the exhaust stroke for this stage and as the piston moves up the cylinder once again the burnt gases are expelled through the open exhaust valve. Then the exhaust valve closes and the cycle begins again.

    What is required of the Cam and Follower System in the Internal Combustion Engine
    In the internal combustion system the cam and follower system are required to control the opening and the closing of the inlet and exhaust valve. This implies that that the followers must open the valves they are connected to once in the four stroke cycle. For instance the follower in contact with the inlet valve opens the inlet valve for the induction stroke and closes it for the other three strokes. The follower for the exhaust valve must open the exhaust valve for the exhaust stroke and close it for the other three strokes.

    Example of a Cam and Follower System at work within an Internal Combustion Engine
    How the cam and follower system in an internal combustion engine work is shown in the animation of the four stroke cycle below.
    Animation of the Four Stroke Cycle


    Another Common Use of a Cam and Follower System
    Another common use of a Cam and Follower system is within a pump, such as an oil pump. In such pumps the cam and follower system is used to suck oil in through one non-return valve and push it out through on other non-return valve. The suck action is achieved by the system because the follower is cylindrically shaped and moves within a tight fitting cylinder so oil is sucked in and pushed out as the follower moves up and down. This is similar to the gaseous mixtures being sucked into and forced out of the engine cylinder as the piston moved up and down in the previous example
    .
    Animation of a Pump Working


    Drum Cam and Roller Follower




    Disc Cam with Flat Follower




    The diagram above shows the an animation of a rotating cam and flat follower. As the cam rotates the the follower is pushed up and down.
    There is some external force pushing the cam back down, so that it remains in contact with the cam profile.
    The cam shown above is also known as a plate cam.
    This type of follower can be found in the cam and follower system used to open and close inlet and exhaust valves in an engine.


    Disc Cam with Roller Follower




    The diagram above shows the an animation of a rotating cam and roller follower. As the cam rotates the the follower is pushed up and down
    There is some external force pushing the cam back down, so that it remains in contact with the cam profile
    The cam shown above is also known as a plate cam

    Disc Cam with Knife Edge



    The diagram above shows an animation of a rotating cam and knife edge follower. As the cam rotates the the follower is pushed up and down
    There is some external force pushing the follower back down, so that it remains in contact with the cam profile
    The cam shown above is known as a plate cam






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