Introduction to CAMS :
A cam and follower are a method of converting rotary motion into linear motion. The most well-known application is in an internal combustion engine, where the cam and follower combination determines the opening times and duration of the valves. (via the pushrod and rocker arm).
Other applications might be on industrial machinery, regulating opening and closing of equipment for filling bottles or containers, toys (for example a “quacking duck” toy, where a cam can be used to move the duck’s beak and make the quack noise), in fact, any application where you want a fairly short linear motion at a regular repeated interval.
In machines, particularly in typical textile and automatic machines, many parts need to be imparted different types of motion in a particular direction. This is accomplished by the conversion of the available motion into the type of motion required. Change of circular motion to the translatory (linear) motion of simple harmonic type and vice-versa and can be done by slider-crank mechanism as discussed previously. But now the question arises, what to do when circular or rotary motion is to be changed into linear motion of complex nature or oscillatory motion. This job is well accomplished by a machine part of a mechanical member, known as cam.
Definition of CAMS :
A cam may be defined as a rotating, reciprocating or oscillating machine part, designed to impart reciprocating and oscillating motion to another mechanical part, called a follower.
A cam and follower have, usually, a line contact between them and as such they constitute a higher pair. The contact between them is maintained by an external force which is generally, provided by a spring or sometimes by the sufficient weight of the follower itself.
Classification of Cams
Cams are classified according to :
(b) Follower movement
(c) Type of constraint of the follower
According to Shape
Wedge and Flat Cams
It is shown in Figures 1.48(a), (b), (c) and (d).
In Figure 1.48(a), on imparting horizontal translatory motion to wedge, the follower also translates but vertically in Figure 1.48(b), the wedge has a curved surface at its top. The follower gets an oscillatory motion when a horizontal translatory motion is given to the wedge. In Figure 1.48(c), the wedge is stationary, the guide is imparted translatory motion within the constraint provided. This results in the translatory motion of the follower in Figure 1.48(d), instead of a wedge, a rectangular block or a flat plate with a groove is provided. When horizontal translatory motion is imparted to the block, the follower is constrained to have a vertical translatory motion.
Further, there is no need to provide a spring in this case as in case (a) and (b). In this case the path of the groove, which causes motion to the follower, constrains the follower to move upward and downward.
Radial or Disc Cam
In radial or disc cams the shape of the working surface (profile) is such that the followers reciprocate in a plane at right angles to the axis of the cam as shown in Figure 1.49(a). It is called radial cam because the motion of the followers obtained is radial (Figure 1.49). A differently shaped radial cam is also shown in Figure 1.49(b).
It is pointed out that the radial cams are very popular due to their simplicity and compactness,
Cylindrical cams have been shown in Figures 1.50(a) and (b). In Figure 1.50(a) the follower reciprocates whereas in Figure 1.50(b) the follower oscillates. Cylindrical cams are also known as barrel or drum cams.
It is shown in Figure 1.51. The cam comprises a plate on the face of which a groove of the form of a spiral is cut. The spiral groove is provided with teeth which mesh with pin gear follower.
This cam has a limited use because it has to reverse its direction to reset the position of the follower. This cam has found its use in computers.
As the name implies, the cam comprises two discs, keyed together and remain in constant touch with two rollers of a follower as shown in Figure 1.52.
This cam is used where the requirement is of high dynamic load, low wear, low noise, high speed and better control of follower.
This cam has two types of surfaces: convex and concave. A helical contour is cut on the circumference of the surface of rotation of the cam as shown in Figures 1.53(a) and (b). The end of the follower is constrained to move along the contour and then oscillatory motion is obtained. In this cam, a large angle of oscillation of the follower is obtained.
In this cam, as shown in Figure 1.54, the cam is of the shape of a sphere on the peripheral of which a helical groove is cut. The roller provided at the end of the follower rolls in the groove causing the oscillatory motion to the follower in an axis perpendicular to the axis of rotation of the cam.
According to Follower Movement
In this type of cam, its profile or contour is such that the cam rises, returns without rest or dwell, and without any dwell or rest, it again rises. Follower displacement and cam angle diagram for this type of cam is shown in Figure 1.55(a).
Dwell, Rise-return Dwell (DRRD)
In this type of cam after dwell, there is the rise of the follower, then it returns to its original position and dwells for sometimes before again rising. Generally, this type of cam is commonly used. Its displacement cam angle diagram is shown in Figure 1.55(b).
It is the most widely used type of cam. In this, dwell is followed by a rise. Then the follower remains stationary in the dwell provided and then returns to its original position [Figure 1.55(c)].
As may be seen in the follower-displacement verses cam angle diagram, shown in Figure 1.55(d) in this cam, the fall is sudden which necessities an enormous amount of force for this to take place.
According to Type of Constraint of the Follower
Pre-loaded Spring Cam
For its proper working there should be contact between the cam and the follower throughout its working, and it is achieved utilizing a pre-loaded spring as shown in Figures 1.48(a) and (b), etc.
Positive Drive Cam
In this case, the contact between the cam and the follower is maintained by providing a roller at the operating end of the follower. This roller operates in the groove provided in the cam. The follower cannot come out of the groove, as shown in Figures 1.52 to 1.54.
Gravity Drive Cam
In this type of cam, the lift or rise of the follower is achieved by the rising surface of the cam (Figure 1.48(c)) and the follower returns or falls due to force of gravity of the follower. Such types of cams cannot be relied upon due to their uncertain characteristics.
Classification of Followers
Followers may be classified in three different ways :
(a) Depending upon the type of motion, i.e. reciprocating or oscillating.
(b) Depending upon the axis of the motion, i.e. radial or offset.
(c) Depending upon the shape of their contacting end with the cam.
Those of followers falling under classification (a) and (b) have already been dealt with as indicated above. Followers of type (c) will be taken up now.
Depending upon the Shape of their Contacting End with the Cam
Under this classification followers may be divided into three types :
(a) Knife-edge Follower (Figure 1.55(a))
(b) Roller Follower (Figure 1.55(b))
(c) Flat or Mushroom Follower (Figure 1.56(c))
Knife-edge followers are generally, not used because of the obvious high rate of wear at the knife edge. However, cam of any shape can be worked with it. During working, considerable side thrust exists between the follower and the guide.
In place of a knife-edge, a roller is provided at the contacting end of the follower, hence, the name roller follower. Instead of sliding motion between the contacting surface of the follower and the cam, rolling motion takes place, with the result that the rate of wear is greatly reduced. In roller followers also, as in knife-edge follower, side thrust is exerted on the follower guide. Roller followers are extensively used in stationary gas and oil engines. They are also used in aircraft engines due to their limited wear at high cam velocity.
While working on the concave surface of a cam the radius of the surface must be at least equal to the radius of the roller.
Advantages of Roller Follower over Knife Edge Follower
a) Roller follower has less wear and tear than knife edge follower.
b) Side thrust is less as compared to knife edge follower.
c) Power required for driving the cam is less due to less frictional force between cam and follower
d) Function is smooth
e) Life of Cam-follower arrangement is more
f) Surface of cam not damaged in roller follower due to rolling
g) No possibility of noise
Flat or Mushroom Follower
At the name implies the contacting end of the follower is flat as shown. In mushroom followers there is no side thrust on the guide except that due to friction at the contact of the cam and the follower. No doubt that there will be sliding motion between the contacting surface of the follower and the cam but the wear can be considerably reduced by off-setting the axis of the followers as shown in Figure 1.56(c)(i). The off-setting provided causes the follower to rotate about its axis when the cam rotates.
Flat face follower is used where the space is limited. That is why it is used to operate valves of automobile engines. Where sufficient space is available as in stationary gas and oil engines, roller follower is used as mentioned above. The flat-faced follower is generally preferred to the roller follower because of the compulsion of having to use a small diameter of the pin in the roller of the roller follower.
In flat followers, high surface stresses are produced in the flat contacting surface. To minimize these stresses, a spherical shape is given to the flat end, as shown in Figure 1.56(d). The curved faced or spherical faced followers are used in automobile engines.
With flat followers, it is obvious, essential that the working surface of the cam should be convex everywhere.
Terminology of Cam and Follower – CAM PARTS
The Cam Profile
The working contour of a cam which comes into contact with the follower to operate it is known as the cam profile. In Figure 1.57, A-B-C-D-A is the cam profile or the working contour.
The Base Circle
The smallest circle, drawn from the center of rotation of a cam, which forms part of the cam profile, is known as the base circle and its radius is called the least radius of the cam. A circle with center O and of radius OA forms the base circle. The size of a cam depends upon the size of the base circle.
The Tracing Point
The point of the follower from which the profile of a cam is determined is called the tracing point. In the case of a knife-edge follower, the knife-edge itself is the tracing point. In roller followers, the center of the roller is the tracing point.
The Pitch Curve
The locus or path of the tracing point is known as the pitch curve. In knife-edge followers, the pitch curve itself will be the cam profile. In roller follower, the cam profile will be determined by subtracting the radius of the roller radially throughout the pitch curve.
The Prime Circle
The smallest circle drawn to the pitch curve from the center of rotation of the cam is called the prime circle. In knife-edge followers, the base circle and the prime circle are the same. In roller follower, the radius of the prime circle is the base circle radius plus the radius of the roller.
The Lift or Stroke
It is the maximum displacement of the follower from the base circle of the cam. It is also called as the throw of the cam. In Figure 1.57, distance B’B and C’C is the lift, for the roller follower.
The Angles of Ascent, Dwell, Descent and Action
- Refer Figure 1.57, the angle covered by a cam for the follower to rise from its lowest position to the highest position is called the angle of ascent denoted as θ1.
- The angle covered by the cam during which the follower remains at rest at its highest position is called the angle of dwell, denoted by θ2.
- The angle covered by the cam, for the follower to fall from its highest position to the lowest position is called the angle of descent denoted as θ3.
- The total angle moved by the cam for the follower to return to its lowest position after the period of ascent, dwell and descent is called the angle of the action. It is the sum of θ1, θ2 and θ3.
The Pressure Angle
The angle included between the normal to the pitch curve at any point and the line of motion of the follower at the point is known as the pressure angle. This angle represents the steepness of the cam profile and as such it is very important in cam design.
The Pitch Point
The point on the pitch curve having the maximum pressure angle is known as the pitch point.
The Cam Angle
It is the angle of rotation of the cam for a certain displacement of the follower.
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