Chapter 2 Cams - Theory of Machine, Mechanical Engineering Mechanical Engineering Notes | EduRev

Chapter 2

CAMS

•  A cam is mechanical member used to impart desired motion to a follower by direct contact. The cam may be rotating or reciprocating whereas the follower may be rotating, reciprocating or oscillating.
•  They are manufactured usually by die-casting, milling or by Punch Press.
•  A cam and follower combination belongs to the category of higher pairs.

ELEMENT OF A CAM

•  A driver member known as the cam
•  A driven member called the follower
•  A frame which supports the cam and guides the follower.

TYPES OF CAMS
(i) According to Shape

•  Wedge and flat cams: A wedge cam has a translational motion. The follower can either translate or oscillate. Instead of using a wedge, a flat plate with a groove can also be used.

• Radial or Disc Cams: A cam in which the follower moves radially from the centre of rotation of the cam is known as a radial or disc cam.

• Spiral cams: A spiral cam is a face cam in which a groove is cut in the form of a spiral. It finds its use in computers.
•  Cylindrical cams: In a cylindrical cam, a cylinder which has a circumferential contour cut in surface, rotates about its axis. It is also known as barrel or drum cams.
  

•  Conjugate cams are preferred when the requirements are low wear, low noise, better control of the follower, high speed, high dynamic loads etc.
•  Globoidal cams: It has two types of surface i.e. convex or concave. It is used when moderate speed and the angle of oscillation of the follower is large.

(ii) According to the movement of follower

•  Rise-Return-Rise (RRR): In this, there is alternate rise and return of the follower with no period of dwells. The follower has a linear or an angular displacement.

•  Dwell-Rise-Return Dwell (D-R-R-D): In this cam, there is rise and return of the follower after a dwell.

•  Dwell-Rise-Dwell-Return-Dwell (D-R-D-R-D): The dwelling of the cam is followed by rise and dwell and subsequently by return and dwell.

•  In case the return of the follower is by a fall, the motion may be known as Dwell-Rise-Dwell (DRD).

TYPE OF FOLLOWER
(i) According to Shape

•  Knife-edge follower: Its use is limited as it produces a great wear of the surface at the point of contact.
•  Roller follower: At low speeds, the follower has a pure rolling action, but at high speeds, some sliding also occurs. In case of steep rise roller follower is not preferred.
•  Mushroom follower: It does not possesses the problem of jamming of the cam. However high surface stresses and wear are quite high due to deflection and misalignment if flat faced follower is used. These disadvantages are reduced if a spherical faced follower is used instead of a flat follower.

(ii) According to movement

•  Reciprocating Follower: In this type, as the cam rotates the follower reciprocates or translates in the guides.

•  Oscillating Follower: The follower is pivoted at a suitable point on the frame and oscillates as the cam makes the rotary motion.

(iii) According to location of line of movement

•  Radial Follower: The follower is known as a radial follower it the line of movement of the follower passes through the centre of rotation of the cam.

•   Offset Follower: If the line of movement of the roller follower is offset from the centre of rotation of the cam, the follower is known as an offset of follower.

TERMINOLOGY OF CAM

•   Base circle: It is the smallest circle tangent to the cam profile drawn from the centre of rotation of a radial cam.
•  Trace Point: It is a reference point on the follower to trace cam profile such as knife edge of a knife, edged follower and centre of the roller of a roller follower.
•   Pitch curve: It is the curve drawn by the trace point assuming that the cam is fixed and the trace point of the follower rotates around the cam.
•   Pressure angle: The pressure angle, representing the steepness of the cam profile, is the angle between the normal to the pitch curve at a point and the direction of the follower motion. It varies in magnitude at all instants of the follower motion. A high value of the maximum pressure angle is not desired as it might jam the follower in the bearings.
•  Pitch Point: It is the point on pitch curve at which the pressure angle is maximum.
•   Pitch circle: It is the circle passing through the pitch point and concentric with the base circle.
•  Prime circle: The smallest circle drawn tangent to the pitch curve is known as prime circle.
•  Dwell is the zero displacement or the absence of motion of the follower during the motion of the cam.
•  The motion of the cam can be represented on a graph, the x-axis of which may represent the angular displacement of the cam and the y-axis, the angular or the linear displacement of the follower.
•  Angle of Ascent (f_a): It is the angle through which the cam turns during the time the follower rise.
•  Angle of dwell (d): Angle of dwell is the angle through which the cam turns while the follower remains stationary at the highest or lowest position.
•   Angle of Decent (f_d): It is the angle through which the cam turns during the time the follower returns to the initial position.
•  Angle of Action: It is the total angle moved by cam during the time, between the beginning of rise and the end of return of the follower.
•  The dynamic effects of acceleration (jerks) usually, limit the speed of the came.