Unit VIII. Cams and Followers

Objectives

1. To identify different types of followers2. To identify different types

of cams3. To identify the notations

of a cam follower system

Objective }

To identify different types of followers

Activities

o Read the material provided below, and as,you xead:4. Classify the followers by their motion.

5. Classify the followers by their physical shape.

6. Classify the followers by their orientation.o Check your answers to the competency items with your instructor.

1. . '

ReadWg Material

A cam is a mechanical member that transmits motion to a follower by directcontact. The driving member is called a cam and the driven member is called thefollower. The cam may rotate, translate, oscillate, or even remain stationary. Thefollower may, on the other hand, have either rotary or translatory motion.

The followers are classified either according to their shapes, the types ofmotions displayed by them, and the actual locations of the line of movement.

The knife edge, the roller, and the flat face followers are shown in Figs. I, 2,and 3. The knife edge follower is quite simple in construction. However, since itproduces extreme wear of surface at the point of contact, its use is limited.

The roller follower shown in Fig. 2 is a practical form of the knife edgefollower shown in Fig 1. The roller follower is a cylindn'cal body free to rotateabout the pin joint at P as shown in Fig. 2. The action of the roller at low speedis pure rolling. However, as the speed increases, the action of the roller is acombination of rolling and sliding. The rolling followers normally become aproblem case when a cam has a steep rise. In this situation, the roller followerwill jam the cam.

The flat face follower shown in Fig. 3 is a further refinement of the rollerfollower. The flat face follower will not jam when the cam has a steep rise. The

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Unit VIII. Carns and Followers 169

Competency Items

o State the advantages or disadvantages that would apply to an offset flat face

follower.o Sketch an offset knife edge followex.* List three different methods of forcing a cam follower to remain in contact

with a cam profile.* Sketch a positive drive cam mechanism for a translating flat face follower.

ObjectiLve 2

To identify different types of cams

Activities

o Read the material provided below, and as you read:7. Classify the cams in terms of follower motion.

8. Classify the cams in terms of their shape.

9. Classify the cams in terms of the manner of constraint of the follower.* Check your answers to the competency items with your instructor.

Reading ~

Cam is usually a driving member of the cam and follower assembly. Hence,cams are classified in any one of the three possible ways: according to theirfollower motion program, according to their shape, and according to theconstraints of their followers.

A cam may have any one of the following three important motion programs.These are (1) rise-return (R-R), (2) dwell-rise-return (D-R-R), or (3) dwell-

rise-dwell-return (D-R-D-R) motion program.

The rise-return motion program of the follower in terms of angulardisplacement of the cam is shown in Fig. 8.

The rise-return program makes it possible for a follower to go up and downaccording to the dispfaceme~program shown ~ Fig~. 8. A follower could have

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the cam rotates 3600 The cycle begins when 0 = 00 , completes the risesegment when 0 = 01 = , and completes the ret urn segment when 0 = 360oFrom Fig. 3, we can write the following boundary conditions for the motion ofthe follower: at 0 = 00 , S = O; at 0 = 01 , S - Sm ax ; at 0 - 3600, S = O,

The dwell-rise-return motion program of the follower shown in Fig. 9 beginswith a dwelling motion of the follower in addition to the rise-return motionprogram. The dweHing conditions of a follower implies that the follower has nonet displacement, has zero velocity, and zero acceleration. The followingboundary conditions can be written to describe the foHower displacements:

qr

unit VIII. Cams and Followers 171

O0 -< q 1 S - 0

q `- q2 S `- Sma:

q - 3600 S - 0for

atNote that = 02 - ql is the angle that describes the amount of cam rotationthat corresponds to the maximum displacement of the follower during its risesegment of the motion.

The dwell-rise-dwell-return motion program of the follower begins with adwelling motion of the foHower; this dweHing motion is foHowed by a rise anddweHing motion as shown in Fig. 10. The following boundary conditions can bewritten to describe the foHower displacements:

f

o

r

0 < 0 q 1S - - 0

f

o

r

02 q <- q3 S - Smax

at

q - 360o S - 0

(|w_IRI|SE

172 Mechanism Synthesis and Analysis

.The cams can be also classified by their shapes. The shape of a cam may

be awedge, cylindrical, spiral, conical, spherical, globoidal, radial, conjugate, or threedimensional. The cam may have either rotational or trans{ational motion. Awedge-shape cam having translatory and osciHating types of foHowers are shownin Figs. I I and 12.

The wedge-shaped cam is simple in its design. The follower is kept in contactwith the cam surface either by spring loading the follower or by providing apositive drive groove.

The radial or the disk cam has followers which move radially from thecenter of rotation of the cam. The two types of disk cams are shown inFig. l3a and b; Fig. 13a has translating roller foHower; Fig. 13b has osculatingroller follower. The followers are kept in contact with the cams by means ofpreloaded springs. The disk or plate cams are more popular cams because of theirsimplicity and compactness.

The conjugate cam has double disk cams which are constantly in contact withone foHower. An example of oscillating roHer foHower conjugate cam is shownin Fig. 14. The conjugate cam is preferred when high speed, high dynamic loads,low noise , low wear, and high degree of control of the foHower are designrequirements.

The spiral cam, shown in Fig. 15, is a face cam with a spiral groove whichcontrols the motion of either osculating or translating foHower. The applicationof this type of cam is limited because the cam has to rotate in the reversedirection to reset the foHower position.

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unit VIII. Cams and Followers 173-

2. ' . . ~

The globoidal or barrel cam is shown in Fig. 16. The motion to the follower is

imported by the circumferential contour cut into the surface of rotation of the

cam. There are two types of globoidal cams. The type is determined by the

surface of the cam. The surface could be either convex or concave as shown in

PINGEA

1 - FOLLOWER

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Unit VIII. Cams and Fallawem 175\

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Fig. 18 Conical cam.

The conical cam is shown in Fig. 18. The follower of the conical cam translatesalong the line which generates the cone if revolved about the axis of the cone.The conical cams are expensive to fabricate and, therefore, have limited use.

The spherical cam is shown in Fig. 19. The cam is made from a sphericalsurface which transmits motion to the follower. The follower oscillates about anaxis which is perpendicular to the axis of rotation of the cam. The sphericalcams are similar to conical cams in cost and fabrication and have limited use.The advantage of using a spherical cam rahter than a disk cam is that with thespherical cam one can obtain oscillatory motion about an axis which is notparallel to the axis of rotation of the cam.

Competency Items

o Name the two main groups into which followers fall according to their

motion characteristics.* Name six different types of cam shapes.* Write an application for a cam of each type.

Objective 3To identify the notations of a cam follower system

Activities* Read the material provided below, and as your read:a. Define all the terminologies of a cam.~b. Lay out a disk cam profile with different types of followers.

,176 Mechanism

Synthesis and Analysis

Reading Material

In order to design a cam and a follower, the student should first define the

necessary terminologies~The final contour or shape of a cam is decided by the displacement program of

its follower. In the previous objective we have studied the different types of

displacement programs a follower may have. For the purpose of constructing a

cam profile, examine a dwell-rise-return motion program, shown in Fig. 20. .

IPITCH POINT

BASE CIRCLE

(-Fig. 20 Displacement program and cam layout.

Unit Vltt. Cams and Fattawem 177

From the displacement program, we can write the following boundaryconditions for the follower motion:

0 - 00 S - 0.0 in.

q - 600 S - 0.0 in.

q -~ 1800 S -~ 1.5 in.

q -, 3600 S - 0.0 in.The cam profile for a roller translating follower is constructed using

thefoHowing steps:

10. Select a radius for a base circle with rb = 0.8 in. and draw a base circle asshown in Fig. 20.

11.Divide the circumference of the base circle into 12 equal parts since weknow from Fig. 20 the displacements of the follower for every 300 of thecam rotation angle. These 12 parts are shown as Co , C I , C2 , . . . , CII .These points Co , CI , . ~ . , Ct"1 are located in the direction opposite to thatof cam rotation. .~

12.Select a radius of roHer foHower Ti = 0.25 in. The radius of the roHerfoHower is required to be smaller than the minimum radius of curvature ofthe cam profile. Hence, a student is required to perform some calculationsbefore a radius of a roHer foHower is selected.

13. Locate centers Po , P I , . . . , Pl I of the roller follower by measuringradially from points Co, CI , . . . , CII distances Yi - Si + rf for i - 0, ,,II.

14. With Po, Pl , . . . , Pll as centers and rt as radius draw circles to representpositions of roHer foHower.

15.Draw a continuous curve which is tangent to the circles obtained in Step 5.This curve describes the cam profile for a translating roHer foHower.

In laying out the cam proflle, we have selected only 12 positions of thefollower. Often, it becomes necessary to have positions of the follower locatedat every one-degree increment. It is important to note that the basic principleemployed in developing the cam surface is the principle of inversion. The cam isheld stationary and the foHower is rotated in the direction opposite to that ofthe cam rotation.

The reader should become familiar with the foHowing notations in describingthe cam profile characteristics:

Base circle-the smallest circle tangent to the cam profile. See Fig. 20.Pitch circle-the circle passing through the pitch point and concentric with the

base circle.Pitch cane-the curve traced by the center of the roHer foHower or the

knife-edge foHower. Thus, the trace point traces the pitch curve.f)iLek point-describes the position of the point on the pitch curve at which the

pressure angle is maximum.

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178 Mechanism Synthesis and Analysis

Pressure angle-the angle between a normal to the pitch curve and the directionof follower motion. The pressure angle changes its magnitude at every instantof follower motion. The maximum value of pressure angle plays a significantrole in the design of cam; thmailer the max m pressure angle, the betterthe design. With too large a pressure angle, it is possible that the followermight jam in its bearing.

me circle-the smallest circle tangent to the pitch curve.ace point-the point coinciding with the krlife edge of the follower or the

center of the roller follower.

Competency Items

o Make up a dwell-rise-dwell-return motion program and lay out a cam for atranslating roller follower.

o Select different values for a prime circle radius and lay out the correspondingcam profile for an assumed displacement program. Observe critically and

state:

16. how the maximum pressure angle changes with different radii of the

prime circle radius

17. how the size of the cam changes with the maximum values of pressure

angles

Performance Test #1

For each of the curves below describing follower displacements,profile. ~ .

0~25 in~

.30o 60o 90o 120o150o 160o 210o 240e270o 300o 330o 360oCAM ROTATION

a . Swinging roller follower

lay out cam-

Unit VIII. Cams and Followers 179

Performance

Test #2

Using the displacement diagram given, lay out the cam profile for the flat facfollower shown. Find the minimum value of angle a so that contact will alwaysexist between the cam and the circular face of the follower

Performance Test #3

18. Sketch an offset knife edge follower.

19. Sketch a positive drive cam mechanism for a trans

20. State the type of cam and follower that

is obtained when the half"cone angle B

o

becomes 90'lating flat face .f oHower.

180Mechanism Synthesis and Analysis

4. On the sketch indicate the pressure angle of tIfnthe cam-follower system when radial line (a) ill~is vertically upward.

If the cam of problem 4 were made with the same motion program but witha larger prime circle radius, state how the maximum pressure angles willchange.Choose correct answer within the parenthesis and explain your choice: Acam has several consecutive points where the

pressure angle is zero. Describeall situations when this pressure angle becomes zero.

Supplementary References

(9.1-9.6); 3 (7.1 , 7.9-7.13); 6 (1.1 -2.2); 7 7.1 -7.2); 8 (8.1-8.3, 8.5)5.

6.

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