Cam Second Lecture.ppt

7/30/2019 Cam Second Lecture.ppt

1/8

Axis of transmission is extended to intersect

effective link 1, which is the ground link.

This intersection is instant center I24 (labeled B)

which by definition has the same velocity in link

2(the cam) and in link 4(the follower) .

Because link 4 is in pure translation, all points on it

have identical velocities vfolower, which are equal to

the velocity of I24 in link 2

We can write the expression for the velocity of I24

in terms of cam angular velocity and the radius b

from cam center to I24

Axis of the motion of the follower does not intersect the center of the cam

Eccentricity: Perpendicular distance between the follower's axis of motion and the centerof the cam

2

3

12

23

34


2/8

It can be expressed in terms of cam angular velocity and the radius b from cam

center to I24

Where s is the instantaneous displacement of the follower from the s diagram and

s dot is its time derivative in units of length/s

Distance b to the instant center I24 is equal to the velocity of the follower, v in

units of length per radian


3/8

Distance b is expressed in termsof cam design parameters, primecircle radius Rp and the

eccentricity

arctanv

s RP2 2

From triangle CDO2

22

PRd

Substitute this in equation for vand rearrange to get pressureangle in terms of camparameters and eccenticity


4/8

arctanv

s RP2 2

As Rp is increased, will be reduced, Practical limitation is the size and cost

If small cam cannot be obtained with acceptable pressure angle, then eccentricity can beintroduced

For positive , a positive value of eccentricity will decrease the pressure angle on the risebut will increase it on the fall. Negative eccentricity does the reverse

This is of little value with a form closed(groove or track) cam, as it is driving the followerin both direction

For a force closed cam with spring return, you can sometimes to afford to have a largerpressure angle on the fall than on the rise because the stored energy in the spring isattempting to speed up the camshaft on the fall

When the cam function is asymmetrical and significant differences exist (with noeccentricity) between maximum pressure angles on rise and fall, introducing eccentricitycan balance the pressure angles and create a smoother running cam


5/8

Overturning Moment-Translating Flat Faced Follower

Pressure angle can be seen to be zerofor all positions of cam and follower

As the contact point moves left andright, the point of application for theforce between cam and followermoves with it

There is an overturning moment onthe follower associated with this off-center force which tends to jam thefollower in its guide just as did toolarge a pressure angle in the roller

follower case

In this case keep the cam as small aspossible in order to minimize themoment arm of the force


6/8

Cam Synthesis

Type Synthesis: Choose the type of cam and follower

Type of follower (translation/oscillation; force/form closed; roller/flat faced)

Type of cam( radial/axial)

Distance between cam shaft and the center of the required oscillatory output

Rotational speed of cam, loading on cam and follower, and masses to be moved

Environmental and economical considerations

Follower Motion

Once a cam and follower pair has been chosen, the follower motion must be synthesized. In

most cases a cam follower is required to be displaced through a specified rise or fall

The shape of the displacement curve not important, but velocity , acceleration are of great

importance

Dimensional Synthesis of the Cam Profile

Except in the case of radially translating point follower, interference of portions of a roller

or flat face follower requires a more detailed cam profile synthesis procedure


7/8

Once the cam profile is synthesized, the design may not be acceptable for

many reasons

The pressure angle may be unacceptable The follower may not be able to follow the cam surface due to local

curvature conditions

Too large a return spring may be required to keep the follower in contact

with the cam surface during the cycle

Excessive dimensions may be required

Usually following parameters can be varied to alleviate such problems

Base circle diameter

The offset of the follower travel (if a straight life translating follower is

used) The relative locations of the fixed pivot of the follower arm to the center

of the cam (if an oscillating follower is employed)

Length of the follower arm

Radius of the follower(if a roller or spherical follower is used)

Cam Analysis


8/8

Choosing the Prime Circle

Start with something about 3 times max lift h.

Compute for all values ofq.

Iterate to an acceptable condition.

Maximum pressure angle for a translating roller

follower should be < = 30 degrees.

Eccentricity can be introduced to correctasymmetry in max and min if desired.

Cam Second Lecture.ppt

Documents

Transcript of Cam Second Lecture.ppt