cams_ism.pptx

8/10/2019 Cams_ism.pptx

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Cam

Follower

The cam and fo l loweris a device which can

convert rotary motion (circular motion) into

linear motion (movement in a straight line).

Followers

(valves)

Cams


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The cam can have

var ious s hapes.

These are know as

cam pro f i les.

Pear Heart Circu lar Drop

Cam

fol lower


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A follower is a component

which is designed to move

up and down as it follows

the edge of the cam.

Knife edge

Follower

Flat foot

follower

Off set

follower

Roller

follower

cam

Follower


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The 'bumps' o n a cam are cal ledlobes.

The square cam il lu strated has fou r

lobes, and l i f ts the fol low er fou r t imes

each revo lut ion .

Square cam

Follower

Examples of oth er rotary cam pro f i les.


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Cams can also be cyl ind r ical in shape

Below a cy lindr ica l cam and rol ler fo l lower.

The cam fol lower does not have to

mo ve up and dow n - it can be an

osc i l lat ing lever as shown above.

Cam ris e and Fall

Max Lift

Min Lift


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Knife edge follower- When the contacting end ofthe follower has a sharp knife edge, it is called aknife edge follower, as shown in Fig(a). The slidingmotion takes place between the contacting

surfaces (i.e. the knife edge and the cam surface).It is seldom used in practice because the small areaof contacting surface results in excessive wear. Inknife edge followers, a considerable side thrustexists between the follower and the guide.


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Roller follower- When the contacting end of thefollower is a roller, it is called a roller follower, asshown in Fig. (b). Since the rolling motion takesplace between the contacting surfaces (i.e. the

roller and the cam), therefore the rate of wear isgreatly reduced.

In roller followers also the side thrust existsbetween the follower and the guide. The rollerfollowers are extensively used where more space isavailable such as in stationary gas and oil enginesand aircraft engines.


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Flat faced or mushroom follower. When the contacting end ofthe follower is a perfectly flat face, it is called a flat-facedfollower, as shown in Fig. 20.1 (c). It may be noted that theside thrust between the follower and the guide is muchreduced in case of flat faced followers.

The only side thrust is due to friction between the contactsurfaces of the follower and the cam. The relative motionbetween these surfaces is largely of sliding nature but wearmay be reduced by off-setting the axis of the follower, asshown in Fig. (f)so that when the cam rotates, the followeralso rotates about its own axis. The flat faced followers are

generally used where space is limited such as in cams whichoperate the valves of automobile engines.

When the flat faced follower is circular, it is then called amushroom follower


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Pressure Angle: The angle between the direction of thefollower movement and the normal to the pitch curve at anypoint is called pressure angle. Pressure angle varies frommaximum to minimum during complete rotation. Higher thepressure angle higher is side thrust and higher the chances of

jamming the translating follower in its guide ways. The pressure angle should be as small as possible within the

limits of design. The pressure angle should be less than350for low speed cam mechanisms with oscillating followers,whereas it should not exceed 300in case of cams with

translating followers. The pressure angle can be reduced byincreasing the cam size or by adjusting the offset.


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Force can only be transmitted from cam to follower or vice versa

along the common normal or axis of transmission which isperpendicular to the axis of slip, or common tangent as shown inprevious. The pressure angle (or alpha ) is the angle between thedirection of motion (velocity) of the follower and the direction of theaxis of transmission.

When = 0, all the transmitted force goes into motion of the

follower and none into slip velocity. When becomes 90 there willbe no motion of the follower. As a rule of thumb, we would like thepressure angle to be between zero and about 30 for translatingfollowers to avoid excessive side load on the sliding follower. If thefollower is oscillating on a pivoted arm, a pressure angle up to about35 is acceptable. Values of greater than these can increase thefollower sliding or pivot friction to undesirable levels and may tendto jam a translating follower in its guides.


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