Cam Pressure Angle, [μ]

Pressure-Angle represents the efficiency with which the cam transfers its motion and force to the follower, and vice versa.

Understanding Pressure Angle.

Imagine pushing a door to open it...

If you push the door handle at 90º to the door (perpendicular[⊥], or normal, to the door), you will open the door easily. You only need to overcome the inertia force of the door and any friction in the hinges.
If you push the door handle at 45º, then you will need to:
oHold on to the door's handle so that your hand does not slide across the door.
oThe reaction force with which the handle stops your hand sliding across the door is also the force pulling on the door's hinges, which also pull on the door frame. You hand also feel this force react against your hand.

The angle with which you push the door is similar to the pressure-angle between a cam-profile and the cam-follower lever.

GA-Cam-PressureAngle1

The three vectors of the 'force-triangle' [usually taken at the centre of the cam follower] that result from the contact between the cam-profile and the cam-follower are the:

Contact Force. [ 'Hypotenuse' of the force-triangle]. Its direction is normal to the cam at the cam-contact point.
Useful-Force. [ 'Adjacent' of the force-triangle]. Its instantaneous direction in which the cam-follower must move.
Useless-Force. [ 'Opposite' of the force-triangle]. Its direction will 'stretch' a Swinging-Arm Follower, or 'bend' a Translating Follower.

From the three vectors, we can define Pressure-Angle

The Pressure Angle, μ, is, by definition, the angle between the direction in which the cam-follower must move and the direction normal to the cam at the point of contact between the cam and cam-follower.

μ

Definition:

Pressure Angle, μ, is the 'angle between the direction in which cam-follower moves and the direction of the force transmission from the cam to the cam-follower'.

Pressure Angle = Cos-1

(

 

Useful Force

)

Contact Force


Pressure Angle Limits: Rules-of-Thumb

When a Cam has a Translating Cam-Follower, limit the Pressure Angle to less than ±30º
When a Cam has a Swinging Cam-Follower, limit the Pressure Angle to less than ±35º

To reduce the Pressure Angle

Improve the motion design. Can you increase the duration of a segment? Can you change the cam-law to one with a lower peak velocity?

Move the position of the cam-follower's pivot axis. Change the pivot-point of the cam-follower. Or, if the follower is a 'translating follower', 'offset translating follower'.

Increase the size of the cam. The pressure-angle will decrease as you increase the size of the cam.


Force Vectors with different Pressure Angles

We can compare Contact, Useful, and useless Forces

If we assume a 'Useful Force' of 100N, then with a:

Pressure Angle is 0º:   Contact Force = 100.0N;     Useless Force = 0N
Pressure Angle is 10º: Contact Force = 101.5N;   Useless Force = 16.7N
Pressure Angle is 60º: Contact Force = 200.0N;   Useless Force =  173.2N
Pressure Angle - 30 degrees

^^ Pressure Angle 30 degrees ^^

Pressure Angle = 30º

Useful Force      = 100N
Contact Force   = 115N
Useless Force    = 57.7N

The force that tries to 'stretch' the Cam Follower arm is 57.7% of the useless force.

The Contact Force is 15% more than the useful force.

Pressure Angle 45 degrees

^^ Pressure Angle 45 degrees ^^

Pressure Angle = 45º

Useful Force       = 100N
Contact Force    = 141.4N
Useless Force     = 100N

The force that tries to 'stretch' the Cam Follower arm is the same as the useless force.

The Contact Force is 41.4% more than the useful force.

Overturning Moment

The Overturning Moment applies to Flat-Faced Translating Followers.

The Overturning Moment is the product of the distance from the contact-point to the sliding axis of the translating-follower.

We define Re-Turning-Moment as the product of the Reaction-Forces and the Distance between Bearings or Length of a Sliding-Bearing

OverturningMoment-1a

Understanding Overturning-Moment

Imagine you want to slide a plank of wood on the floor, but between two walls. The plank is 2m long, 1m wide, with the 2m length along the walls. There is a small gap.

The Plank is uniform. However, it is cut so that it contacts the wall only at its ends, 2m apart.

Ff [against the floor] = Friction-Coefficient [μ] × Mass × Gravity [m×g]. [Note: do not confuse Friction-Coefficient, μ, and Pressure-Angle, μ].

If the plank is 'uniform', the Friction-Force vector will be through the Centre-of-Mass and parallel with the wall - nominally 0.5m away from the wall.

No Over-turning Motion

If you slide the plank [slowly] with a hand that is in line with the centre-of-mass, the plank will not tend to rotate.

The wall does not need to react against the sides of the plank. The wall does not, theoretically, need to be there.


 

OverturningMoment-2a

Over-turning Moment

Now, push the plank at a point nearer to one wall [so that the contact is offset from centre-of-mass 'axis' and friction-force vector

'Overturning Moment' = Frictional force[Ff] × perpendicular distance to your hand from plank's centre-of-mass 'axis'.

Assuming the Friction-Force remains the same, the further away from the plank's centre-of-mass you push, the greater the over-turning moment.

Re-turning Moment [to coin a term]

The overturning-moment will tend to rotate the plank. The wall prevents it rotating, of course.

The wall resists the plank at opposite corners of the plank [nominally]

Re-turning moment = Reaction Forces from the walls against the plank × length of plank = –ve overturning table.

If you use a longer plank, the reaction-forces from the wall become less, but the overturning-moment remains the same.

 

Over-turning moment = Re-turning moment

Over-turning moment = Contact-Force × distance to the 'sliding-axis' from the contact-point.

Re-turning moment = Reaction-Forces at Points at each end of the Lines in the Sliding-Joint × distance between Points in Sliding-Joint.

GA-OverturningMoment

In the image: we can see the contact-force at the top of a 'cam' between the cam and the flat-faced-follower.

The contact force is 20N, and it is 50mm from the vertical sliding axis of the cam-follower.

Thus:

Overturning Moment [1000N.mm] = Contact Force [20N] × Distance to Contact Point from Sliding-Axis [50mm]

Overturning-Moment = Re-turning-Moment

Re-turning Moment [1000N.mm] = Distance between Sliding-Bearings [or Length of a Sliding-Bearing] [30mm] × Reaction Forces [33.33N]

 

Notes:

During the motion of the follower, the contact point continually moves across the follower to one side then the other of the face.

This analysis ignores any extra friction-force in the sliding-bearings and friction-force between the cam and the cam-follower.

How to reduce the Over-turning Moment:

Improve the motion design.

Can you increase the duration of a segment? Can you change the cam-law to one with a lower peak velocity?

Move the cam-follower's sliding-axis relative to the cam's centre.

Change the the 'translating follower' to an 'offset translating follower' as alternative [Move the Sliding-Axis towards the Contact-Point.

This only applies when the contact force is is different for each direction of the followers motion.

Increase the size of the cam

This does not make any difference to the contact-force or overturning moment. However, the cam's radius of curvature increases, which will reduce its Hertzian Contact-stress.

Increase the length of the 'Sliding-Joint' - move bearing apart as far as possible.

The contact force at each of the sliding-joint is inversely proportional to the length of the sliding-joint.

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