Design Checklist with the Cam-Data FB

Cam Data FB icon

Add a Cam-Data Function-Block (FB)...

...edit the FB to link it to a 2D-Cam...

...use the data available at its output-connectors to check various 2D-Cam design parameters.

See Add Cam-Data FB and Edit Cam-Data FB

In MechDesigner, it is very easy to add cams that are 'good' and 'bad'.

'Good' or 'bad' is whether the cam has the capacity to perform its duty for a specified life.

We can do:

Geometric Analysis : or, as Kinematic Analysis.

Force/Stress Analysis  : or as Kinetostatic Analysis.


Geometric Analysis:

The Geometric Analysis reviews the:

Cam Size
Pressure Angle / Overturning Moment
Radius-of-Curvature

The cam's size, pressure-angle and radius-of-curvature are related to each other. They are a function of the dimensional layout of the cam mechanism, given a fixed motion. It is critical that you review these parameters early in your cam design. The strongly influence the contact force and contact stress.

Force Analysis:

Before looking at Contact-Force and Contact-Stress... the model must represent the power flow in the machine correctly.  

You may need to 'Configure the Power Source' to make sure that the force between the cam and cam-follower is correct.

See: Configure the Power Source for Payload Analysis

The Force Analysis reviews the:

Contact-Force

Contact-Force is mostly used to find the Life of the Cam-Follower.

Contact-Stress (Hertzian Stress)

Contact-Stress is mostly used to find the Life of the Cam.

Generic Payload Types and 'Signatures'

Payload: is the superposition (addition) of the different payloads that act on the Cam-Follower - the total load of the output transmission.

Payload Signature: is how the payload varies over one machine cycle.

Payload Type: are payloads types that are a function of the motion-derivative, and thus, they are a function of machine-angle and machine-speed.

Payload Signatures vs Motion-Derivative

Payload Signatures

Motion-derivative and Payload Type

Examples

CamForce-constant

Not a function of the motion

Constant Load

Gravity, 'Friction', Spring Pre-load

CamForce-ProportionalPosition

First Motion-Derivative

Load Proportional to Displacement

Spring Displacement

CamForce-ProportionalVelocity

Second Motion-Derivative

Load Proportional to Velocity

Viscosity, Air-Cylinder

CamForce-ProportionalAcceleration

Third Motion Derivative

Load Proportional to Acceleration

Inertia Forces

CamForce-ProportionalSpecial

A different function of Machine Angle, for example an engine's piston load.

'Power' stroke of an Engine

Importance of Machine Speed

In the example below, the payload is the superposition of three load types:

Constant Force (Green), for example: Gravity, Air-Cylinder.
Displacement Force (Blue), for example: Spring Force.
Acceleration Force (Magenta), for example: Inertia Force.
Cam forces at 'Low Speed'.

Cam forces at 'Low Speed'.

'Low' Machine Speed:

The maximum payload is at the maximum displacement of the cam 1s
The acceleration force is 'low'.
Cam loads at 'High Speed'.

Cam loads at 'High Speed'.

'High' Machine Speed:

Accelerations increase to machine speed squared.
The maximum payload is during the motion 'rise' 2s, which is during the positive acceleration phase is also at its maximum positive value.
The minimum payload is also during the motion 'rise', but during the negative acceleration phase. The minimum force is less than 0N 3s, which means the cam-follower roller will lift off the cam-flank.

Cam-Follower: Payload 'Signature'

Typical Load Signature of a Cam-Follower

Cam-Follower: Payload Signature of 'Force-Closed Cam'.

 

In this example, the Cam-Follower Roller is contact is 'active' for all of the machine cycle.

First: review whether the contact-force is active against the cam for the complete machine cycle or only part of the machine cycle.

The Contact-Force Signature maybe different for a Force-Close Cam when compared to a Body-Closed Cam.

Body Closed Cam - Conjugate-Cam types

Cam-follower '1' is active during the acceleration phase of the 'Rise', and the deceleration phase of the 'Return'.
Cam-follower '2' is active for the other phases.

Body Closed Cam - Type 2: Groove-Cam types

One roller is active for the complete machine cycle.
The roller is in contact with one cam-flank as the cam-follower accelerates and the other cam-flank as it decelerates.
The roller must reverse its rolling direction when it changes cam-flanks. As it changes rolling direction, it must must scuff when the linear speed of the roller-surface and speed of the cam-flank are not equal.
The contact force is zero [0] while the roller moves between flanks within the backlash zone. There must be some backlash with Groove-Cam types.

Force-Closed Cam

The roller is 'active' for the complete machine cycle, assuming the Contact-Force does not become ≤ 0N.

Cam: Payload 'Signature'

Cam: Payload Signature of Body-Closed Cam

Cam: Payload Signature of Body-Closed Cam

Mean-Stress = (Max+Min)/2;

Stress Amplitude = (Max-Min)/2;

Max Stress = 2*Mean;

Minimum Stress = 0 MPa

Each point along the cam surface (and just below) becomes stressed at one 'machine angle'* in the machine-cycle

The stress that the point experiences is zero at all other machine angles.

For each point along the cam, the stress is said to be 'pulsatile' (like heart-beat pulses) with a 'period' of 2.π or 360º*.

* Assumes the cam rotates one time per machine cycle.


This image shows the 'stress pulse' at a single position on the cam's surface


 

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd