Contact-Force and Contact-Stress Analysis
Why compare the Cam-Follower with Contact-Force and the Cam with Contact-Stress?
Cam-Follower: Payload Signature
Typical Load Signature of a Cam-Follower
First: review whether the contact-force is active against the cam for the complete machine cycle or only part of the machine cycle. [The payload signature in the image to the left does not become less than 0N, thus the contact-force is always active].
Conjugate-Cam (Form Closed Cam)
|•||One of the cam-follower bearings is active during the acceleration phase of the 'Rise', and the deceleration phase of the 'Return',|
|•||The other bearing is active for the other phases of the Rise and Return.|
Groove-Cam (Form Closed Cam)
|•||The cam-follower bearing is active for almost the complete machine cycle.|
|•||But, it must reverse between acceleration and deceleration and thus scuff while it gets up to the rolling speed in the opposite direction.|
|•||There is zero contact force while the roller transfers between flanks within the backlash zone.|
|•||The cam-follower bearing is active for the complete machine cycle because the contact-force does not become less than 0N|
See Contact Force and Life Analysis
|•||Payload: 'pseudo-continuous rotation' and 'pseudo-continuous load'; similar to a 'normal' bearing.|
|•||Loaded Elements [inner-race and rolling elements]; similar to a 'normal' bearing, outer-race has larger section to limit distortion|
|•||Material Properties: tight tolerance and quality control; identical to a 'normal' bearing|
|•||Geometric Properties: radius of outer-ring, inner-ring and the balls/rollers are known.|
The loading of a cam-follower is similar to that of a 'normal' bearing.
Thus, we will use the Basic Static Load Rating, Basic Dynamic Load Rating and Fatigue Limiting Load to calculate the life of the cam-follower bearing.
Pulsatile Payload 'Signature' for a Cam
CAM-STRESS: 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) is stressed one time* in each machine cycle.
Thus, the stress that each point along the cam 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.
The image shows the Payload 'pulse' at a single point on a cam.
Every single-point on the Cam experiences a stress pulse similar to that in the image, one time per rotation. The amplitude of the stress pulse changes continually.
|•||Payload: pulsatile, each point on the cam is loaded once per cam rotation.|
|•||Material: Decided by design engineer, material supplier quality control, heat treatment|
|•||Geometric Properties: Radius continuously changing|
|•||Material: surface finish, heat treatment, hardness|
|•||Lubrication: Oil or Grease|
We cannot use the Basic Life Equation to find the life of a Cam.
We must compare the 'working contact-stress' with the 'permissible contact-stress' of the cam material.