What is chordal action or the polygon effect?
We often receive questions concerning the chordal action, also known as the polygon effect. Today we will try to break down the meaning of this action and suggest a couple ways to reduce its effect.
Polygon means ‘multi-angle’. Module and chain links moving around the radius of the sprocket causes the linear belt speeds to vary. The pivot rod travels on the pitch diameter of the sprocket while the module moves through the smaller chordal radius causing a horizontal rise and fall of the module. This polygon effect is typical of all modular belt systems. The magnitude of speed variation is dependent on the number of sprocket teeth.
Sprockets with a low number of teeth show a high speed variation (5 teeth up to over 14%). This is why we advise using a sprocket with a minimum of 12 teeth in order to limit the polygon effect. As the graph shows, more teeth will further reduce the effect. By the way, this not only effects the sprockets. You will find the same result among all idler-, transfer- or support rollers. To minimize this effect, choose the diameter for the roller based on the nominal pitch of a 12 teeth sprocket of the belt series that you prefer.
In general, we suggest designing the conveyor with the biggest possible diameter in mind and taking care that the distance between the center of the rollers is not a multiple of the belt pitch.
If you have any further questions regarding the polygon effect, download our HabasitLINK Engineering Guide or contact us!