Vibratory bowl feeders, with their hopper and
inline cousins (All called "feeders" here for convenience) are the workhorse of assembly automation. They have only
been around a bit over 50 years but the assembly process without one, or dozens, of them is rare. Unfortunately, in most plants
today, feeder performance is frequently below what it should be.
The problem is, simple as they are, most people
do not understand them. The easiest way to understand them is to realize that the base, springs and bowl form a pendulum system.
It is usually an upside down pendulum but a pendulum nonetheless.
For an example of a pendulum, push a child
on a swing.
The first thing to know about pendulums and feeders is that a given pendulum will always cycle at
the same frequency, regardless of distance traveled. A child on a swing might take 1 second to complete a cycle from 1'
behind the centerpoint to 1' in front and return to the starting point. If the swing is pushed harder so that it moves
6' instead of 1', it will move faster but will still take 1 second to complete the cycle. This cycle time is the natural
period or frequency of the pendulum. It can be changed only by changing the weight or length of the pendulum.
The
second thing to know about pendulums is when to push. Once the child is swinging steadily, a small amount of energy must be
added on each cycle to overcome friction from air and bearings. If the push is timed properly, it can be done with a fingertip.
The properly timed push will occur exactly at the top of the cycle, just as the swing changes direction. That means that there
will be a push every 1 second.
If the push is made early, before the top of the cycle, significant force must be expended
to stop the backwards motion before any forward motion can be imparted. If the push is made late, the swing is already moving
forward again and energy is wasted catching up before any pushing can take place.
So how does this apply to feeders?
In the case of the child’s swing, the push is timed to the match the natural frequency of the pendulum. For most feeders,
the cycle is fixed. Most feeders are driven by AC electromagnets or solenoids which pull the springs back and release them
120 times per second. This frequency is determined by the frequency supplied by the power company. (60hz US, 50hz in Europe
and elsewhere) If the natural frequency of the feeder is 120 cycles/minute, this will be fine. If the pulses and the frequency
don’t match, this will not be fine. The feeder will not work properly, if at all. Feed rates will be slow. It will be
noisy. Power consumption will be high. The solenoid can overheat and burn out. Parts may bounce around more than they should
preventing proper orientation.
When the feeder frequency and the line frequency do not match, the feeder is said
to be out of tune. A feeder may be brought into tune by adding or removing springs or by adding or removing weight. Alternately,
a variable frequency controller such as Service Engineering’s AccuTune may be used to provide the proper frequency to
the solenoid.
Whichever means are chosen, the feeder and line frequency must match for good performance.
Properly tuned, a feeder will vibrate properly but, by itself, vibration will do little more than make the parts bounce
around. In order to make the parts move, the vibration must be focused in the desired direction. This is done by mounting
the springs on an angle rather than vertically. As the feeder bowl rotates back and forth on its springs the angle causes
it to move vertically as well. As it moves forward and up, friction pushes the part forward. As it moves back and down, the
part falls vertically. This sawtooth motion can move parts gently at astonishing speeds.
That’s really
all there is to it.
That is the fundamental principle of operation of vibratory feeding equipment. There are
a lot more details but once the basic principles of frequency and directed vibration are understood, the rest is much easier
to understand.
I hope that this brief primer can serve as a helpful starting point. For those wanting to learn
more, Changeover.com offers training workshops in your plant covering feeders in depth. For more information, please fill
the form below or call 787-550-9650 during eastern business hours.
The above article is copyright 2008 by
John R Henry. Permission is freely granted to circulate or republish it provided that it is circulated or republished in its
entirety including this notice. If republished, we would appreciate a copy of the publication.
