A bar horn is a rectangular horn that is either unslotted or slotted
only through the thickness. Special design techniques give optimum face amplitude
uniformity. Bar horns generally has low-to-moderate gain (1:1 to 4:1). Bar horns are used
for plunge and scan welding.
Example
The following example shows a 20 kHz 12" wide bar horn. The horn's thickness has
been reduced in the blade section in order to provide reasonable gain. The horn is one
half-wavelength long at the axial resonance (the desired resonance), as indicated by the
single node that is generally transverse to the principal direction of vibration.
For all images, the output surface (face) is at the top and the input surface is at the
bottom. The warmest colors indicate the highest amplitudes. The darkest color traces the
axial node(s).
All results are from finite element
analysis.
Original design
The following shows the original (unoptimized) design and the resulting amplitude
distribution.
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Original design -- No optimization. |
 |
Axial resonance, relative amplitudes -- The
amplitude at the end of the face is much lower than at the center. This will cause reduced
welding at the ends or over- welding at the center. View
actual vibration
|
Improved design
The following shows an improved design that has substantially better amplitude
uniformity across the horn's face.
 |
Improved design -- Uses
optimized slots, back masses, and flutes. |
 |
Axial resonance, relative amplitudes
-- The amplitude is very uniform across the horn's face.
View actual vibration
|
Design considerations
For bar horns that are wider than about 0.35 * wavelength (about 3.5" at 20 kHz),
longitudinal slots must be used in order to reduce the transverse coupling due to the
Poisson effect. The maximum distance between adjacent slots should not exceed about 0.3 *
wavelength (about 3" at 20 kHz). Without such slots the horn will either have very
uneven amplitude across the face or may even resonate in a nonaxial manner.
Although slots help to improve the face amplitude uniformity, additional horn
refinements are often necessary to further improve the uniformity, depending on the
particular application. Unfortunately, the required slots can introduce additional
problems, although these can be reduced through careful design.
Secondary resonances
Slots often introduce additional secondary resonances. The following image shows a
typical secondary resonance, although many others are possible.
Such secondary resonances may interfere with the vibration of the axial
resonance. In some cases, the power supply may prefer to start on a secondary resonance or
may jump to a secondary resonance during the weld cycle. The effects of secondary
resonances can be minimized by designing the horn so that the secondary resonances are
sufficiently far from the axial resonance.
Stresses
In bar horns, the stresses are generally highest at the end of the slots or at the
termination of the nodal radius. High cyclic stresses can cause the horn to fail by
fatigue. This problem can be reduced by proper design and by machining the horn from
high-strength materials.
