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Bar horns

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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.

Ultrasonic horn -- slotted bar, no optimization Original design -- No optimization.
Ultrasonic horn -- slotted bar, axial resonance, relative amplitudes 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.

Ultrasonic horn -- slotted bar, optimized

Improved design -- Uses optimized slots, back masses, and flutes.

Ultrasonic horn -- slotted bar, axial resonance, relative amplitudes

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.

 

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Krell Engineering
212 E. Medwick Garth    Baltimore, MD  21228    USA
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