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Reconditioning the Ultrasonic "Stack"

The ultrasonic "stack" is made up of three components: the transducer, booster and ultrasonic horn. The quality of the mating surfaces between these three units will have a major impact on the operating efficiency of your ultrasonic welding equipment. If they are not flat, if they become corroded or they make poor contact, they become a weak link in the transmission of ultrasonic vibration, and power output is wasted, tuning is difficult, noise levels will increase and heat damage to the transducer is possible.

First let's look at each component and reasons for possible failure:

Ultrasonic horns crack due to:

  • Metal to metal horn contact.
  • Fatigue over time.
  • Poor original design (sharp corners, slot design and finish, drastic change in cross section and tuning to improper frequency, i.e., radial versus axial).
  • Improper horn material selection (titanium is best up to approximately 4 in. dia or 2-1/2 in. thick, while D-2 steel should be used only for lower-amplitude applications).
  • Being too highly driven by the booster, which results in exceeding the elastic limits of the horn material, causing more rapid fatigue.
  • Over-tightening the horn on the boosters.
  • Over-tightening the stud.
  • Flaws in the horn material.
  • Applying uneven welding pressure to the horn face.

Boosters crack or fail due to:

The same reasons as above for horn failure, as well as:
  • Operating ultrasonic horns that are poorly designed and/or tuned to the improper frequency..
  • Out-of-balance horns or applying uneven welding pressure to the horn.

Transducers fail due to:

  • Dropping the transducer and breaking the ceramic.
  • Poor horn design and/or out-of-frequency horns.
  • Over-tightening of the booster.
  • Moisture from air lines seeping into housing.
  • Shock from the application (insertion). This is more likely to happen on older units without transducer protection.
  • Tightening booster or horn without a spanner wrench on the transducer front slug.
  • Heat developed by problems already listed.

Reconditioning the ultrasonic stack

It is important to check the stack regularly to make sure the components are in good working order. In addition, there are several steps you can take to recondition the stack:

  1. Disassemble the transducer/booster/horn stack and wipe the mating surfaces with a clean cloth or paper towel.
  2. Examine the surfaces. If they appear to be in good condition, skip to Step 9. If any surface is corroded or shows a dark, hard deposit, it should be reconditioned (Steps 3 - 8). If the mating surface of any component shows evidence of crowning, cupping, or any other out-of-flatness condition, contact an ultrasonics industry professional for advice. Very small, isolated pits in the mating surfaces are generally not a serious problem.
  3. If necessary, remove the mounting studs.
  4. 4. Tape a clean sheet of #400 (or finer) emery cloth to a clean, smooth, flat surface. A piece of plate glass is usually suitable.
  5. Hold the component at its lower end and carefully stroke it in one direction across the emery cloth. Do not apply pressure as the component's weight alone will suffice. NOTE: Use extreme care to avoid tilting the component. Loss of flatness on interface surfaces may render the welding system inoperative.
  6. Perform a second stroke, then rotate the part one-third turn and repeat.
  7. Turn the part the final one-third and perform the same two strokes. Be certain to perform the same number of strokes (two) at each location.
  8. Re-examine the mating surfaces, and repeat steps 5 through 7 until most of the contaminate has been removed. This should not take more than two or three complete rotations.
  9. Before reinserting a stud, examine it to make sure the threads have not been damaged. Clean all foreign material, grease and oil from the threads of the stud and the threaded hole using a clean cloth or towel.
  10. Replace worn or damaged studs with those specified by the manufacturer. Ordinary steel set screws are not properly heat treated for use as stack studs.
  11. Very lightly coat the flat mating surfaces with high-pressure silicone grease or insert a high-temperature polymer film washer (not both) to promote good transmission of ultrasound and prevent the stack components "cladding" together.
  12. Torque studs and mating surfaces properly, as indicated in the accompanying table showing Correct Torque Values for Stack Component Assembly. Loose studs or joints will cause overloads or intermittent operation, while excessive tightening results in material distortion that shortens component life.
  13. Install the stack in the welder and test ultrasonic operation.

A Note About Flatness

It is essential that the mating faces between an ultrasonic transducer/booster and a booster/horn be flat and parallel. If any air gap remains, there will be a resultant loss in power output and efficiency. Coupling may be so poor as to prevent the starting of an ultrasonic stack.

The condition of excessive crowning, or uneven contact surfaces, is normally evidenced by a burnished appearance only around the bolt area of the contact surfaces. This indicates that contact between the members is occurring only at the burnished area and not around the periphery of the surfaces.

The following flatness tolerances are typically specified:

  • for transducers:  0.0005 in. for 20 kHz and 0.0005 for 40 kHz
  • for boosters:  0.0010 in. for 20 kHz and 0.0005 for 40 kHz
  • for horns:  0.0010 in. for 20 kHz and 0.0010 in. for 40 kHz.

Correct Torque Values For Stack Component Assembly

Studs in horns and boosters:

12-18 in.-lb 1-1.5 ft.- lb 1.4 - 2 Nm 1/2 in. x 20 tpi studs
12-18 in.-lb 1-1.5 ft.- lb 1.4 - 2 Nm 3/8 in. x 24 tpi studs
12-18 in.-lb 1-1.5 ft.- lb 1.4 - 2 Nm 8 mm studs

Transducer/booster/horn assembly:

540 in.- lb 45 ft.- lb 61 Nm 15 kHz Stack
420 in.- lb 35 ft.- lb 47.5 Nm 20 kHz Stack
216 in.- lb 18 ft.- lb 24.4 Nm 30 kHz Stack
216 in.- lb 18 ft.- lb 24.4 Nm 40 kHz Stack

Replaceable tips on horns:

360 in.- lb 30 ft.- lb 40.7 Nm 1/2 in. x 20 tpi tip threads
336 in.- lb 28 ft.- lb 38 Nm 3/8 in. x 24 tpi tip threads
300 in.- lb 25 ft.- lb 33.9 Nm 5/16 in. x 24 tpi tip threads
240 in.- lb 20 ft.- lb 27.1 Nm 1/4 in. x 28 tpi tip threads

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