Using Nitinol for Vibration Damping

Purpose: Mechanical vibrations are the primary cause of premature product failure.  They can arise from inaccurately made parts, varying forces within the system, or outside input.  The vibrating components generate heat, excess wear, dropping efficiency and shortening the service life of the product. 

Typically, there are two methods to eliminate vibration: increase accuracy or use damping materials.  Increasing the accuracy of the parts also increases the cost of the parts—usually exponentially.  For damping materials, there is a general rule that there is an inverse relationship between damping effectiveness and structural strength.  Nitinol is different in this matter in that it has the structural strength of titanium but the damping effectiveness of silicone rubber.

1. A Simple Test for Vibration Damping: There is a very simple test to qualify how effectively a metal damps vibration—the drop test.  If the metal returns a sharp ring, that means that it is supporting standing wave vibrations—the super destroyer.  If the metal returns a dull thud, then the metal is very effectively damping vibrations.  Aluminum returns a pronounced ring.  Magnesium returns a less pronounced ring.  Nitinol returns a dull thud.

Background: What is Nitinol: Nitinol is the trade name associated with the family of alloys based on the mixture of nickel and titanium which exhibit the shape memory effect (SME).  The name nitinol is synthesized from Nickel-Titanium Naval Ordinance Laboratory—the organization that discovered it.  The shape memory means that it can be deformed at a low temperature and then the shape is recovered upon heating.  This apparently magical property is caused by the existence of a solid state phase transformation.

The low temperature phase, martensite, has bonds that easily rotate, absorbing deformation without breaking.  This causes the material to feel soft and malleable, similar to solder.  The high temperature phase, austenite, is a rigid cubic structure.  Since the bonds were not broken in the martensite phase, they very forcefully snap back to the original matrix form when heated.

The temperature at which this transformation occurs can be tuned to meet application requirements.  For binary nitinol alloys, the range of possible transition temperatures lies from -100°C to 100°C.  By adding in a third metal, the range of possible transition temperatures broadens out to -200°C to 250°C and, if materials like platinum fit the budget, the upper limit can be pushed up to 600°C.

When nitinol operates above its transition temperature, a different phenomenon is observed: superelasticity.  Superelasticity has two basic properties that are notable: very high strain recovery and plateau stress.  The plateau stress means that, once the deformation reaches a high enough value, the force supplied by the nitinol element changes very little with the increasing or decreasing deformation.  A superelastic nitinol element can elastically recover up to 50% strain. 

• Vibration Damping: One of the side effects of the shape memory effect and superelasticity is that nitinol is very effective at damping vibrations.  The switching of the material from one microstructure to the other and then back again results in dissipating enormous amounts of energy.  Most materials that are effective at shock hardening are very soft—meaning that they cannot be used in structural applications.  Nitinol, on the other hand, has the structural strength of titanium.  This means that a part of the cost of damping with nitinol can be written off by not needing an additional structural material.  This is especially important for space restrictive applications.
  • Vibration Damping Metals: It is important to note that nitinol is not the only metal that is useful for shock hardening.  Magnesium is very effective at shock hardening due to the high degree of twinning in the microstructure.  The impact causes the microstructure to detwin, creating internal friction, converting the   However, with damping efficiencies of 90-95%, a nitinol damper is far more effective at vibration damping than magnesium (as well as having some side benefits).  This increased effectiveness comes from higher degrees of twinning and the hysteretic nature of nitinol’s microstructure.
• Nitinol and Printing: Drop placement is one of the core problems affecting printers and vibrations are one of the major causes of incorrect drop placement.  The vibrations are caused by the carriage running back and forth across the frame.  Here are a few isolation techniques that may work well to isolate the printhead from the vibrations of the carriage.
  • Belleville Springs: Belleville springs (or washers) are a cone shaped washer, which acts as a spring.  There are two advantages to Belleville springs.  First, they have a significant preload once they are installed.  This preload makes better use of the hysteretic nature of nitinol.  Second, they can damp large amplitude vibrations.  If the Belleville washer isn’t completely compressed, then some translation may be allowed, thereby flattening large amplitude vibrations—such as those caused by a carriage crash.  In most cases, Belleville springs are best made from superelastic nitinol.
  • Bushings: Using bushings would allow a more rigid attachment than Belleville springs.  This would maintain positive control over the printhead position while dramatically reducing the vibration.  The bushings can be as simple as washer replacements, resulting in very minimal product change.
  • Framework: The greatest benefit comes from replacing frame components with nitinol.  This reduces the natural frequency of the carriage while dissipating the vibration energy across a large area.  This method usually has the highest cost—in exchange for the greatest benefit.