Large Nitinol Actuators – Kellogg’s Research Labs

Nitinol Helical Springs

Nitinol has been used as an actuation technology for about 20 years.  However, nearly all applications have been with thin wires. In the world—especially in the world of energy production and valve control—there are often needs for actuators that supply hundreds of tons or thousands of tons of force.  Our customers are often surprised to learn that, yes, nitinol actuators can be produced in very large sizes.

With operating forces up to 25,000psi (175 MPa) for multi-use actuators and 100,000psi (700MPa) for single use actuators, the force density vastly outpaces any other actuation technology, so it is very lucrative for space constrained applications.

So, what is the largest actuator that can be made from nitinol?  This really is limited by the largest nitinol that can be produced.  As of the writing of this article, we maintain a constant inventory of 25mm (0.984”) diameter, 2m long round bars for the purpose of producing large scale actuators.  The largest actuator we have made to date was 6” (152.4mm) diameter and 3m long.  If you need something even larger produced, all that it would take is for us to locate a steel mill that is ready, willing, and able to run the bar stock through its reduction.

Designing Large Actuators: The design constraints of working with nitinol generally hold true when scaling into the region of large diameters, but the difficulties are generally proportionate to r2.

  1. Attachment: For tension actuators, making a sufficiently strong attachment is a major undertaking.  One option is to weld on a ring terminal and then secure it with a bolt.  However, this may not be sufficiently strong for very high force applications.  Another method might be to bend a hook or loop into the end of the bar.  However, bending the rod can cause strain points that lead to premature failure.
  2. Heat transfer: The rate of heat exchange with the environment is generally related to .  To increase the heat transfer in and out of the actuator, we often overlay a conductive polymer to allow a high surface area, thereby increasing the heat transfer.  However, the heat transfer is still related to .  The only way to break this relationship is to have multiple actuators.
  3. Microstructure: This is a problem that we at KRL have to solve rather than something that you need to solve.  However, as the diameter of the nitinol increases, the grain size of the center of the rod also increases.  Large grain size leads to lower pressures and shorter fatigue life.  There are methods for reducing the effects of this so that the actuators do everything that you need them to do.

If you have a high force application, one question to ask is if multiple, smaller actuators can be used in place of the single larger actuator.  Sometimes, there is an intersection of cost and benefit when counting the number of actuators and identifying that intersection can be challenging.

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