Selection Guide

Given the complex nature of Nitinol and of course its spring form, we have listed following steps as your Nitinol spring selection guide.  Feel free to contact us anytime to talk about your product needs, and our experienced engineers will be happy to recommend or design the spring that will fit your needs.

One Way or Two Way

The benefit of one-way springs is that they are lower cost than two-way springs.  However, a one-way spring needs to be deformed by an external force when cooled.  So, if your application does not readily provide this biasing force, additional components may need to be added to the system, increasing cost, size and weight.

A two-way spring automatically resets itself when cooled, eliminating the need for a biasing force.  This allows actuators to be put into extremely tiny packages.  For many applications, the reduction in components offsets the higher cost of the two-way spring.

We can customize everything, but if you need a one-way, spring, we have more than one million part numbers in stock and ready to be purchased online: Micro-Spring and Helical Spring.

Compression or Extension

A tension spring is a spring where the pitch is equal to the diameter of the wire (closed wrap).  This spring can only exert tension on a system because the closed wrap cannot be compressed in any way.  Of course, the exception to this is a two-way spring, because it can exert force in both directions.

A spring with a pitch greater than the wire diameter can be either extended or compressed.  These springs are referred to as compression springs or extension springs, depending on their application.

Increasing the pitch of the spring increases the stiffness and decreases the travel of the spring.

We can custom make extension spring, please feel free to contact us.

Transition Temperature

Choosing the proper transition temperature for your application is critical for your project to be a success.

Shape memory:

For electrically actuated springs, choosing a higher transition temperature yields a faster cycle rate while a lower transition temperature consumes less electricity.  For applications that use a heat source and sink to actuate the spring, the transition temperature should be a little lower than that of the heat source.

Superelastic:

For superelastic springs, the stiffness of the spring increases as the temperature increases away from the transition temperature.  Additionally, the amount of recoverable deformation is maximum close to the transition temperature.

Alloy

Nitinol is extremely sensitive to chemical composition, so choosing the proper alloy is very important.  Here is some basic information on some of the alloys available:

NiTi: The basic nitinol alloy.  NiTi exhibits the largest deformation recovery and is used in the vast majority of nitinol springs that we manufacture.

NiTiCu: Adding copper to the nitinol reduces hysteresis and improves fatigue properties.  The tradeoff is a dramatically reduced recoverable deformation.

NiTiFe: Adding iron to the nitinol reduces transition temperature and increases tensile strength.  A mild increase in hysteresis is also observed.

NiTiCo:  Adding cobalt to the nitinol increases the stiffness of the nitinol.

NiTiHf:  Adding hafnium to the nitinol allows transition temperatures up to 150C

NiTiPd or NiTiPt: Adding either palladium or platinum to nitinol allows transition temperatures up to 600C.  These metals are very expensive, so the cost of parts is also very high.

Mandrel Size

The mandrel is the tool or fixture on which the spring is wrapped.  For standard helical springs, the mandrel size is constant and is the inside diameter of the spring.   Increasing the mandrel size decreases the stiffness and increases the range of motion of the spring.

We can also produce conical, spherical, and many other various forms of springs to meet your design needs. These springs are custom made, so if you wish to use them, please submit a request for quote.

Wire Size

Springs are constructed from nitinol wire.  Increasing the wire size increases the stiffness of the spring while decreasing the range of motion.  Decreasing the wire size reduces the cycle time for shape memory springs.

Pitch Size

Pitch is defined as the distance between the centers of two consecutive wraps of wire.  Clearly, the smallest pitch possible for a helical spring is the diameter of the wire used.

Increasing the pitch increases the stiffness of the spring while reducing the travel.  Compression travel is optimized when the wire is wrapped at a 45 degree angle.

Ends

The ends of the spring control how the springs interact with your product.  Some of the possible endings that we offer are:

As cut: This is the lowest cost spring as the spring is simply cut from the stock.  This results in a pressure point on the end of the wire where it makes contact with your system.  Whether or not this matters will largely depend on your application.

Flat: The last one or two wraps of wire are wrapped with a closed pitch to give the spring a flat surface to mount onto your product.  Note: for closed wrap springs, as cut and flat ends are identical.

Ground: After the springs have been flatted, the final wrap of wire is ground perfectly flat.  This provides the best possible seating with your product but comes at an extra cost.

Hooks: The last half wrap is bent ninety degrees into a hook that is easily attached to your system.

Loops: The last one or two wraps are bent ninety degrees so that the spring can be secured with a bolt or screw.

Custom: Do you have a special ending that you need for your springs?  We have made many custom ended springs.  Just send in a request for quote to get the conversation started.

Special Shapes

Do you need a variable pitch spring or a conical spring or something that seems to wild to manufacture?  We produce parts like these every day of the week!

Force and Modulus

We often get requests for the spring constant or Youngs  Modulus of our nitinol springs.  While nitinol springs do have a spring constant, almost all applications of nitinol springs operate outside the linear-elastic zone.  This means that the spring constant is not constant, it is dependent on strain.  Unfortunately, the “spring constant” is also dependent on temperature, further complicating the problem.  The best bet is to capture the full stress/strain curve.

Force and displacement are easily determined from the stress/strain curves.  The force generated is determined by the distance between the martensite and austenite curves when strain is held constant.  The displacement can be determined by the distance between the martensite and austenite curves when stress is held constant.

One-way, compression spring order-able online:

Micro-Spring

SHOP

Helical Spring

SHOP

Flat Spring

SHOP

Torsion Spring

SHOP

Two Way Shape Memory Spring

KRL will customize two transition temperature, two set shape, with required Dimension, force and travel.

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