Shape Memory Alloys
How much ‘expansion’ can I expect from SMAs?
SMAs do not operate based on volumetric expansion as in paraffin or wax type actuators. Rather Shape Memory Alloys are able to recover from “deformation” by heating from one state to another. The mode of deformation and subsequent recovery can be in all directions (i.e. tensions, compression, bending, twisting, or any combination thereof). Recoverable strains for binary Nitinol is in the order of 4% where single crystal derivatives can exceed 10%.
Is the recovery of SMAs linear with temperature?
No, unlike bi-metallic actuators or devices which operate based on material CTE (Coefficient of Thermal Expansion) SMAs recovery from one state to another occurs over a narrow temperature range (approximately 10C). This is analogous to water boiling when it gets to 100C (i.e. there is a “state change” which occurs relatively abruptly when the material hits a critical temperature).
How high can you make the actuation temperature of SMA elements?
Titanium Nickel based alloys can generally go up to 100C. Copper based alloys (such as our SCSMA) can go up to 200C.
Does the applied load or stress affect the transition temperature?
Yes, the transition temperature can be raised with load or stress. The value is typically in the order of 1°C/ksi. As an example, an element that has a free recovery transition temperature of 70C will exhibit an In-Situ actuation temperature of 100C if under a load of 30ksi.
What is meant by ‘Super-Elasticity?’
Super-Elasticity refers to the ability of Shape Memory Alloys to spring back after exposure to significant deformation. For SCSMAs this can be 9% strain (or more) which is approximately 20 times greater than normal metals.
Is the SMA used to retract the Pin Puller output Pin?
No, the Pin is retracted under the force of a “Compression Spring” made of conventional materials. Rather, the SMA (or TiNi™) is used to release the latch which initiates movement.
Can you raise the resistance of your Pin Pullers (or ERMs) so they can be operated from higher bus voltages (such as 20-70 Vdc) without exceeding your current limit?
Yes, we routinely do this for many applications by simply incorporating an inline resistor which is capable of dissipating the power required. Generally, this is not a problem because the function times are sub-second and therefore result in very little energy (Joules) dissipated.
How does EBAD determine the rated force of the Pin Pullers?
As an example, the TiNi™ P10 Pin Puller provides a minimum force available at the start of the travel which is 10 lbf. The retraction force at the end of travel however may be significantly lower at approximately 5 lbf. This is consistent with the requirements of most applications where the maximum required force is at the start of travel when overcoming static friction.
Are the Pin Pullers vented?
Yes, unless otherwise required, all EBAD’s TiNi™ products are vented for space use.
Does it matter where on the Pin the load is applied?
As with all such applications, it is best to apply the load as close to the supported section (Pin Puller Flange) as possible. However, the quoted specification is based on worst case situations where the load is applied at the end of the Pin forming the greatest moment.
How does EBAD make the Pin Pullers (and ERMs) operate so fast?
The TiNi™ element is “Joule Heated” (i.e. the current is passed directly through it such that it can be heated very quickly).
What establishes the upper maximum temperature of the Pin Puller, ERM, or Frangibolt®?
This is limited by the self-actuation temperature of the device (i.e. the maximum operating temperature must not exceed the temperature at which point the device risks functioning without the application of power). All EBAD’s TiNi™ standard products maintain a conservative margin of 20C from this temperature.
Why is it that I measure only 80C at actuation when the quoted actuation temperature is supposed to be 100C?
This is because the embedded temperature sensor (RTD element) does not read the exact temperature of the TiNi™ cylinder which governs actuation (i.e. it happens to be at a colder spot within the heater assembly). Note that this also depends on power applied and starting temperature (i.e. the temperature distribution may differ from cycle to cycle depending on these external conditions).
Can I machine my own Frangibolt® Actuator Bolt or Fastener?
No, the Frangibolt® Actuator Fastener is an integral part of the overall mechanism and must be manufactured by EBAD. This is done to very high standards requiring a significant amount of inspection and testing of every Lot produced. Furthermore, the notch profile and material used are key in providing the proper performance.
What are the typical shut off methods used with the Frangibolt® Actuators?
Power is typically shut off based on either a timed pulse, maximum temperature read, or using an inline shut off switch.
What maximum temperature does EBAD recommend for me to base my shut off?
Generally, the Frangibolt® Actuator temperature (as read by the embedded RTD) should not be allowed to exceed 150C. However, it is highly recommended that the customer perform worst case system level testing to quantify both the function time and max temperature for the particular conditions of the application. This is typically enveloped by high voltage/temperature and low voltage/temperature which result in the fastest and slowest function time, respectively.
Can I reset the Frangibolt® Actuator immediately after actuation?
Yes, as long the Actuator is allowed to cool back down to room temperature 23 +/- 5 C.
How is the Frangibolt® Actuator retained after actuation?
The Actuator is typically housed in an enclosure which keeps it in a confined cavity after actuation. The opposing bolt segment may be automatically secured by threading directly into the deployable or similarly retained by using a retainer.
Does the Frangibolt® Actuator joint have to be preloaded to work?
No. The Frangibolt® Actuator has adequate stroke to function even if the bolt or Fastener is not preloaded. Joint preload is generally an application requirement to “hold down” the item being secured to avoid “rattle” under high G conditions such as launch. It is important however that the Frangibolt® Actuator joint does not have, or develop, any gaps or compliancy which can rob the Actuator of stroke needed to fracture the bolt.
How do I know the Frangibolt® Actuator has been reset correctly?
Verification that the Frangibolt® Actuator has been reset correctly is by measuring its compressed length. This value should be within +0.005″ of previously recorded value for the particular Serial Number. Maintaining a log of all firings is therefore very important for critical applications.
Is there any harm (or benefit) in compressing the Frangibolt® a 2nd time?
No, the Frangibolt® Actuator can be exposed to the compression force repeatedly without any effect on performance. Generally, once the Actuator has been reset, the application of the same force will not result in any further compression.
Why is it that I measured the length of my Frangibolt® Actuator after a successful firing and do not see the 3% stroke noted?
The fully recovered length of the Actuator can only be measured while hot (at approximately 100C). Any other measurement will be less than that due to either incomplete transformation or self-shortening which can occur during cooling.
What type of locking feature is recommended for use with the ERM hold down bolt?
Locking inserts (Heli-coils) installed into the Coupler are the most common approach. Also, other methods such as locking fluid or patches are acceptable. Aeronautical or military applications may benefit from the use of lock down wire as used on any other bolt or fastener.
What is the function of the Kick Off Spring and how much force does it apply to the coupler upon release?
Depending on the ERM size, the kick off spring applies a force of 2 to 10 lbf (i.e. very light). Its function is to eject the coupler in the event that no external load or preload is applied.
Can your ERM (and Pin Pullers) be sealed for underwater applications?
Yes, we have done this for many ERM and Pin Puller embodiments and found it be effective for pressures up to 1000 psi. However, if higher pressures (depths) are expected then the ERM (or Pin Puller) needs to be housed in a pressure compensating enclosure.
Can you change the ERM resistance as in question #2 relating to Pin Puller?
Yes, the internals of our ERMs and Pin Pullers are nearly identical so many (if not all) Q+A relating to our Pin Pullers also applies to our ERMs.
How does your ‘Load Feedback’ ERMs work and can it be applied to both your ‘top mount’ as well as ‘bottom mount’ configuration?
Load feedback is offered by installing calibrated strain gages directly in the ERM enclosure. This offers very accurate measurement of preload during torque up. This option is readily introduced into the bottom mount configuration where the Actuator enclosure is in the load path. It has also been incorporated into the Top Mount configuration where the enclosure is not in the load path by use of a “transfer tube”.
What is the difference between your Pin Pullers and ERMs?
The ERMs are nearly identical to our Pin Pullers in that they share more than 90% of the same parts. However, by adding a ball lock at the coupler interface the load rating is increased by more than one order of magnitude to provide a unique and powerful Non Explosive Actuator.
How quickly can I reset your ERMs (or Pin Pullers)?
These can be reset within seconds although it is best to wait at least 3 minutes to ensure that the TiNi™ element has cooled back to room temperature.
Is the ERM (coupler) designed to react to any side or shear load?
No, the ERM is similar to our Frangibolt® Actuator in that it should be exposed to axial loads (resulting from joint preload) only. Of course the friction in any such bolted joint is capable of reacting to some amount of side load. However, a cup-cone interface between the structure and deployable is recommended if significant side loading is anticipated.
Can I fire the ERM with no external load and does the ejected coupler represent a safety hazard?
Yes, you can fire the ERM with nothing attached to the coupler. Generally, you will find that the force is light enough such that the ejected coupler is thrown into the air by only 5 feet or so. Therefore, cupping your hand over it is a common and acceptable method for retaining it. The only safety precaution should be to not put one’s head (or eye) directly in the path of the coupler.
Do I need to support the Coupler or ERM body during torque up?
No, EBAD’s early literature suggested that a “backing wrench” should be used but this is no longer the case. All ERMs are designed to be able to react to the recommended installation torque (i.e. as long as the ERM body is bolted to the structure, the coupler can be torqued into just like any other Nut or Threaded Hole).