ENERGETICALLY DRIVEN PISTON ACTUATORS

Potential Hurdles

First let’s address an area of concern that often comes up with our customers in the initial technical discussions. If you have never used an energetically driven product before, there are hurdles to overcome in both facility licensing and in shipping licensing for your end product. Whether you were unaware of licensing requirements or you’re well on your way implementing the process in your facility, we have the subject matter experts you need to get you through the process as quickly and painlessly as possible, and in parallel with the development efforts of the design teams. This allows your design team and ours to focus on the right solution for your application. So, what is the right solution?

The Simplest Solution

The simplest solution would be a piston driven by the pressure generated when an energetic is activated. Sometimes called “pin puller” or “pin pusher” (depending on, as you can guess, which way the “pin” moves relative to the body when it is actuated), these devices can be as little as three primary mechanical components and represent the highest potential reliability of all the solutions. They also represent the smallest potential packaging. A pin puller is a force-generating device with almost unlimited force output capability and very large temperature range capability, which enables the designer of the end-use application to simplify the actuated mechanism because obstacles such as jamming, tolerance issues, and material CTE issues become less influential in the overall system performance. In the case where the pin is reacting a shear force as in a door latch, it is almost a no-brainer that this technology can be applied. However, with the simplicity of the design comes some drawbacks to the technology that must be carefully considered.

Pin Puller vs. Mechanical Actuator

As previously stated, a simple pin puller is a force-generating device. A common mistake for a designer implementing such a device for the first time is to confuse this device with a mechanical actuator, such as a linear screw or hydraulic piston, both of which are motion generating devices. What’s the difference? Let’s oversimplify and assume all conditions are ideal. A common linear screw or hydraulic piston will move at a constant rate regardless of the load (a “tractor”). This means the resistance presented to the actuator can be variable and ideally does not affect the overall behavior. However, a simple piston actuator is a force-output device (a “rocket”), and its motion is mainly governed by the familiar Force = Mass x Acceleration. It doesn’t take long to discover there is a big difference in the acceleration when the resistive forces vary from assembly to assembly, or even as the mass varies through iterations of design. When the mechanism is allowed to accelerate without restriction, the energy input into the driven system can be damagingly large (there is typically a shock load induced both at initiation and at end-of-travel). Compare the situation to pulling a plow with a tractor vs. a rocket. The tractor will push forward regardless of the resistance the plow encounters and overall the system’s behavior is easy to predict and model. The rocket can be made to work, but the resistance the dirt presents to the plow must be carefully balanced with the output of the rocket. Without that balance, the plow either never starts or it accelerates out of control, with the ideal behavior balanced precariously in between. As in the plow analogy, the resistances encountered in your mechanism aren’t always controllable.

Design Features, Developments and People

Simple actuators have been around for a long time, and you can bet the industry has come up with design solutions to overcome all of the potential drawbacks. We have successfully implemented many different design features to overcome what may be drawbacks to the simple pin puller in our customer’s applications, from as simple as incorporating repeatable frictional resistances to prevent runaway situations, soft-start to mitigate the initial shock, crush features to reduce the end-of-travel shock, and oil-orifice dampening (to name a few). We have successfully developed constant-speed actuators that behave like motion generating devices, and have exceeded the performance of the more traditional hydraulics or linear screw actuators in both force capability and temperature range. More often than not, and even in the case of a constant speed actuator, a solution can be found that does not require specialized energetic formulation or special grain shape that will generate a specific gas output or burn profile, and does not require complicated math models to marry the behavior of the energetics and mechanics in all environmental conditions.

If an energetic development solution happens to be appropriate, we have the best chemists and ballisticians in the industry to advance the best design for your application. We recognize that special energetic development can be time-consuming. The preferred solution might be one that diminishes the role of the energetic to simply an energy source and not a fundamental driver to the performance of the actuator. Wee also have the best mechanical and systems design engineers to advance this particular kind of solution. All of these solutions, while very different from one another, still have the same fundamental advantages of the simple pin puller in terms of reliability, motion energy output capability vs. size and weight, and simplicity of implementation in the next-higher assembly. What this means to you and your design organization is that your actuation solution will not be pigeon-holed into a technology that happens to be the sole specialty of the partner you choose, but rather a solution that marries well with your own capabilities and design philosophies.