TNO’s functional AM spring design cuts stress and weight 70%
March 27, 2012 - As part of a scientific instrument development project, TNO developed an actuator for high accuracy positioning. Critical components in this actuator assembly are metal springs designed by TNO. To demonstrate Additive Manufacturing (AM) possibilities, TNO designed a dedicated AM version of the spring that was prototyped by LayerWise. AM allowed the original thick coil to be replaced with multiple thin hollow coils, drastically reducing internal material stresses and overall weight. As LayerWise masters the technology of building parts layer by layer, it stretches the limits of traditional metalworking in favor of functional designs offering peak performance.
Investigating AM technologies with LayerWise
“We contracted LayerWise to investigate Additive Manufacturing (AM) technology capabilities with regards to optimising the performance and weight of the metal actuator springs,” says John Van der Werff, Mechanical Designer for TNO’s Rapid Manufacturing department in Eindhoven, The Netherlands. “We met LayerWise specialists at one of the TNO-hosted rapid manufacturing workshops. Since then, LayerWise has successfully completed several Titanium component AM projects for TNO.”
Van der Werff explains that the original high-tensile steel spring prototype is precision-machined. “Although the spring stiffness is sufficient for its specific purpose in the actuator assembly, the spring’s internal stresses rose as high as 1200 MPa. TNO engineers collaborated with LayerWise to re-think the spring design with AM in mind, and explored what weight and performance gains were within reach.”
The spring is produced directly from its STL design files using a powerful yet high-precision laser beam. The laser pinpoints metal powder particles to selectively build up a 20 to 40 micron horizontal layer. As the layers are built successively, AM hardly faces any restrictions to build the most complex shapes, which often cannot be produced any other way.
“The first step in dealing with the spring’s peak internal stresses is the use of Titanium,” marks Van der Werff. “It’s a stiff lightweight material that offers a low E-modus and is available in multiple grades.” The engineers selected TiAl6V4, which reliably withstands 300 MPa. This value is not good enough when considering the original spring design, but TNO and LayerWise redesigned the spring geometry to keep stresses below this threshold value.
Unlimited design freedom creates a completely new spring
Van der Werff explains that the design freedom associated with AM allowed TNO to opt for a spring coil design that’s half as thin. “Building a coil with a cross section half the size reduced internal stresses 50% when considering equal spring compression. We found out that a hollow coil with a triangular instead of rectangular cross section distributes the stresses more evenly over the entire coil surface. Altogether, the new spring concept faces stress levels that are 4 times lower compared to the original design.”
“To compensate for the spring’s reduced stiffness, we implemented a design comprising of 8 identical parallel spring coils. This had no impact on production cost because AM produces the entire spring, including all spring coils, as a single part in one production step. The Titanium spring part incorporating perforated top and bottom disks with 8 thin hollow spring coils in between enabled us to cut spring weight 70%. Such impressive results underline AM’s superior lightweight design capabilities.”
Custom center support was added to the AM design to avoid support attachments on critical part surfaces or hard-to-reach part locations. Special side-support structures, which easily can be removed afterwards, were added to keep the coils locked in position during post-processing. Furthermore, the design incorporates 3 tiny angled surfaces that center the spring during post-processing of the mounting surfaces.
Throughout the spring project, TNO and LayerWise collaborated intensively. Through positive interaction, multiple prototypes have been built and the production process tuned to achieve the best results. “AM proved to be suitable for functional prototyping in the design and test phases,” concludes Van der Werff. “Although no real cost reduction could be realized in this case, AM’s design freedom resulted in major weight savings. By changing design and production rules, AM stretches design limitations in favour of extreme part optimisation. Material properties of AM products is one aspect that need further investigation, in particular fatigue on high-stress AM parts is uncharted territory.”
“Cutting spring weight 70% underlines AM’s superior lightweight design capabilities.”
John Van der Werff
Mechanical Designer for TNO’s Rapid Manufacturing department in Eindhoven
More info : www.layerwise.com