FEATURE LINEAR MOTION NAVIGATING THE PATH TO OPTIM W
hen machine designers need to translate motor torque into linear thrust, they often specify a stepper motor with
an integrated lead screw and nut. Known as a stepper motor linear actuator (SMLA), this configuration can provide precise control, reliability and versatility along a linear path, but if there is potential for even a small moment or side load, it will be necessary to add additional linear guidance. A well- designed guidance mechanism can prevent any binding, bending, buckling or other dysfunction that side loading might introduce. On the surface, adding guidance to an SMLA
appears to be so easy that it may tempt experienced engineers to design it themselves, but given the complexity of calculations that may be involved, in even the smallest applications, the DIY strategy might not be worth the risk. Determining what is right for any application requires careful consideration of the motion profile, available time and budget, space considerations, integration and support.
WHY SMLAs An SMLA typically consists of a stepper motor and an integrated lead screw that moves a load in a back-and-forth linear motion. SMLAs are highly reliable, economical, workhorses that drive numerous applications in which precise stopping and starting, and high programmability, are desired – such as microscope stage adjustments, 3D printer head movement and pipetting.
THE NEED FOR GUIDANCE An SMLA provides thrust along a linear axis but cannot tolerate side loading. Undesirable loading
Hans Dahlen, product line manager – Linear Motion Systems for Thomson, looks into designing compact linear motion systems, and why working with
an experienced vendor’s engineering team may get you the optimised product you need faster and with less risk
perpendicular to this axis can result from any of the following factors: • Bending moment force from misaligned loads • Vibration and shock loads, especially in environments with mechanical impacts or oscillations • Thermal expansion differences among components
• Gravity. Such forces, independently or in combination,
can wear out or deform components. They can cause bending and buckling, premature failure, and increased stress on bearings or lead screw nuts and threads. Misalignment, increased friction and material fatigue, also contribute to overall performance degradation and shorter system life. Avoiding the negative impact of moment or side
loads requires supplemental guidance to protect against external forces impacting the linear motion of the load. Guidance often involves deploying additional round or square rails and
bearings external to the lead screw. The challenge for either the user or OEM is to determine the most robust guidance, in the smallest space, and at the lowest cost.
ADDING THE GUIDANCE The larger the load and the higher the speed, the more complicated it is to add guidance in an optimal way. The following steps are involved: • Assessing the spacing, budget and delivery parameters
• Determining whether round rail, square rail or another guiding mechanism is best • Running optimisation calculations, which can take days • Implementing the final design in CAD • Assembly • Testing.
It is sometimes possible to put together a
compact linear motion system from off-the-shelf components by yourself, integrating thrust and guide rails in a single, small-scale unit. However, an additional degree of expertise is usually needed to arrive at the optimal solution. Machine designers are left to choose between assembling a linear motion system themselves, potentially with unknown risks, or relying on a motion control manufacturer with decades of application expertise to create a highly configured solution, most often available with standard components.
DIY?
Designing a compact linear motion system by yourself can result in a more bespoke solution at a lower cost and could bring economies of scale in high-volume applications, but there are many potential downsides as well. Machine designers who do not have extensive
experience with system design may err on the side of caution and over-engineer. While over- engineering might reduce risk, it usually adds complexity as well. For example, oversizing the rails would add more support but would also add costs and eat up space. It is not uncommon for engineering projects to fail in their first iteration, which can entail going back to square one, adding further costs and delays. Many, if not most, DIY projects integrate
Motion engineers have myriad standard configuration options for adding guidance to an SMLA
26 DESIGN SOLUTIONS NOVEMBER 2024
components from different vendors, which are less likely to work together optimally. If there is a problem, there may be finger-pointing among the component suppliers. Likewise, maintenance and
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