Leadscrews in medical equipment

Leadscrews are essential to surgical robotics, patient-monitoring systems, diagnostic imaging devices, and rehabilitation equipment.

By Chris Nook | Chairman and CEO • Helix Linear

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Leadscrews are those spiral shafts that (upon application of torque) convert rotational motion into linear motion — axially advancing and receding a mating nut to deliver accurate positioning. They’re well suited to medical equipment as they come in a variety of materials, dynamic load capacities, and output arrangements. Leadscrew assemblies are also quiet because of the sliding contact between the screw and nut — a particularly valuable trait in settings where patients need quiet environs.

Leadscrews can be manufactured in various diameters, leads, lengths, materials, and coatings to meet specialized requirements.

For example, thread geometry can be optimized to complement the operation of the motor, gearbox, and encoder in the assembly.

Selecting an appropriate screw diameter and lead is key to achieving intended design load capacity, stiffness over time (to avoid drift), and life.

Prioritizing efficiency or greater mechanical advantage is suitable where leadscrews are destined for integration into battery-powered equipment.

Where leadscrews install on vertical axes, it’s sometimes beneficial to select screw diameter and lead to yield self-locking (non-backdriving) behavior.

The design of the screw and nut can be customized to fit into confined design footprints.

Leadscrew and nut materials can improve performance and ruggedness against environmental challenges.

Finally, leadscrew assemblies can (with anti-backlash nuts) be specified to ensure superior positional accuracy. Take-up mechanisms within such nuts ensure motion isn’t lost when loads are suddenly applied or the travel direction changes.

Leadscrews for robotic surgery dexterity

Robotic surgery has accelerated in recent years, providing minimally invasive options for complex procedures such as knee replacements, coronary artery bypasses, and tumor removals. Leadscrews are vital to the precision of the robotic arms involved in such procedures.

For example, the da Vinci surgical system (now in use for more than three decades) lets surgeons make tiny incisions for minimal tissue trauma and speedy patient recovery. To address the way in which small incisions narrow view and access, the da Vinci’s multiple agile robotic arms wield miniaturized instruments such as scalpels and scissors. Each arm employs leadscrew assemblies to pivot, rotate, and extend and navigate tight spaces. Emerging leadscrew applications in surgical robotics enable increasingly complex operations.

Leadscrews in imaging resolution for diagnostics

Leadscrews are common in medical equipment such as MRI, PET, CT, and X-ray equipment. The screw assemblies serve as drivetrains and move patients as well as critical equipment subassemblies. In X-ray equipment especially, leadscrews satisfy unique configuration requirements on axes for scanning images with anti-backlash nuts that deliver bidirectional repeatable (zero-backlash) motion.

Tackling medical-device size and performance challenges

The manufacturability of leadscrews and nuts allows for diameters down to two millimeters — enabling engineers to design the most miniaturized medical devices. What’s more, specialized coatings minimize particulate residue and friction in leadscrew systems for medical devices while optimizing lubricity between components.

Hard-coated lubricant based on polytetrafluoroethylene (PTFE) is a leading coating choice as no particulate transfer between the coating and mating components is necessary to reach a low coefficient of friction. In addition to PTFE, various synthetic polymers incorporated into this coating can enhance its adherence to the leadscrew, thereby increasing wear resistance and improving its capacity to withstand high temperatures.

While options abound for lubricating leadscrew assemblies, a plastic nut with Teflon infused into the base material combined with a leadscrew coated with a PTFE-based hard lubricant balances reliability, load capacity, speed, corrosion resistance, and maintenance requirements.

Helix Linear | helixlinear.com