Lead Screw Precision in Medical Applications

Lead Screw applications for medical devices and laboratory automation

As medical diagnostics, devices, and laboratories have become more reliant on automation and laboratory processing, a need has grown for more refined and dynamic linear motion systems. PBC Linear has emerged as a leader in advanced lead screw systems, providing higher quality screws with tighter tolerances, and a better overall product solution to the standard belt drives. Depending on the application, our lead screws can provide an appropriate combination of accuracy, positional repeatability, and consistent linear motion speeds that meet application requirements, and ultimately help to balance cost with overall effectiveness.

First, it is important to understand the type of application that you’re going to build on. We break these down into three main categories within the medical genre: precision, transport, and set-up applications. Each of these application categories, as further explained below, has its own unique requirements to operate successfully in medical environments. The value to OEMs, medical institutions, and ultimately patients depends on how lead screw systems are selected, implemented, and eventually how they can dramatically improve the overall equipment effectiveness (OEE) within these medical devices and automated systems.

High-precision applications

Before we dive deeper into high-precision applications, it is best to first define how precision is quantified with regards to lead screws. As a general industry rule, lead screw accuracy is defined using a 300 mm (V=₃₀₀) or 12-inch length of lead screw. This method works fine for lengths that are over 300 mm, however, this does not provide accurate tolerances for smaller travel applications. For more precise needs under the 300 mm threshold, the standard metric by which miniature screws are rated has become lead accuracy per-revolution (V_2∏). This is especially true with applications that demand very low increments of travel, with only a slight fraction of a revolution.

In medical applications that require exceptional precision, a high deviation in the thread geometry can be potentially catastrophic or even fatal. Therefore, the most important requirement in this application is accuracy. PBC Linear uses sophisticated equipment to measure and record 100% positional accuracy along the entire travel length of the lead screw. This lead accuracy per-revolution metric means that, if for example, your application requires a 2-mm/revolution lead with 50 pitches over 100 mm, there will need to be 200 measurements. With tolerances at 0.003 inches per foot, our lead screws are much tighter than the competitor standards, which typically measure in at 0.0072 - 0.0100 inches per foot and above.

Other typical requirements within these high-precision applications include:

• Fine screw leads: These micro-travel processes are extremely reliant on sophisticated lead screw technologies. Screw leads as fine as 1mm/revolution are necessary in device applications such as syringe and dosing pumps, where lifesaving medications and crucial control doses are output in microliters. On the laboratory side, the rigid controls of fine leads benefit high density pipetting in well plate dispensing systems, programmable and DNA sequencing equipment, and blood analyzers that tally cells and platelets within blood samples.
• Smooth and constant velocity: Our automated equipment and detailed inspection methods result in higher control and quality over competitors, producing consistent sub-rotation accuracy within the high and low points of individual threads. This is especially important for scanning applications where non-contact sensor cameras or laser 3D topography are used, such as medical records or laboratory functions. These, along with image stitching of radiographs, requires steady linear motion speed to avoid ripples and anomalies in digital image output.
• Minimized backlash: Traditional anti-backlash nuts use a multi-piece design that is susceptible to failure. In contrast, PBC Linear uses a patented and simplified 2-piece design referred to as Constant Force Technology™ (CFT). This design succeeds by using a self-lubricating nut for enhanced efficiency, a constant force spring that ensures consistent pre-load, and greatly reduced backlash (2 to 4 times the performance vs conventional designs). This makes the CFT nut a critical component in reducing positioning settling times and significantly improving positional repeatability.
• Sophisticated electronic controls: PBC Linear actuators can be equipped with a multitude of motor, feedback devices, and controller options for precision application performance. We utilize smart motor technology, with a controller and encoder integrated onto the back of the motor. This offers valuable space savings, reduced labor wiring, faster communications, and often substantial cost savings. These types of electronic control devices can provide an increased level of control and performance, including closed-loop control for position confirmation in demanding high-precision positioning applications.

Transport systems: lead screw vs belts

Transport systems, as they relate to medical devices and laboratory applications, can also be described as cartesian mechatronics. They are often found in low-density well plate pipette dispensing applications, as well as pick-and-place linear motion systems that are common in automated laboratory workstations. These transport medical applications typically require medium precision and agile acceleration and deceleration rates to reach peak speeds. Optimal screw straightness is a very important factor in reducing vibrations and minimizing polymer nut wear. This combination of speed and medium precision can be achieved by using lead screws with tested straightness, aggressive leads, and our patented anti-backlash nuts.

Many manufacturers are not aware that lead screws can provide a much better option than standard belt drives in transport systems. While belt drives offer high speeds over long travel, screw drive actuators from PBC Linear typically outperform belt drives in short-stroke motions under 500 mm. Our lead screws provide higher accuracy, greater positional repeatability, smoother and quieter motion, and lower costs than the common belt drive. This makes them ideal for automated cartesian designs in medical and laboratory transport systems. Other advantages over belt drives include:

• Large range of diameters and leads to match application requirements.
• Self-lubing, lightweight polymer nuts that provide long life and low maintenance.
• Smaller motors due to less input torque required.
• Non-corrosive materials that are less susceptible to contamination
• Superior anti-backlash design with constant force nut option.
• Reduced or eliminated load drift in vertical applications with appropriate fine lead selection.

Set-up access applications: slow and low duty

Set-up access systems in medical applications can include ergonomic adjustable stands for computers and radiology equipment, along with positioning cameras, labelers, and printers. These lower precision medical applications are very often reliant on vertical linear motion systems that move slow, but are quiet and aesthetically appealing, and will hold a load in place without back driving (also referred to as load drift). In these applications, the best choice is a lead screw with fine leads, wide diameter, and preferred tension loading that will hold the payload steady on the vertical axis. Also, because of the slower speeds, they only require a low-torque motor to operate.

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For more technical information relating to lead screws from PBC Linear, please read our article Linear Motion Design Elements of Lead Screws