Alex Margolin, Engineering Manager, Nook Industries, Inc.
Important Factors for Using Ball Screws in Medical/Laboratory Applications
Controlling linear motion is a common requirement in the development of medical devices and laboratory equipment, and design engineers have numerous linear drive options available to them. However, the nature of medical/laboratory applications presents unique challenges, such as ensuring precise, reliable, and repeatable movement; handling varying dynamic loads within increasingly smaller spaces; operating in clean environments that require minimal amounts of wear debris; and reducing noise, vibration, and mechanical disruptions to instrumentation during operation.
To meet these challenges, designers of medical/laboratory equipment predominantly choose precision ball screws for linear motion control because they exhibit the necessary performance characteristics, are tested for durability, and are available in the variety of size/style options required to support these specialized needs. That's why ball screws are integral parts of many blood pumping devices in medical settings and automated sample conveyance systems in laboratory environments. In short, ball screws have become the linear motion control solution of choice for medical/laboratory applications.
At its most fundamental level, a ball screw comprises a screw, a nut, and a ball-bearing return mechanism, all packaged within an assembly that, when driven by a motor, both translates rotary motion into linear motion and controls linear positioning of the attached object, whether it's a table, platform, pump actuator, sample rack, etc. The screw has a helical groove along the length of its shaft, and the nut includes a matching groove. These grooves act as the inner and outer races along which precision metal balls travel to produce linear motion. Ball screws are more efficient, have a lower coefficient of friction, and support better torque control than other types of linear control screws, making them a better fit for the challenges inherent to medical applications. For example, with torque control in both the driving and driven directions, ball screws allow for better position control in applications with reversing load directions, such as piston-style metering systems.
Yet not all ball screws are the same, and there is a range of important factors that design engineers should consider when using ball screws in designs for medical devices or laboratory instrumentation.
Precise Linear Motion Control
Although ball screws offer higher levels of precision than other linear motion control devices, lead accuracy can vary across different models. The lead of the ball screw is the axial distance traveled when advancing the ball screw one revolution. Lead accuracy relates to the variation in distance traveled per revolution over the length of the stroke or shaft.
Theoretically, the lead should equal the product of the pitch, the axial distance between each screw thread, times the number of screw starts, the number of threads on the screw shaft. Lead accuracy is the difference between the actual distance traveled vs the theoretical distance. The International Organization for Standardization (ISO) and Deutsches Institut für Normung (DIN), the German national standards organization, have set lead accuracy standards for precision ball screws for both positional (P) and transport (T) applications. For example, lead accuracy of P5 or T5 is within ±23 microns over a stroke of 300 millimeters; P7 or T7 is ±52 microns over 300 millimeters; P10 or T10 is ±210 microns over 300 millimeters.
Thus, design engineers in the medical/laboratory field need to account for lead accuracy as it applies to achieving the positional and transport accuracy required by their application. When positional/transport accuracy is paramount, select the most precise level of lead accuracy available.
Assessing Load Conditions
Another critical factor to consider when using precision ball screws for medical/laboratory applications is how well the device can handle the required loading conditions, including the higher dynamic loads that occur within the small spaces required for many types of instrumentation. Design engineers must assess a range of different loading scenarios, particularly when today's applications require smaller and smaller ball screw assemblies.
Designers need to consider static loads — will the thrust applied to the ball nut damage the assembly — as well as dynamic loads associated with travel of the nut along the screw, linear and rotational speeds, acceleration and cycle rates, and the drive torque involved. They need to make sure that the ball screw that they select will not succumb to tension loads by stretching, or compression loads that squeeze the shaft. Are side or overturning loads part of the mix that need to be evaluated? Is a preloaded ball screw assembly — a ball screw that is preloaded with an internal force between the ball nut and screw assembly to eliminate axial and radial lash — required for a specific application? With many precise medical manufacturing processes, an engineer may need to control assembly stiffness to maximize positioning and stability control in the servo drive control system. Is the ball screw assembly stiff enough for a particular application?
Developing a detailed load profile for the ball screw per its use in a device or mechanism will enable designers to avoid costly missteps and select the appropriate ball screw assembly for their particular application.
Rolled vs Ground Ball Screws
Until recently, design engineers had only one option for achieving the exacting precision and higher dynamic load-bearing linear motion demands of many of today's medical devices and pieces of laboratory equipment: they had to resort to manufacturing custom-ground ball screw assemblies. While this approach satisfies basic performance criteria, it has notable drawbacks.
Grinding custom ball screws is more expensive than purchasing standard manufactured products. Costs can rise even higher if the custom-ground ball screw wears out or needs to be replaced, requiring another round of costly grinding and production. Fortunately, advances in thread-rolling manufacturing technology have made manufactured ball screws with the same levels of precision, lead accuracy, efficiency, and performance as custom-ground ball screws a reality. For example, Nook Industries recently released a line of thread-rolled PMBS™ Precision Metric Ball Screws with a mechanical, no-slip connection that permanently times the input to the output, providing a more cost-effective, high-precision linear actuation alternative.
Thread-rolled ball screws offer the same degree of fine adjustment, accuracy, and repeatability for controlling positioning as custom-ground ball screws, but carry several advantages. Because they do not require custom production, delivery is often faster. They also are more affordable — typically, a fraction of the cost of custom-ground ball screws — and are interchangeable, making replacements quick and easy.
Reliability, Repeatability, and Efficiency
Another reason why most medical device designers prefer precision ball screws for linear motion control is the greater efficiency, dependable repeatability, and operational reliability that they provide. The ball screw design is much more efficient for translating rotary motion into linear movement and positioning control, converting roughly 90% of a motor's torque into thrust. Balls screws achieve this high level of efficiency because the bearing balls transfer and share the load between the nut and the screw. The basic arrangement of this mechanism is ideal for minimizing mechanical wear; ensuring repeatable, predictable performance; eliminating the risk of overheating and jamming; and extending service life and reliability. Due to the high efficiency of the ball nuts, torque control in the driving and driven direction is more consistent than other types of screws, allowing for better position control for medical applications with reversing load directions.
That's not to say that there are not possible problems to consider with ball screws. One potential issue that designers should watch out for is backlash. Backlash relates to the degree of axial and radial motion or play between the screw and nut when the ball screw is not moving. As mentioned previously, preloading the ball screw with an internal force between the ball nut and screw assembly can eliminate axial and radial lash.
Range of Diameters, Leads, and Options
Of course, using precision ball screws for medical/laboratory linear motion control applications is not a one-size-fits-all proposition. Separate uses will require different-sized diameters and leads, and may demand alternative materials, thread styles, flange types, and/or mounting options. Diameters for precision rolled ball screws range from 6 to 65 millimeters; leads typically range from 1 to 50 millimeters. However, for medical applications, the diameter range is generally 6 to 16 millimeters and the leads usually are between 2 and 12 millimeters.
Although the screw, nut, and ball bearings are generally available in carbon, alloy, stainless, hardened steels, other materials are often used to support specialized requirements. Threads come in a variety of styles — such as V-thread, cylindrical keyed, and integral-flange — and usually come with either right hand or left hand threads.
When using a flange to mount the ball screw, take note of the available flange types to ensure mounting flexibility. Flanges come in a variety of styles, including DIN-style flanges, with most configurations offering either 4, 6, or 8 mounting holes. Nook also complements ball screws offering end bearing mounting supports, providing complex solution for application.
Profile and Ball Return Mechanisms
When choosing precision rolled ball screws, design engineers in the medical/laboratory field also should consider the type of ball return mechanism, external profile, and wiper system that best fits their particular application. The basic design of a ball screw requires a way to transfer the bearing balls that have reached the end of the groove inside the nut back to the start of the groove at the beginning of the nut. Traditionally, this was accomplished with external tube returns that run along the outside of the nut to return the balls from the end of the nut back to the beginning.
While the external return works just fine, the orientation of the return tube or tubes, which must be installed in a certain way, often creates a wider profile for the ball screw that can get in the way and create clearance issues, especially in the increasingly smaller spaces required for medical devices and laboratory equipment. To overcome this problem, Nook precision ball screws use internal return inserts, which deflect and move the bearing balls within the ball nut body boundaries, providing the most compact solution.
Quiet, Dependable Operation
Because smooth, frictionless, rolling movement characterizes ball screw performance, small motors are generally adequate for driving them. Small motors are much quieter than large motors, and the precision ball screw's reduced noise and vibration make them the perfect linear control solution for medical and laboratory settings.
Ball screws are also extremely dependable and virtually maintenance-free. Wiper kits or integral wipers are recommended for use with ball nuts to prevent internal contamination by foreign, potentially damaging materials. Lubricants are necessary for maintaining optimal ball screw performance. Without proper lubrication — typically with an oil- or lithium-based grease — 90% of a ball screw's anticipated service life can be lost. Standard lubrication processes for antifriction bearings apply to ball screws. Depending on the actual ball screw application, corrosion-resistant coatings and/or lubricated finishes may also be warranted.
Always make sure to operate ball screws within the recommended temperature range. Thermal expansion of the screw can cause positioning errors. By keeping the screw operating temperature within the prescribed range, design engineers can avoid this scenario, ensure that lubrication is working properly, and realize dependable, reliable performance.
Precision ball screws remain the "go-to" solution for handling the unique linear motion control requirements of medical/laboratory applications. However, choosing the most suitable ball screw configuration for a particular need demands an objective assessment of application requirements and corresponding ball screw features. By understanding the range of critical factors discussed in this article, design engineers will not only satisfy their linear actuation needs, but will also implement the most efficient, cost-effective, and reliable solution for controlling motion in their medical device or piece of laboratory equipment.
