By Alex Margolin, Engineering Manager, Nook Industries, Inc.
Take heed of these important factors when designing ball screws into medical or laboratory equipment.
Controlling linear motion is a common requirement of medical devices and laboratory equipment, and there are many linear drive options available. 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.
Precision ball screws have become a common choice for medical and laboratory equipment for linear motion control because they meet these challenges. They also have the necessary performance characteristics, are tested for durability, and are available in a 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.
Ball Screw Basics
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. 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.
Important Factors to Consider
The first factor to account for is lead accuracy as it applies to achieving the positional and transport accuracy required by the application. When positional/transport accuracy is paramount, select the most precise level of lead accuracy available.
Although ball screws offer higher levels of precision, 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 versus 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 mm; P7 or T7 is ±52 microns over 300 mm; P10 or T10 is ±210 microns over 300 mm.
Another critical factor to consider 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. You need to assess a range of different loading scenarios, particularly when today’s applications require smaller and smaller ball screw assemblies.
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. Make sure that the ball screw won’t succumb to tension loads by stretching, or compression loads that squeeze the shaft. Are there side or overturning loads that are part of the mix and 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, you 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 let you avoid costly missteps and select the appropriate ball screw assembly for your particular application.
Until recently, there was 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; resorting 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.
Thread-rolled ball screws offer the same degree of fine adjustment, accuracy, and repeatability for controlling positioning as custom-ground ball screws, buy carry several advantages. Because they don’t 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.
Another advantage of using 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 with ball screws to consider. 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. Preloading the ball screw with an internal force between the ball nut and screw assembly can eliminate axial and radial lash.
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.
Although the screw, nut, and ball bearings are generally available in carbon, alloy, stainless, or 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 four, six, or eight mounting holes.
When choosing precision rolled ball screws, you should also consider the type of ball return mechanism, external profile, and wiper system that best fits the 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, newer designs of precision ball screws use internal return inserts, which deflect and move the bearing balls within the ball nut body boundaries.
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 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.
Nook Industries, Inc.
Filed Under: Ball screws • lead screws, Design World articles, Linear motion • slides, Motion control • motor controls