Advertisement
Precise motion control is critical in a number of medical devices. Unfortunately, with the many options available to manufacturers, it is often difficult to know which solution is ideal for a particular application. This report will provide an overview of the indications and benefits of using ball screw technology to fulfill certain motion control needs.
By Tarek Bugaighis

A "miniature" ball screw ideally suited for lab instrumentation
At a Glance
• Physical attributes
• Selection variables
• Addressing backlash
• Example applications

Ball screws have long been prescribed as solutions to enable precise positioning in medical instrument applications, including syringe and other medical pumps, pipettes, coagulation analyzers, and liquid-handling systems. Whether performing pick-and-place or other automated operations, these anti-friction devices serve vital functions by converting rotary motion into smooth, accurate, and reversible linear motion to quickly and reliably position loads.

Over the years, ball screws have given competing technologies a run for their money and will often provide the ideal method for linear drive systems. This is especially true when compared with sliding screws, which present higher friction coefficient and, consequently, have lower efficiency. Innovations in ball screw design technology have further advanced their appeal.
Ball Screw Fundamentals
Ball screws consist of a screw shaft, nut, balls, and a ball recirculating system. Components are usually made from a variety of hardened or stainless steels, well-suited for medical applications.


A cross-section of a typical ball screw (with circulating balls)
Ball screws can be designed in a variety of configurations in both inch and metric dimensions. Standard inch series ball screws can range from 3/8 in. diameter with leads from 0.1 to 1.0 in. and in varying lengths. Metric series screws are made in sizes from 6.0 mm and have leads from 2.0 mm. Preloaded and non-standard sizes, configurations, and special materials, including composite inserts, expand the possibilities.

Their configuration consists of a shaft with precision ground or rolled concave helical groove (acting as the inner race) and nut with internal grooves (acting as the outer race). Circuits of precision steel balls circulate in the grooves between the screw shaft and nut. Depending on the application, either a rotating screw shaft or nut will then translate in a linear direction. The ball screw has a natural ability with a high efficiency to convert about 90% of a motor’s torque into thrust, which results in minimal mechanical wear and constant performance throughout the life.

Return devices are intended to create smooth and efficient recirculation of the balls from the end of their load-carrying path back to the beginning to complete the circuit. A relatively new ball-return design features a robust “no-tubing” internal system, whereby balls in each circuit are removed from the raceway and returned to it by solid metal deflector pins. This eliminates all external tubes (and the possibility of damage upon installation) and delivers high-speed operation without interference from tubes.

Lab instrument applications exert particular demands on linear drive systems and related issues abound. Systems must be engineered to operate quietly, accurately, and safely; accommodate higher dynamic loads to facilitate compact instrument design; and serve reliably over time. Ball screw technology has, in turn, kept pace.

As an example, “miniature” precision rolled ball screws ideally suited for lab instrumentation have been developed with standard diameters ranging from 6.0 to 16 mm and leads ranging from 2.0 mm to 0.5 in. Among the noteworthy advantages:


A cross-section of the "miniature" type ball screw
• Optimized cylindrical nut geometry significantly reduces noise• Smaller leads produce extremely high levels of positioning accuracy• Balls eliminate any potential for overheating and jamming (a potential risk with sliding screws) to extend service life• Capability to handle higher dynamic loads (despite their reduced size) translates to an opportunity to specify even smaller ball screw assemblies to fit the needs of even smaller instruments
Diagnosing the Parameters
When selecting a ball screw for a medical equipment application, a variety of critical parameters should always be evaluated. These include (but are not limited to) load profile, linear and rotational speed, rates of acceleration, cycle rate, drive torque limits, environmental issues, required life, lead accuracy, and system stiffness. All will have an influence on performance and as much information as possible for an application should be predetermined and communicated before ball screw selection.

In matching a ball screw to an application, users may want to pay special attention to other important issues, such as backlash and the type of support bearings when making the decisions.

Backlash (with a maximum of 70 µm and even less if required) is the relative axial motion between the screw and the nut when the motor is not turning. In a vertical motion application, where the load constantly pushes down on the nut, backlash is not an issue, because load will enable permanent contact to keep the highest accuracy in the motion.


A variety of ball screw assemblies
The high efficiency also exists in indirect mode and it is necessary to brake the shaft with the motor to prevent any backdriving. The same load to support the driving torque downwards is then much less than the torque needed to move the load upwards. Both torques will also be much lower than the torque needed to drive an Acme screw of the same pitch. This promotes motor downsizing.In non-vertical motion applications, backlash may result in positioning errors if load direction changes.

Backlash can be avoided by opting for ball screws with preloaded nuts. Preload can be applied with plus-size rolling elements or with an axial force applied to a split/tandem nut. The applied preload eliminates any axial play and increases the stiffness of the assembly. High preloaded nuts are subject to less elastic deformation than non-preloaded types, which increases reliability and accurate positioning under load.

The degree of support on each end of a ball screw in a lab instrument will determine how fast the screw can spin and how much load can be handled. Simple supports are typified by deep groove ball bearings, which offer good radial stiffness but poor axial stiffness, while fixed supports (such as pairs of angular contact bearings) will provide stiffness in both directions. Shafts can also be left “free” when coupled with a fixed configuration on the other end, which means no support. Application demands will help guide choices.
Optimizing Performance

Ball screw assemblies in all their varied types and styles can provide long predictable life and efficient service in the medical setting. But a good part of their promise hinges on their design and actual operating conditions.

For example, thermal expansion of the screw shaft can lead to systematic positioning errors, which needs to be accommodated in the system software and with the mounting conditions. Keeping the operating temperature of the screw constant is critical to high repeatability of positioning. A control of the temperature helps also to define the optimum lubricant that will allow the screw to perform at its best and with stable performance.

With the required demand for precise positioning, it is important to control the lead error of the screw. Lead precision of a screw can be classified as the difference between the theoretical and actual position on a given number of points along the working stroke. When two screws are used in parallel, units with matched leads should be employed, unless they can be controlled independently with a linear encoder and different servomotors.

To create optimum system stiffness or eliminate backlash, a preloaded screw may be required, but at the same time the drive torque might also be increased if the output force is low compared to the preload level. It is therefore always recommended to calibrate the preload force with accuracy to minimize some side friction effects introduced by the preload on one hand and to achieve the stiffness expected on the other hand. In addition, it may be necessary to include the preload torque as part of the total useful torque when sizing the motor.

Whenever evaluating ball screws for a particular medical equipment application, the first rule of thumb is to understand that a solution for one application may be inappropriate for another.

Beyond the standard life calculation, torque requirement, and output force evaluation, many other parameters must be considered, such as noise level, smoothness, repeatability, speed and acceleration, lubrication, and coatings. All should be evaluated during the design phase so that the product will operate with the expected reliable performance. Perfect traceability during the manufacturing process is also a “must” for all high-tech applications.

Partnering at the outset with an experienced manufacturer can help chart the most appropriate decision-making course.

ONLINE
For additional information on the technologies and products discussed in this article, visit SKF USA Inc. at www.skfusa.com.

Tarek Bugaighis is the director of medical business development for SKF USA Inc., 1530 Valley Center Pkwy., Bethlehem, PA 18017. He is responsible for supporting the medical industry with the company’s assortment of linear and rotary motion products and services. SKF is a leading global supplier of products, customer solutions, and services in the rolling bearing, linear motion, and seals business. Bugaighis can be reached at 610-861-3705 or tarek.bugaighis@skf.com.

Advertisement
Advertisement