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Guidelines for Ball Screw Selection

Fri, 08/10/2007 - 8:03am

Ball screws can be useful for a variety of medical devices, but ensuring that an application’s requirements are being properly met can be slightly less clear cut. Designers must be familiar with the capabilities of this technology in order to be certain it will fulfill their needs. This article reviews the critical items to know when selecting a ball screw.

By Tarek Bugaighis
Introducing a linear drive system into new or existing medical or laboratory applications is a challenging task for two reasons. First, medical and laboratory applications are usually accompanied by multiple application demands, ranging from reliability and repeatability, to restrictions on size and limitations on noise. Second, there are now so many different types of linear motion devices to consider.
Design simulation of SKF ball screw sectioned to show recirculating balls and device.


However, a close look at past linear motion solutions in medical and laboratory equipment shows that one type of linear motion device—the ball screw—has an outstanding record for meeting instrumentation requirements. Ball screws have provided so many effective solutions that many designers have put them ‘first on the list’ when considering linear motion devices in medicine and research.

Due to continuous development, ball screws have maintained their position as a leading solution. For example, precision rolled ball screws, ideally suited for laboratory instrumentation, are now available with standard diameters ranging from 6.0 to 16 mm and leads ranging from 2.0 to 12.7 mm (other ball screws with diameters up to 63 mm and leads up to 50 mm are also available) . These versatile ball screws have optimized nut geometry to significantly reduce noise. They have smaller leads to produce extremely high levels of positioning accuracy and balls that extend service life and reduce the potential for overheating and jamming (a potential risk with sliding screws). Also, their capability to handle higher dynamic loads (despite their reduced size) enables designers to specify even smaller ball screw assemblies to fit the needs of smaller instruments.

Some recent examples of where ball screws have been successful in medical instrumentation include the pump for a blood separation device used in cardiac surgery, movement of a sample rack in an automated lab sample analyzer, and an axial pump used for blood movement through a dialysis unit. So, the question is, can a ball screw meet most application demands?

To answer this question, this article takes a brief look at ball screw basics, then goes on to look at ball screw options and some recent developments. This is followed by some general advice on matching a ball screw to an application and continues with a checklist of critical factors and some key pointers for ensuring optimum performance.
Ball Screw Basics
The basic ball screw assembly consists of a motor driven screw, an associated nut, and a ball re-circulation device. Unlike sliding screws that have a higher coefficient of friction and lower efficiency, a ball screw usually converts about 90% of a motor’s torque into thrust.
SKF ball screws are available from 6.0 mm and 1/4 in. diameters.


It does this by having a shaft with a precision rolled or ground helical groove along its length and an associated nut with a matching internal groove. The groove on the shaft acts as an inner race while the groove in the nut acts as an outer race for precision steel balls. The balls circulate in the groove between the shaft and nut to provide linear motion from the shaft or the nut depending on the application requirements. It’s an arrangement that ensures minimal mechanical wear and lifetime reliability.A key design element for any ball screw is the means provided to take balls that have reached the end of their circuit inside the nut back to the beginning of the nut to be ready for re-circulation. Usually this is done by an external tube arrangement that completes the circuit from nut end to nut beginning. However, because external tubes can be damaged during installation, alternative methods are now being developed. One effective method is to provide the ball screw with an internal “no-tubing” system, called “inserts.” With this method, deflector pins speedily remove balls from the end of the nut and return them to the beginning to complete the ball circuit.

Because ball screws provide an efficient and effective solution to a wide variety of linear motion applications, they are available in various materials and various configurations. For example, although the screw shaft, nut, balls, and ball circulating system are usually made from carbon or stainless hardened steels, other special materials and composite inserts are sometimes used to meet special application requirements.
SKF miniature ball screw; full view


They are also available in inch and metric dimensions. Metric ball screws are available in sizes from 6.0 mm and have leads from 2.0 mm. Ball screws with inch dimensions have diameters starting at 1/4 in. with leads from 0.1 to 1.0 in. Other options include screw length, non-standard sizes, preloaded nuts, special configurations, and special materials.
Selecting the Right Ball Screw
When preparing to select a ball screw for a proposed linear motion application, it is always possible to overlook a critical requirement. Any such oversight can affect performance and be costly to rectify, so it pays to be aware of every critical factor associated with an application. The checklist for an application’s critical requirements greatly simplifies this process. It’s a reminder of what ideally should be known when designing-in a ball screw systems.
Other Factors
There are other factors, such as the type of lubricant or whether the assembly needs to be coated, that might be considered at this stage. But for now, they are secondary to the critical performance requirements listed previously. However, there are two other factors that need special consideration: backlash and bearing support.
Backlash
When the ball screw is at rest, there will typically be some degree of axial motion between the screw and the nut. This is known as backlash and is usually in the order of 70 µm. If required, it can be made even less. Backlash usually occurs when load direction changes and the resulting displacement produces positioning errors.
SKF miniature ball screw; cutaway


The usual method for overcoming backlash is to introduce some type of preloading into the ball screw. This will increase stiffness and eliminate any axial play so that reliability and accurate positioning are improved. Preloading is achieved by the use of a preloaded nut. This can apply an axial force by using a split/tandem nut or the nut can be made to operate with plus-size rolling elements.

In vertical motion applications, backlash is not as much of an issue because the load pushes down on the nut keeping it in constant contact with the screw. Accuracy is maintained whether the load is being raised or lowered. Another advantage with vertical motion applications is that the torque needed to lower the load is less than that required to raise it. This means that there are sometimes opportunities for downsizing the motor. However, it is usually necessary to have a brake for the screw shaft with the motor to prevent any backdriving.
Support Bearings
When a ball screw is installed in any medical or laboratory equipment, the speed at which the shaft can rotate and the maximum load are both determined by the degree of support provided by the bearings. Deep groove ball bearings offer good radial stiffness but poor axial stiffness. Stiffness in both directions can be provided by the use of fixed supports using sets of angular contact bearings.
Checklist for an Application’s Critical Requirements
  • Load: a detailed load profile for the application
  • Speed: the required linear and rotational speeds
  • Acceleration: the required rates of acceleration
  • Cycle rate: the required cycle rate
  • Drive torque: the required drive torque limits
  • Environmental: the environmental requirements that need to be met
  • Lead accuracy: the required lead accuracy
  • Life: the required life for the ball screw
  • Stiffness: the required system stiffness
  • Repeatability: the required repeatability
  • Noise: the maximum noise level


Some types of fixed support allow the shaft to be supported at one end and have the other end left free (unsupported). Usually, it is the demands of the application that determine the type of support required for optimum performance.
Ensuring Positioning Performance
Even if a designer has worked through the checklist and considered the two additional factors previously covered, there are three other items worthy of their attention if they want precise and repeatable positioning performance.
Thermal Expansion
Systematic positioning errors caused by thermal expansion of the screw shaft are usually overcome by keeping the operating temperature of the screw constant. An associated benefit from keeping the temperature constant is that it enables a lubricant to be specified that will improve stability and give top performance.

Another way to accommodate the error is by making changes to the software and modifying the mounting arrangements.
Lead Error
Lead precision of a ball screw is defined as the difference between the theoretical and the actual position on a given number of points along the working stroke. It can be particularly problematical when working with two ball screws used in parallel. If the two screws can be controlled independently with a linear controller and different servomotors, the problem is overcome; otherwise, it will be necessary to select two screws with matching leads.

System Stiffness
The usual way to increase stiffness or eliminate backlash is to use a ball screw with a preloaded nut. However, this can result in the drive torque being increased if the output force is low compared to the preload level. This is why selection of preload type and magnitude is a delicate balance to minimize side friction effects from the preload on one hand and to achieve the necessary stiffness on the other.

Conclusion
Being aware of the critical requirements for an application and the other recommendations made in this article will put designers in a position where they can speak confidently to a manufacturer, and in most cases, be sure of getting a ball screw that will match their application requirements. To be even more certain that nothing has been overlooked, keep in mind that partnering with an experienced manufacturer to get the product and the performance needed is always a good idea.OnlineFor additional information on the technologies and products discussed in this article, visit SKF Group at www.skf.com.

Tarek Bugaighis is a global segment manager for SKF Linear Motion and Precision Technologies. He is responsible for Medical and Healthcare. Bugaighis can be reached at 610-861-3705 or tarek.bugaighis@skf.com.
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