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Ball screw selection for medical and laboratory applications

1st April 2013


Introducing a linear drive into some new, or existing laboratory application is a challenging task for two reasons. Firstly, laboratory applications are usually accompanied by a cluster of application demands, ranging from reliability and precise repeatable movement, to restrictions on size and limitations on noise. Secondly, there are now so many different linear motion devices to consider.

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.

And because of continuous development, ball screws continue to maintain their prime position. For example, precision rolled ball screws, ideally suited for laboratory instrumentation, are now available with standard diameters ranging from 6mm to 16mm and leads ranging from 2mm to 12.7mm. These versatile ball screws have optimised cylindrical nut geometry to significantly reduce noise. They have smaller leads to produce extremely high levels of positioning accuracy and balls that extend service life by eliminating any 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 even 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 analyser; and an axial pump used for blood movement through a dialyser. So, can a ball screw meet your particular cluster of application demands?

In reply, 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. Working through the article should bring you closer to answering your question and make it much easier for you to discuss your requirements with a supplier.

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 per cent of a motor’s torque into thrust.

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 is 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 journey inside the nut back to the beginning of the nut 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 its beginning to complete the ball circuit.

Because ball screws provide an 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.

They are also available in inch and metric dimensions. Metric ball screws are available in sizes from 6mm and have leads from 2mm. Ball screws with inch dimensions have diameters starting at 1/4in. with leads from 0.100in. to 1.000in. Other options include screw length, non-standard sizes, preloaded nuts, special configurations and special materials.

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 your application. That’s the reason for the simple checklist that follows. It’s a reminder of what you ideally should know when designing-in a ball screw.

Checklist for your application’s critical requirements should cover: 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

There are other factors such as the type of lubricant and whether the assembly needs to be coated for some reason that might be considered at this stage. But for now they are secondary to the critical requirements listed above. However, there are two other factors that need special consideration: the first is backlash and the other is bearing support.

When the ball screw is at rest there will always be some degree of axial motion between the screw and the nut. This is known as backlash and is usually in the order of 70µm and if required it can be made even less. Backlash usually occurs when load direction changes and the resulting displacement produces positioning errors.

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 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 there are sometimes opportunities for downsizing the motor. However it is always necessary to brake the screw shaft with the motor to prevent any backdriving.

When a ball screw is installed in a 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 pairs of angular contact bearings. 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.

Even if you’ve worked through the checklist and considered the two factors above there are three other things worthy of your attention if you want precise and repeatable positioning performance.

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 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. 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. That’s why it is recommended to calibrate the preload force with accuracy to minimise side friction effects from the preload on one hand and to achieve the necessary stiffness on the other.

Being aware of the critical requirements for your application and other recommendations in this article will put you in a position where you can speak confidently to a manufacturer, and in most cases be sure of getting a ball screw that will match your application requirements. To be even more certain that you have not overlooked anything remember that you can always consider partnering with an experienced manufacturer to get the product and the performance that you need.u

Tarek Bugaighis is with SKF. www.skf.com





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