MAKING THE RIGHT MOVES in Ball Screw Selection for Medical and Laboratory Applications

Introducing a linear drive into some new, or existing, medical or laboratory application is a challenging task for two reasons.

First off, medical and 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.

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.

Because of continuous development, ball screws still 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 optimized 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 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 dialyzer. So, can a ball screw meet your particular cluster of application demands?

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.

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 the 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 journey inside the nut back to the beginning of the nut ready for re-circulation. This is done by an external tube arrangement that completes the circuit from nut end to nut beginning.

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.

Ball screws are available in inch and metric dimensions. Other options include screw length, non-standard sizes, preloaded nuts, special configurations and special materials.

MATCHING A BALL SCREW TO AN APPLICATION

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. The simple checklist that follows is a reminder of what you ideally should know when choosing a ball screw.

BACKLASH

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. 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. It is always necessary to brake the screw shaft with the motor to prevent any backdriving.

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 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.

THREE WAYS TO ENSURE POSITIONING 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.

1) 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.

2) 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 problematic 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

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

HELPING HAND

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.