Introducing a linear drive into a new or existing medical or laboratory application is challenging for two reasons. First, 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. Second, there are so many different linear-motion devices. Take ball screws for instance. They have been successful in a variety of medical instruments, such as the pump in a blood-separation device used in cardiac surgery, movement of a rack in an automated sample analyzer, and an axial pump that moves blood through a dialysis machine.

The basic ball-screw assembly consists of a motor-driven screw, a nut that rides on the screw, and a ball-recirculation 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 with a shaft rolled or ground with a helical groove along its length, and a nut machined 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. These circulate in the groove between the shaft and nut to provide linear motion from the shaft or the nut depending on the application. It's an arrangement that ensures minimal mechanical wear and lifetime reliability.

When selecting a ball screw for an application you will need to know its load, speed, acceleration, cycle rate, drive torque, environment, lead accuracy, life, stiffness, repeatability, and noise. Other factors could be the type of lubricant and whether the assembly needs to be coated. Two other factors, backlash and bearing support, need special consideration.

When the ball screw is at rest there will always be some degree of axial motion between it and the nut. This is known as backlash and is usually on the order of 70 µm. When 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 preloads the ball screw in one of several ways. The end result increases stiffness and eliminates axial play to improve reliability and accurate positioning. A preloaded nut is one way to eliminate backlash. The nut can apply an axial force by using a split/tandem design 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. Of course the torque needed to lower the load is less than that required to raise it. This is an advantage because it means there are opportunities for downsizing the motor. However it is always necessary to brake the screw shaft with the motor to prevent any backdriving.

The speed at which a ball-screw shaft can rotate in medical or laboratory equipment and the maximum load are both determined by the degree of support provided by its bearings. Deep-groove ball bearings offer good radial stiffness but poor axial stiffness. Stiffness in both directions can be provided by using fixed supports with pairs of angular contact bearings.

The shaft can have fixed supports at one end and the other free. The demands of the application will determine the type of support needed.

Systematic positioning errors caused by thermal expansion of a screw shaft are usually overcome by keeping the screw's operating temperature constant. A benefit for doing so allows specifying lubricant that will improve stability and give top performance. Another way to accommodate positioning errors 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 problematic when working with two ball screws used in parallel. The problem is overcome when the two screws are controlled independently with a linear controller and different servomotors. Otherwise it will be necessary to select two screws with matching leads.