Demand for miniature motion components follows trends.

Not too long ago, the motion systems used in medical and lab automation equipment had technical requirements that were easy to satisfy. These lightly loaded applications generally required simple point-to-point moves with low to moderate positioning accuracy requirements.

Fig. 1 - Even when preloaded, linear guides can run smoothly, as shown by the uniform frictional resistance data.
With the exception of surgical robots and some diagnostic systems, many medical machines still have modest positioning accuracy requirements, at least compared to applications such as semiconductor and electronics assembly. Yet the motion axes in medical machines do have to run smoothly and quietly, sometimes at high speeds.

Medical motion systems have had to become more sophisticated in other respects to keep pace with two unfolding trends in the medical machine marketplace.

Choosing the Right Linear Guide

One of these trends is miniaturization. Diagnostic equipment, DNA sequencers, and other types of automation systems occupy less space than in years past, and these machines increasingly require streamlined mechanical designs. This ongoing shift creates a strong need for miniaturized motion components, especially linear guides.

The other trend is an increasing demand for reliability and low cost of ownership. Here too, choosing the right linear guide can make a big difference in how well the machine runs–and how much it will cost to keep running.

The next generation of medical machines, then, will need linear guides that are compact relative to the loads they carry. They will also need to run smoothly with adequate precision. And finally, they will also need design features that ensure that the machine has a long, trouble-free life.

Trends in Customer Demands

Compact: Like many types of consumer and industrial products, medical machines of all kinds are shrinking. To take one example, lab automation systems have been scaled down to meet the needs of smaller laboratories that have less floor space—and budget—to spare.

There is an extensive range of miniature linear motion products available that can meet the requirements of sizeconstrained medical applications. Among them are the world’s smallest recirculating ball linear guide, which has a track rail width of just 1mm and a cross sectional height of 2.5mm, and a tiny ball-spline guide, with a shaft diameter of 2mm and a cylinder diameter of 6mm.

Smooth: In medical applications, one of these functional requirements is smoothness. Many guides can move from point to point quickly, but not all can do so smoothly. Medical robots and lab automation systems in particular can be especially sensitive to jerky motion. In many medical applications, smoothness counts for more than maximum speed. Smoothness also translates to less noise, and quiet motion components are strongly preferred in any medical machine or diagnostic system used in proximity to patients.

When selecting smooth guides for medical machines, look for products that have a low, uniform sliding resistance over their travel distance (See Figure 1).

Maintenance-free: The cost of maintenance, particularly lubrication needs, drives up the cost of ownership for many types of moving machines. Medical and lab automation machines are no exception.

Manufacturers can supply linear guides with proprietary technology that allow the units to operate for more than 20,000 kilometers or 5 years without the need to replenish the lubricant. A polymer reservoir can be positioned within the guide’s slider so that it comes in contact with the recirculating balls or rollers. Surface tension in the porous polymer would then continually bring lubricant to the surface of the reservoir, allowing lubricant to transfer to the balls or rollers as they pass by. This method can be much more cost-effective and cleaner than other maintenance-free methods that apply lubricant directly to the guide rails via a lubricating plate.

Lubricating plates, which remain in contact with the rails, can also have another downside. The plate can increase the drag forces on the slider, driving up the guide’s overall resistance.

Reliable, long life: There are many reasons why a linear guide can fail to live up to its projected life cycle. Unabated contamination, for example, can shorten the life of a linear guide. So can excess temperatures. So can mechanical design or installation errors that cause misalignment between the sliders and rails. All these failure modes are possible in medical applications, but the most common and easily avoidable premature failures result from under- or over-lubrication of linear motion components.

This article was written by Yuichi Ikeuchi, Engineering Manager, IKO International, Parsippany, NJ. For more information, Click Here .