Better Automation Critical to Winning Design

You can understand Immunicon Corp.'s satisfaction at bringing to market its automated, design-award-winning CellTracks^® Analyzer II which facilitates important cancer research and diagnostic work.

As Frank Modica, manager of mechanical engineering for the Huntingdon Valley, PA company, describes the design challenge, "There are four stages that we use. Two of them position the sample. The third one is the focus stage. The fourth one moves the proper filters in place, which produces the imaging in the three colors."

Critical to the automation was the linear bearings the company would choose. These bearings would have to meet specific requirements in order to achieve perfect positioning. These were determined by the amount of space within the product envelope, distance to be traveled and load to be carried. This instrument's profile — or envelope — dictated miniature bearings be chosen for their reduced height. At 300 mm, for a medical application, the instrument has a large stage, but it is not high load. However, the alignment and accuracy of the stage was strategic. A cross roller type linear bearing provides the most accurate form of linear motion. "The cross roller bearing is in the X coordinate," says Art Marlin, the principal manufacturing engineer and project manager. "We needed that to keep the X and Y stationery because any movement between frames could distort the images. We have two, 375-millimeter slide guide bearings that are in the long (Y axis) length."

The cross roller bearing they chose comes from NB Corp. It is the SV type slide way that consists of two R type roller cages, which have precision rollers in a cross arrangement and four rails having V-shaped raceway surfaces. Rollers provide a larger contact area than ball bearings. That reduces elastic deformation resulting in consistently precise movement. Since the rollers do not recirculate, they are all carrying the load, which produces greater rigidity, as well as high load capacity. Another benefit of not recirculating is less fluctuation of frictional resistance. In fact, there is little or no difference between static and dynamic frictional resistances — even under low-load conditions — making them well-suited for minute motion.

Marlin claims they are getting greater positional accuracy of an overlay of three images of the cell taken in three different colors. So if those images are not registered, one on top of the other, you really do not get an idea of what the cell looks like.

For the three remaining axes, Immunicon chose NB's SEB type slide guide, which consists of a block and a guide rail, both of which have two precision ground raceway grooves. The two raceway and four-point contact structure of the SEB types minimize their height, making them well-suited to installations with limited space. Being relatively wide, the SEB-W type provides greater moment load — more than the project actually required. More importantly, the SEB's standard radial clearance is twice as accurate as conventional slide guides.