Nut cuts backlash

Leadscrew assemblies fitted with XCSeries antibacklash nuts from BSA Inc., San Jose, eliminate backlash in an imageprocessing machine. A patented ActiveCam uses a stainless-steel biasing cam to boost axial stiffness and minimize drag torque.

The nut is used in the UHR2001 scanning acoustic microscope (SAM) from Sonix, Springfield, Va. The SAM is a nondestructive way to inspect wire bonds. Similar to Sonar, it generates images based on variations of reflected ultrasonic energy.

The machine inspects flat surfaces in the X-Y direction to 0.5 m. The antibacklash-nut equipped leadscrew drives the Y axis through an open-loop stepper motor. No feedback means precision, accuracy, and repeatability are critical. And smooth motion is key to sharp image resolution.

Other methods of reducing backlash use a preloaded compliant component placed between two half nuts to ensure contact on both screw flanks. During operation, spring preload force can be overcome by the driven load. This could move one of the nut halves out of tight engagement with the screw threads, increasing backlash. Not to mention the high preloads needed to minimize backlash also accelerate wear.

ActiveCam, in contrast, self-compensates for wear by virtue of a solid member that maintains high stiffness with minimal deflection without high preloads.

The XC-Series nut threads are made of an internally lubricated polymer minimizing maintenance. PTFE is mixed in with the resin then injection molded. Mechanical noise is minimized because there are no recirculating ball bearings.