San Jose, CA--Until now, designers looking to prevent backlash in light-duty motion-control applications had little to choose from but the traditional ball screw. While that method can be effective, it is primarily aimed at heavy loads, making it a more expensive approach than necessary in many cases.
Now, a new line of plastic nut assemblies has been introduced that features a patented device, known as ActiveCAM, that automatically compensates for backlash, or play, between machine parts. Aimed at light-duty applications like semiconductor, packaging, medical, and other industries, it also provides a relatively high level of axial stiffness with a reduction in drag torque compared to other solutions. This recently patented concept features few parts, making it less expensive to make than conventional anti-backlash devices. The manufacturer, Ball Screws & Actuators Inc., estimates a cost of $30 to $60 for the assembly, compared to $200 to $300 for an antilash ball screw.
The product does not provide as much stiffness as the ball screw, but for loads up to 250 lb the stiffness is more than adequate. The plastic components are also impregnated with internal lubricants, making them self-lubricating. That makes the product ideal for use in clean rooms or hard-to-access locations. An optional coating is available that provides a lower, more-consistent torque.
The design also provides some other advantages. Traditionally, two split nut-halves are biased into engagement with the screw threads, typically by a compression spring. The spring pre-loads the nut assembly in an axial direction, forcing the threads of the nut portion against the screw threads. In operation, however, the amount of preload force applied by the spring may be overcome as loading increases. This could result in one of the nut halves moving out of its tight fit with the screw threads, producing backlash and diminishing the precision with which the position of the nut assembly can be controlled.
The captured preload induced by the typical mechanisms also increases the drag torque between the nut and the screw, resulting in increased thread wear and the need for a larger motor to overcome the increased torque. ActiveCAM features a rigid stainless steel sleeve that sits between the two plastic nut ends. The steel sleeve increases axial stiffness and improves positioning accuracy.
A small torsion spring sits inside the sleeve. The spring supplies a preload that forces the two nut portions apart to compensate for any backlash that may occur, either on initial installation of the assembly on the screw or due to thread wear from prolonged use. The production of minimal torque drag, due to the rotational nature of the preload force, means that a smaller motor can be used whenever the device is applied in place of a conventional anti-backlash solution.
| A small torsion spring that sits inside the sleeve supplies a preload that forces the two nut portions apart to compensate for any backlash.
|| One end of the sleeve features a cam surface (see drawing) that abuts the rim of the nut with a corresponding cam surface.
Additional details…Contact Ball Screws & Actuators Co. Inc., 3616 Snell Ave., San Jose, CA 95136.