High duty cycles and temperatures were once the realm only of ball screws. No more.
| Materials with a greater PV value better withstand dynamic loads.|
Leadscrews use a threaded nut to convert rotary motion to linear motion. Internally lubricated plastic nuts let the devices run smoothly, quietly, and stiction-free, without periodic ( external) lubrication. Nuts of acetal-based plastics blended with 15% PTFE (Teflon) lubricant work for light to medium loads and speeds, but can overheat when subjected to high duty cycles and and elevated operating temperatures.
But leadscrew nuts made of selected engineering plastics can work in these harsh environments. Such plastics are typified by a high pressure-velocity PV value. PV is a measure of a material's ability to withstand dynamic loads. An increase in applied pressure load, P must be met with a lowering of sliding velocity, V so as not to exceed the PV limit. PV limit is defined as the point at which frictional heat causes permanent deformation of the plastic.
Torlon polyamide-imide and Vespel polyimide engineering plastics, for example, have PV limits five to 10 times that of acetals. Both can be compounded with lubricants, though their high melting temperatures make them difficult to process. Vespel cannot be injection molded and is instead direct formed or compression molded. Here, powdered material is compressed into a "green" compact and heated to form net-shape parts or rods from which parts can be machined by traditional methods. Torlon is also compression moldable but can be injection molded using special equipment.
Another engineering plastic called PEEK ( polyetheretherketone) has mechanical properties resembling those of polyamide-imide. PEEK has a deflection temperature of nearly 400°F and can be blended with PTFE. Unlike polyamides, PEEK is injection moldable at high process temperatures.
In general, plastic nuts machined from bar stock make sense for low-volume applications. All of the above plastics come in bar form. Conversely, injectionmolded nuts get the nod for high-volume applications where both reduced cycle time and part cost are significant.
Rapid prototyping machines create solid, plastic, 3D objects from CAD drawings. Software "slices" the CAD model into multiple layers. A spray nozzle mounted on a three-axis gantry table driven by leadscrews then moves in the required geometry and deposits a thin bead of heated, liquid plastic to form each layer. The plastic hardens immediately after being squirted from the nozzle and bonds to the layer below, forming a 3D model.
The hot plastic raises ambient temperatures to about 175°F, which combined with frictional heating, rules out acetal-based nuts. The additional needs of high position accuracy and volume production pointed toward PEEK/15% PTFE XC-Series antibacklash nuts. XC-Series nuts have high axial stiffness, zero backlash, and minimal drag torque. A patented Active-CAM mechanism automatically adjusts for wear, ensuring zero backlash for the life of the nut.
In other applications, electromechanical actuators can replace pneumatic cylinders, eliminating the need for compressed air. Engineers redesigning a pick-and-place assembly, for example, traded a pneumatic cylinder for a 3/8-in.-diameter, 0.100-in.-pitch leadscrew and high-rpm motor (4,500 rpm). The high rotational speeds combined with a load of 40 to 50 lb raised the temperature inside the nut to 400°F, well beyond the 180°F maximum operating temperature of acetal. Positioning accuracy was also important. A Torlon XC-Series nut eliminates backlash in the assembly and withstands the high internal temperatures.
In yet another application, a leadscrew-driven gantry positions high-intensity stage lighting. The leadscrew nut locates close to the light itself, raising nut temperature to about 250°F. Positioning accuracy was not critical, however. In this case, a standard Torlon nut provides minimal backlash and withstands the elevated temperatures.
ELIMINATING BACKLASH, HIGH DRAG TORQUE
Leadscrew nuts are made with larger pitch diameters than their mating screws to accommodate thermal expansion, tolerance variations, and to leave room for lubricants. The amount of play between the two components limits repeatability.
Antibacklash lead nuts eliminate freeplay and boost repeatability. Most designs place a compression spring or other compliant member between the two halves to remove axial clearance. Preload between the nut halves must equal exceed applied axial load — in the direction in which the assembly is loaded through the take-up mechanism — to prevent lost motion. The result: high drag torque (typically requires that motors be upsized), reductions in load capacity and duty cycle, and added frictional heating of the nut.
In contrast, XC-Series nuts from Danaher Motion, Saginaw, Mich., use a proprietary Active-CAM to remove freeplay between a nut and screw without introducing excessive drag torque. Cam surfaces on a biasing halfnutlet it rotate and translate relative to the other nut half, accommodating clearances from wear of nut threads. The angles at which the cam surfaces are let the two parts self-lock, preventing "ramp down" under load. A torsional spring inside the cam between the half-nuts provides sufficient torque for nut biasing. Only when wear at the nut-thread interface creates clearance will the cam advance to eliminate backlash. The self-locking action prevents cam movement except during load reversals.
XC-Series lead nuts are designed to run on standard and precision-grade rolled leadscrews from Ball Screws & Actuators Co. Inc., San Jose. Lead accuracy of these leadscrews is typically better than 0.0003 in./in. But even slight lead and pitch variations can cause variations in reflected torque in a solid preloaded nut. A high Durometer washer placed in series with the take-up mechanism minimizes the effect but introduces a small but acceptable amount of compliance into the system.