How It's Made – Lead Screw Assemblies

Our goal was to create a better lead screw assembly

As today’s technological capabilities have become more sophisticated, the working parts within machine systems have increasingly required greater performance. That need for more precise linear motion has driven PBC Linear to pinpoint substantial resources along each step of its engineering and manufacturing of lead screws, and simultaneously employ quality metrology processes. The result is an integrated lead screw system, with precision focus on three points including the screw, the nut, and the motor.

We began by building a more sophisticated lead screw

As our lead screw infographic illustrates, the manufacturing of a lead screw requires a complex manufacturing process. Initially, that process begins with the O.D. grind of raw material down to tolerances measured to a maximum of 0.0005 inches (12 µm). After the initial grind-down, we then utilize a precision German CNC roll threader to ensure strong and accurate thread forms.

During the roll threading operation, our CNC controlled machinery incorporates precision process adjustments and controls while employing automated in-feed and outfeed features. Together, these provide consistency over the full length of the screw stock. In addition, our roll threading machine’s progressive dies control raw material flow by manipulating speed, skew, and temperature. These key process variables are precisely monitored to ensure the highest level of lead accuracy.

Next in line on the production chain is an automated straightening process. This is a fine-tuning point in the operation, which has eliminated errors that were inherent in older manual processes. PBC Linear can now minimize runout and its potential negative effects, such as vibration, noise, and premature wear.

In order to increase lead screw efficiency and extend its life, we use a PTFE coating on our lead screws, thereby reducing the coefficient of friction. PBC Linear developed its own custom PTFE coating process along with the necessary equipment. The result has been an increase in the quality of finish, including the elimination of surface pitting or flaking over time.

We added an innovative anti-backlash nut

PBC Linear developed an anti-backlash nut using its own Constant Force Technology.

This innovative design utilizes a spring that applies uniform radial pressure to the nut at all stages of the motion profile. The nut is available in a wide variety of optimized geometries, offering customers the ability to quickly customize their lead screw assemblies. This patented design boasts:

• A greater than two times the superior backlash compensation. (Confirmed by leading lab automation, customer validation testing).
• A consistent preload over life (Important for system level tuning and consistent performance over the entire life).
• Two to four times better performance than traditional designs, as validated by customer testing.
• A self-lubricated feature (Special PTFE nut formulation developed from thirty-plus years of plain bearing knowledge).
• A simple two-piece design.

We incorporated high-end, lead screw motors along with machined end journals

Fixed, flat, keyways, and threaded journals can be machined for multiple lead screw uses. PBC Linear offers customers a wide variety of optimized nut geometries, along with the ability to quickly customize screw journals. The final addition to the lead screw assembly is the drive system. The lead screw is first matched to a hollow shaft motor and then precision mounted. The motor’s hollow shaft concentricity serves to minimize runout to values less than 0.003 inches (75 μm). In the final step, the screw is then permanently fixed near the back of the motor using an industrial adhesive, creating a uniquely harmonious unit. Closer inspection of the drive system reveals important features and options, including:

• Larger bearings that increase thrust capacity and add longer life.
• Preloads on bearings that remove axial play and reduce system backlash.
• Optional smart motors.
• Connection via EtherNet/IP, EtherCAT® P, or Modbus®.

We built in a more comprehensive metrology at each stage of lead screw production

PBC Linear implements microscopic inspections of the surface finish at each level of the manufacturing and coating process. At the threadform stage, our acute inspections validate major and minor profile diameters, with additional emphasis on pitch, flank angle, etc. The resulting maximum screw runout is less than 0.001 inches (24.4 μm).

When comparing the PBC linear lead screw to industry competitors, we find our lead accuracy is 0.003 inches/feet (76 μm/300 mm), which is three times better than the level of typical industry specifications. Further comparisons reveal that PBC Linear inspects one hundred percent of its screw length, which on the finest leads is up to twenty-thousand points over seventy-two inches. In contrast, our competitors only take their data points every six inches over the same length of screw.

During the automated straightening process of the lead screw, PBC Linear consistently yields the highest straightness tolerances available. At the finish coating stage, each screw is thoroughly inspected with a digital microscope to ensure there is no flaking or pitting on the surface coating. This smoother finish, coupled with a precise and uniform length, makes for a much longer product life.

Along with lead screws, our intensive metrology standards are applied to anti-backlash nuts and journaled ends. Customers can rely on PBC Linear for reliable products that have been inspected to ensure backlash tolerances meet precise specifications. These high standards are applied to screw journals as well, allowing then to be ground to less than 0.0005 inches (12um).

Overall, PBC Linear dedicates two separate test labs that are focused on establishing high performance characteristics within their product line. One lab is used for load and life, where assemblies are required to run in excess of 2,500 miles without failure. The other test lab is sound-proofed, which allows our technicians to effectively analyze product noise levels. At the end of the day, it is plain to see that PBC Linear has achieved its goal of creating a better lead screw assembly.

View our comprehensive lead screw how it’s made infographic

Discover your ideal linear motion solution:

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