Wood Dale, Ill.
Leadscrews are one of the most common and cost-effective tools for converting rotary motion into linear motion. With the help of advanced materials and coatings, they serve in a multitude of applications such as semiconductor manufacturing equipment, medical-treatment devices, and rapid-prototyping machines.
One particularly useful function leadscrews provide is the ability to reduce or even eliminate the need for preventive maintenance. They can work without manually applied oil or grease in many cases as the majority of leadnuts produced today use polymers with integral lubricants. However, simply omitting lubrication reduces leadscrew life and load-bearing capacity.
An excellent alternative adds a coating to the screw. The best coating to complement a polymer leadnut is a polytetrafluroethylene-based (PTFE) solid lubricant. In addition to the PTFE, this type of coating includes other engineered polymers that help it adhere to the leadscrew. They also cut down on wear and let them work well at high temperature. A 0.0005 to 0.0010- in. layer on the lead-screw provides several advantages including less friction, reduced wear, and resistance to corrosion.
A PTFE-based coating is usually applied by the leadscrew manufacturer rather than by the end user. Manufacturers must carefully control the process to develop the correct thickness and material properties. First, they thoroughly degrease the leadscrew and mask any noncoated part of the screw, such as end journals. The coating is then applied with air-spray techniques, which can be either manual or automated. Several passes are made to develop a uniform thickness. Finally, the coating cures in an oven. Typically, several parts are sprayed and then cured simultaneously in a batch oven.
The tough coating formed on the leadscrew greatly enhances performance over running without lubricant and has several advantages over oil or grease. The friction coefficient of PTFE-based coatings ranges from 0.06 to 0.12, typically higher than the friction coefficient of oil or grease. But the latter types of lubricants need maintenance and don't work well in harsh and high-particulate areas. While high-particulate environments shorten the life of any leadnut, the use of solid rather than liquid lubricant greatly reduces the degradation. The solid coating does not attract and retain particles like an oil or grease.
One quality that makes PTFE-based coatings a better choice than other solid lubricants is their ability to perform consistently over a wide range of conditions. For example, the friction coefficient remains relatively constant regardless of the magnitude of axial load.
PTFE-based coatings display good resistance to acids, bases, solvents, and other environmental aspects such as saltwater, road chemicals, and ultraviolet radiation. They're useful for temperatures from about 420 to 550°F. The coating even helps eliminate some noise and vibration problems.
Office machines are one area well suited to leadscrews with PTFE-based coatings. For example, copiers expose leadscrews to airborne toner particles yet still must work for a long time at relatively high-duty cycles without much maintenance. Coated leadscrews also played a role in developing maintenance-free rapid-prototyping equipment small enough for offices.
Other places to consider lead-screws with PTFE-based coatings is in areas where cleanliness is important. Such areas include life sciences and semiconductor manufacturing. For example, liquid lubricants in DNA-sampling machines could raise sample-contamination issues.
Though not as widely used, there are two other leadscrew coating options: modified tungsten disulfide and chrome plating. Modified tungsten disulfide performs like PTFE-based coatings but is used primarily with leadnuts made of bronze rather than polymers. Chrome plating, on the other hand, provides strong corrosion resistance in marine settings.
All in all, internally lubricated polymer leadnuts along with a PTFE-based leadscrew coating offer a balance of reliability, life, load capacity, speed, corrosion resistance, and low maintenance. This combination has met many past needs and continues to meet more.
Danaher Motion, (866) 993-2624, danahermotion.com