Quando você deve lubrificar um parafuso de avanço?

When should you lubricate a lead screw?

A ball screw, with metal-to-metal bearing contact between the balls and races, is expected to require periodic lubrication with grease or oil. One of the advantages of ball screws over ball screws is that they can often operate without lubrication, eliminating maintenance and the potential hazards of using grease or oil: process or environmental contamination, lubricant preservation, and material attraction. particulate for assembly. But just because some lead screws don't require lubrication doesn't mean lubrication should never be used. In fact, in some lead screw applications and operating conditions, lubrication can benefit both performance and service life.

It is important to note that the “zero maintenance” benefit of guide screws applies to those that use a plastic or polymer nut. Bronze nuts, which are used for higher flow rates and in some cases better corrosion resistance, always require lubrication as they involve metal-to-metal sliding contact between the bolt and nut.

But plastic or polymer nuts, which make up a significant portion of lead screw applications, have self-lubricating properties, meaning that lubricants are absorbed by the nut material and are released as the nut and screw slide over each other. the other. However, internal lubricants are not released consistently and this fluctuating level of lubrication can cause uneven operating torque and wear. Externally applied lubrication solves this problem by ensuring a more consistent application of the lubricant.

External lubrication also reduces friction, which can be significant in lead screw operation due to the sliding contact between the screw and nut. This is especially critical for preloaded spindle assemblies, as preload increases contact and therefore friction between the bolt and nut.

One of the main criteria for sizing and selecting a nut is its PV value, which is the highest combination of pressure and speed that the bolt and nut can withstand. The PV value depends on the amount of heat generated when the screw and nut surfaces slide against each other.

Because lubrication reduces friction and heat generation, manufacturers often provide FV values ​​that assume the presence of a film of grease or dry lubricant. Be sure to check with the manufacturer that the PV value does not indicate whether external lubrication is assumed.

pressure-velocity graph

The PV rating of a material depends on heat or, in particular, the material's ability to dissipate heat. Adding external lubrication to a lead screw assembly reduces heat and improves lead screw PV. Manufacturers typically publish PV ratings that assume the application of an external lubricant, whether grease/oil or solid lubrication.

When external lubrication is considered advantageous, the designer or engineer can choose between grease/oil or a solid lubricant. Whether you choose grease or oil, suitable formulations are available for lead screw applications that require high travel speeds, cleanroom compatibility and a variety of temperature ranges. But keep in mind that these lubricants, especially grease, can attract and trap dust and other fine particles that are abrasive to the lead screw assembly and can reduce the life of the screw and nut.

lead screw

Grease and oil options for spindle lubrication offer a wide range of temperature ranges and chemical compatibility.

The term “solid lubricant” generally refers to a PTFE-type coating (known by the brand name “Teflon”), which is by far the most popular solid lubricant for lead screws. Some manufacturers offer their own formulations, but most use PTFE as the main lubricating component. Solid lubrication is generally applied by the manufacturer with a spray method to achieve a very precise and uniform thickness and then cured.

Solid lubricants provide better protection than oil or grease against contamination by liquids – water, acid or alkaline – and generally allow for a wider range of operating temperatures. They are also the best option in situations where grease and oil are not permitted, such as medical and optical applications. Although the coefficients of friction of solid lubricants are higher than those of grease or oil (PTFE has a coefficient of friction range of 0.06 to 0.12), the coefficient of friction remains constant regardless of the preload or load applied , which ensures consistent torque requirements and smooth operation of lead screw life.

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