By Alexander Beck
Executive Vice President
Edited by Kenneth Korane
Staying competitive in the global marketplace requires production equipment that operates around the clock with little, if any, time for routine maintenance. This places substantial demands on machine components, especially the drives.
Machine tools, for instance, are part of this trend. The newest machines remove metal faster, have the versatility to perform several machining operations, and offer outstanding productivity. Reduced machining time, however, means quicker accelerations, higher cutting speeds, and rapid traverses, all which place greater demands on linear drives. High-speed cutting and dry-machining operations also generate fine particulates. Unchecked, these particles hasten wear and can quickly destroy even the best of drives.
Precision-ground ball screws continue to be the preferred drive mechanism for machine tools. They are compact, economical, and efficient — key advantages, especially for high-speed machines. However, drive-screw longevity no longer depends solely on rated load capacities in relation to mean applied load. Engineers must now consider two other factors when designing modern high-speed machines: lubrication and contamination-related wear.
Generally, three types of friction relate to steel balls rolling between ground steel surfaces. With no lubricant, there is direct contact or so-called solid friction. With lubricant, there is either a constant oil film between ball and race (elastohydrodynamic or EHD friction) or a mixture of oil film with some direct contact (mixed friction).
EHD is preferred, but compared to other machine components such as deep-groove or angular-contact ball bearings, or even plain bearings, ball screws face performance demands that make clean and lubricated operation a must. There is more mixed friction due to lower relative speeds. Inclined contact lines — similar to angular-contact ball bearings — cause twisting and sliding, and the ball returns in the ball nut produce nonuniform rolling. Another factor is sliding friction between balls that run without retainers or separators. Ball chains or plastic spacers meant to reduce friction between adjacent rolling balls are subject to design limitations, because the ball chain still must pass through returns.
Ball screws are usually sized based on the Hertzian pressure of an applied load and the number of load cycles, using the classic L10 life equation. Applied load eventually causes material fatigue, reflected in the equation for life expectancy:
where L10 is the bearing life in revolutions. Reducing the mean equivalent load Fm in relation to the dynamic load rating Ca should dramatically increase useful life. This holds for conventional, slower machines. However, for modern high-speed machines that require a long life, it is not valid. Two other wear factors, abrasion and adhesion, are not included in conventional life calculations. Contamination inside the ball-screw nut causes abrasive wear, and lubricant-film breakdown results in microwelding and adhesive wear.
These wear mechanisms lead to a loss of preload and premature failure. Avoiding preload loss is especially important with the latest high-dynamic servodrives, which are sensitive to changes in friction. Therefore, to extend the life of ball screws, cleanliness and lubrication must be considered, in addition to dynamic load capacity.
LUBRICATION AND SEALING
Keeping ball screws clean and well lubricated is no easy task. It is normally not possible to cover ball screws sufficiently to completely isolate them from dust and fine particles, so wipers must seal the nut from abrasive attack. But regular, noncontacting wipers (labyrinth seals) will not keep small particles from entering the nut because there is a gap of several tenths of a millimeter between wiper and screw. Brush wipers also fail to effectively protect ball nuts from such contamination.
Elastic lip seals work substantially better. They are similar to shaft seals and provide a tight fit. But they, too, have their drawbacks. For instance, they wear because one lip typically slides on the screw surface with sufficient pressure to ensure a tight seal. And in ball screws with large pitch, the lip may flex significantly in the axial direction, which compromises effectiveness.
Segmented plastic wipers overcome these problems. They have several edges approximately oriented at right angles with the direction of motion. The wipers (mounted on both ends of the nut) must fit closely, because effectiveness depends largely on line contact with the screw surface.Segments must be stiff enough to remove tough dirt from the screw but still be flexible enough to maintain contact through the spring preload.
They work best when wipers with protruding "fingers" do not contact elastic end-of-travel bumpers. In such cases, flush-mounted wipers are necessary. Wedge-shaped segments or beveled edges are recommended where applications see large amounts of dirt.
Wipers that store and distribute lubricant are becoming increasingly popular, especially as environmental regulations on oil disposal become more restrictive. This type of wiper can substantially reduce lubricant consumption yet ensure proper lubrication of ball nuts. Although such systems use significantly less lubricant than conventional auto-lubrication systems, regular replenishment of the reservoir is still necessary — and close attention must be paid to cleanliness. As a general rule, reducing lubricant flow in turn requires a better seal, as fresh lubricant no longer continuously flushes dirt from the ball nut.
One solution combines sealing and lubricant dispensing within a single polymer ring. However, the polymer material requires heat to operate properly. The ring, often spring preloaded and impregnated with oil, forms a seal around the thread. As the plastic ring rubs against the rotating thread, friction raises the material temperature. This causes oil to flow from the polymer. It may reabsorb some oil as it passes over the thread, but there is no reservoir to refill. There is also no possibility to choose between grease and oil, because the manufacturer determines the oil impregnated in the plastic.
On the other hand, segmented plastic wipers combined with felt rings have proven to be an especially effective combination. The first stage in these combination wipers is plastic, with the necessary mechanical properties to provide a required line contact and divert contaminants away from the screw. The second stage is a continuous felt ring that stores lubricant and distributes a thin, even film. Another advantage (in addition to lower oil consumption) is that combination wipers can use any liquid lubricant, whether oil or liquid grease. The latter is particularly advantageous in applications with short-stroke oscillating moves, or low speeds. Felt can store up to 75% of its volume with lubricant, and the reservoir can be refilled.
As an added bonus, the felt acts as a second wiper absorbing tiny particles that get past the plastic wiper. This feature is especially useful in applications with fine, abrasive dust or sludge, such as grinding machines. However, felt alone cannot be used as a wiper, except in relatively clean conditions, because it will absorb coolant and particles. Thus, felt ensures trouble-free operation and reduces lubricant consumption by up to 90%, but only in combination with a good plastic wiper.
The cost for this option is minor compared to downtime due to unexpected and premature failures. And, in some cases, long-term lubrication or even "for-life" lubrication may be feasible with the proper lubricant.