Machine tools spend about 70% of their time cutting metal. The rest of the time, they're either making a tool change or positioning for the next machining function. It's possible, however, for machine tools to spend more time cutting and less time moving and positioning by employing rolling-element guideways. These pre-engineered machine paths not only offer long life, but also reduced deflection for rapid, accurate motion in many applications.
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Rolling-element guideways include balls or rollers to support loads. With just a 0.005 coefficient of friction, they redue the force needed to move and accelerate machine-tool masses, thereby decreasing horsepower. Low friction also translates to less wear. While these physical conditions all influence bearing life, mathematical formulas can accurately estimate it:
Lrollers = (C/P)(10/3) × 105
Lballs = (C/P)3 × 105
Where L = life
C = dynamic-bearing capacity
P = effective load
Preload on the carriages can also affect bearing life. If,
F• 3 × Fpreload, then P = Fpreload + (2/3F)
Where F = actual load
F = 3 × Fpreload, then P = F.
This means that if the applied load to the carriage is greater than or equal to three, then it can be considered the effective load. Including a dynamic equivalent for inconsistent loads adjusts calculated service life to a more realistic value.
Prior to installing roller-bearing guideways, end users must read and understand assembly guidelines. These instructions advise keeping rails wrapped in original packaging until installation and cleaning them with light machine oil or spray, such as WD-40, after unwrapping. This is necessary because acid from a person's skin can cause corrosion on the rails. End users should then align rails precisely. Depending on preload class and accuracy grade, misalignment as little as 0.0001 in. can dramatically increase internal loading and shorten life.
If installation becomes difficult, end users must never strike or hammer carriages, as this dents the running surfaces and causes false brinelling. False brinelling consists of hollow spots, like Brinell dents, that result from vibration and swaying where rolling elements contact the raceway. It leads to premature failure and ruins a linear bearing's running accuracy.
Grease adheres to rolling elements readily, but attracts contaminants and disperses heat slowly. When end users employ it to lubricate carriages, they should pump in new grease continuously and slide the carriage to purge old grease. However, oil is preferred in most applications because it readily removes heat and tends to flush contaminants away. But, due to its low viscosity, oil easily escapes from linear bearings, requiring constant replenishment. Many older machine tools provide for oil loss by incorporating sheet metal drip pans or channeling in the cast base.
After selecting a lubricant, end users must decide how to apply it to the carriage. The simplest and most cost-effective method is attaching a grease or oil gun to the carriage's fitting. In many applications, however, the carriage is difficult or dangerous to reach. To solve this, end users can install automated-centralized systems, which dispense lubricant in measured amounts and predetermined intervals.
Because most linear bearings contain an internal lubrication path, carriages must move during lubrication. Manufacturers recommend sliding carriages at least three times their length for thorough lubricant dispersion. When applications do not permit this, end users should lubricate from both ends of the carriage. Lubrication frequency varies among manufacturers, but as a guide, when C/P is greater than two, the lubrication interval is calculated as:
Li = C/P × 100,000
For example, if C/P = 15, then frequency is 15 × 100,000 = 1,500,000. Therefore, this bearing slide requires lubrication every 1,500,000 m.
However, carriages may require more frequent lubrication, depending on velocity and environment. For slow and medium speeds (0.5 to 1 m/sec), NLGI 2 grease is recommended. An example of NLGI 2 is automotive wheel-bearing grease. If a machine runs regularly at 1 m/sec or more, end users should apply a grease thickness of NLGI 1 or less. Abrasive or water-based environments wash lubricants away and cause wear more quickly — so in these environments, linear bearings need more frequent lubrication.
Running accuracy is one way to classify linear bearings. It is defined as a single carriage's variation of travel measured against the rail's reference edge. Suppose a design calls for a rail length of 2,200 mm with an accuracy grade of G0. On the graph shown here, the vertical axis defines a tolerance band, 5 µm, that the carriage moves through while traversing the rail. This means that A and B2 vary by more than 5 µm as the carriage travels down the rail.
As a rule, GO and GI accuracy should be used for grinding and precision-milling machines. For less demanding applications, such as tool changers, pallet changers, or handling equipment, G2 or G3 accuracy grade should be specified.