Tom Kamada
Senior Engineering Manager
IKO International
Parsippany, N.J.
Manufacturers need production equipment that runs faster than ever, yet with higher precision to turn out the lowest-cost, highest-quality components their customers demand.
Linear guideways, one of the most important components in many manufacturing machines, play a key role in meeting such demands.
As a result, companies are replacing conventional sliding guideways with rolling linear guideways in many machines. Major advantages of machine tools equipped with rolling guideways include faster run times and the ability to hold tighter tolerances.
Designers are also turning to cylindrical rollers instead of balls as the rolling elements in linear guideways. They offer several benefits, but engineers must keep several issues in mind when designing linear guideways. These include:
Load capacity of linear roller guideways depends on the diameter, length, and number of rollers, among other factors. To take full advantage of rollers, designs should use many small-diameter rollers with long effective contact lengths. This results in a large contact area between rolling element and raceway and, in turn, a high load rating. For instance, IKO linear roller way Super X Series have 1.8 to 2.1 times the load rating of ball-type guideways of the same size.
Rigidity, the extent to which a linear guideway elastically deforms under load, is important when machining parts to tight dimensional and geometrical tolerances.
Rolling elements inherently have a higher rigidity than balls, so overall rigidity of a roller-type guideway is greater as well. For instance, the Super X has about 2.6 times the rigidity of a ball-type guideway of the same size.
Vibration damping is better because roller-type guideways deflect less under repeated and varying loads. Less vibration improves cutting capacity and part surface finish, and results in longer tool life.
For instance, tests show a surface grinder spindle head using linear roller guideways provided a finer surface finish, and reduced the wheel wear rate by half, compared with a machine equipped with ball-type guideways.
Low rolling friction stems from antifriction roller elements in the linear guideways. Friction coefficients are as small as 0.002 even with large preloads. This permits precise response of machine tools to NC commands and, in turn, yields better positioning accuracy.
For example, low-friction roller guideways stabilize dimensional accuracy in surface grinding; improve the taper accuracy in internal grinding; and increase contouring accuracy in the circular milling operations of machining centers. Low friction also equates to less wear, so machine tools maintain accuracy for longer intervals without adjustment.
Durability extends linear roller guideway life and reduces maintenance. In life tests using a lightly preloaded guideway carrying a 4,000-kg load at an average speed of 32 m/min, flaking was not evident on the raceway surface until 1,700 to 2,300 hr of operation. This represents 2.8 to 3.8 times the calculated life.
Durability of linear roller guideways helps minimize variations in kinetic friction and rigidity over the life of the bearings, to maintain long-term accuracy. Two other attributes are that high-speed operation produces little temperature increase, and temperature changes have minimal effect on preload. Both ensure long-term accuracy.
Tests with Super X linear roller ways substantiate that linear roller guideways show such behavior. High-speed durability tests used a guideway carrying 1,500 kg at an average speed of 95 m/min and maximum acceleration of 3 g. Kinetic friction initially decreased during the break-in period, then remained nearly constant over a total travel distance of 10,000 km. Decreases in rigidity were minimal as well. This indicates that high-running speeds have little effect on durability.
Tests for heat generation at high speed involved heavily preloaded guideways carrying a 1,500-kg load, at an average speed of 62 m/min and maximum acceleration of 1.5 g. Sensors measured temperature at several points on the slide unit and track rail. Average temperature increase was approximately 10°C. The corresponding decrease in the preload dimension was about 0.002 mm, indicating that high-running speed has a small effect on preload.
The practical effect of the mechanical characteristics of roller guideways is seen in actual machine-tool performance. In one case, end-milling operations were conducted on two vertical machining centers, one incorporating linear roller guideways in the X, Y, and Z axes, the other with ball-type linear guideways. An acceleration sensor attached to the spindle head monitored vibration during cutting. Straightness and inclination of the milled surfaces were measured as an indication of machining accuracy.
For peripheral milling, in the machine with roller guideways, the amplitude of vibration affecting accuracy was about one-third that of the machine with ball-type guideways.
The amplitude when milling the central work area was one-fifteenth the level on the ball-guideway machine. The actual amplitude under heavy cutting conditions was as small as 0.01 mm, indicating excellent vibration absorbing characteristics of roller-type guideways.
Straightness and inclination of the milled surface were checked after peripheral milling to a 2-mm depth. Deviations from straightness were one-twelfth to one-half that of the surface obtained using the machine with ball-type guideways.
On all surfaces, the inclination of the cutting tool (rectangularity) was as small as one-ninth to one-fifth that obtained with ball-type guideways, or less than 0.05 mm. This results in time savings for finish cutting, thanks to the rigidity of roller-type guideways.
In heavy face-milling and end-milling tests conducted on gantry-type machining centers equipped with linear roller guideways, results were much the same. Results indicate that linear roller guideways have the rigidity and damping characteristics needed for matching cutting capacity to the maximum power of the motor, in addition to maintaining precise geometry of work pieces at high feed speeds.