Ken Mizutani
President
IKO International
Parsippany, N.J.
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Linear guideways position workpieces and tooling, and are key components in most machine tools. |
Linear guideways are one of the most-important parts in the power-transmission train of machine tools. And as accuracy requirements for automotive, electronic, and other precision-machined parts continue to increase, machine builders are demanding guideways that offer higher rigidity and accuracy, and lower friction.
There has been a transition in recent years from conventional sliding guideways to rolling guideways in machine tools. Resulting benefits include higher feed speeds and accuracy, and simpler guiding structures that reduce machine assembly time and labor.
Rolling guideways are available with balls or rollers, and each has different characteristics in terms of load capacity, rigidity, vibration damping, accuracy, friction, life, and noise.
Most machine builders prefer ball guideways due to lower costs and more-widespread use, even though roller guideways can offer higher stiffness, load capacity, and precision.
To emphasize the advantages roller guideways hold, here is an in-depth look at performance characteristics of two "competing" linear guideways. One is IKO's Linear Roller Super X, a new product developed to meet the latest machine-tool requirements. The other is a dimensionally interchangeable, IKO ball-type linear guideway.
ROLLER ADVANTAGES
Roller-type linear guideways use cylindrical rollers, rather than balls, for the rolling elements. This provides a number of benefits in linear guideways:
- Greater contact area between the rolling element and raceway increases load ratings over ball-type guideways of the same size.
- Rolling elements are more rigid than balls, which translates to higher rigidity for roller guideways.
- Optimizing the diameter and effective number of rollers minimizes deflection during travel.
- Due to the small deflection under repeated and varying loads, coupled with excellent vibrationdamping characteristics, roller guideways improve machine-tool cutting capacity and machining quality (surface quality of finished parts), and extend tool life.
- A low friction coefficient — as small as 0.002 even with large preloads — provides accurate response to NC control of machine tools. This yields better positioning accuracy and, in circular milling, better contouring accuracy. Feed speed can also increase.
- Reduced wear means machine tools maintain stable accuracy for longer periods.
- They generate low noise levels in the high-frequency range to which people are sensitive, resulting in quiet machine operation.
BEARING CHARACTERISTICS
The Linear Roller Way Super X guideway (LRXG) has four rows of recirculating rollers in a rigid casing. Collars accurately guide the cylindrical rollers to ensure smooth running in a fashion similar to roller bearings.
Two pairs of V-form raceways provide a 45° contact angle to bear the vertical (downward and upward), lateral, and moment loads. The LRXG comes in flange and block types, with mounting dimensions identical to respective ball guideways. Several bearing characteristics deserve elaboration. For example:
Load capacity of linear guideways depends on the diameter, length, numberof rollers, and other factors. To take full advantage of the rollers, the LRXG has many small-diameter rollers and a long effective roller-contact length.
The Load capacity chart compares the basic static load rating of roller and ball (LWHG) guideways with the same mounting dimensions, indicating that roller guides have a load capacity 1.8 to 2.1 times that of the ball types.
Rigidity or elastic-deformation characteristics of loaded linear guideways are important for dimensional and geometric accuracies of machined workpieces.
The Rigidity chart shows measurements of actual elastic deformation of heavily preloaded (T3) guideways subject to downward loads. Comparing the two types indicates that roller guides are 2.6 to 3.0 times more rigid than ball variations. The Moment-deflection curves indicate rigidity under moment loads, again showing the advantages of linear guides.
Vibration-damping characteristics of linear guideways are important considerations for improving machining capacity and quality, and extending tool life.
Vibration damping curves show linear-guideway behavior for vertical and lateral applied vibration.Roller guideways produce-a smaller vibration amplitude and damp vibration in a shorter time interval, and thus demonstrate better characteristics than ball-type guides. Tests show that a grinder equipped with roller guideways provides a finer surface finish and less wheel wear, to as low as one-third to one-half, than a machine with ball-bearing guideways.
Rolling-friction characteristics are rated excellent for roller elements. Thus, roller guideways improve machine-tool response to NC commands. For instance, the guideways stabilize dimensional accuracy in surface grinding, improve taper accuracy in internal grinding, and increase contouring accuracy in circular milling operations.
Kinetic friction diagrams show the relation between load, speed, and friction in linear guideways. For a T3 preload, the coefficient of kinetic friction of an LRXG45 is 0.002 or less. Sliding-speed tests indicate that the LRXG45 has lower kinetic friction than the ball guide, and it remains nearly constant over the entire speed range.
DURABILITY
In addition to long life, roller guideways must minimize any variation in kinetic friction and rigidity over its life span to guarantee a machine tool maintains mechanical accuracy within a tight range. It is also important to minimize temperature increase during high-speed operation, because a temperature change will change the preload. The following tests relate to these characteristics.
Life test: A crank-drive, life-test machine was used with a lightly preloaded LRXG45 sample. Downward load = 4,000 kgf, maximum speed = 64 m/min and average speed = 32 m/min, acceleration = 0.5 g, and the lubricant was AV-EP2 grease.
Results showed flaking on the raceway surface after running 1,700 to 2,300 hr. This is 2.8 to 3.8 times the calculated life, so the unit can be classified as a long-life linear guideway.
High-speed durability test: A belt-drive, highspeed durability tester was used with a standard pre-loaded, LRXG45. Downward load = 1,500 kgf, stroke length = 400 mm, maximum speed = 140 m/min and average speed = 95 m/min. Acceleration = 3 g and AV-EP2 grease was the lubricant. Target travel distance was 10,000 km (1,750 hr).
The Durability tests charts show the change in kinetic friction and rigidity as travel distance increases from 0 to 10,000 km. Kinetic friction initially decreases and then becomes nearly constant after break-in. The decrease in rigidity is minimal, and high running speed seems to have little effect on durability.
Heat generation at high speed: Using a belt-type high-speed durability tester with a heavily preloaded roller guide, downward load = 1,500 kgf, maximum and average speeds are 120 and 62 m/min, acceleration = 1.5 g, and lubricant was AV-EP2 grease.
The stabilized temperature at highspeed operation was measured at several points on the slide and track rail. Results show a temperature rise about 10°C and a corresponding decrease in the preload dimension of about 0.002 mm, indicating that a high running speed has little effect on preload.
MAKE CONTACT
IKO International,
(800) 922-0337,
www.ikont.com
RESULTS OF CUTTING TESTS
To verify test results under real-world conditions, end-milling operations were conducted in two vertical-machining centers, one with roller ways on the X, Y, and Z axes, the other with ball-type linear guideways. The object was to compare spindle-head vibration and accuracy of the milled surfaces.
3 450-mm table. The cutting tool was an OSG 20-mm-diameter end mill with four cutting edges, and the workpiece was 1503 1053 105-mm JIS SKD-11 tool steel. Vibration during cutting was monitored with an acceleration pickup attached to the spindle head and signals sent to an FFT analyzer. The straightness and inclination of the milled surfaces were the measures of accuracy.
The Spindle vibration table indicates cutting conditions for end milling and spindle-head vibration. In the machine equipped with roller guideways, results showed that for peripheral milling of the work, the vibration amplitude that affects accuracy was one-third that of a corresponding machine equipped with ball-type guideways. And the amplitude was one-fifteenth when milling the central work area. The actual amplitude under the 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 was measured for peripheral milling at a 2-mm cutting depth. Deviation in straightness of the milled surface was one-twelfth to one-half that of the surface with the ball-guideway machine. On all surfaces, cutting-tool inclination ( rectangularity) was as small as one-ninth to one-fifth of that with the ball-type guideway machine, or <0.05 mm. This indicates the potential for significant time saving when finish cutting, and excellent rigidity of the roller guideways.
Test results compare the straightness and inclination of milled surfaces for a 2-mm cutting depth. Measurement values in parenthesis are for the machine with ball-bearing guides. |
| SPINDLE VIBRATION | ||
Vibration Amplitude, um |
5.7 |
1.7 |
| Cutting speed, m/min. | ||
| Spindle rpm | ||
| Feed/revolution, mm/rev | ||
| Feed speed, mm/min | ||
| Cutting depth, mm | ||
| Coolant | ||