LM guide boasts the following outstanding features.
a. Smooth movement with no clearances
b. High running precision with ease
c. High positioning accuracy
d. High rigidity in all directions
e. High permissible load rating
f. High long-term precision
g. High speed operation
c. High productivity
d. Substantial energy-savings
e. High efficiency in machine design
f. Easy maintenance
LM Guide with caged ball technology
In the conventional LM guide their existing contact between the balls (rolling elements) resulting in friction between balls which produce noise. Fig. 4.16.
The above drawback of conventional guide has been over come using caged ball technology. Caged ball technology eliminates contact between balls. This prevents friction between balls thereby ensuring low-noise, maintenance free and high speed operation for a long period of time.
Friction between balls
In the conventional LM guide, the rotary ball bearing of the LM part are not caged. Friction between balls caused loud noise, made high-speed rotation impossible and shortened the service life.
Later after 20 years of the above invention the caged ball design was developed for rotary ball bearings. The new design reduced the number of balls used while enabling high-speed rotation at low noise level and increasing the service life. This marked a major break through in the history of rotary ball bearings. Similarly the quality of needle bearings was significantly improved by the caged ball structure.
What happens in bearings that are not ball caged ? (Disadvantages of uncage ball design).
a. Balls contact one another at various points and rotate in opposite directions.
b. The sliding contact two adjacent balls occur at a speed twice the ball-spinning rate.
c. As the balls are metallic, they cause severe wear and load noise, resulting in short series life.
d. Normally, an oil film between the balls used as lubrication breaks under a bearing stress of 3 kg/mm2. Uncaged ball create point contact, resulting in infinite bearing stress, breakage of the oil film and sliding contact between balls.
Advantages of caged ball design
a. The contacts of the ball is over a wide area, hence oil film will not break.
b. The noise level is low.
c. The balls can rotate at high speed
d. Long service life.
The service life is prolonged (long service life) due to elimination of wear caused by friction between balls.
The lack of friction between balls results in reduced heat generation during high-speed rotation. Lack of friction between balls results in lack of ball-to-ball impact noise.
Even spacing of balls enables them to move smoothly.
The retaining of lubrication oil between balls ensures longer service life.
These type of antifriction linear motion bearing can reduce stick- slip to a very great extent.
Advantages of antifriction guideways
a. Reduced stick-slip motion of slides
b. Low static and Kinetic frictional resistance
c. High durability
d. High accuracy of movement of slides
e. High load coming capacity because of reduced friction.
f. High traverse speed.
g. East of assembly.
h. Operation without external hydrodynamic lubrication.
Disadvantage
a. Low damping Capacity
b. Very expensive.
Use of ball and rollers requires highly finished and hardened guiding surface because contact iscurved line or point, where the waviness of the surface impairs the accuracy of the motion. To combine the advantages of both sliding (as regards to the accuracy of guided motion) and rolling
tools.
Frictionless guide ways: This is featured by solidless contact between the surfaces of the guide ways and slides. In the case of friction and antifriction guide ways there exists physical contact of metals or solids which are proved to wear developing friction. This necessitate replacement of the guide ways and slide after a long usage and another major disadvantage of antifriction guide ways is its low damping capacity. The above demerits of antifriction guide ways can be over come by frictionless guide ways.
Frictionless guide ways do not have any solid or metal contact between the guide ways and the slide and the two are separated by a thin film of fluid. Frictionless guideways are of two types depending on the fluid employed to separate the guide ways and the slide. They are.
a. Hydrostatic guide ways
b. Pneumatic guide ways (Aero static guide ways)
Hydrostatic guide ways Hydrostatic guide ways are those wherein the surfaces of the guide and the slide are separated by a film of oil supplied under a pressure. These type of guide ways are increasingly finding its application is precision machine tools. Friction and stick -slip are completely eliminated. The clearance between the two surface is maintained between 15 to 25 microns. Fig 4.19 shows the principle of hydrostatic guide ways. A high degree of dynamic stiffness and damping can be achieved with these types of guide ways contributing to good machining capabilities.