In addition to air bearings, we also design and manufacture motion stages that use precision mechanical bearings like the crossed roller and linear guide (see description below). This reason is simple; for many applications they have an advantage in terms of load capacity, compact size and cost. However, each technology whether its air or mechanical bearing has its limitation which needs to be well understood. Often the limitation is obvious but sometimes it’s an artifact that’s not indicated on the data sheet and only seen through testing. We have the expertise and impartiality to help you select the right technology so your application can be successful.
V-Groove or Crossed Roller Bearings
The V-groove bearing utilizes a rectangular rail with a V shaped cut and a ball, cylindrical roller or needle bearing as the load carrying element. If using a cylindrical roller as the load element they are known as “crossed roller bearings” (see figure 1 below). The payload is supported by direct contact between the rectangular rail and load element which allows them to have high load capacity and high stiffness in a very compact design which is their main advantage. Other advantages include extremely low friction (μ≅.003) and very smooth motion since the rolling element does not recirculate. These features allow crossed roller bearings to be used in static nano-positioning applications when paired with non-contact linear motors and linear encoders. However, please note the smoothness of crossed rollers is not equivalent to air bearings and micro-vibrations will be present when moving at a constant velocity.
Figure 1: Crossed Roller Bearing
In a typical crossed roller stage the moving plate or carriage will extend over the stationary plate during its motion so they are not used for long travel applications (<600mm for 6mm diameter roller). As shown in Figure 2, this extension will be 1/2 the total travel in the forward direction and 1/2 the total travel in the reverse direction. The payload will be cantilevered at extreme ends of travel which affects motion errors such as flatness and pitch. In general, the parallelism (horizontal straightness or vertical flatness) of the crossed roller bearing is quite good with most vendors between 10 microns per meter for the normal grade to about 2.5 microns per meter for the super precision grade.
Figure 2: Travel of Crossed Roller Stage
However, the parallelism specification of the bearing may not be indicative of the final stage tolerances. First, the mounting plates must be manufactured to tight tolerances to avoid misalignment and distortion of the bearing rails. Second, great care must be given to preloading the crossed roller stage. Normally, a preload is applied with set screws that run along the length of the outer rail (see figure 3). This process must be done by a skilled technician, otherwise the bearing friction will be inconsistent and the motion errors affected.
Figure 3: Preloading of Crossed Roller Stage
Using the super precision grade is really pushing the limits of the crossed roller technology and should only be used in certain applications where air bearings are not practical such as vacuum envrionments (Scanning Electron Microscopes) or space constraints (wire bonders). The super precision grade requires great skill to manufacture and among high quality vendors will include mirrored guide surfaces and rollers matched in diameter to extreme tolerances. However, the penalty for using the super precision grades are higher cost, much longer lead times and more stringent manufacturing requirements on the mounting plates. In general, its important to know when you are approaching a point of diminishing returns and to find another technology in which the desired tolerances are more within reach.
Linear Guides
The linear guide is probably the most common precision mechanical bearing and used quite often in CNC machines, high speed pick and place machines or general transport applications. There are a number of reasons this bearing is so widely used. First, the linear guide has high load capacity and high stiffness in a very compact design but with much better misalignment capabilities than the crossed roller bearing. Second, the load carrying element recirculates within the module providing much longer travels (up to 3 meters) and the ability to support the load throughout the travel range. Third, the linear guide stage is straightforward to design and assemble primarily because the preload can be set with great uniformity at the factory using oversized ball bearings. Lastly, they have fairly good parallelism specifications with most vendors between 20 microns per meter for the normal grade to about 2.5 microns per meter for the super precision grade. Again, using the super precision grade comes with all of the caveats seen with crossed roller bearings.
The reason you will not see linear guides in critical scanning applications or nano-positioning applications is due to friction and smoothness. Linear guides have low friction (μ≅.003-.01) when not using seals but this is not often the case. The seals serve an important function in keeping out contaminants and keeping lubrication in. Bearing smoothness issues are due to the movement of the ball bearing in and out of the load area many times a second. The micro-vibrations can be felt by hand at slow speeds and heard when moving at high speeds. Of course, the vibrations vary by vendor quality, retainer technology, preload level, mounting plate precision and assembly misalignment.
The drive selection varies for stages with linear guides. For general transport applications, it’s not uncommon to use high quality leadscrews rather than precision ballscrews or even linear motors in an effort to keep cost down. For CNC applications, the precision ballscrew is most common because of its high rigidity and mechanical advantage. The linear motor is widely used with pick and place machines because of the need for high speed and high acceleration. We can select the right components to deliver you a cost effective solution that meets your performance requirements.
Figure 4: Linear Guide Design