Linear Bearings or linear guides are support mechanisms that are designed to allow you to easily move significant weight along a single axis. One advantage of linear bearings over other linear supports, like drawer slides, is that they are also able handle torques causes by uneven loads which will protect other actuating components. This is why you will often see both torque and force specifications listed for linear bearings, particularly for roller style linear bearings. You will also often see two force specifications given as well; one for compression and the other for tension. This blog will aim to explain all of these specifications so that you may have a better understanding of them and be able to identify the right linear bearing for your next project. If you want to learn more about linear bearings or need a refresher, check out our Linear Bearing 101 blog.
Compression is when forces push down onto an object, like in the diagram above, and is probably the most commonly needed specification. If the compression force specification is exceeded it could lead to excessive wear or complete failure of the bearing. The compression force specification will generally always be higher than the tension force specification as it is much harder to crush a solid mechanism, like a linear bearing, compared to pulling it apart.
Tension occurs when forces are pulling or stretching an object, like in the example above. The amount of tension a linear bearing can handle is generally lower than compression due to the design of linear bearings. For roller-style linear bearings, like our FA-SGR-35 Series, tension puts stress on the roller bearing shafts used to connect the rollers to the cartridge and may cause cracks which will lead to failure. While compression forces also put stress on these shafts, the pulling apart due to tension will lead to these cracks propagating much faster. For sliding contact linear bearings, like our FA-MGR-15 Series, the above principle is also true, but the stress occurs on the rail and is impacted by its design.
In your application, you will need to determine all the forces involved to identify whether your linear bearing will be experiencing tension or compression. Making use of free body diagrams, like the ones above, can be used to identify all forces and their directions that your linear bearing will experience. You can then sum up all the forces to determine the direction and magnitude of the resultant force on your linear bearing which can be used to determine the minimum size of linear bearing required, although you should always add a safety factor to ensure your application doesn’t fail. If your loading condition is quite dynamic, you may be required to determine the resultant force on your linear bearing at multiple loading conditions as it is possible to have your linear bearing experience both tension and compression at different points in a single application.
A torque is a turning force that causes rotation and is equal to the force applied times the perpendicular distance to the point of rotation. Torques can be caused by off center and/or unbalanced loads. A torque specification refers to how much of an unbalanced torque can the cartridge of the linear bearing can handle before failing. Torque specifications are generally listed for each axis and refers to the max torque about that axis. If the torque specification about each axis is different, you need to confirm how the supplier has labeled each axis. For the examples below, the x-axis is the axis of motion for the linear guide, the y-axis is the side to side axis, and the z-axis is the up and down axis. Again, this may not be how your supplier has labeled their axis and you should confirm their labeling.
Torques About X-Axis
The above example shows a situation where a torque about the x-axis would occur. As the center of gravity of the load is not aligned with the center of gravity of the cartridge, the load will try and cause the cartridge to rotate causing a torque. The center of gravity of the load would still be perpendicular in this case. A torque about the x-axis would also occur if there was an unbalance force that was acting on the load in a similar orientation to the example above.
Torques About Y-Axis
A torque about the y-axis can also be caused when the center of gravity of the cartridge and the load do not align, but in this case, the center of gravity of the load is still parallel with the rail of the linear bearing, like shown above. This torques will try and flip the cartridge. A torque about the y-axis would also occur if there was an unbalance force that was acting on the load in a similar orientation to the example above.
Torques About Z-Axis
A torque about the z-axis would most likely be caused by an unbalance force that is off centered from the center of gravity, like above. This torque will try and cause the cartridge to spin or be dislodged from the rail.
Like with forces, you’ll need to determine all of the forces involved in your application as well as how far they act away for the center of gravity of the cartridge to determining the torques involved. Again, you can also used free body diagrams, like the ones above, to visualize the forces and determine whether or not it will cause a torque as well as the direction of that torque. While the above examples are simple, your application may be more complex and involve multiple torques. Like with forces, you will need to sum up all the torques for each axis to determine the minimum torque specification for your linear bearing, although you should always add a safety factor.