Linear Components Design Considerations
PowerTrax™ Linear Slide Systems are available in a variety of configurations. The following factors should be considered when choosing the slide system which best suits an application.
Single Or Double Shaft Systems
The majority of applications require double shaft systems in order to restrain the load in two planes. Single shaft systems may be used for hanging or vertical loads where rotation of the bearing around the shaft is allowable.
Fully Supported Or Unsupported Shafts
Fully supported systems are used to eliminate shaft deflection. Full shaft supports must be attached to a machined mounting base. Openstyle bearings used with this system are sensitive to load orientation.
End-supported systems are generally used to span a gap or where some deflection is allowable. This system uses closed-style bearings that achieve higher load capacities. The shaft must be selected so that deflection does not exceed self-alignment capability of the bearing.
Linear Bearing Pillow Blocks
Two bearings must be used to support a load on a shaft. Single blocks allow for custom spacing and wider load bearing stances. Twin pillow blocks have a compact, one-piece design.
Carriage plates are designed in two styles for linear system packages. Carriage 1 is designed for two pairs of single bearing blocks. Carriage 2 is designed for two twin bearing blocks and has a shorter overall length.
For fully supported systems the bearing size needed for the application is determined by the load and life requirements.
For end-supported systems, both the bearing diameter that meets load and life requirements and the shaft diameter that results in an allowable deflection must be determined. The correct choice of shaft/bearing diameter is the larger of the two.
Linear systems require at least three bearings to define the plane of motion. It is necessary to identify and understand which of following load conditions affect the application:
- Centered Loads
- Offset Loads
- Side Loads
- Vertical Loads
- Gravity Effects
- Reaction Forces (i.e., cutting tool reaction).
- Dynamic loading (acceleration, deceleration and inertial loads).
Apply the actual load to the appropriate load condition in the figure below to calculate the resulting bearing loads. (See Image 1 in Figure 1 Below)
Use the equation in the following figure to determine the additional forces developed due to acceleration. If impact or impulse loads are anticipated, these forces must also be considered when selecting the appropriate bearing size. (See Image 2 in Figure 1 Below)
Shaft deflection should be considered when choosing the proper bearing and shaft diameter for end-supported systems. Deflection is directly related to the diameter of the shaft, the unsupported length of the shaft, and the type of shaft end mounting that is used.
- Typical Shaft End Mounting (See Image 3 in Figure 1 Below)
- "Simple" - the end allows some of the shaft deflection slope through the fastening point.
- "Fixed" - the ends are constrained from deflection.
- NOTE: Fixed end mounting can be accomplished by capturing the shaft end with a length of engagement equal to or greater than 1 ½ times the shaft diameter.
Use the formula:D = N × W × L3
- N = value from the chart below
- W = load in pounds
- L = length (in inches) of unsupported shaft section
|"N" VALUE FOR NOOK SHAFTS|
|SHAFT DIAMETER (in.)||SIMPLE||FIXED|
|1/4||3620 x 10-9||905 x 10-9|
|3/8||715 x 10-9||179 x 10-9|
|1/2||226 x 10-9||56.6 x 10-9|
|3/4||44.7 x 10-9||11.2 x 10-9|
|1||14.1 x 10-9||3.54 x 10-9|
|1 - 1/2||2.79 x 10-9||.698 x 10-9|
|2||0.866 x 10-9||.0220 x 10-9|
|3||0.168 x 10-9||.432 x 10-10|
|4||0.052 x 10-9||.136 x 10-10|
Calculate Misalignment Angle
PowerTrax™ linear bearings allow for ½ degree misalignment. To determine the amount of misalignment due to shaft deflection use the formula:θ = sin-1 (D/L)
- θ = angle in degrees
- D = shaft deflection
- L = length (in inches) of unsupported shaft section.
- If misalignment is greater than ½ degree, then:
- Reduce the Length of the shaft
- Use a larger shaft diameter