Selecting the proper ballscrew package for a specific application involves consideration of many interrelated factors. Some of these factors are: load, speed, travel rate, positioning accuracy, repeatability, mounting rigidity, and design life. Changing one of these factors often influences another and the final decision regarding the proper ballscrew package for the application.
In some cases, additional factors must be considered such as the operating environment, available drive torque, or side loading. Overturning or cocking type loads should be avoided whenever possible. If you have any questions regarding your specific application, please contact Comptrol Incorporated.
DEFINING THE APPLICATION
Before starting the selection process, Comptrol suggests defining some basic operating parameters to determine the design requirements. These parameters include:
- Type of application, horizontal or vertical
- Total weight of the load
- Maximum travel rate
- Length of stroke
- Cycle rate
- Design life objective
- Type of end mounting
- Tension or compression loading
Proper lubrication and protective covers should also be considered with respect to the operating conditions and environment.
END MOUNT BEARING SUPPORTS
The end bearing support arrangement is usually the one design parameter left to the discretion of the designer. It is also one of the most important because it affects critical speed, load capacities, and overall system stiffness.
Whenever possible, the ballscrew should be fixed at both ends using bi-directional thrust bearings and held in neither tension nor compression for maximum stiffness. If the screw must be held under stress, tension is always more desirable than compression.
BALLSCREW SELECTION
In most applications, there are four main factors that must be considered when selecting the proper size ballscrew for an application. These factors include: the applied load, life expectancy, compression loading, and critical speed.
Remember the ballscrew you select should meet or exceed the application requirements. This may require selecting a larger ballscrew to meet the life expectancy, compression loading, and/or critical speed requirements even though a smaller ballscrew has sufficient static load capacity.
EFFECTS OF CHANGE IN PARAMETER ON BALLSCREW CHARACTERISTICS
|
INCREASE IN: |
AFFECTS |
HOW |
|
Screw Length |
Critical Speed Column Load |
Decreases Decreases |
|
Lead |
Torque Load Capacity Positioning Accuracy Angular Velocity Ball Diameter |
Increases Increases Decreases Decreases Increases |
|
Angular Velocity Load Number of Balls |
Critical Speed Life System Stiffness |
Decreases Decreases Increases |
|
Preload |
Positioning Accuracy System Stiffness Drag-torque |
Increases Increases Increases |
|
Screw Diameter |
Critical Speed Inertia Stiffness Spring Rate Load Capacity Column Load |
Increases Increases Increases Increases Increases Increases |
|
Mounting Rigidity |
Critical Speed System Stiffness |
Increases Increases |
|
Nut Length (7½ turns max.) |
Load Capacity Stiffness |
Increases Increases |
|
Ball Diameter |
Life Stiffness Load Capacity |
Increases Increases Increases |
The alternative is to change the design parameters, such as the travel rate, end mounting, or life expectancy to allow a smaller diameter screw to be used. Any decision to change the design parameters must be based on good engineering practices to maintain the safe operating conditions of the overall system.