Impact on Speed, Force, and Precision
The lead of a lead screw is a key parameter in screw drives. It significantly influences how rotational motion is converted into linear motion and has a direct impact on feed rate, transmissible forces, efficiency, and positioning accuracy.
Especially with ball screws and trapezoidal screws, choosing the correct lead is crucial for a machine’s performance.
What is meant by the lead of a screw?
The lead describes the linear distance a nut travels during one complete revolution of the screw. It is usually specified in millimeters per revolution (mm/rev).
Example:
A lead screw with a pitch of 10 mm moves the nut 10 mm in the axial direction with one full rotation of the lead screw.
For multi-start threads, a distinction is made between pitch and thread rise. Multiple thread starts increase the resulting pitch without changing the thread pitch.
Relationship Between Rotational Speed and Feed Rate
The pitch directly determines the feed rate of a lead screw. The greater the pitch, the greater the linear travel per revolution.
The feed rate is calculated as follows:
Feed rate = Rotational speed * Pitch
A larger pitch therefore results in a higher linear speed at the same rotational speed.
Example:
- Spindle A: Pitch 5 mm
- Spindle B: Pitch 20 mm
- Rotational speed 1000 min-1
The resulting feed rates are:
- Spindle A à 5 m/min
- Spindle B à 20 m/min
This makes a larger lead particularly suitable for dynamic axes with high travel speeds.
Effect on the Transmissible Force
In addition to speed, the lead also influences the mechanical ratio between torque and axial force.
A small lead acts like a high force ratio. As a result, the required axial force can be generated with less torque.
A large lead, on the other hand, reduces the ratio, meaning that a higher torque is required to produce the same axial force.
Typical Effects
|
Pitch |
feed rates |
Torque |
Axial force |
|
small (↓) |
↓ |
↓ |
↑ |
|
big (↑) |
↑ |
↑ |
↓ |
Effect on Efficiency and Self-Locking
The pitch also affects the friction behavior and thus the efficiency of the lead screw
In trapezoidal lead screws, a smaller pitch often results in self-locking. This means that the nut does not move backward on its own under load when the drive is turned off. This property is desirable, for example, in lifting spindles or clamping systems.
As the pitch increases, self-locking decreases.
In ball screws, self-locking generally plays no role, as the rolling contact results in very high efficiency. Here, the focus is more on the relationship between speed and force.
Effect on Positioning Accuracy and Resolution
The pitch also affects the positioning resolution of a drive system.
For a servomotor or stepper motor with an encoder, the linear resolution is calculated from the motor’s rotational resolution and the lead of the lead screw.
A smaller pitch therefore results in a finer linear positioning resolution, since the nut moves less per angle of rotation.
However, a steeper slope reduces the resolution.
Typical Pitch Ranges in Practice
Different pitches are used depending on the application.
Small pitches (e.g., 2–5 mm)
Are frequently used in high-precision positioning tasks, such as in measurement systems or fine-adjustment mechanisms.
Medium pitches (e.g., 10–20 mm)
Are typical for machine tools and standard linear axes, offering the best balance between force transmission and positioning accuracy.
Large pitches (e.g., 25–50 mm and above)
Are primarily used in highly dynamic machines or for longer travel distances, such as in high-speed handling systems or pick-and-place applications.
Conclusion
The lead of a spindle is a critical parameter for the design of a lead screw system. It significantly determines the relationship between speed, force transmission, and positioning accuracy.
A small lead enables high forces and fine resolution, while a large lead allows for high travel speeds and dynamic motion. The optimal lead is therefore always determined by the specific requirements of the application in question.
Careful design of the lead is thus an essential step in the construction of high-performance and efficient linear axes.
