What is the preload force of a ball screw?
What is the preload force of a ball screw? This is a question frequently asked by many customers. As a manufacturer specializing in the R&D and supply of ball screws, we have observed through our technical services that numerous customers hold misconceptions about ball screw preload force. Some believe that "higher preload equals higher precision," blindly increasing preload and accelerating screw wear. Others neglect preload entirely, relying solely on default settings, which can result in excessive transmission clearance and compromised positioning accuracy. The preload of a ball screw is not a fixed value but a "dynamic parameter" that requires comprehensive calculation based on the screw's specifications, load characteristics, and precision requirements. Improper preload adjustment can lead to reduced transmission efficiency at best and shortened screw lifespan at worst. Today we'll thoroughly dissect how to determine ball screw preload and outline selection strategies for different scenarios.
First, the fundamental calculation logic for ball screw preload: Starting from the "rated dynamic load"
The core basis for ball screw preload is its rated dynamic load (C) - the maximum dynamic load the screw can withstand over its rated lifespan. Preload is typically set as a percentage of this rated dynamic load, balancing precision requirements and service life to avoid excessive tightness or looseness.
Formula for Matching Preload and Load
In practical applications, preload must simultaneously address both "external load" and "clearance elimination" requirements, satisfying: Preload Fp ≥ (Maximum External Axial Load Fa × 1/3).
This prevents load exceeding preload and causing clearance recurrence. For example:
For a ball screw in an automated device with maximum external axial load Fa = 15kN, calculating Fp ≥ 5kN per formula; if the screw's rated dynamic load C = 50kN, taking 10% C = 5kN proportionally, both values match, setting preload to 5kN suffices;
If the external load Fa = 24kN, the formula yields Fp ≥ 8kN. Since the ball screw's C = 50kN (10% C = 5kN), the preload must be increased to 8kN (verify the ball screw's tolerance to avoid exceeding its upper limit) or replace it with a ball screw having a higher rated dynamic load capacity (e.g., C = 80kN, 10% C = 8kN).
Second, Core Factors Influencing Ball Screw Preload Selection
1. Load Characteristics: Determining the "Lower Limit" of Preload
For a vertical lifting device with a ball screw (C=40kN) and a unidirectional load of 12kN, setting the preload to 4kN (10% C) is more reasonable than 5.6kN (14% C) in a bidirectional load scenario, extending the service life by 30%.
2. Accuracy Requirements: Determining the "Upper Limit" of Preload
Positioning Accuracy Demands: Higher accuracy requirements necessitate greater preload - increasing positioning accuracy from 0.1mm/m to 0.01mm/m requires raising preload from 8%-12% C to 15%-20% C. For a coordinate measuring machine's ball screw with a positioning accuracy requirement of 0.005 mm/m, a preload of 18% C (C=100 kN), i.e., 18 kN, is applied to eliminate backlash error during measurement. If the preload is reduced to 10% C=10 kN, positioning accuracy degrades to 0.02 mm/m, failing to meet requirements.
Repeatability Requirements: For repeatability accuracy of ±0.001mm, preload must remain stable (fluctuation ≤5%) to prevent accuracy drift from preload decay. A semiconductor packaging machine's lead screw maintains repeatability within ±0.0008mm by controlling preload fluctuations below 3% through monthly calibration.
3. Service Life: Balancing Preload and Wear Rate
Preload and service life exhibit an inverted U-shaped relationship: insufficient preload leads to increased clearance and reduced accuracy; excessive preload intensifies friction and shortens lifespan. Experimental data shows:
At 10% C preload, screw life reaches 15,000 hours;
Increasing preload to 20% C reduces life to 8,000 hours (47% decrease);
Lowering preload to 5% C extends life to 18,000 hours but causes significant accuracy degradation, failing to meet medium-to-high precision requirements.
For a ball screw in an automotive component machining line, the original preload set at 18% C (C=70kN) yielded only 7,000 hours of service life. After adjustment to 12% C=8.4kN, service life extended to 12,000 hours while maintaining positioning accuracy meeting the 0.03mm/m machining requirement.
Third: Key Considerations for Preload Control in Different Methods
Ball screw preloading methods include "Positioning Preload," "Constant Pressure Preload," and "Dual Preload."
Each method has distinct preload adjustment logic and applicable scenarios, requiring targeted control:
1. Position-Based Preload (e.g., double-nut shim preload, single-nut pitch shift preload)
Principle: Preload is fixed via shim thickness or pitch offset. Once set, it cannot automatically adjust to load changes. Suitable for stable-load, high-precision applications.
Applications: Precision grinding machines, laser processing equipment, and other high-accuracy devices with stable loads.
2. Dual Preload (Positioning + Pressure Combination Preload)
For a lead screw in an aerospace component machining device:
- Base preload: 15kN (15% C=100kN)
- Compensating preload: 5kN
When load exceeds 20kN, the compensating preload automatically activates to ensure precision and safety.
Fourth, Examples of Ball Screw Preload in Different Application Scenarios
1. General Automation Equipment (e.g., Conveyor Lines, Lifting Platforms)
Screw Specification: C7-grade screw, C=40-60kN;
Preload Selection: 8%-10% of C, e.g., C=50kN yields 4-5kN preload, using positioning preload (single-nut preload);
2. CNC Machining Equipment (e.g., CNC lathes, milling machines)
Operating conditions: Moderate precision requirements (positioning accuracy 0.03-0.08mm/m), moderate load fluctuations (Fa=10-20kN), bidirectional loading, service life requirement 10,000-15,000 hours;
Lead Screw Specifications: C5-C6 grade lead screws, C=60-100kN;
Preload Selection: 12%-15% of C, e.g., C=80kN yields 9.6-12kN preload, using positioning preload (double nut shim preload);
Example results: For a CNC lathe lead screw (C=80kN, Fp=10kN), machined part dimensional tolerances stabilized at ±0.02mm.
3. Heavy-duty fluctuating equipment (e.g., presses, injection molding machines)
Screw specifications: C6-grade screw, C=100-200kN;
Preload selection: 10%-12% C (base preload) + 5% C (compensation preload). For example, C=150kN, total preload 15-25.5kN, using constant-pressure preloading (spring preload);
Example Effect: For a stamping machine ball screw (C=150kN, base Fp=18kN, compensation Fp=7.5kN), when load increases from 25kN to 45kN, preload automatically compensates to 25kN. No overload damage occurs, and positioning accuracy remains stable at 0.07mm/m.
Summary
There is no "universal standard" for ball screw preload. The core principle is to "calculate based on rated dynamic load ratio, adjust according to load characteristics, and determine based on precision requirements"-low-precision scenarios use 5%-10% of C, medium-precision uses 10%-15% of C, high-precision uses 15%-20% of C. Simultaneously, adjust based on preload method and operational fluctuation. Blindly increasing preload shortens lifespan, while neglecting it compromises accuracy. Balancing the relationship between "accuracy, load, and lifespan" enables optimal ball screw performance.
As a supplier, we recommend customers provide three critical parameters during selection: maximum axial load (static + dynamic), positioning accuracy requirements, and expected service life. Our professional team will calculate the matched ball screw specifications and preload. During operation, regularly inspect preload (every 3-6 months), promptly replenish or adjust it to prevent equipment failure caused by preload decay.
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