2025-10-13
To address the frequent fracture of feed springs in nut machines, a systematic adjustment from four aspects—spring selection, installation position, mechanical coordination, and environmental control—is required. The specific solutions are as follows:
I. Spring Selection Optimization: Matching Load and Compression
Load Matching
Root Cause: If the spring's allowable compression is 30% but the actual compression reaches 40%, it leads to plastic deformation and fracture.
Solution: Recalculate the required spring stiffness (K-value) to ensure the compression does not exceed 80% of the allowable compression.
Example: For ejecting a 20mm product, the clamp width when unclamped should be ≥19mm, reserving a 0.5-1mm clearance to prevent excessive spring force from forcing the clamp open.
Prioritize die springs (e.g., rectangular section springs), whose load capacity is 30%-50% higher than ordinary springs.
Material Upgrade
Use high-carbon steel (e.g., 65Mn) or stainless steel springs, which have better fatigue resistance than ordinary spring steel. Avoid materials with excessive impurities to prevent stress concentration fractures.
II. Installation Position Adjustment: Precise Positioning and Mandrel Fit
Mandrel Dimension Calibration
Root Cause: An undersized mandrel causes wear between the spring and mandrel, leading to fracture; a mandrel that is too short and unchamfered increases friction.
Solution: The mandrel diameter should be ≥95% of the spring's inner diameter, and the end should be chamfered (R0.5-1mm) to reduce stress concentration.
Example: If the spring inner diameter is 10mm, the mandrel diameter should be ≥9.5mm.
Verticality and Parallelism
Ensure the spring axis coincides with the mandrel axis, with a deviation ≤0.1mm. The flatness of the mounting surface should be ≤0.05mm, and the parallelism of the two end locating surfaces should be ≤0.1mm to prevent compression distortion.
III. Mechanical Coordination Optimization: Reducing Friction and Foreign Object Interference
Clamp Design Improvement
The clamp opening width should be 0.5-1mm larger than the product diameter to prevent the spring from hitting and opening the clamp during ejection.
Example: A 20mm product requires a clamp opening ≥20.5mm.
Foreign Object Removal
Regularly check for foreign objects like metal chips or grease between spring coils. Clean and apply a dry film lubricant (e.g., molybdenum disulfide) to reduce friction.
Standard Practice for Series Connection
Avoid springs in series bending beyond the mandrel or counterbore length, which causes uneven load distribution. If series connection is necessary, add guide rods to ensure linear motion.
IV. Environment and Operation Control: Extending Spring Life
Temperature Management
The operating temperature should be ≤ the maximum allowable temperature for the spring material (typically ≤150°C). In high-temperature environments, switch to heat-resistant spring steel (e.g., 50CrVA).
Compression Monitoring
Install displacement sensors to monitor compression in real-time and trigger automatic shutdown if limits are exceeded.
Example: If the spring's allowable compression is 30mm, the working compression should be ≤24mm.
Regular Maintenance
Check the spring free height every 500 hours; replace if the decrease is ≥5%.
Perform shot peening every 2000 hours to increase surface compressive stress and delay fatigue fracture.
V. Emergency Repair Solutions (Temporary Measures)
If immediate spring replacement is not possible, consider:
Reduce Compression: Adjust the limit block to reduce compression to 70% of the allowable compression.
Increase Preload: Add a shim at the spring bottom to reduce initial clearance and lower working stress.
Local Lubrication: Apply silicone-based grease to worn areas to reduce friction.
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