Introduction: failure symptoms of the gas pressure unit on the stamping line
During a scheduled audit on the line for precision sheet metal forming at a plant in Dnipro, the chief engineer detected systematic positioning deviation of the actuating mechanism by 0.35 mm. Preliminary checks of the Siemens Sinumerik CNC system excluded electrical errors of the encoders and play in the couplings. Mechanical diagnostics of the feed axis revealed axial play in the ball-screw drive (BSC) type Festo LSNG-63. The calculated nominal service life of the unit was 35 000 hours, however, a catastrophic increase in dead travel occurred after 14 200 hours of operation.
Similar failures stop high-precision equipment and lead to defective products. In this material, we will conduct a full criminalistic analysis of the causes of accuracy loss in CNC machines, based on the requirements of ISO and DSTU standards.
Component overview: construction and operating conditions Festo LSNG-63
Ball-screw drive Festo LSNG-63 converts the rotational motion of a servo motor into linear movement with high efficiency (up to 90-98%). The construction includes a precision screw made of hardened steel (hardness 58-62 HRC according to ISO 683-17), a nut with internal ball circulation and a pre-tensioning system to eliminate axial play.
Technical characteristics of the unit:
- Nominal screw diameter: 63 mm
- Thread pitch: 10 mm
- Accuracy class according to ISO 3408-3: T7 (permissible pitch deviation ±52 µm per 300 mm)
- Working temperature range: from -20°C to +80°C
- Maximum dynamic load: 125 kN
- Calculated MTBF: 35 000 hours under conditions of regular lubrication
The node operates under conditions of cyclic axial loads with variable sign at a displacement speed up to 1.5 m/s and a reversal frequency up to 40 cycles/min.
Failure evidence: measurement data and visual inspection
Disassembly and inspection of the unit in the UNITEC-D workshop allowed to collect an accurate picture of the mechanism degradation:
- Bearing clearance measurement: A dial indicator (accuracy 0.01 mm) showed an axial play of 0.28 mm, exceeding the limit of not more than 0.02 mm.
- Vibration analysis (spectral analysis): Peaks have increased by 14 dB at the roller frequency (BPFO / BPFI) compared to the baseline, indicating damage to the raceway.
- Visual inspection (10x): Micropitting and fatigue spalling were found on the screw raceway. The ball surface has lost its mirror finish, with abrasive scratches present.
- Lubricant condition: The grease inside the nut has mixed with metallic dust and abrasive particles from the slag, turning into a gray paste with increased viscosity.
Systematic analysis of root causes (5 Whys Method)
The root cause was determined using the "5 Why" tool in combination with an Ishikawa diagram:
- Чому виник люфт у КГП? Зносилися доріжки кочення та зменшився діаметр кульок через абразивне тертя.
- Why has abrasive wear occurred? Foreign hard particles (dust and scale) entered the nut housing.
- Чому бруд потрапив усередину захищеного вузла? Пошкодилися торцеві ущільнення гайки КГП.
- Чому ущільнення передчасно зруйнувалися? Гумові манжети втратили еластичність і порвалися через роботу в умовах сухого тертя та підвищеної температури (+75°C).
- Why did dry friction and overheating occur? The centralized lubrication system did not supply lubricant due to clogging of the fine filter.
Defined root causes (ranked list)
As a result of the investigation, the factors that led to the accident are distributed by degree of influence:
- Lubrication system failure (Probability 55%): Contamination of the lubricant channel and absence of lithium-based grease with anti-wear additives (EP) resulted in limiting friction and destructive heating of the node up to +85°C (exceeding the limit by 15°C).
- Contamination by foreign particles (Probability 30%): Penetration of slag through degraded seals. Particles acted as a grinding paste between the balls and the screw groove.
- Loss of pre-tension (Probability 15%): The pre-tension force of the nut was reduced due to thermal shock and micro-cracks, which accelerated impact loads during reversal.
Corrective actions: immediate elimination and long-term prevention
Emergency Measures (Urgent Repair):
- Full replacement of the CGP node Festo LSNG-63 with a new one according to the spare parts catalog.
- Cleaning and purging of lubrication lines under pressure 6 bar.
- Alignment of guides using a laser interferometer (parallelism tolerance according to ISO 10791-7: not more than 0.015 mm per 1000 mm).
Long-term prevention:
- Installation of additional telescopic protective covers and modernization of the air curtain around the hazardous cutting zone.
- Switching to synthetic grease class NLGI 2 with high load capacity and implementation of an automatic lubricator with pressure control.
- Integration of vibration and temperature sensors for continuous monitoring of bearing units and gearboxes.
Quick diagnostic checklist for technical specialists (Tablet-compatible)
This checklist is intended for monthly inspection of the condition of the KPP directly in the workshop:
- [ ] Check the presence and condition of the lubricating film on the screw's working surface. The lubricant should be even, without black spots.
- [ ] Measure axial play with a dial indicator (critical value > 0.05 mm).
- [ ] Evaluate the condition of the nut face seals for absence of cracks, breaks and deformations.
- [ ] Check the nut body temperature with a non-contact pyrometer (norm up to +65°C).
- [ ] Listen to the node using an electronic stethoscope for the presence of foreign metallic noises.
- [ ] Check the operation of the automatic lubrication system: actuation of the dispensers and pressure in the line (working pressure 30-50 bar).
- [ ] Clean open areas of the screw from residues of process liquid and dust using a special certified cleaner.
- [ ] Check the mounting of the screw bearings for axial play.
Preventive damage strategy
To ensure reliable operation of equipment on Ukrainian manufacturing enterprises, it is recommended to follow these maintenance intervals:
- Щоденно: Візуальний огляд цілісності захисних гофр та наявності мастила.
- Кожні 500 годин: Контроль рівня та якості мастила у гайці, перевірка люфтів.
- Кожні 2000 годин: Повне оновлення консистентного мастила з попереднім промиванням розчинником за стандартом ISO 12925-1.
- Кожні 10 000 годин: Плановий перерозподіл або заміна вузла КГП згідно з регламентом виробника.
Conclusion
Early failure of the ball-screw drive Festo LSNG-63 was caused by chain reaction: contamination of the lubrication system led to dry friction, overheating and damage to the seals, allowing abrasive particles to enter the nut. Adherence to maintenance schedule, control of lubricant cleanliness and use of high-quality original components allow avoiding equipment downtime. Select the necessary spare parts, precision screws and related elements in UNITEC-D E-Catalog.
References and Standards
- ISO 3408-3:2006 — Ball-screw drives. Part 3: Requirements for accuracy and measuring tolerances.
- ISO 281:2007 — Rolling Bearings. Dynamic Load Rating and Rated Life.
- DSTU EN 60204-1:2015 — Machinery safety. Electrical equipment of machines and machine tools.
- ISO 12925-1:2018 — Lubricants, Industrial Oils and Related Products.
- Festo Technical Documentation: LSNG Series Ball Screw Assemblies Maintenance Manual.
