1. Introduction
During a scheduled inspection of the hot stamping line at one of the Ukrainian metallurgical enterprises, a critical leakage of working fluid was detected through the piston rod of the Bosch 200737119 hydraulic cylinder. The nominal working pressure in the system is 210 bar, peak values reach 315 bar. The mean time between failures (MTBF) for this unit is specified by the manufacturer as 15 000 hours. However, an emergency failure occurred already after 4 200 hours since the last major overhaul. Similar incidents lead to production stoppages, pressure loss in the pipeline and contamination of the environment with mineral oil lubricant HLP ISO VG 46.
2. Component Overview
Hydraulic cylinder Bosch 200737119 is a double-acting piston type actuator with single rod. It is used in heavy machinery for clamping forces up to 250 kN.
- Робоче середовище: мінеральна олія за ISO 11158 (HLP 46)
- Oil temperature range: from -20°C to +80°C
- Maximum rod speed: 0.5 m/s
- Ущільнювальний вузол: багатокомпонентна система з поліуретановими брудоспівачами, манжетами штока та напрямними кільцями з бронзованого фторопласту (PTFE)
Operating conditions on Ukrainian plants are often accompanied by a high level of dustiness with abrasive particles of slag and cyclic overloads.
3. Failure Evidence
Visual and metrological inspection of the disassembled unit in the workshop revealed the following defects:
- Wear of the primary seal edge of the rod with an asymmetric profile over 360 degrees with wear tilt upwards by 0.15 mm.
- Presence of longitudinal microgrooves on the working surface of the chrome-plated coating up to a depth of 0.02 mm.
- Extrusion signs (extrusion) of the support ring into the gap between the piston rod and the cap from the high-pressure side.
- Oil analysis by ISO 4406 showed a cleanliness class of 21/19/16, which exceeds the permissible limit of 18/16/13 for high-pressure hydraulics.
4. Root Cause Analysis
For determining the source of the fault, the Fault Tree Analysis method was used. Three main hypotheses were considered: abrasive contamination, mechanical tilt, and hydraulic shocks (pressure spikes).
| Hypothesis | Probability | Verification Evidence | Proofing evidence |
|---|---|---|---|
| Contamination of working fluid | High | Increased content of solid particles due to ISO 4406, microscratches on the piston rod | Irregular wear pattern around the perimeter |
| Mechanical stem tilt | High | Asymmetric wear of bushing rings, signs of fretting corrosion | No axial runout on the test stand |
| Hydraulic pressure jumps | Average | Extrusion of polymer support rings, pressure peaks on the pressure gauge | Absence of cylinder body rupture |
5. Root Causes Identified
- Abrasive wear due to degradation of the dust seal (Probability: 85%): Entry of dust and debris through the damaged protective seal resulted in accumulation of abrasive material under the seal.
- Radial misalignment of the unit (Probability: 70%): Misalignment between the rod eye and the guide ways caused a local pressure on the seal edge exceeding 15 N/mm².
- Hydraulic shocks (Probability: 45%): Rapid switching of proportional distributors generated pressure waves up to 340 bar, causing cyclic deformation of the polymer.
- Removal of Bosch cylinder 200737119 and replacement of seal kit with certified ones according to DIN ISO 3601.
- Grinding and re-chroming of the rod or its replacement with f7 tolerance.
- Hydraulic reservoir cleaning and filter element replacement with a filtration accuracy of β10 >= 75.
- Installation of additional telescopic protective housings to prevent splatter from reaching the piston.
- Laser alignment of the cylinder according to the requirements ISO 10720 (parallelism tolerance not more than 0.05 mm per meter).
- Modernization of the hydraulic accumulator system for damping peak hydraulic pressure surges.
- [ ] Check the external condition of the chrome-plated rod coating for the presence of scratches and corrosion.
- [ ] Measure the cylinder housing temperature with a pyrometer (limit up to +65°C).
- [ ] Assess the vibration level on the cylinder bushings (vibration velocity not exceeding 2.8 mm/s per ISO 10816-3).
- [ ] Check the condition of the bladder and air filter of the hydraulic accumulator.
- [ ] Carry out an oil express test for the presence of free water and mechanical contaminants.
- [ ] Read the pressure sensor readings for the presence of peak jumps.
- [ ] Check the bolt tightness of the flange and cylinder bushing with a torque wrench.
- [ ] Assess the condition of the wiper for elasticity and tightness of contact with the rod.
- Monthly monitoring: Oil sample analysis for cleanliness according to ISO 4406; filter element replacement when pressure drop exceeds 2.5 bar.
- Quarterly inspection: Inspection of seals for microleakage; checking of fastenings for vibration resistance.
- Annual audit: Full geometry check of the rod travel and measurement of radial clearances in the sliding bearings.
- ISO 4406: Hydraulic fluid power - Fluids - Method for coding the level of contamination by solid particles.
- ISO 10720: Hydraulic fluid power - Cylinders - Mounting dimensions.
- DIN ISO 3601: Fluid power systems and components - O-rings.
- Bosch Rexroth Technical Documentation: RE 17038 / 05.11 - Hydraulic Cylinders for Industrial Applications.
6. Corrective Actions
Urgent Measures
Long-term prevention
7. Quick Diagnostic Checklist for Field Engineer
This checklist is intended for use on a tablet directly next to the equipment:
8. Failure Prevention Strategy
To achieve the design MTBF of 15 000 hours on Ukrainian production sites, the following maintenance program must be implemented:
9. Conclusion
Premature failure of hydraulic cylinders in heavy industry is the result of the combined effect of abrasive contamination and mechanical misalignment, which are intensified by pressure surges. The use of quality components and adherence to maintenance schedules guarantee stable operation of the equipment. To select original spare parts, seals and hydraulic components, please visit UNITEC-D E-Catalog.
