System Architecture
Four Hydraulic Systems, One Shared Oil Quality Dependency
A rolling mill hydraulic plant serves four systems with different precision requirements, pressure levels, and failure mode consequences — all supplied from the same HPU. The AGC system demands the cleanest oil in the plant. The coiler cylinders are most tolerant of contamination. But the contamination level is set by the weakest link in the filtration chain, not by the most demanding consumer. A contamination event at any point in the shared reservoir circulates through the entire plant.
AGC
Automatic Gauge Control
200–315 bar · servo-hydraulic
Highest precision — 3–5 µm spool clearance
LPR
Looper Hydraulics
100–200 bar · proportional valve
Threading impact — accumulator critical
CLR
Coiler Systems
150–250 bar · cylinder-driven
Seal wear — cycle-count tracking required
AUX
Auxiliary Hydraulics
80–160 bar · directional valve
Lower precision — accumulator sizing key
Oil Cleanliness Requirement vs Typical Mill Operating Level
AGC servo valves
Target: 16/14/11
Typical mill: 18–19/16–17/13–14
Looper proportional
Target: 17/15/12
Typically within range with good filtration
Coiler cylinders
Target: 18/16/13
Typically achieved with standard filtration
The AGC cleanliness gap is the single highest-return maintenance improvement available in most rolling mills. Closing it from ISO 19 to ISO 16 at the 4µm channel doubles servo valve service life. OxMaint tracks weekly cleanliness per circuit.
AGC Hydraulic Maintenance
Automatic Gauge Control: Servo Valve Performance and Cylinder Monitoring
AGC servo valve degradation is a silent failure — the valve continues to control, but with progressively increasing dead band, hysteresis, and gain loss that produce thickness deviation in the strip before any alarm trips. Detecting this degradation requires two production-data methods: command signal trending (the signal required to maintain target position at consistent load) and periodic linearisation testing (flow vs command at defined setpoints). Both require consistent logging in OxMaint to produce the trend that makes the degradation visible 4 to 8 weeks before it produces a quality event. Sign up free and configure your first AGC servo valve trend today.
AGC
AGC Hydraulic System — Monitoring and PM Tasks
200–315 bar · ISO 16/14/11 target
Daily
Critical
Servo valve command-to-position ratio logging per stand
Log the servo valve command signal required to maintain target roll gap at a consistent rolling load. A rising command-to-position ratio indicates spool wear reducing valve gain — the first detectable degradation signal, visible 4 to 8 weeks before thickness deviation appears. Log per stand, per shift, at consistent rolling force.
Weekly
Critical
Servo valve linearisation test — flow vs command at 25%, 50%, 75%
Step each servo valve command to three setpoints and measure actual cylinder velocity. Compare against the commissioning baseline. Flow gain drop above 10% is the replacement threshold — schedule valve change at next planned roll change, not at next quality rejection. Log all three setpoints per stand per week in OxMaint.
Weekly
Critical
AGC accumulator nitrogen pre-charge verification
Verify pre-charge at all AGC accumulators with system at low pressure. Target: 60–70% of working pressure. A pre-charge below 50% of target removes the accumulator's pressure absorption function — pressure spikes on load changes transmit directly to servo valve spools, accelerating spool wear. Recharging takes 15 minutes.
Monthly
High
AGC cylinder rod seal inspection and position drift test
Inspect rod seal areas for weeping and apply maximum rolling force with zero servo command. Cylinder position drift above 0.1 mm per minute indicates piston seal bypass — the cylinder cannot hold position against rolling force, appearing as slow thickness drift that the servo valve partially compensates until it cannot.
Outage
High
Servo valve bench test and filter screen inspection
Bench test each valve against OEM specification — hysteresis, threshold, linearity, null bias. Inspect valve-mounted and last-chance filter screens. Analyse captured particles to identify contamination source: silica indicates external ingress, iron indicates internal wear, fibres indicate seal material breakdown.
Looper Hydraulic System
Looper Hydraulics: Accumulator, Proportional Valve and Cylinder Maintenance
The looper hydraulic system is the most dynamically loaded system in the rolling mill — it must absorb threading impact forces in milliseconds and then maintain precise torque control during the coiling sequence. These two requirements — fast energy absorption and precise proportional control — depend on different maintenance parameters. Energy absorption depends on accumulator pre-charge. Precise control depends on proportional valve performance. A maintenance program that addresses both on their correct frequency covers the full looper failure risk. OxMaint schedules looper PM at the correct frequency for each parameter.
LPR
Looper Hydraulic System — Monitoring and PM Tasks
100–200 bar · ISO 17/15/12 target
Weekly
Critical
Looper accumulator nitrogen pre-charge verification
Verify pre-charge at each looper accumulator with system depressurised. Target: 60–70% of working pressure. An accumulator at 40% pre-charge cannot absorb the threading impact force — the strip loses tension momentarily at stand entry, causing threading cobbles. Recharging costs 15 minutes. A threading cobble costs 2 to 4 hours of mill stop plus strip loss.
Monthly
Critical
Proportional valve linearisation and solenoid resistance check
Step the proportional valve command from 0 to 50% and measure cylinder response time and velocity. Compare against commissioning baseline. Also measure solenoid coil resistance — rising resistance in the hot mill environment indicates coil degradation preceding coil failure. Log both readings per looper per month.
Monthly
High
Looper cylinder rod seal and mounting pin inspection
Inspect rod seals for weeping — oil dripping on a hot looper in a hot rolling environment is a fire risk requiring same-shift repair. Check clevis pin wear by rocking the cylinder with system depressurised. Pin wear above 0.5 mm causes positioning error at the looper tip, appearing as camber variation in the rolled strip.
Quarterly
High
Accumulator bladder condition assessment
Check for bladder extrusion through the poppet valve port, rapid pre-charge loss between weekly checks (more than 0.5 bar per week), and oil in the nitrogen side. Oil in the nitrogen side indicates bladder perforation — replace immediately regardless of the quarterly schedule. Log all three checks per accumulator per quarter.
Coiler Hydraulic Systems
Coiler Mandrel, Wrapper Arm and Pinch Roll Hydraulic Maintenance
Coiler hydraulic cylinders operate at high cycle rates in the most thermally challenging part of the mill — near the strip exit where temperatures and steam exposure are highest. The mandrel expansion cylinder completes thousands of cycles per campaign; the wrapper arm cylinder engages and retracts on every coil. Seal wear in this environment follows a cycle-count pattern, not a calendar pattern — a campaign that produces 200 coils per shift degrades seals four times faster than one producing 50 coils per shift at the same calendar time. OxMaint tracks both cycle count and calendar time and alerts on whichever criterion reaches the threshold first. Configure your coiler cylinder seal life alert in OxMaint today.
CLR
Coiler Hydraulic Systems — Monitoring and PM Tasks
150–250 bar · ISO 18/16/13 target
Daily
Critical
Mandrel expansion pressure and final position logging per coil
Log mandrel expansion pressure and final position at each coil start. Rising pressure required to achieve full expansion at constant mandrel diameter indicates piston seal wear or contamination ingress. Position variation above 2 mm at full expansion means incomplete mandrel expansion — producing oval coil inner diameters and coil transfer car handling problems.
Daily
High
Wrapper arm position consistency at head engagement
Log wrapper arm final position at strip head engagement for each coil. Position variation above 3 mm between coils at the same gauge indicates cylinder seal bypass or valve gain drift. Consistent wrapper arm position is required for uniform initial wrap tension — variation produces loose inner wrap defects causing coil collapse during transit.
Monthly
Critical
Mandrel cylinder internal bypass test at full expansion pressure
Apply full expansion pressure and hold for 5 minutes with the supply isolated. Drift above 1 mm indicates piston seal bypass requiring seal replacement before the next campaign. A bypassing mandrel piston seal partially collapses the mandrel under coil weight during rotation — invisible during normal operation until the coil is stripped and the oval inner diameter is measured.
Monthly
High
Pinch roll gap symmetry check — drive vs operator side
Measure pinch roll gap at identical pressure setpoints on both sides. A gap difference above 0.5 mm indicates unequal cylinder bore wear or unequal supply pressure. Asymmetric pinch roll loading causes strip edge damage and inconsistent tension in the initial coiling phase — producing edge wave in the first few wraps of every coil.
Per campaign
High
Rod seal replacement at cycle-count or calendar interval — whichever fires first
Replace mandrel and wrapper arm rod seals at the configured cycle interval (typically 15,000 to 25,000 cycles depending on oil cleanliness) or at the calendar interval, whichever is reached first. Log removed seal condition in OxMaint — particle embed marks on seal faces indicate wiper seal failure as the ingress path, requiring simultaneous wiper seal replacement.
Oil Quality and Contamination Control
Hydraulic Oil Analysis, Filtration and Contamination Source Management
The single most impactful hydraulic maintenance improvement available in most rolling mills is not a new servo valve, a better accumulator, or an upgraded cylinder. It is achieving and maintaining ISO 16/14/11 oil cleanliness in the AGC circuit. Most mills operate at ISO 18/16/13 to 19/17/14 in the AGC circuit and accept the resulting servo valve service life of 12 to 18 months as normal. ISO 16/14/11 is achievable — mills that maintain it consistently achieve servo valve service lives of 36 to 60 months on identical valve models. The improvement comes entirely from filtration discipline, contamination source elimination, and weekly oil sampling that detects degradation before it crosses the servo valve action threshold.
Contamination Sources — Entry Points and Countermeasures
01
Worn cylinder wiper seals — water spray zone
AGC and looper cylinders operating in the roll cooling spray zone draw process water and mill scale through degraded wiper seals on each extension stroke
Monthly wiper seal inspection on all spray-zone cylinders. Replace before visible damage — wiper seal failure precedes rod seal failure by 2 to 4 weeks. Contamination from this source raises both particle count and water content simultaneously.
02
New oil addition without pre-filtration
Bulk hydraulic oil delivered to site is typically ISO 21/19/16 — far above servo valve requirements. Adding it directly to the reservoir introduces a contamination spike that takes weeks of system filtration to reduce
All oil additions must pass through a portable filtration cart to ISO 15/13/10 before entering the system. Log every oil addition event in OxMaint with the volume added and pre-addition cleanliness reading to correlate additions with cleanliness trend changes.
03
Saturated reservoir breather element
Humid mill atmosphere is drawn into the reservoir through the breather as oil level drops during operation. A saturated desiccant breather passes both moisture and particles
Replace desiccant breather monthly regardless of visual condition. The element that looks clean is already saturated internally — colour change indicators on cheap breathers are unreliable. Monthly replacement eliminates this ingress source entirely at negligible cost.
04
Oil cooler pinhole tube failure
A pinhole failure in the oil-to-water heat exchanger tubes allows cooling water to enter the oil circuit at the cooler operating differential pressure — detectable only by water content measurement
Monthly water content measurement from the reservoir is the only practical early detection method. A result that rises from 50 ppm to 150 ppm between monthly samples indicates an active cooler leak requiring investigation before the next sample — not at the next monthly review.
Oil Analysis Parameters — Monthly Sampling Program
Viscosity @ 40°C
VG 46: 41–51 cSt
Alert: ±15% from new oil
Thermal degradation, water contamination, wrong oil topping
Total Acid Number
< 0.3 mgKOH/g
Alert: > 0.5 mgKOH/g
Oil oxidation — plan oil change at next outage if trending above 0.5
Water content
< 100 ppm
Critical: > 200 ppm
Wiper seal failure, cooler tube leak, or condensation ingress
Iron particles
< 10 ppm baseline
Alert: > 30 ppm or rising trend
Internal component wear — correlate with vibration and valve performance data
ISO cleanliness class
AGC: 16/14/11
Alert: AGC above 18/16/13
Filter bypass, breather saturation, or new oil addition without pre-filtration
Foam tendency
ASTM D892 < 25/0 mL
Alert: increasing tendency
Antifoam depletion — excessive foaming causes pump cavitation at high demand
Hydraulic Power Unit
HPU Pump, Filtration and Cooling System Maintenance
The HPU is the origin of all hydraulic system condition — its pump output, filtration performance, oil temperature, and reservoir cleanliness determine what arrives at every servo valve and cylinder in the mill. A plant that maintains excellent servo valve performance records but neglects the HPU is building on sand: every downstream maintenance investment is conditional on the HPU supplying clean, cool, correctly pressurised oil at all times. HPU maintenance is not glamorous. It is foundational. OxMaint auto-schedules all HPU PM tasks with daily threshold alerts on pump performance, filter loading, and oil temperature.
PMP
Pump System
DailyOutlet pressure, flow and motor current vs baseline at constant system demandRising current at constant demand indicates volumetric efficiency loss — the earliest pump wear signal before pressure drop.
MonthlyPump bearing vibration at drive end and outlet port housingAxial vibration increase indicates shaft bearing wear; outlet vibration increase indicates vane or gear wear.
QuarterlyVolumetric efficiency test — flow at zero vs rated pressureBelow 92% efficiency requires pump overhaul or replacement before the next campaign.
FLT
Filtration System
DailyFilter differential pressure — all banksSchedule element change at 70% of bypass pressure. A filter at bypass allows unfiltered oil to circulate through the entire system.
Per changeFilter element particle analysisRinse removed elements and send for particle type analysis — silica, iron, fibres each indicate a different contamination source to investigate.
MonthlyDesiccant breather element replacementReplace regardless of visual condition — internal saturation precedes visible colour change. Monthly replacement eliminates this ingress source at negligible cost.
CLR
Cooling and Reservoir
DailyOil temperature at reservoir — inlet/outlet delta to coolerRising approach temperature indicates cooler fouling. Above 85°C servo valve seal degradation accelerates exponentially.
MonthlyWater content measurement — cooler tube leak detectionA jump from 50 ppm to 150 ppm between monthly samples is the only practical early indicator of a cooler tube pinhole failure.
AnnualReservoir internal inspection and sludge removalAccumulated sludge resuspends during flow surges and causes downstream filter loading spikes. Log sludge volume and composition against previous years.
OxMaint Platform Features
OxMaint Features for Rolling Mill Hydraulic Maintenance
Sign up free and deploy your first hydraulic system inspection program within 48 hours. No IT integration, no hardware, no implementation project — runs on the smartphones your hydraulic and process engineers already carry.
Servo Valve Trending
Command Signal and Linearisation Records
Daily command-to-position ratio logged per stand. Weekly linearisation test results stored per valve identity and trended. When the trend shows a valve approaching the 10% gain loss threshold, OxMaint generates a replacement work order for the next scheduled roll change — preventing quality rejection events from servo valve degradation.
Per-valve trendGain alertPlanned replacement
Oil Cleanliness
ISO Class Tracking Per Circuit
Weekly ISO cleanliness results logged per sample location — AGC circuit, looper circuit, coiler, reservoir. Individual alert thresholds per circuit. When AGC circuit crosses ISO 18/16/13, OxMaint schedules an offline filtration campaign work order within the same week. No waiting for the next monthly maintenance meeting.
Per-circuit samplingFiltration trigger
Accumulator Management
Pre-Charge Trending and Bladder Life
Weekly pre-charge readings per accumulator trended for loss rate. An accumulator losing 0.5 bar per week is flagged for bladder inspection — the maintenance action is bladder replacement at the next access, not weekly recharging of a failing bladder. Quarterly bladder condition check results stored per unit.
Loss rate alertBladder life tracking
Cylinder Seal Life
Cycle Count and Calendar Dual Trigger
Coiler mandrel and wrapper arm cylinder seal replacement tracked by both production cycle count and calendar time — whichever threshold is reached first triggers the work order. Monthly bypass test results logged per cylinder. Removed seal condition recorded on replacement to identify upstream contamination causes.
Dual-trigger alertBypass test records
HPU Automated PM
Filter Delta-P and Pump Performance
Daily pump pressure, filter delta-P, and oil temperature auto-compared against configured baselines. Filter element change work order auto-created at 70% of bypass pressure. Pump volumetric efficiency test scheduled quarterly with structured result logging. Breather replacement auto-created monthly regardless of condition.
Pre-bypass filter alertEfficiency scheduling
Mobile at HPU
Offline-Capable Floor Execution
All daily readings completed on smartphone at the HPU or valve cabinet — servo valve command signals, filter delta-P, accumulator pre-charge, cylinder positions. Offline for network-limited HPU rooms and below-grade pump installations. Numeric threshold validation flags deviations in real time before the technician leaves the equipment.
iOS and AndroidOffline capableReal-time alerts
FAQ
Rolling Mill Hydraulic Maintenance: FAQ
What ISO oil cleanliness is required for AGC servo valves and why is it so hard to achieve?
AGC servo valves require ISO 16/14/11 or cleaner — a demanding standard that requires a disciplined filtration program to achieve consistently. The difficulty comes from four compounding factors: high particle generation from the rolling mill environment, multiple contamination ingress points (wiper seals, breather, new oil additions), inadequate standard filter ratings for servo valve circuits (most standard return-line filters are rated beta 10 rather than beta 3 at 3 µm), and the lack of weekly sampling that would detect degradation before it crosses the servo valve threshold. Mills that achieve ISO 16/14/11 consistently use dedicated high-beta filtration on the AGC circuit, pre-filter all oil additions, replace breather elements monthly, and sample the AGC circuit weekly rather than monthly.
OxMaint tracks weekly ISO cleanliness results per circuit with circuit-specific alert thresholds.
How is AGC servo valve degradation detected before it causes a quality rejection?
Two production-data methods detect servo valve degradation without removing the valve. Command signal trending logs the servo valve command required to maintain target position at a consistent rolling load — a rising ratio over 4 to 8 weeks indicates gain reduction from spool wear, detectable before any thickness deviation. The linearisation test steps the valve command to 25%, 50%, and 75% of rated flow and measures actual cylinder velocity, comparing against the commissioning baseline. A 10% flow gain reduction is the replacement threshold. Both methods require consistent logging in a CMMS to produce the trend — a single reading is not diagnostic, but 8 to 12 weekly readings in OxMaint reveal the degradation curve clearly.
Why does accumulator pre-charge loss cause looper threading failures?
The looper accumulator absorbs the impact energy when the strip front end reaches the next rolling stand during threading. This impact applies a sudden force that tries to collapse the looper — the accumulator must supply additional oil to the looper cylinder fast enough to maintain the looper angle against this force. An accumulator at full pre-charge responds in milliseconds. An accumulator at 40% of design pre-charge has far less stored energy and responds much more slowly — the looper partially collapses, the strip loses tension momentarily at stand entry, and the threading angle changes, causing a cobble that requires 2 to 4 hours to clear. The fix is 15 minutes of nitrogen recharging per week. OxMaint creates this task automatically every week and escalates immediately if it is overdue — because the cost asymmetry makes deferral economically irrational.
How should coiler cylinder seal replacement be scheduled?
Coiler cylinder seal replacement should be scheduled by whichever of two criteria is reached first: production cycle count (typically 15,000 to 25,000 expansion cycles for mandrel cylinders, depending on oil cleanliness and temperature exposure) or calendar time (typically 12 to 18 months). Using cycle count alone ignores the thermal and chemical degradation that occurs at rest. Using calendar time alone ignores the actual mechanical wear accumulated during high-production periods. OxMaint tracks both simultaneously and fires the replacement work order on whichever trigger fires first. Log the removed seal condition at every replacement — seals showing particle embedding indicate wiper seal failure as the ingress path, requiring simultaneous wiper seal replacement to prevent recurrence.
How quickly can OxMaint be deployed for rolling mill hydraulic maintenance?
Most rolling mill hydraulic maintenance teams are running digital daily rounds — pump pressure, filter delta-P, oil temperature, accumulator pre-charge, servo valve command readings — in OxMaint within 48 hours of account creation. The hydraulic asset register (HPU, each AGC cylinder and servo valve per stand, looper assemblies, coiler mandrel and wrapper arm cylinders) is configured from the existing equipment list in a few hours. PM templates for daily monitoring, weekly servo valve checks, and monthly oil analysis are built from current paper procedures. No IT integration and no hardware required — OxMaint runs on the smartphones your hydraulic and process engineers already carry on the mill floor.
Sign up free and register your first hydraulic system today.
