Heavy-duty rolling mill hydraulic gap control system
Hydraulic gap control (HGC) for rolling mill stands · Servo-valve-driven gap cylinders in closed-loop position or force control, with AGC, tension and flatness offsets from Level 2, eccentricity compensation, redundant position transducers, side-to-side differential control and safe fault modes
A hydraulic gap control (HGC) system sets and holds the gap between the work rolls of a rolling mill stand with hydraulic cylinders driven by servo valves, so strip or plate thickness can be corrected during rolling. Ask three things first: which stand, roll force and cylinder arrangement it must control, which Level 2 functions (gauge, tension, flatness, eccentricity) will send corrections, and how position feedback, side-to-side balance and fault handling will work.
Hydraulic gap control for flat rolling mill stands: gap cylinders, servo valves, position transducers, hydraulic supply and the Level 1 control that regulates position and force. Mill housings (see mill housing), work rolls (see heavy-duty rolling mill work roll), main drives and plate rolling machines are outside this page.
CNFX editorial research · Sources checked October 11, 2026 · 4 sources
TelePro describes its hydraulic mill stand controller as Level 1 control of the cylinders for work roll gap positioning, roll bending and backup roll balancing. The gap cylinders sit at the top or bottom of the stand, and the HGC control performs closed-loop position and pressure regulation of them depending on the operating mode. [1]
During setup it receives preset position and roll-separating force references from the Level 2 setup system or the operator; during rolling, Level 2 sends average and differential position or force offsets for automatic gauge control (AGC), interstand tension control (ITC) and automatic flatness control (AFC). Roll eccentricity compensation offsets need a 1 ms scan and may arrive through hardwired analogue inputs. [1]
Its HGC modes are position mode (closed-loop position), force mode (closed-loop force), velocity mode (cylinders retract at a fixed rate on HGC stop), vent mode (full-open reference to the servo valves to dump pressure) and fault mode (null reference to hold the cylinders), with the mode selected automatically from the mill stand state. [1] Bosch Rexroth supplies complete hydraulic systems and components for steel, aluminium and other rolling mills, in which stand functions such as automatic gap control are implemented hydraulically. [2]
A 2023 study of a 5,500 mm medium and heavy plate finishing mill gives the scale of one installed HGC. Its HGC cylinder is mounted at the bottom of the stand, under the lower backup roll, one of the two installation positions (upper or lower) the authors describe. The electro-hydraulic servo valve opening sets oil flow to the cylinder, and a displacement sensor feeds back piston position. The authors tuned the position loop on a semi-physical simulation using a Siemens S7-1500 PLC; against Ziegler–Nichols tuning, their improved particle-swarm PID cut overshoot by 14.26%, brought the peak 0.31 s earlier and shortened settling by 3.71 s for the same roll-gap step. [4]
Give the supplier the mill type and stand arrangement, maximum roll force, cylinder position (top or bottom) and stroke, hydraulic pressure available, required modes, Level 2 interfaces (setup, AGC, ITC, AFC, eccentricity compensation), scan times, position transducer type, roll bending and balancing scope, and the commissioning and data recording needed.
02
Specify feedback, side-to-side control and protection
In TelePro's design, independent position or force loops regulate the operator-side and drive-side cylinders, and a supervisory differential loop holds the required offset between the sides, correcting differences in dynamic response; excessive differential force puts the HGC into fault mode and stops further movement. [1]
Each cylinder carries two position transducers, inside and outside, with separate signal conditioners; their signals are averaged, one is used alone if the other faults, and the system goes to fault if both fail. The same controller handles work and intermediate roll bending and backup roll balancing in closed-loop pressure control, and records trend data at 1 ms (1,000 samples per second per point). [1]
03
Cylinder, servo valve, transducer and control data to ask any HGC supplier for
Ask any supplier for cylinder bore, stroke and force rating, servo valve type and response, position transducer type and resolution, hydraulic pressure and flow, oil cleanliness requirement, control hardware and scan times, Level 2 interface, operating and fault modes, references, and commissioning scope.
04
Put servo valve pressure, response and oil cleanliness on the order
The gap cylinders are only as precise as the servo valves metering oil to them, and a servo valve catalog shows which figures to fix. Moog's D661-G....A catalog describes two-stage servo-proportional flow control valves for electrohydraulic position, velocity, pressure or force control, and lists heavy industry equipment among their uses. [3] It rates ports P, A and B up to 350 bar, gives 0 to 100% stroke response times of 8.0, 14.0 and 18.0 ms across its rated-flow groups (20 to 200 l/min at 35 bar per land), and states that these figures are measured at 210 bar, 32 mm²/s viscosity and 40 °C fluid temperature. [3] Ask any HGC supplier for valve response measured under stated conditions like these, not a bare number.
Moog states that fluid cleanliness greatly affects spool positioning, resolution and wear, and recommends ISO 4406 < 18/16/13 for normal operation and < 16/14/11 for longer life, with filter ratings of ß15 ≥ 75 (15 µm absolute) and ß10 ≥ 75 (10 µm absolute) respectively, using a high pressure filter without bypass but with a dirt alarm, upstream of the valve if possible. [3]
The same catalog limits ambient temperature to -20 °C to +60 °C and fluid temperature to -20 °C to +80 °C, recommends 15 to 100 mm²/s viscosity (5 to 400 mm²/s allowed) on mineral oil to DIN 51524 parts 1 to 3, and rates the valve IP 65 with its mating connector fitted. Its 24 VDC electronics provide a 4-20 mA spool position signal usable for cable break detection. [3] Use these as the fields to fill for the valves in your HGC quotation.
05
Gap cylinders, servo valves, position transducers, Level 1 controller, Level 2 interface and hydraulics
Redundancy sits in the transducer row: TelePro fits two position transducers per cylinder with separate signal conditioners, averages their signals and falls back to one if the other faults. [1]
Stroke, roll force, hydraulic pressure, response, oil grade and cleanliness for an HGC order
The hydraulic and electrical rows come from one servo valve catalog rather than a complete HGC: Moog rates the D661 ports to 350 bar and asks for ISO 4406 < 18/16/13 oil, so check them against the valve actually offered. [3]
Field
What it means and published values
Ask the supplier for
Maximum Roll Gap Adjustment (mm)
85 mm effective piston stroke on one 5,500 mm plate mill. [4]
Stroke.
Positioning Accuracy (μm)
Not stated in the reviewed sources.
Accuracy.
Maximum Hydraulic Pressure (bar)
Servo valve ports rated up to 350 bar in one catalog. [3]
Pressure.
Response Time (ms)
Eccentricity offsets at a 1 ms scan in one design [1]; servo valve natural frequency 73 Hz in one mill model. [4]
Loop response.
Maximum Roll Force (kN)
100,000 kN (100 MN) on one 5,500 mm plate mill. [4]
Force.
Operating Temperature Range (°C)
Servo valve ambient −20 to +60 °C, fluid −20 to +80 °C in one catalog; system range not stated. [3]
Range.
Hydraulic Fluid Viscosity Grade
Mineral oil to DIN 51524 parts 1 to 3, 15–100 mm²/s recommended for one servo valve. [3]
Oil grade.
Fluid Cleanliness Level
ISO 4406 < 18/16/13 (normal) or < 16/14/11 (longer life) for one servo valve. [3]
ISO code.
Electrical Supply Voltage (V DC)
24 V DC servo valve electronics in one catalog; system supply not stated. [3]
Supply.
Control Signal (mA)
Hardwired analogue inputs for eccentricity offsets. [1]
Signal type.
Ingress Protection Rating
IP 65 for one servo valve with its mating connector fitted; system rating not stated. [3]
Rating.
System Weight (kg)
Not stated in the reviewed sources.
Weight.
07
Send the mill, force and control data together
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What does hydraulic gap control regulate?
TelePro's HGC performs closed-loop position and pressure regulation of the work roll gap cylinders. [1]
Which corrections reach the gap control during rolling?
Level 2 offsets for automatic gauge, interstand tension and flatness control, plus eccentricity compensation. [1]
How is a transducer failure handled?
With two transducers per cylinder, one is used alone if the other faults; if both fail, HGC goes to fault mode. [1]
What oil cleanliness do HGC servo valves need?
Check the valve maker's figure: Moog's D661-G....A catalog, for example, recommends ISO 4406 < 18/16/13 for normal operation and < 16/14/11 for longer life. [3]
How large are the cylinders and forces in a heavy plate mill HGC?
One 5,500 mm plate finishing mill studied in 2023 has a 100 MN maximum rolling force, a bottom-mounted HGC cylinder with an 85 mm effective stroke and a 1,500 mm piston rod, and a 73 Hz servo valve. [4]
Sources and what still needs a direct check
Sources checked 2026-10-11. TelePro documents a mill stand HGC controller; Bosch Rexroth supplies rolling mill hydraulic systems; Moog publishes servo valve data; a 2023 Xi'an University of Architecture and Technology paper gives the parameters of a 5,500 mm plate mill HGC.
