Mill-type hydraulic cylinder for hot rolling mill auxiliaries: 250 bar eye-mounted cylinders, their seals near hot steel, and stroke speed by bore

01

Rod seals beside the hot strip: the 80 °C and 120 °C rod-zone bands, water glycol and ester fluids

Bosch Rexroth's mill-type data sheet RE 17332 (CDH1/CGH1/CSH1, 2021 edition, marked blocked for new applications) splits its seal selection table by the temperature around the piston rod, not only the oil: one band for ambient and rod temperature of –20 to +80 °C, and an extended band of +80 to +120 °C with its own rating for each seal version, from suitable to inappropriate. The same table has a row for robust application conditions in steel works and mining, and recommends a medium temperature of about 40 °C. [1]

Danfoss notes that its T high-temperature seals function up to 200 °C but that the wear band strength reduces accordingly; the rated range it prints for the T system is –10 to 80 °C. [2] Read the two figures separately: the printed range is the one the catalogue rates, and the 200 °C figure is a limit on seal function with weaker wear bands.

Rexroth's ordering code reflects the same split: a standard seal system for mineral oil, HFA and water glycol HFC, and a separate one for HFDR phosphate ester and HFDU polyol ester. [1] Rexroth also lists the hazards to keep away from the cylinder, among them aggressive fine particles, vapours and high temperatures. [1]

Where the rod zone runs hotter than the seal band, Milwaukee Cylinder's engineering leader Douglas Lacina, reported by Fluid Power World, ranks the measures: move the cylinder away from the heat or put walls, barriers or insulation between them; then change seal materials, citing nitrile and polyurethane at 180 to 200 °F, ethylene propylene and Viton to 350 °F and PTFE to 500 °F; then water cooling, with a water jacket in the barrel and cooling at the rod bearing for rods that extend into a furnace. He also advises keeping the rod retracted except for short extensions and adding reflective boots against radiant heat. [3] Editorial calculation: 180 °F is about 82 °C and 350 °F about 177 °C, so Lacina's standard-seal figure sits at the edge of Rexroth's +80 °C rod-zone band.

State the highest temperature at the rod where it passes the head, the radiant source and its distance, how long the rod stays extended, and the fluid type with its operating temperature. Ask which seal code covers that combination and whether shielding, a rod boot or water cooling is included.

02

Plain eye or spherical eye on a swinging mill linkage, and how long a pinned cylinder may be at 250 bar

Danfoss's IM25 catalogue gives the rule for the eye at the cylinder base: a plain eye (MP3) is for a machine member that swings in one plane, and any misalignment causes excess side loading and premature bearing failure; a spherical-bearing eye (MP5) is for the same one-plane swing where some misalignment is unavoidable, within the tilt angle listed for that mounting. Both carry push and pull. [2] Rexroth adds that the own weight of an eye- or trunnion-mounted cylinder may have to be considered, and that side forces on the rod and piston guides are to be avoided. [1]

The lengths in the Danfoss buckling chart include stroke, rod accessories and extra rod projection, are for push strokes, and do not consider side loading or stop tubes; above 250 bar the factory is to be consulted. [2] Editorial calculation from the table: at 250 bar a 100/70 cylinder pinned at both eyes may be 1,850 mm long, about 0.71 of the 2,616 mm allowed with a fixed flange or foot mount, so a long-stroke eye-mounted cylinder on a guide or carriage reaches its buckling limit sooner.

Danfoss recommends the largest rod offered for a bore for maximum reliability and fatigue life of the rod, keeping the smaller rods for short push strokes or reduced pressure, and limits the force lost to seal friction and piston inertia to 5 % or less of the theoretical force. [2]

Give the pin-to-pin length retracted and extended, the swing angle and whether it stays in one plane, the push and pull loads, and the expected misalignment, so the supplier can choose between a plain and a spherical eye and check buckling.

03

Stroke speed against bore size, force at 250 bar and the 375 bar test

Large mill-type bores move slowly through their standard ports. Rexroth's guideline values, based on a recommended flow velocity of 5 m/s in the line connection, fall from 0.31 m/s for a 40 mm piston on a G1/2 port to 0.11 m/s at 200 mm and 0.06 m/s at 280 mm on G1 1/2, with higher speeds on request. [1] Danfoss gives a maximum rod speed of 1.0 m/s for the IM series, and 0.5 m/s for its normal seal system. [2]

Editorial calculation from the Danfoss table: a 200/140 cylinder pulls with 400.55 kN, about 51 % of its 785.40 kN push, so a linkage that does its heavy work on the retract stroke needs a larger bore or a smaller rod than the push figure suggests. At Rexroth's 0.11 m/s guideline for a 200 mm piston, a 1,000 mm stroke takes about 9 s.

Both series are proof-tested above the 250 bar rating: Danfoss gives a proof pressure of 375 bar and tests to ISO 10100; Rexroth gives a static test pressure of 375 bar and a reduced test pressure of 315 bar, and states that operating pressure must not exceed the nominal 250 bar, including pressure intensification from the annulus-to-piston area ratio and throttling. [2] [1] Rexroth also asks for a minimum pressure of about 10 bar, fluid cleanliness to ISO 4406 class 20/18/15 and viscosity between 12 and 380 mm²/s, with 20 to 100 mm²/s as the optimum. [1]

For outdoor bays and wet scale areas, Rexroth's corrosivity classes cover only primed or painted surfaces and not the piston rod or trunnions, which need special measures. [1] Danfoss offers case-hardened chrome-plated rods in carbon or alloy steel, heavier plating as a custom option and stainless steel rods including 17-4. [2]

Send the required stroke time, both directions' loads, the supply pressure at the cylinder port, and whether flanged ports or a valve mounted on the cylinder are wanted to reach the speed.

04

What a mill auxiliary cylinder enquiry needs

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Duty: [side guide, roll-change carriage, cover, manipulator; push and pull loads]
Mounting: [plain or spherical eye, flange, trunnion, foot; pin-to-pin length; swing angle]
Size: [bore; rod; stroke; stroke time]
Heat: [rod-zone temperature; radiant source and distance; time extended]
Fluid: [mineral oil, HFC, HFDR or HFDU; operating temperature]
Protection: [shield; rod boot; water cooling; rod material and plating]
Tests: [proof pressure; acceptance test standard]

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