Float glass annealing lehr roller (lift-out and lehr rolls)

01

Match the roll to its lehr zone and temperature

US patent 3,963,469 (Glass sheet annealing lehr) describes a lehr of about 120 m in which float glass enters at about 600 °C and leaves at about 150 °C; the temperature falls very slowly down to 450 °C, to dissipate stresses, and much faster thereafter. In the hot part the rollers sit at the surrounding temperature and the patent says heat exchange with the walls "occurs primarily by radiation", keeping the rollers at a homogeneous temperature; in the cooler part heat is exchanged by convection. [1]

US patent 4,293,335 (Roll for manufacturing float glass) explains that the lift-out roll, the first one to three conveyor rolls at the tank outlet, contacts the ribbon at about 550–650 °C and touches molten tin carried on the glass, so it needs high-temperature strength and oxidation and corrosion resistance. It says "the lift-out roll is usually made of a product of JIS-SCH22 or SCH21 heat-resistant cast steel", corresponding to ASTM HK40 or HK30 (25Cr-20Ni), with 0.2–0.45% C, 19–22% Ni and 23–27% Cr, and proposes adding 0.01–2.0% rare earth metals and 0.1–3.0% Al for better resistance. [2]

US patent 4,260,404 states that "temperatures of 900° to 1600° F. (482° to 871° C.) are not uncommon in the glass making operation", that stainless rolls of 20–25% Cr and 12–20% Ni may be used, and that "typical roll life may vary from about a week" for take-off rolls at 649–871 °C "to several months" for lehr rolls at 93–316 °C, depending on location. [3]

Editorial calculation: cooling from about 600 °C to 150 °C over about 120 m is an average of 450 / 120 ≈ 3.75 °C per metre, though the patent states the drop is much slower above 450 °C; with rolls spaced about 0.6 m apart (the spacing that patent gives for its cooler section, assumed here for the whole lehr), a 120 m lehr would hold about 200 rolls. [1]

Give the supplier the lehr zone and roll position (lift-out, hot zone, cold zone), glass temperature at that position, ribbon width and thickness, roll body length and diameter, journal and drive end drawings, line speed, current alloy and roll life, marking defects seen, and the straightness, runout and surface finish required.

02

Prevent roll imprint, marking and bending

US 4,293,335 explains that lift-out rolls in contact with tin at 500–600 °C corrode and oxidise to a rough surface, and that this roll imprint is transferred to the glass as a surface defect; frequent roll changes then cut productivity and raise cost. [2] US 4,260,404 adds that metal oxide and tin oxide deposits on rolls can imprint the glass, and that asbestos rolls deform with continuous hot contact, leaving out-of-round rolls that scuff the glass. It proposes coating stainless rolls in place with an aqueous boron nitride composition whenever marking appears, so the rolls can stay in service. [3]

US 3,963,469 describes how, in the cooler zone, uneven convection cooling makes the same parts of each roll run hottest, so rolls bend and the resulting temperature differences may fracture the ribbon. It proposes stainless rolls with annular or helical ridges in that zone, staggered from roll to roll so heat losses are spread evenly, with smaller diameters (preferably below 305 mm for 4 m rolls spaced about 0.6 m apart) to improve convection. [1]

Editorial note: agree with the supplier how straightness and surface will be checked on delivery and after heating, and whether rolls are supplied ready for in-place coating or regrinding.

03

Lift-out, stainless, non-asbestos and donut rollers on the listed page

The supplier reviewed lists annealing lehr rollers, lift-out rollers, stainless and non-asbestos lehr rollers and donut rollers; ask for the alloy and its standard, casting route, machining tolerances, surface finish and reference lines.

04

Know the annealing range the lehr rolls carry the glass through

In a 2015 Lehigh University IMI-NFG glass processing lecture, Mathieu Hubert of CelSian Glass & Solar explains that internal stresses are relieved in about 15 minutes at the annealing point but about 15 hours at the strain point. Stresses remaining from above the strain point are permanent unless annealed, so cooling must be slow between the two points; below them, faster cooling causes only temporary stress. He gives the annealing range of soda-lime-silica glass as about 20–30 °C (about 515–545 °C). [4]

For a permitted permanent stress of 1 MPa, his worked example limits the cooling rate in that range to 6 K/min for 10 mm plate glass cooled on both sides and 36 K/min for 4 mm plate, and he states that these cooling-rate limits determine the length and temperature profile the lehr needs. [4]

Editorial calculation: crossing a 30 °C range takes 30 / 6 = 5 minutes at 6 K/min and about 0.83 minutes at 36 K/min. Lift-out rolls run at about 550–650 °C [2], above this range, and bent rolls can fracture the ribbon [1]; ask for straightness and runout at each roll position's temperature. [4]

05

Roll body, surface, alloy, journals and internal medium

The surface and alloy rows carry the glass-quality risk: US 4,293,335 traces roll imprint to oxidised, tin-corroded surfaces, and US 3,963,469 adds staggered ridges in the cooler zone so rolls lose heat evenly and do not bend. [2] [1]

06

Diameter, length, alloy, temperature rating and straightness for a lehr roll

Diameter, length, temperature and alloy have published examples from the three patents; load capacity, speed, thermal shock data and roll weight have to be quoted for your line.

07

Send the zone, size and quality data together

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

Position: [lift-out; hot zone; cold zone]
Temperature: [glass at roll]
Size: [body diameter; length; journals]
Alloy: [grade; chemistry]
Surface: [Ra; ridges; coating]
Tolerances: [straightness; runout]
History: [current roll life; marking defects]

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