Purlin Rollforming Machine

Overview

A purlin rollforming machine turns a flat coil of galvanized steel into the C and Z section purlins that carry the roof and wall sheets of steel buildings. A purlin is a long thin beam with a folded section: a web with two flanges and a small return lip on each flange, which together give a light strip of steel enough stiffness to span between rafters. The machine makes the section by pulling the strip through a line of paired rolls, each of which bends the metal a little more than the last, so that by the end of the line the once-flat strip has taken its full C or Z shape. The same coil that enters as a flat band leaves as a finished structural member with bolt holes already in place, cut to the exact length the building needs.

The line is built around a few large subsystems. A Decoiler and Coil Car holds the coil and pays it off. A Feeding and Leveling Unit flattens and aligns the strip, a Pre-Punch Press stamps the holes, the Forming Stations do the bending, a Drive System turns every roll, a Hydraulic Shear cuts each piece to length, and a Run-Out Table catches the finished purlins. A Control Cabinet ties the sequence together, a Lubrication System system keeps it running, and all of it bolts to a heavy Machine Frame.

How it works

The strip travels in one direction through the whole machine, so the work is done in a continuous pass rather than in separate setups. The Expanding Mandrel of the decoiler expands inside the coil bore and rotates as the line pulls strip off it, while a Tension Brake holds back overrun so the strip stays taut. The Leveling Roll bend the strip back and forth in a staggered set to take out the curl it kept from the coil. The flat strip then runs into the punch press, where a Punch and Die Set driven by the Punch Cylinder stamps bolt holes and end notches while the strip is still flat and easy to pierce. From there the strip enters the forming line, where each Forming Stand adds a few more degrees of bend until the section is complete. A Length Encoder counts how much strip has passed, and when the set length is reached the Hydraulic Shear cuts the piece free.

Forming stations

The forming line is the heart of the machine and the part that defines what section it can make. Each Forming Stand carries an upper and a lower Forming Shaft, and on those shafts sit profiled Forming Roll pairs. The first stands lift the flanges only a little, the middle stands bring the flanges most of the way up, and the last stands roll the return lips and set the final angles. Bending the metal gradually keeps the surface smooth and stops the strip from cracking or twisting. Because purlins come in several depths and the two flanges are not always equal, the line is adjustable: a Spacing Screw moves the roll columns in or out, and a Width Spacer Set set repositions the rolls on the shafts to change the web width between jobs.

Drive and hydraulics

Every forming shaft must turn at the same surface speed, or the strip would buckle between stands. A Main Drive Motor run from a Variable Frequency Drive feeds a Reduction Gearbox, and from the gearbox a single Drive Chain runs the length of the line, turning a Sprocket on each shaft so all the rolls move together. A Chain Tensioner keeps the long chain tight. The punch and shear need short bursts of high force rather than steady rotation, so they run on hydraulics: a Hydraulic Power Pack with its Hydraulic Pump, Valve Block, and Oil Reservoir supplies oil to the Punch Cylinder and the Shear Cylinder.

Materials

The strip itself is usually galvanized structural steel of 1.5 to 3.0 mm thickness, since the zinc coating protects the purlin inside an unheated building without any paint. The forming rolls take the most punishment: they are made from bearing steel such as GCr15, hardened and ground to the profile, then hard chromed so the strip slides over them without picking up. The Forming Shaft is a tough alloy like 42CrMo4 because it carries both the forming load and the drive torque. The punch and shear blades are D2 tool steel, which holds a sharp edge against thousands of cuts in coated steel.

Variants and use

Most lines are described by the section they make. A dedicated C purlin line and a dedicated Z purlin line share the same layout but differ in the late stands, while a combined CZ line uses interchangeable Forming Roll sets or quick column moves so one machine can switch between both. Higher production lines add a Servo Axis axis and a flying shear that cuts without stopping the strip, which keeps output up at the cost of a more involved Control Cabinet. The machines live in steel building plants, where an operator enters the section, length, quantity, and hole pattern on the Touchscreen HMI, loads a coil with the Coil Car, and the line runs out finished purlins bundled by length and ready to bolt straight onto a building frame.