Large Crusher Gear Processing Technology

• 1
  What makes a crusher gear different from a small gear
• 2
  Blank preparation and stress relief before cutting
• 3
  Split-half coupling: why two halves are machined as one
• 4
  Cutting teeth on a ring that will not stay round
• 5
  Measuring a gear that is taller than the inspector
• 6
  Heat treatment order and its effect on final accuracy
• 7
  When this process is the wrong choice
• 8
  Step by step: the working sequence we use
• 9
  Tooth cutting methods for large crusher gears

A crusher girth gear is not a scaled-up version of a gearbox pinion. A 6,000 mm gear with a 400 mm face is a large, thin-walled ring. Its own weight bends it. The rim cools faster than the web after casting, so the blank already carries internal stress before a single cut is taken. Those two facts drive every later decision.

The service duty is heavy too. Crusher gears turn slowly, carry high torque and see shock loads every time a lump of ore drops into the chamber. Tooth breakage at the root and pitting on the flank are the two failure modes that matter. Both are sensitive to how the gear was machined, not only to how it was designed.

Size changes the machine. A 500 mm gear can be cut on a universal mill. A 4,000 mm gear needs a machine with enough travel or a rotary table, plus a way to measure a diameter that no caliper will reach. That is why large crusher gear processing is usually a sequence of separate setups, not one continuous operation.

This page covers the machining side: blank form, split-half coupling, tooth cutting methods, heat treatment order and inspection. It does not cover gear design or lubrication selection.

Rough machining removes the casting skin and brings the blank to a uniform stock allowance, typically 5–10 mm per side on the rim and web. Then the part is stress relieved. Skip this step and the gear will move after the first tooth is cut, and no amount of finish machining will bring it back.

After stress relief, semi-finish turning establishes the bore or the mounting face, the rim outside diameter and the reference faces. For a split gear, each half is machined separately at this stage, leaving stock on the split faces for the coupling operation.

Casting defects matter here. Porosity or shrinkage in the rim will show up as a hard spot or a soft spot during tooth cutting, and a cutter that meets a hard inclusion can chip. Ultrasonic or dye penetrant checks on the rim before cutting are cheap insurance on a part this size.

In our shops, blanks up to 4,000 mm are handled on machines with 4,000 × 400 × 150 mm travel. Larger rings are usually split at the foundry and coupled in-house before tooth cutting.

Crusher gears above roughly 4,000 mm are cast in two halves because a single ring will not fit a furnace, a lathe or a truck without special permits. Each half is rough machined, then the pair is bolted and doweled together into a full ring. From that point the two halves are machined as one part.

The split faces are the critical joint. They are milled flat and matched so that the two halves meet with no visible gap under a light band, then located with dowel pins and clamped with high-tensile bolts. Any mismatch in the joint becomes a tooth spacing error at that position on the pitch circle.

After coupling, the ring is turned as a unit. Bore, mounting face and rim are all referenced from the assembled geometry, so runout is controlled against the joint rather than against one half. The gear is then marked across the joint so it can be reassembled in the same orientation on site.

This is the single biggest difference between large crusher gear processing and ordinary gear cutting. Coupling first, then cutting, keeps the joint from becoming a local error zone.

A large ring sags under its own weight. Set it on three or four support pads and the rim deflects between them. Cut the teeth in that state and the pitch circle is elliptical, not circular. The fix is to support the ring on a full-face fixture or a set of evenly spaced jacks that mimic the mounting condition on the mill.

Thermal drift is the second problem. Cutting a 6,000 mm gear takes many hours. The workpiece, the fixture and the machine column all move as the shop warms up. Roughing passes in the morning and finishing passes in the afternoon can differ by more than the tolerance band. We rough, let the part stabilize, then finish in a controlled sequence.

Cutter wear shows up faster on large modules because the chip load per tooth is high. A worn hob produces a lead error that builds along the face. Operators check the first and last tooth of each pass and re-dress or index the cutter before the error reaches the limit.

On hardened gears, distortion from heat treatment is corrected by profile grinding. On soft girth gears that are too large to grind economically, the finish cut is planned so that heat treatment is done before final tooth cutting, or the gear is cut in the soft state and run-in on site.

You cannot put a 6,000 mm gear on a gear measuring center. Large crusher gear processing relies on portable metrology: laser trackers, portable arms, chordal measurement over pins or balls, and templates. Each method has a different uncertainty, and that number has to be smaller than the gear tolerance or the measurement tells you nothing.

Runout is checked first. Radial runout on the pitch circle and axial runout on the mounting face are measured with an indicator or a tracker while the gear turns on its own bearing or on a fixture that reproduces the mounting. Typical acceptance for a large girth gear sits in the 0.1–0.3 mm range for runout, while the tooth profile itself may be held to ±0.005 mm on smaller crusher gears.

Tooth thickness is verified over pins or balls at several positions around the ring, not just one. Split gears get extra points at, and next to, the joint. Base tangent length or span measurement is used where the face width allows.

All records go into a report with the measurement positions marked on the gear. If a customer needs the report, we provide it. For a part this size, the report is often more useful than the certificate.

There are two common routes. Cut soft, then case harden and grind the flanks: this gives the best accuracy because grinding removes distortion. It also costs more and needs a grinder that can handle the ring. Cut after hardening, using a carbide or ceramic cutter on a hard but stable blank: this is cheaper but limits the achievable accuracy grade.

For most crusher girth gears, the teeth are cut in the soft state and the gear is not flank ground. Distortion is controlled by a slow, uniform heat treatment cycle and by leaving enough stock for a light correction pass. If the design calls for hardened flanks, plan the grinding allowance into the tooth thickness from the start.

Never heat treat after final turning of the bore and mounting face without re-checking them. A ring that was round before the furnace is often oval after it. Finish machining of the bore and face belongs after heat treatment, not before.

The rule is simple: the last operation that can move the part must come before the last operation that defines its accuracy.

If the gear is under about 800 mm in diameter, split-half machining adds cost and a joint without any benefit. A solid blank cut on a 5-axis center will be more accurate and cheaper.

If the required accuracy grade is fine-pitch precision gearing with ground flanks, a large cast girth gear is the wrong architecture. At that point a smaller, hardened and ground gearbox stage does the work, and the girth gear only transmits torque at a coarse pitch.

If the ring has to be cut in the field because it cannot be moved, the shop sequence above does not apply. Field machining uses portable equipment, a different support scheme and looser achievable accuracy. Plan for that in the drawing, not after the part is cast.

And if the blank arrives with porosity in the rim, no machining sequence will fix it. Reject the casting before cutting starts. That decision is much cheaper at the foundry than at the machine.