How to choose
a girth gear manufacturer.

02

Check 1 — The blank, and the certificate that comes with it

A girth gear starts as a cast, forged or rolled ring. Castings are common at large diameters; forgings and rolled rings give better directional properties where the load has to run around a thin rim. The choice has consequences, so it should be named in the quotation rather than left to the supplier’s convenience.

What matters more than cast-or-forged is what the blank arrives with: the mill certificate, carrying the heat number, the chemical composition and the mechanical properties, plus the nondestructive test the grade requires. Stress relief before the teeth are cut is not optional. A blank that relaxes after machining takes the tooth alignment with it.

Our practice is to work from castings or forgings supplied with the mill certificate, stress-relieve before the teeth are cut, and check the part again after hardening. Where a customer supplies the blank, it is inspected and recorded before it goes on the machine.

Where the drawing calls for ultrasonic testing, ask what volume was tested and to what acceptance level. A certificate that says UT was performed, without an extent and a level, tells you almost nothing: scanning the whole rim is a different job from a spot check on two faces, and the price of the blank moves accordingly.

03

Check 2 — Can the machine take your diameter?

Large girth gears run from two to eight metres outside diameter. The limit that decides who can make yours is not the lathe’s nominal capacity alone but its swing, its height under the tool, and whether the workshop has the crane and the fixturing to hold the ring while it is cut.

Ask three numbers: maximum outside diameter, maximum face width and maximum weight. Then ask a fourth: how is the ring supported while the teeth are cut? A large ring that is not supported evenly will spring back when the supports are released, and the runout is already wrong before the part ships.

Our numbers are an 8 m maximum gear outside diameter, a 2.5 m maximum turned diameter, a 12 m horizontal lathe for shaft work, and 35 t for a single casting or forging. If your part is bigger than that, we say so at drawing review rather than after the quotation.

Large diameters are also measured differently. Past four metres, shops commonly check with a pi tape or a laser tracker rather than a micrometer, so ask which method the inspection report uses and how the ring is levelled when it is measured. Runout taken on a ring that is not sitting flat is runout of the setup, not of the gear.

04

Check 3 — Tooth cutting: hobbing, shaping, and the grade they can hold

Hobbing and shaping are different machines for different teeth. Hobbing is the faster route for external spur and helical teeth. Shaping is what gets into internal teeth, herringbone designs, and gears with a shoulder a hob cannot reach.

The question to ask is not whether they can hob a gear — almost everyone can. It is what accuracy grade they can hold, and how they measure it. A tooth profile is not a single number: pitch deviation, total cumulative pitch error, profile form, helix deviation and runout are measured separately, and the grade on the drawing applies to all of them.

A supplier who answers with a grade and the instrument used is telling you something. A supplier who answers high precision is telling you something else. At review we confirm the module and accuracy grade the drawing calls for, because those two decide the machine, the cutter and the inspection long before they decide the price.

One more question hides in the same answer: how many gears of this size the shop has cut. A cutter that has run a hundred modules of work behaves differently from one set up for the first time, and a workshop that cuts one large gear a year will quote a long lead time, honestly or not.

05

Check 4 — Heat treatment, and the order in which it happens

Two heat treatments matter on a gear: quenching and tempering of the blank, and hardening of the tooth surface. The first makes the steel machinable and tough; the second gives the flank the hardness that decides how many years it lasts.

Order is where the quality is won or lost. In our route the blank is quenched and tempered before finishing, the teeth are cut afterwards, and the tooth surface is induction hardened as the last cutting-related step. Hardening after cutting distorts the tooth slightly, so the gear is re-inspected afterwards: pitch, runout and fit.

A quotation that does not name the heat treatment, or that hardens before cutting the teeth to save a step, is asking you to accept whatever the distortion does. Ask for the sequence, the hardness range and the material grade together — all three are needed to judge the other two.

Ask for the hardness as numbers, too. A quenched and tempered blank and an induction-hardened flank are specified in different units — HB for the core, HRC for the surface — and a quotation that mixes them was written by someone who did not read the drawing. Where the flank hardness carries the wear, ask whether a case depth was specified with it.

06

Check 5 — Split gears: the joint is the part that fails first

Most girth gears on kilns and mills are split into two halves or four segments, because they have to go around a shell that is already standing. That makes the joint the most sensitive place on the part: two tooth pitches that do not agree across the split will knock on every revolution.

The halves have to be machined as a set, with the joint faces and the dowels in place, so the tooth pitch runs continuously across the split. Pre-assembly before dispatch is the only way to prove it, because the joint is then checked on the supplier’s floor and not on yours.

Ask whether the gear is pre-assembled, whether match marks are stamped, and whether you receive a record of the joint check. Our split gears are machined as a set and pre-assembled before dispatch; the joint is checked before the crate is closed.

A four-segment ring adds two more joints, and a question of sequence: which segment goes onto the shell first, and how the ring is closed. Ask for the assembly sequence in writing, with the bolt torque values, and make sure it travels with the crate to the site.

07

Check 6 — Inspection: what is measured, and what you receive

Inspection is the part of the order buyers most often leave unspecified, and it is the cheapest place to add certainty. The basics are chemical composition and mechanical properties from the mill certificate, ultrasonic testing of the blank or the finished forging, hardness of the tooth surface, and dimensions and geometric tolerances to the drawing.

Then come the gear-specific checks: runout of the pitch circle against the bore, tooth thickness and backlash, and pitch measurements across the joint. All of them should produce numbers rather than a stamp.

Ask to see a sample report before you order. It shows what the supplier habitually measures and in what language it is written. Ours ship in English for export orders, with the material certificate and packing list in the same envelope, because the parts that arrive without documents cost the most to accept.

On a first order, ask whether the first article can be inspected before the rest of the batch is finished, and whether you or your inspector may witness it. Remote witnessing by video has become ordinary, and it costs a supplier nothing to offer.

08

Check 7 — Datums, runout, and the fit on the shell or shaft

A gear can be right to the drawing and still run badly, because the bore, the mounting face and the teeth are referenced to one another. Ask which datum tooth runout is measured against: the bore that fits the shaft, or the face that bolts to the flange. If the answer is vague, the gear will be right in the inspection room and noisy on the machine.

The same applies to keyways, bolt holes and spigots. The dimension chain should run from the mounting surface outward, so a small error at the bore does not become a large one at the rim.

It is also why we cut gears and their shafts as a set where the drawing allows it: tooth form, tooth lead and journal datums are checked on the same route, and the parts that meet in service are measured against each other rather than against two separate reference blocks.

Where the gear has a shrink fit or a taper, ask how the bore was checked. A bore measured at three heights is a different claim from one reading at the entry, and the difference shows up on the first hot assembly.

09

Check 8 — Packing, lifting, and the voyage

A six-metre ring is a heavy, thin part, and the sea is unkind to both steel and timber. Rust protection has to survive weeks of humid air inside a container; lifting points have to be marked where the rigger will actually find them; the cradle has to carry the ring on its face, not on its teeth.

For large rings, ask for the packing drawing, the shipping weight and the dimensions before the crate is built. Out-of-gauge freight is quoted differently, and a supplier who has not shipped one before will learn on your order.

Our packing follows the destination: rust prevention, bracing and shipping marks, then a full container, a shared one or out-of-gauge, quoted to your port under your trade term.

Ask, finally, who arranges the freight and who insures it. Under FOB the risk passes at the ship’s rail; a supplier quoting CIF is selling you freight and insurance you may prefer to buy elsewhere.

10

Check 9 — Lead time, and what the quotation actually covers

Four to eight weeks from the approved drawing is a normal range for a single large gear or shaft, depending on diameter, module and heat treatment. Anything substantially shorter is worth a question, because a quench-and-temper cycle, tooth cutting and re-inspection after hardening cannot be compressed indefinitely.

Ask what the price includes: material, heat treatment, inspection reports, packing, and freight to which port under which trade term. Then ask what happens if the drawing changes after the blank has been ordered — that is where the money goes when a project slips.

We confirm the date with the quotation rather than ahead of it, because the route sets the date, not the calendar.

And keep production time apart from transport time. Four to eight weeks is the workshop; sea freight to a European or American port adds another three to six weeks on top, and a quotation that merges the two hides the number you will actually have to manage.

11

What a quotation should name, in one page

A quotation built from this list can be set beside another quotation and compared. One that cannot is a price, not an offer.

• The blank: cast, forged or rolled, and who supplies it
• Material grade, hardness, and the certificate that comes with it
• The route: which operations are in-house and which are subcontracted
• Accuracy grade, and the instrument used to prove it
• Heat treatment: sequence, hardness range and any case depth
• Inspection plan, and the documents that ship with the part
• Packing method, shipping weight and dimensions
• Delivery date, trade term, and what the price excludes

12

Red flags in a quotation

A few patterns repeat in quotations that later turn into problems:

• A price far below the others, with no explanation of where the blank comes from.
• A promise of certificates later: a mill certificate belongs to one heat of steel and cannot be produced at will.
• No accuracy grade, or a grade with no instrument named.
• A split gear with no pre-assembly and no match marks.
• A quotation that never mentions heat treatment or inspection.
• A lead time shorter than the heat-treat cycle for that size.
• We can do everything, without naming a single machine.

13

The list to send with your enquiry

Send these in the first message and every quotation you receive becomes comparable, which is what makes the differences in price mean something.

• Outside diameter, face width, and weight if known
• Module, number of teeth, tooth form: spur, helical, herringbone or internal
• Accuracy grade, and the standard it is written to
• Material and required hardness
• Split or solid, number of segments, and whether the joint is dowelled
• Bore, keyway, bolt hole pattern and the datum face
• Drawing file in DWG, DXF, STEP or PDF
• Quantity, required date, destination port and trade term