How to Order a Replacement Gear from a Worn Sample: What to Measure

A gear fails, the machine stops, and nobody can find the drawing. The machine may be twenty years old, the original maker may have closed, or the gearbox came from abroad and spares take months. In most of these cases the gear can still be made again from the worn part itself. What decides whether the new gear fits first time is how well the old one, and the parts around it, are measured.

This guide covers what we ask customers to measure or send when they need a replacement spur gear, helical gear or pinion shaft made from a sample.

Start with What You Can Count and See

Before picking up a vernier, note down the simple things. They sound obvious, but a missing tooth count or a wrong helix hand is the most common reason a copied gear does not mesh.

• Type of gear: spur (straight teeth), helical (angled teeth), bevel (cone-shaped), worm wheel, internal gear or rack.
• Number of teeth: mark one tooth with chalk or a paint pen and count all the way round. Count twice.
• Hand of helix for helical gears: hold the gear with its axis vertical. If the teeth slope up to the right, it is right hand; up to the left, left hand. A mating pair of external helical gears has opposite hands.
• Bore, keyway and hub: bore diameter, keyway width and depth, hub length, any spline, set screw or taper lock.
• Face width and overall length (for shafts, every step diameter and length).

Working Out the Module or DP

The module (metric) or diametral pitch (inch) sets the size of the teeth. For a standard spur gear it can be estimated from the outside diameter and the number of teeth:

• Metric: module = outside diameter ÷ (teeth + 2)
• Inch: DP = (teeth + 2) ÷ outside diameter in inches

Take a gear with 42 teeth and an outside diameter of 110 mm: 110 ÷ 44 = 2.5, which is a standard module. If the answer lands on an odd figure such as 2.54 or 3.17, try the inch formula. Many older and imported machines use DP gears, and 3.175 mm (8 DP) or 2.54 mm (10 DP) are giveaways.

Measure the outside diameter across the tips at several points and on an even number of teeth, directly opposite each other. Worn or burred tips will read small, so use the least damaged area.

Helical Gears Need Two More Numbers

On a helical gear the same formula does not work directly, because the teeth are inclined. You also need the helix angle and to know whether the module is quoted in the normal or transverse plane. The helix angle can be found by rolling the gear on a sheet of paper with a little ink on the tips and measuring the angle of the marks, but the more reliable route is to give us the centre distance and the tooth count of both gears. From those, the helix angle can be calculated and then confirmed on the sample.

Pressure Angle and Profile Shift

Most modern gears use a 20° pressure angle. Some older machines use 14.5°, and a few special drives use 25°. Gears are also often made with profile shift, where the teeth are cut slightly deeper or shallower than standard to adjust strength or centre distance. When a gear has profile shift, the outside diameter formula gives a module that is close but not exact. This is one of the reasons we ask for the centre distance and the mating gear, not just the broken one.

Measure the Mating Parts, Not Only the Broken Gear

The single most useful number for a replacement gear is the centre distance: the distance between the two shaft centres in the housing. Measure it from bore to bore on the casing, or between shaft centres if the gearbox is still assembled. With the centre distance and both tooth counts, the gear geometry can be worked out and checked against the sample.

If the mating gear is in reasonable condition, measure it as well, or send it with the sample. If both gears are badly worn, it is usually better to replace them as a pair. A new gear running against a heavily worn partner wears fast and is noisy from day one.

Material and Hardness

A replacement should be at least as strong as the original. If you know the original material, tell us. If not, a few simple observations help:

• A dark or blue-grey band on the teeth or journals, with a brighter core visible on a broken face, usually means the surface was hardened.
• If a hand file skates over the tooth flank without biting, the surface is hard. If it cuts easily, the gear is probably unhardened or only through-hardened to a moderate level.
• Bronze or brass colour on a worm wheel means a bronze rim. It should be replaced with bronze, not steel.

For gears and pinion shafts we commonly work with EN8, EN9, EN19 and EN24 steels, and with case-carburising grades such as EN353 where the drawing calls for it. Where the teeth or journals need a hard surface, we carry out induction hardening in-house. Our separate guide on what to specify for induction hardening explains what to write on the order.

What to Send with Your Enquiry

If you can send the old part to our unit, that is the most reliable option. If not, clear photos and measurements are often enough to quote and, in many cases, to make the gear. Please include:

1. Photos of the gear from the front, the side and the bore, with a ruler or vernier in the picture for scale.
2. Tooth count, outside diameter, face width, bore and keyway sizes.
3. Details of the mating gear (tooth count and outside diameter) and the centre distance.
4. Machine type, motor power and speed, if known.
5. Quantity required, and whether you want the mating gear made too.

Common Mistakes to Avoid

• Measuring only the broken gear. Without the centre distance, a gear with profile shift can be made to the wrong geometry.
• Assuming the gear is metric. Check the inch formula whenever the module comes out as an odd number.
• Replacing one gear of a badly worn pair. The worn partner will quickly damage the new gear.
• Ignoring the helix hand. A helical gear with the wrong hand will not mesh at all.
• Leaving out hardness. A soft copy of a hardened gear will wear out far sooner than the original.

How We Handle Sample-Based Orders

At our unit in Gandevi, Gujarat, we regularly make gears and shafts to a sample or a worn part. We measure the sample, check it against the mating gear or centre distance, confirm the geometry with the customer, and then cut the teeth by hobbing or shaping. Where needed, teeth and journals are hardened before a final check and dispatch.

If you have a worn gear and no drawing, send the details through our contact page and we will tell you what else is needed. You can also browse our industrial transmission gear range to see the types we make.