Bevel Gears — Straight and Spiral for Right-Angle Drives

Power that has to turn a corner needs a bevel pair. The two shafts intersect — usually at 90° — and the teeth have to carry full torque through a geometry that is far harder to cut and measure than a spur or helical tooth.

PinyiGear produces straight bevel and spiral bevel gears, milled and, where the class demands it, ground after heat treatment. Spiral pairs are matched and their contact pattern checked before dispatch. This page covers the choice between the two tooth traces, the blank geometry that has to be fixed before machining, and where the accuracy actually comes from.

Straight or spiral: the choice that decides everything downstream

Straight bevel gears are simple to cut and cheap to blank. Their limitation is contact: one or two tooth pairs carry the load at any moment, so each tooth engages with an impact rather than a gradual transfer. At low speed and moderate load that is acceptable — farm machinery, general-purpose drives, hand-adjusted equipment. At higher speed the impact becomes noise, and the noise is a symptom of load concentration rather than a cosmetic problem.

Spiral bevel gears cut the tooth along an arc instead of a straight line. Each tooth enters the mesh diagonally and sweeps across the face width, so more teeth are in contact at once and the load spreads. The results are a higher torque capacity for the same size, quieter running and less sensitivity to small mounting errors. The cost is a much harder geometry to cut and inspect — which is why grinding matters so much on these gears.

The usual split:

Spiral bevel gears: what the geometry does

The arc-shaped tooth trace means the contact starts near one end of the face and rolls across to the other as the pair rotates. Two consequences follow.

First, the load is never concentrated at a single line across the whole face, so the tooth can carry more torque for its size and the mesh is tolerant of small deflections under load.

Second, the tooth surfaces are three-dimensional curves. A small deviation in tooth form or lead does not merely change a dimension — it moves the contact pattern, and the contact pattern is what decides noise, temperature and life. This is the reason a spiral bevel pair is judged by where the contact sits, not only by whether the tooth measures within tolerance.

Blank geometry: four numbers that interlock

Before any machining can be discussed, four dimensions on the blank have to be consistent:

• Pitch cone angle — fixed by the tooth-count ratio of the mating pair
• Large-end pitch circle diameter
• Cone distance — the large-end pitch radius divided by the sine of the pitch cone angle
• Face width — normally held to no more than about one third of the cone distance

They are not independent. Change the ratio and the pitch cone angle moves; change the cone distance and the face width limit moves with it. A drawing that fixes three of the four and leaves the fourth to the shop will produce a part that either does not fit the housing or does not carry the load the design intended.

Working ranges

Material and heat treatment

Carburising is what makes a spiral bevel pair survive: a hard case resists wear and pitting while the core stays tough enough to absorb shock without the tooth snapping. It also distorts the part, which is why a ground tooth is normal on anything above moderate duty.

Manufacturing sequence

1. Blank and geometry check — the four interlocking dimensions above, verified against the mating part.
2. Tooth cutting — a bevel gear cutting machine roughs the tooth slots, leaving stock for hardening and grinding.
3. Carburising and hardening — case depth and surface hardness per the drawing.
4. Tooth grinding — rough and finish passes restore tooth form and lead. This is the step that decides whether the class holds in production, not just in a sample.
5. Contact pattern check — marking compound on the pair, contact position and size read against the specification.
6. Pair matching and lapping where specified.
7. Final inspection — tooth form, lead, pitch, hardness, case depth, and the contact pattern record.

Two failure modes are specific to this process and worth naming: grinding burn, where the surface is overheated during grinding and the hardness drops without any visible change in dimension, and contact drift, where the pattern moves out of position because the hardened part distorted more than the grinding stock allowed. Both are caught by checking hardness after grinding and by reading the pattern, not by measuring tooth size alone.

Why the pair is the unit of supply

A spiral bevel gear is not a standalone part. Backlash, contact pattern and running quietness are properties of the pair, set by how the two members were cut and matched. Supplying one member of a pair to a drawing produces a part that measures correctly and may still run badly against the existing mate.

We cut and grind bevel pairs as sets, check the contact pattern on the assembled pair, and ship them together with the report. Where a customer needs a single member, the mating part drawing is the minimum input.

Where bevel gears are used

• Automotive differentials and axles — the classic application: high torque, shock load, quiet running expected
• Right-angle industrial reducers — where motor power has to turn to a vertical axis
• Construction and mining machinery — slewing and travel drives under repeated shock
• Machine tools — right-angle heads and auxiliary drives with defined accuracy targets
• Agricultural and general machinery — straight bevel gears where speed and noise are not critical

What to send

• Drawing (PDF, DWG, DXF, STEP) with the four blank dimensions, tooth geometry and accuracy class
• Mating gear drawing, when only one member is required
• Duty: torque, speed, environment, expected life
• Material and heat treatment if fixed; if open, the duty decides
• Quantity, and whether the order is a prototype, a first batch or repeat production

An engineer reviews the drawing for manufacturability before a price is quoted, because whether the class can be held consistently is decided by the geometry and the process route, not by the machine alone.

Send a bevel gear drawing for quotation

FAQ

Can you supply one gear of a pair? Yes, but the mating part drawing is required. Contact pattern and backlash belong to the pair, so a single member cannot be checked properly without its mate.

Is grinding always necessary on a spiral bevel gear? Not always. Below a certain class, milled and tempered gears are adequate. Where the duty involves speed, noise targets or a tight class, carburising without grinding leaves distortion in the tooth and the pattern moves.

How is the contact pattern verified? With marking compound on the assembled pair, then checking position and size against the specification. The result is recorded and travels with the pair.

What about grinding burn? It is checked by hardness measurement after grinding, not by looking at the surface. Burn reduces the hardness of the very layer that resists pitting.

Can you cut shaft angles other than 90°? Yes. The angle is part of the geometry review; it changes the blank and the cutting setup, so it has to be on the drawing.

Related pages

• Spur and helical gears
• Worm gears
• Custom gear manufacturing
• CNC machined parts