Home › Guides › Straight vs spiral bevel gears

Straight vs spiral bevel gears

Both transmit motion between intersecting shafts, both are cut on a cone, and a straight and a spiral pair of the same size fit the same space. What changes is the tooth trace - a straight line to the apex, or a curve - and with it the way the teeth meet, the forces on the bearings and the way the pair is made.

How the teeth meet

A straight bevel tooth runs straight towards the cone apex. A pair of teeth comes into contact along its whole face width at once and leaves it at once, so the load passes from tooth to tooth in steps. That is fine at moderate speeds, but it is where the noise and the dynamic load of a straight pair come from.

A spiral bevel tooth is curved - on a face-milled pair it is an arc of the cutter circle - and set at a spiral angle to the cone generatrix. Contact starts at one end of the tooth and travels along it, and before one pair of teeth leaves, the next has already begun. This overlap is the face contact ratio: zero for a straight bevel, and for the spiral pair below about one full extra tooth. More teeth share the load at any instant, the engagement is gradual, and the pair runs smoother and quieter.

One pair, designed both ways

To compare like with like, here is the same envelope worked both ways in the designer: 20/40 teeth, outer module 5 mm - pitch diameters 100 and 200 mm - face width 25 mm, rated to ISO 10300 at 250 Nm and 1 500 min^-1 (K_A 1.25, case hardened steel at 700 HV, accuracy grade B6). The spiral pair has a 35° mean spiral angle and a 114.3 mm cutter from ISO 23509 Table E.1.

Three things to read from it. The flanks gain most: pitting, the usual limit on a hardened pair, improves by about a fifth. The root does not simply improve - at the same outer size the spiral tooth is smaller in its normal section, and S_F drops a little - but ISO 10300 asks less of a spiral root (1.3 instead of 1.5), so the margin over the minimum is still larger. And the bearings see considerably more force, which is the next section.

These are the values for one pair; the balance moves with the spiral angle, the ratio and the face width. The designer rates any pair both ways in a few seconds.

Forces on the bearings

On a straight bevel the axial force always pushes each member away from the cone apex, out of mesh, and it is modest. On a spiral bevel the spiral angle adds a component whose direction depends on the hand of spiral and the direction of rotation: for one combination the pinion is pulled towards the apex, into mesh, for the other it is pushed out, and the size changes with it. In the example above the pinion is pulled in with 2 407 N, and its radial force more than doubles. The usual practice is to choose the hand so that, in the main direction of rotation, the pinion is pushed out of mesh; either way the bearings and the housing have to be sized for the spiral pair's forces.

Making them

Straight bevel gears are generated by a tool that stands for a basic rack - the ISO 53 profiles - on straight bevel generators, and are also forged or formed in volume. Spiral bevel gears are cut with a rotating face-mill cutter head (or face hobbing) on dedicated spiral bevel machines, and are often lapped or ground in matched pairs. That needs more specialised machines and setup, and it is why a spiral pair usually costs more for the same size.

For the geometry this means two more inputs on a spiral pair: the mean spiral angle and the cutter radius, which together set how the spiral angle changes from the toe to the heel. The designer takes the cutter radii from ISO 23509 Table E.1.

Which to choose

• Straight when speeds are moderate, noise matters little, the axial load has to stay small and simple, or the parts are made in volume by forming.
• Spiral when the pair runs fast or has to be quiet, when the flanks are the limit and the space is fixed, or when a high ratio needs a small pinion to run smoothly - and the bearings can take the thrust.

Questions