Custom Spur Gears
Precision CNC Machined

A spur gear is the simplest and most common type of cylindrical gear. It has straight teeth cut parallel to the gear's rotational axis, and meshes with other gears in the same plane to transmit power between parallel shafts. Spur gears are widely used because they are easy to manufacture, highly efficient (97–99%), and available in a vast range of sizes and materials.

Look for three key features: (1) straight teeth that run perfectly parallel to the shaft axis — not angled or twisted; (2) a cylindrical body (not conical or worm-shaped); and (3) external or internal tooth profiles on a flat disc. If you view the gear face-on, the teeth appear as straight radial lines. Helical gears look similar but their teeth are visibly angled.

Per ISO 54 / DIN 780, the preferred standard modules for spur gears are: 0.5, 0.8, 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20 (Series I). Module 1 to 3 covers most robotics and small industrial applications; module 4 to 6 dominates automotive and machine-tool gearboxes; module 8 and above are used in heavy industrial drives. Module is calculated as pitch diameter divided by tooth count (m = d / z). HXC machines spur gears across the full metric module range plus inch-system diametral pitches from DP 2 to DP 48.

For metric spur gears: d = m × z — pitch diameter equals module multiplied by tooth count. Example: a 24-tooth gear with module 2 has a pitch diameter of 48 mm. For inch spur gears: d = z / DP — tooth count divided by diametral pitch. Example: 48 teeth at DP 16 = 3 inch pitch diameter. Pitch diameter defines where two meshing gears effectively contact and is the single most critical dimension on a spur gear engineering drawing. Center distance between two meshing spur gears equals (d₁ + d₂) / 2.

Spur gears appear in almost every industry: automotive (transmission idler gears, window lift motors), medical (surgical robots, infusion pumps), aerospace (actuator drives), robotics (servo gearheads), industrial machinery (pumps, compressors, rack-and-pinion linear drives), and consumer electronics (camera autofocus mechanisms, printers). They are the default choice whenever parallel-shaft power transmission is needed at moderate speeds.

"Regular gear" is not a technical term — in engineering, spur gear is itself the baseline gear type. Other gear families (helical, bevel, worm, planetary) are defined by how they differ from spur gears. Compared to helical gears, spur gears have straight teeth and generate no axial thrust force; compared to bevel gears, they work on parallel rather than intersecting shafts; compared to worm gears, they are reversible and far more efficient.

Spur gears are also called straight-cut gears or straight gears, referring to the orientation of their teeth. In racing and performance automotive contexts you will often hear "straight-cut gearbox," meaning a transmission built with spur (not helical) gears. In some older technical literature they are simply called cylindrical gears, though today that term covers both spur and helical variants.

The main advantages are: high efficiency (97–99% per mesh — no axial thrust losses); simplicity — the easiest gear type to design, manufacture, and inspect; low cost — hobbing is fast and economical at any volume; no axial load on bearings, simplifying shaft and housing design; precise speed ratios — suitable for positioning and indexing applications; and wide material choice — from hardened steel to engineering plastics.

The main limitations are: noise at high speed — tooth engagement is an impact, not a gradual mesh, producing a characteristic whine above ~3 m/s pitch-line velocity; lower load capacity per unit width compared to helical gears (contact is a line rather than a diagonal band); parallel shafts only — cannot transmit power between non-parallel axes; and vibration from the same impact engagement that causes noise. For high-speed, low-noise applications, helical gears are generally preferred.

At low to moderate speeds they are quiet; at high speeds they produce a distinctive whine because all tooth contact starts and ends simultaneously — an impact rather than a gradual engagement. Noise increases with pitch-line velocity, tooth error, and backlash. To reduce noise: use profile grinding to DIN 5–6, apply tip relief or crowning, choose POM or nylon for light-duty applications, and ensure precise alignment. Helical gears are inherently quieter at high speed due to overlapping tooth contact.

Yes — spur gears are generally the most economical gear type to manufacture. Gear hobbing is fast and requires no special tooling per profile; one hob covers a range of tooth counts at the same module. CNC hobbing can produce hundreds of gears per hour in production. Cost drivers are material grade (alloy steel vs. stainless), required DIN accuracy class (each class step roughly doubles finishing time), and order quantity. At HXC, prototype quantities start from a competitive per-piece price with no tooling fee.

20° is the global standard pressure angle for spur gears today and is used in over 95% of new designs. It offers the best balance of load capacity, contact ratio, and minimum tooth count (17 teeth before undercutting). 14.5° remains in older imperial drive trains and pre-1960 equipment — quieter but weaker and harder to source replacement cutters for. 25° is used where high load capacity is needed and where as few as 13 teeth are acceptable without undercutting. All HXC spur gears are 20° pressure angle by default; 14.5° and 25° are available on request.

External spur gears have teeth on the outside of a cylinder and mesh with another external gear — the rotation direction reverses between input and output. Internal (annular) spur gears have teeth cut on the inside of a ring and mesh with an external pinion sitting inside them — the rotation direction stays the same and the meshing gears share axial space, giving a more compact design. Internal gears are essential for planetary gearboxes, integrated wheel hubs, and harmonic drives. They cannot be hobbed and must be made by gear shaping, broaching, or wire EDM, which makes them more expensive but irreplaceable for compact reduction systems.

Yes. "Straight-cut" is the defining characteristic of spur gears — their teeth are machined parallel to the gear's rotational axis, giving a perfectly straight tooth profile when viewed from the side. This is what distinguishes them from helical gears (angled teeth), herringbone gears (double-angled), and spiral bevel gears (curved). The straight cut is also why they generate an impulsive engagement and no axial thrust.

Yes — all gears have teeth by definition, and spur gears are no exception. Their teeth have an involute profile: a mathematically precise curve that ensures a constant velocity ratio regardless of slight center-distance variations. The key parameters are module (tooth size, metric standard), pressure angle (typically 20°), and tooth count. Tooth count divided by module gives the pitch diameter. HXC machines spur gears from module 0.5 (micro gears) to module 20 (large industrial gears).

For the same tooth size and face width, helical gears typically carry 20–40% more load than spur gears, because the diagonal tooth contact distributes load over a longer contact line. However, helical gears generate axial thrust forces that must be absorbed by bearings, adding complexity and cost. For moderate loads, the strength advantage of helical gears is rarely needed and spur gears are the practical choice.

Spur gears are better when: cost, simplicity, and efficiency matter most; speed is low to moderate; axial space is limited; or you need no thrust loads on bearings. Helical gears are better when: operation is at high speed (above ~3 m/s pitch velocity); low noise is critical; or you need higher load capacity in the same envelope. In practice, most industrial and automotive gearboxes use helical gears for main stages and spur gears for ancillary drives, idlers, and instrument mechanisms.

For maximum strength, case-hardened alloy steels are the standard choice: 20CrMnTi (carburized to HRC 58–62) and 42CrMo4 (through-hardened to HRC 50–55) dominate automotive and industrial gearing. For very high contact stress, M2 tool steel ground to DIN 4 gives the highest surface hardness. For corrosive environments, SUS316L stainless trades some strength for corrosion immunity. Grade 5 titanium offers the best strength-to-weight ratio for aerospace. Engineering plastics (PEEK, POM) are strongest among non-metallic options.

Worm gears are self-locking when the worm lead angle is less than the friction angle — typically below about 5°. At low lead angles, the reaction force on the worm tooth is directed more into the thread flanks than along the thread, so friction prevents the wheel from rotating the worm backward. This is why worm gear lifts and door locks hold position with no brake. Spur and helical gears can always be back-driven because their tooth normals are nearly perpendicular to the shaft, creating low friction relative to the driving force.

The 17-tooth rule states that a standard 20° pressure-angle spur gear must have at least 17 teeth to avoid undercutting — a condition where the cutting tool removes material from the base of the tooth, weakening it and distorting the involute profile. Below 17 teeth, the tooth root is notched, reducing bending strength and contact ratio. Solutions for fewer teeth include: profile shifting (moving the rack cutter outward), using a higher pressure angle (25° allows as few as 13 teeth), or stub tooth profiles. HXC's engineers can advise on the optimal approach for your pinion design.

Six proven approaches: (1) Higher accuracy class — grinding to DIN 5 or 6 reduces tooth error and impact; (2) Tip relief — a small chamfer at the tooth tip eases engagement; (3) Crowning — a slight barrel shape on the tooth face reduces edge loading; (4) Plastic mating gear — pairing a steel driver with a POM or nylon driven gear damps noise significantly; (5) Increased contact ratio — more teeth in simultaneous contact (achieved by larger module or addendum modification) smooths transmission; (6) Lubrication and alignment — correct oil viscosity and precise shaft alignment eliminate most noise in well-designed spur gear pairs.