In almost every spur gear transmission, the same fundamental question arises early on: Helical gearing or straight-toothed gears? Both designs transmit torque reliably, but differ fundamentally in geometry, running characteristics, and operating limits. In spur gears, the teeth run exactly parallel to the gear axis; in helical gears, they are inclined by the helix angle β – typically between 8° and 20° in practice.
This seemingly minor geometric change has far-reaching consequences: Helical gears run significantly quieter, handle higher loads, and operate at higher speeds—but in return generate an axial force that the bearing assembly must absorb. This comparison, based on specific performance metrics, shows which design is the right choice in which situation.
Key takeaway: Spur gears are simple, cost-effective, and generate no axial force—making them ideal for slow, straightforward drives. Helical gears offer up to +30% load capacity, significantly lower noise levels, and greater efficiency at high speeds—at the cost of an axial force that must be accounted for in the bearing design.
Spur Gearing: Simple, No Axial Force, Cost-Effective
Straight-toothed gears are among the oldest and most widely used machine components. Their strengths lie in their simple manufacturing process and their ability to operate without generating any axial force—two characteristics that are decisive in many applications.
- Simple, low-cost production: Straight-toothed profiles can be manufactured on simple gear hobbing machines. Tooling costs, setup times, and scrap rates are lower than for helical gears. This has a direct impact on part costs.
- No axial force: Since the tooth line runs parallel to the axis, only a radial force is exerted on the shaft. Standard radial bearings are sufficient; axial support is not required. This simplifies the gearbox design and reduces bearing costs.
- Noisy and impact-prone: During gear meshing, the entire tooth flank comes into sudden contact with the mating gear. The resulting periodic impacts generate noise and vibrations that are transmitted through the housing and the structure. This is particularly problematic at higher speeds.
- Profile overlap only: The total overlap of straight-toothed gears consists solely of profile overlap (typically εα ≈ 1.3–1.8). Consequently, fewer teeth engage simultaneously than with helical gears, which limits the maximum load-carrying capacity.
Spur gearing is the first choice for simple, low-speed, and lightly loaded drives where manufacturing costs and design simplicity take precedence over smooth operation and maximum power density. Background information on gear geometry fundamentals and pressure angle can be found in the article Basic Concepts of Gear Technology.
Helical Gearing: Quieter, Higher Load Capacity, Faster
The helix angle β is the key design variable for helical gears. It determines the amount of tooth overlap and, consequently, the extent to which running and load-carrying performance improves compared to straight-cut gears—as well as the magnitude of the axial force.
Gradual, progressive tooth engagement: Because the tooth profile runs at an angle to the axis, contact begins at one end of the tooth and moves continuously across the entire tooth width. The force is applied smoothly and dissipated just as smoothly—no jolts, no sudden surges in force. The result is significantly smoother and quieter operation, which is particularly noticeable at medium to high speeds.
Greater total contact ratio and load capacity: In helical gearing, the profile contact ratio εα is supplemented by the overlap contact ratio εβ. The total contact ratio εγ = εα + εβ is therefore always greater than for a comparable spur gear. On average, more teeth share the load simultaneously, flank pressure decreases, and transmissible torque increases. As a rule of thumb: up to approx. +30% compared to a geometrically equivalent spur gear. Those who wish to go deeper into design dimensioning can find the fundamentals in the article Spur Gearboxes: Fundamentals and Design.
Axial force – the key drawback: In addition to the radial force component, the angle of the tooth profile inevitably generates an axial force component, the magnitude of which depends on the torque and the tangent of the angle of inclination. This axial force must be absorbed by the shaft bearings. A simple deep-groove ball bearing is sufficient only for small axial forces; in many cases, angular contact ball bearings or tapered roller bearings are required. If gear backlash and bearing preload are not carefully matched, the axial force will cause bearing drift and premature failure.
Efficiency: The efficiency of helical gearing is marginally lower than that of spur gearing. The reason is the sliding motion along the tooth helix, which produces greater flank-friction losses than the near-pure rolling contact of spur gears. In practice the difference is small—often under one percentage point—but becomes meaningful at high power and speed levels.
Special Form: Herringbone and Double-Helical Gearing
Herringbone gearing combines two mirror-image helical halves on the same gear. The axial forces of the two halves cancel each other out—delivering all the advantages of helical gearing (smooth running, high load capacity) without placing any axial load on the bearings. Manufacturing is more complex and expensive; this design is particularly worthwhile in large gearboxes under high loads, where the bearings cannot or should not sustain axial forces.
Calculating the forces on a helical gear
The inclined tooth line splits the tooth force into three components. From the torque T and the pitch diameter d, the tangential, axial and radial forces follow:
Ft = 2 · T / d
Fa = Ft · tan β
Fr = Ft · tan αn / cos β
Ft = tangential force [N] | Fa = axial force [N] | Fr = radial force [N] | T = torque [Nmm] | d = pitch diameter [mm] | β = helix angle | αn = normal pressure angle (usually 20°)
Worked example:
Given: T = 100 Nm = 100,000 Nmm, d = 100 mm, β = 15°, αn = 20°
Ft = 2 · 100,000 / 100 = 2,000 N
Fa = 2,000 · tan 15° ≈ 536 N
Fr = 2,000 · tan 20° / cos 15° ≈ 753 N
The axial force of 536 N (≈ 27 % of the tangential force) must be carried permanently by the bearings.
Applicable standards: The load capacity of spur/helical gears (pitting and tooth-root strength) is verified to DIN 3990 or ISO 6336; the terms and parameters of cylindrical gearing are defined in DIN 3960.
Direct Comparison: Spur Gearing vs. Helical Gearing
The following table directly compares the key features of both designs. Further background on material selection and gear quality can be found in the article A Comparison of Gear Materials.
| Feature | Spur gearing | Helical gearing |
|---|---|---|
| Tooth line | Parallel to the axis | Helix angle β (8–20°) |
| Smooth operation / Noise | Loud (sudden impact) | Significantly quieter (smooth operation) |
| Load capacity | Standard | Up to ~30% higher |
| Axial force | None | Present (must be absorbed by bearings) |
| Efficiency | Very high | Slightly lower |
| Manufacturing / Costs | Simple, affordable | More complex, higher cost |
| Typical speed | Low – medium | Medium to high |
| Application | Simple, low-speed drives | Quiet, heavy-duty, high-speed gearboxes |
The exact numerical values depend on the module, number of teeth, material, and gear quality. The gear ratio also determines which design is the more cost-effective choice for the overall transmission.
Selection & Decision-Making Guide
The choice between straight and helical gearing depends on the specific design requirements. The following key questions can help narrow down the options. Engineers designing a gear from scratch will find a structured methodology in the guide How to Select a Gear.
Choose spur gears if …
- the application is simple and undemanding (low speed, moderate load)
- no axial force is permitted, and providing axial support would be structurally impossible or costly
- noise and smooth operation are not critical requirements
- production costs or tool availability are the primary consideration
- easy disassembly and reassembly in the field are required (no need to adjust axial play)
Choose helical gearing if …
- smooth operation and low noise are important (e.g., machine tools, vehicle transmissions, industrial gearboxes near workstations)
- high loads and/or high speeds are present and maximum power density is required
- the bearing arrangement can already accommodate axial forces (angular contact ball bearings or tapered roller bearings are already in place)
- installation space is tight and the higher load capacity allows a more compact design
Practical tip from TEA: How to properly design axial force bearings
The axial force of helical gearing increases with torque and the tangent of the helix angle. For a preliminary estimate: Fa ≈ Ft · tan(β), where Ft is the tangential force. At β = 15°, the axial force is approximately 27% of the tangential force—a value that must be factored into bearing sizing from the outset. Herringbone gearing remains the axial-force-free alternative when the bearing layout cannot provide axial support.
If your application calls for special tooth profiles — such as herringbone gearing, internal gearing, or non-standard modules — TEA offers custom gears to drawing. Basic terms such as module, profile shift, and gear quality are explained in Basic Concepts of Gear Technology.
Need to design a gear system for your application?
Our engineers provide guidance on choosing between straight and helical gears, determining the helix angle, and selecting the appropriate custom gear design—from the initial consultation through to the finished component.
View custom gear solutions →Related articles
Selecting a Gear: Step-by-Step Guide
A systematic approach to gear selection based on torque, speed, material, and operating conditions.
A Comparison of Gear Materials
Steel, gray cast iron, plastic, and sintered metal – a comparison of materials for gears based on strength, wear, and cost.
Gear Technology: Topic Overview
An overview of all guides, fundamentals, and tools related to gear technology, spur gears, and gear design.