Gears 101: Spur Gears vs. Helical Gears

Spur gear and worm drive.Few drivers spend time to appreciate the gears in their automobile’s transmission, but without them, they couldn’t get to where they’re going. In fact, gears have been the backbone of mechanical power transmission for millennia and with the exceptions of materials and geometry, the principle behind their operation remains elementary.

Undoubtedly, the spur gear has become the most prominent type of gear, even though there is an argument that the less common helical gear is better suited for important power transmission applications. Ultimately there are numerous factors that influence the decision to use one over the other.

Spur gears are characterized by cogs or ‘teeth’ that extend radially from the gear axis. All of the contact surfaces on the teeth are parallel with the shaft axis. Since adjacent spur gears mate just one tooth at a time the entire transmission load is carried by that one tooth. (Multiple spur gear teeth can be engaged by a worm drive or gear rack.) This results in lower loading capacities when compared to a gear style that transmits power across multiple teeth simultaneously. Spur gears are optimal for transmitting power between shafts at slow-to-moderate degrees of speed and torque. Spurs gears are also considerably noisy, especially at higher speeds, due to the surplus space between gear teeth, known as backlash.

Spurs gears are typically the easiest type of gear to manufacture, also making them the least expensive. They are typically easier to replace in a gear train assembly when the need arises. The inside and outside diameters of gears in a gear train do not need to match. In all gear trains, mating gears of different sizes is the basis for a mechanical advantage. An increase in gear outside diameter between the input shaft and output shaft produces higher output torque at the expense of speed; decreasing the outside diameter produces higher speed but with less torque.

However, to ensure efficient power transmission, gears in a gear train must have the same circular pitch and pressure angle in order to effectively mate. Circular pitch is the value of distance between the center of one gear tooth to the next tooth center. In order to find this, first the diametral pitch needs to be calculated. Diametral pitch is the ratio of the number of teeth to the pitch diameter of the gear.Source: Gear_words / CC BY-SA 3.0 Pitch diameter is the distance between opposing tooth centers on a gear.

Diametral pitch (DP)

DP = N/PD

Where:

PD = pitch diameter

N= number of gear teeth

Circular pitch (CP)

CP = Π/DP

Pressure angle: The angle between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh. Common pressure angles are 14.5, 20 or 25°.Source: GearHeads / CC BY-SA 3.0

Spur gears most commonly mate with other spur gears, but may be used with a worm drive, internal spur gear, face gear or as the pinion half of a rack and pinion setup.

For applications that require higher speeds, higher torque transmission or quieter operation, spurs gears can often be replaced with helical gears. At the name suggests, the teeth on a helical gear do not run parallel to the gear axis, as with a spur gear. Instead the teeth are angled, representing a segment of a helix, and can only mate with other gears with angled teeth.

Helical gear transmission.There are several advantages to this design. Meanwhile helical gear teeth gradually accept more load as more of the tooth face is engaged. Multiple teeth are typically engaged at once, distributing the load across more surfaces. This reduces the shock loads that cause stress and noise with spur gears. It’s often possible to quickly replace spur gears with helical gears if the transmission specifications remain the same.

Another distinct benefit is the ability to use two helical gears for a gear transmission with perpendicular axes, sometimes called a crossed or skewed orientation. A spur gear would need a compatible face gear in order to accomplish the same. Helical gears are labelled with a ‘handedness,’ determined by the orientation of the teeth. Helical gears with opposite handedness transmit torque between parallel axes; gears with similar handedness transmit torque between perpendicular axes.

Circular pitch and pressure angle are important considerations for helical gears, but so too is helix angle. This is the angle of the gears relative to the gear axis. Helix angles of 15, 23, 30 and 45° are most common. Helical gears in a parallel-axis configuration must have the same helix angle but must be off opposite handedness. Those in a perpendicular-axis configuration must be of the same handedness, but can have different helix angles provided that the sum of the helix angles is 90. Crossed helical gears are not as mechanically efficient as those in parallel.Herringbone gear set. Source: Andy Dingley / CC BY-SA 3.0

Yet there are some drawbacks to helical teeth. First, they are more difficult to machine and more expensive to acquire. There is an immense amount of dynamic friction on the surfaces of the gear teeth, so helical gears’ work surfaces often need lubrication and may need to be replaced quicker than a spur gear. Such a replacement is additionally troublesome to install within an existing gear train. There is a fair amount of axial thrust produced by helical gears that must be addressed, typically with thrust bearings.

It’s worth mentioning that axial thrust can also be nullified by using one of two helical gear variants, a double helical gear or a herringbone gear. These gear types have teeth that resemble a shallow letter V and intermesh with opposite-handed pairs in a parallel-axis orientation. As with helical gears, they are excellent for transmitting high speed or torque. The gear sets can be manufactured to contact tip-to-tip or tip-to-trough. They can be offered in a perpendicular-axis orientation only if one of the gears is a face gear. The only difference between herringbone and double helical gears is that double helicals have a centered groove or void machined into the circumference of the gear face.

Gears are an ancient technology—they’ve been around since the 4^th century B.C. and were invented by the Chinese as a type of rudimentary dead reckoning technology. However, no matter how old gears become, they find important, modern applications every day and are in no danger of becoming obsolescent.