A spur gear is a cylindrical gear with straight teeth cut parallel to its rotational axis. It’s the simplest and most common type of gear, used to transmit power between two parallel shafts. If you’ve ever pictured a gear in your mind, you almost certainly imagined a spur gear: a flat disc with evenly spaced teeth around its edge, meshing with another disc to transfer motion.
How Spur Gears Work
Two spur gears mesh by interlocking their teeth, transferring rotational force from one shaft to another. The shafts must be parallel to each other, and the gears spin in opposite directions. When a smaller gear (called the pinion) drives a larger gear, the output shaft spins slower but with more torque. Reverse the arrangement, and you get higher speed with less torque. This is the basic principle behind gear ratios.
What makes spur gears particularly effective is that their tooth surfaces mostly roll against each other rather than slide. This rolling contact generates less friction and heat, which translates to remarkably high mechanical efficiency, often reaching 99 percent. Measured mesh losses in testing range from about 0.3 to 1.7 percentage points depending on speed, meaning very little energy is wasted in the transfer.
Key Parts and Sizing
Every spur gear has a few critical dimensions. The teeth themselves are the most obvious feature, and their size is defined by a measurement called the diametral pitch (common in the U.S.) or the module (common internationally). Diametral pitch is the number of teeth divided by the pitch diameter of the gear. Module is the inverse: the pitch diameter divided by the number of teeth. A higher diametral pitch means smaller, finer teeth. A larger module means bigger, chunkier teeth that can handle more force.
The pressure angle describes how steeply each tooth leans. Modern spur gears almost universally use a 20-degree pressure angle, though older designs sometimes used 14.5 degrees. A steeper angle produces a stronger tooth that handles higher loads, which is why 20 degrees became the standard. Beyond this, the face width (how thick the gear is from front to back), the bore (the hole in the center that fits onto the shaft), and the hub (the raised section around the bore) round out the main anatomy.
External vs. Internal Spur Gears
Most spur gears are external, meaning the teeth point outward from the center. When two external spur gears mesh, their shafts rotate in opposite directions. Internal spur gears flip this arrangement: the teeth are cut on the inside rim of a ring-shaped wheel, and a smaller external gear (the pinion) sits inside it. Both shafts then rotate in the same direction, eliminating the need for an extra idler gear to reverse rotation.
Internal gears also offer a more compact package because the centers of the two gears can be much closer together. They produce a stronger drive with more surface contact and less sliding between teeth. The tradeoff is that they need more complex housing to support the pinion inside the ring. Internal spur gears are a core component of planetary gear systems, the type found in automatic transmissions and many power tools.
Where Spur Gears Are Used
Because they’re simple to design and relatively inexpensive to manufacture to tight tolerances, spur gears appear in an enormous range of machinery. Automotive transmissions, machine tools, electrical appliances, conveyor systems, and pumps all commonly rely on them. Clocks and precision instruments sometimes use a variation with a different tooth profile (called a cycloid curve) that’s suited to very light loads, but the underlying gear type is still a spur gear.
Their main limitation is noise. Because the entire width of each tooth engages all at once (rather than gradually, as with helical gears), spur gears produce a characteristic whine that gets worse at higher speeds. They also generate no axial thrust, the sideways force along the shaft that helical and bevel gears create, which simplifies bearing design and keeps costs down. This makes spur gears ideal for moderate-speed applications where cost and efficiency matter more than quiet operation.
Noise at Higher Speeds
Spur gears are noticeably louder than other gear types, and the physics behind this is straightforward. As teeth mesh, small manufacturing imperfections create tiny oscillations that overlay the smooth rotation. Friction between tooth surfaces heats the surrounding air, and air or oil trapped between meshing teeth gets ejected with each engagement. All of these effects intensify as speed increases.
Testing on steel spur gear pairs illustrates the scale of the problem. At 500 RPM with friction present, sound pressure levels stabilized above 100 decibels measured one meter away, roughly the volume of a lawnmower. At 3,000 RPM, levels climbed to between 100 and 110 decibels. For comparison, frictionless conditions at 500 RPM produced only 80 to 85 decibels. This is why spur gears are typically reserved for low to moderate speeds, while helical gears take over in high-speed or noise-sensitive applications. Even a small increase in the gap between teeth (called backlash) can reduce noise from trapped air and oil, which is one reason gear designers sometimes build in slightly looser tolerances for high-speed spur gear pairs.
Lubrication and Maintenance
Spur gears in enclosed gearboxes typically run in a bath of gear oil. For most industrial setups, extreme pressure gear oils are recommended because they protect teeth from scuffing under heavy loads. Lightly loaded gearboxes without shock loads can get by with simpler rust-and-oxidation-inhibited oils, as long as the viscosity is matched to the gear set’s operating speed and temperature.
Open spur gears, where the teeth are exposed rather than sealed inside a housing, need a different approach. Tacky semi-fluid greases containing solid lubricant additives like graphite or molybdenum disulfide can be brushed or sprayed directly onto the teeth. High-viscosity asphaltic or synthetic oils in semi-closed systems, where one gear dips into an open pan of oil before meshing, are another option. For gears that run infrequently, grease alone provides adequate protection between uses.
Routine maintenance for enclosed systems means checking oil levels, changing filters, repairing leaks, removing any water contamination, and keeping the system clean. The single most important variable is getting the oil viscosity right for your specific gear set, speed, and operating temperature. The machine’s manual or nameplate is the best starting point for those specifications.