The problem is worm gears are typically bronze, a soft metal, and the gear’s sliding surfaces generate friction that causes wear. This mandates frequent oil changes and means relatively short gear life. (Ironically, hardening worm gears accelerates wear.) In comparison,experience minimal wear and are practically maintenance-free.
Spur-gear surfaces have rolling contact, but the teeth are cut straight across on a face; one or two teeth are always in contact with another gear. This incomplete face engagement means spur gears are noisier and vibrate more than helical gears.
Another advantage of helical gears over spur gears is in torque capacity. Spur gears, by design, are weaker than helical gears because loads are transmitted over fewer teeth. Helical gearing is machined with angled teeth, then hardened and ground, which is complex but necessary to achieve a high-efficiency gear mesh. Because teeth are cut at an angle, the gears gradually mesh. Two or three teeth of each gear are always in contact with other gears. This alleviates the load on each tooth and creates a smooth transition of forces from one tooth to the next. The result: less vibration, wear, and noise, and longer life.
Some helical gears are virtually maintenance-free. Tooth profiles are so precise there is practically no wear — and units packaged in sealed, oil-filled housings may require no oil changes, which is a boon for the environment as well as the bottom line.
Helical-gear reducers also come in many shapes, sizes, and configurations, letting machine designers eliminate high-wear and high-maintenance parts such as belts, pulleys, chains, and sprockets.
So, although worm and spur gears are less expensive, they have much shorter life spans than helical gears. After about three years they are usually replaced. And frequent replacement requires more capital and drives up maintenance, downtime, and waste-disposal costs.