Gears and gearboxes

Why it matters

Motors like to run fast, typically 1,200 to 3,600 rpm, but conveyors, mixers and hoists need slow, strong motion. Gearboxes (also called speed reducers) bridge that gap. A mechanic needs to know what kind of gears are inside, how to work out the speed and torque at the output, and what a healthy gearbox looks, sounds and smells like.

Gear types

• Spur gears have straight teeth parallel to the shaft. Simple and efficient, but noisy at speed. They produce no thrust load.
• Helical gears have angled teeth, so they mesh gradually and run quieter. The angle produces thrust along the shaft, which the bearings must carry.
• Bevel gears connect shafts at an angle, usually 90 degrees. Spiral bevel gears are the quieter, curved-tooth version.
• Worm gears use a screw-like worm driving a wheel. They give large reductions in one stage (for example 40:1) and can resist being back-driven, but they are less efficient and run hotter.
• Planetary gears have a central sun gear, planet gears around it and an outer ring gear. They pack a high ratio into a small space.
• Rack and pinion turns rotation into straight-line motion.

Gear ratio

The gear ratio of one pair is the number of teeth on the driven gear divided by the teeth on the driver:

ratio = driven teeth / driver teeth

A 20-tooth pinion driving a 60-tooth gear has a ratio of 60 / 20 = 3, written 3:1. The output turns once for every three turns of the input.

Speed goes down by the ratio:

output rpm = input rpm × driver teeth / driven teeth

Worked example 1. A 1,750 rpm motor drives a 20-tooth pinion meshing with a 60-tooth gear.

output rpm = 1,750 × 20 / 60 = 583.3 rpm

Torque goes up by the ratio, minus losses:

output torque = input torque × ratio × efficiency

If the motor delivers 10 N·m and the gear pair is 98 percent efficient, the output torque is 10 × 3 × 0.98 = 29.4 N·m. Power is conserved apart from losses: speed down, torque up.

Multi-stage trains

Gearboxes often have two or three stages. Multiply the stage ratios to get the overall ratio.

Worked example 2. Stage 1 is 18:54 (ratio 3) and stage 2 is 20:80 (ratio 4).

overall ratio = 3 × 4 = 12
output rpm    = 1,800 / 12 = 150 rpm

An idler gear between two others changes direction but not the ratio, because its teeth cancel out.

Worm gears

For a worm, the ratio is the wheel teeth divided by the number of starts (threads) on the worm. A 40-tooth wheel with a single-start worm gives 40:1. A two-start worm with the same wheel gives 20:1. Worm efficiency can be well under 90 percent, so check the nameplate before assuming output torque.

Reading a gearbox nameplate

A reducer nameplate usually lists the ratio, input and output speed, rated input power or output torque, service factor, mounting position and lubricant. The mounting position matters: the oil level and breather location are set for a specific orientation, and mounting it differently can starve upper bearings of oil.

Inspecting a gearbox

During a PM or a fault call, look and listen for:

• Oil level and condition: check the sight glass at the correct mounting position. Milky oil means water; dark, burnt-smelling oil means overheating; metallic glitter means wear.
• Leaks at shaft seals and gaskets.
• Temperature: compare with the same unit on a sister machine or with past readings.
• Noise: a steady whine is normal for some gears; a knock once per revolution may mean a damaged tooth.
• Backlash: the small free play between teeth. Too much suggests wear.
• Breather: a blocked breather builds pressure and pushes oil past seals.

Common mistakes

• Dividing the wrong way. If the output should be slower, the answer must be smaller than the input speed.
• Forgetting that torque rises as speed falls, so a slower output shaft, key and coupling must be sized for much higher torque.
• Topping up with the wrong oil. Gear oils differ in viscosity and additives; use the one on the nameplate or lubrication schedule.
• Ignoring the mounting position when refitting a gearbox.

Safety

Gear meshes are severe nip points. Inspect running gearboxes only from outside the guards, and open covers only after lockout. Hot oil can burn. Your employer must train you on their equipment before you service a gearbox.

Recap

• Ratio = driven teeth / driver teeth; multiply stage ratios for a gear train.
• Output speed = input speed × driver / driven; output torque = input torque × ratio × efficiency.
• Worm ratio = wheel teeth / worm starts.
• Check oil, leaks, temperature, noise, backlash and the breather.

Try it

Open the Do tab and finish a gear train calculator by writing the output speed formula.