To correctly determine the gear ratio when there is no documentation, which method is correct?

To correctly determine the gear ratio when there is no documentation, which method is correct?

• A. Count the teeth on both pinion and ring gears and then divide the number of teeth on the pinion gear (the smaller number) into the number of teeth on the ring gear (the larger number).
• B. Count the teeth on both pinion and ring gears and then divide the number of teeth on the ring gear (the larger number) into the number of teeth on the pinion gear (the smaller number).
• C. You can't determine ratio accurately by counting teeth; you must reassemble the unit, mark and rotate the pinion until the ring gear completes one revolution.
• D. Contact the manufacturer and ask for the correct documentation.

Answer: (A)

When you need the gear ratio from a ring gear and pinion without any documentation, use the tooth counts directly. The ratio is simply the number of teeth on the ring gear divided by the number of teeth on the pinion gear, because the ring is the driven gear and the pinion drives it. Counting both teeth and dividing the larger (ring) by the smaller (pinion) gives the exact reduction: more teeth on the ring than on the pinion means a reduction greater than one. If you swapped the division (pinion over ring), you’d get the reciprocal and misstate the ratio. You don’t need to reassemble or rotate the gears to determine this, though you can check by rotating one gear and observing how many turns the other makes if you’re unsure. Asking the manufacturer is an option for documentation, but it isn’t required to determine the ratio from the gears themselves.

When you need the gear ratio from a ring gear and pinion without any documentation, use the tooth counts directly. The ratio is simply the number of teeth on the ring gear divided by the number of teeth on the pinion gear, because the ring is the driven gear and the pinion drives it. Counting both teeth and dividing the larger (ring) by the smaller (pinion) gives the exact reduction: more teeth on the ring than on the pinion means a reduction greater than one. If you swapped the division (pinion over ring), you’d get the reciprocal and misstate the ratio. You don’t need to reassemble or rotate the gears to determine this, though you can check by rotating one gear and observing how many turns the other makes if you’re unsure. Asking the manufacturer is an option for documentation, but it isn’t required to determine the ratio from the gears themselves.