The RPM of "A" is 150 and hobbed with 78 teeth. If gears "B", "C", and "D" have 60, 32, and 42 teeth respectively, the RPM of "D" in the gear train illustration is __________. See illustration MO-0088.
• Relationship between gear speed (RPM) and number of teeth when two gears mesh • How to handle a compound gear train (one shaft carrying two gears together) so that their RPM is the same • Overall speed ratio from first driving gear A to final driven gear D using step‑by‑step ratios
• Identify which gears are on the same shaft in the illustration and therefore must turn at the same RPM • Write the gear ratio for each pair that actually meshes (A with B, B with C, C with D, etc.) using teeth counts, then multiply those ratios together to get the overall ratio from A to D • Check whether the final RPM of D should be faster or slower than A based on whether the gear train overall is a speed increase or decrease
• For each meshing pair, use RPM₁ × teeth₁ = RPM₂ × teeth₂ and be sure you assign driver vs driven correctly • Make sure you do not cancel teeth for gears that do not mesh directly; only use ratios for actual contact pairs • After calculating, compare the magnitude: should RPM of D reasonably be greater than, less than, or about the same as 150 RPM? Eliminate answer choices that don’t match that trend
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