Choosing Between Two Gearmotor Concepts Under a Cost Cap
A design team must pick a drive for a compact conveyor. Requirements: 12 N·m at the output, output speed 30 rpm ±10%, continuous duty, 20,000-hour target life, unit cost at 5,000/year no more than $85, and operation from 24 V DC. Two concepts survive screening. Concept A is a brushed DC gearmotor with a three-stage spur gearbox, quoted at $62. Concept B is a brushless DC motor with a planetary gearbox, quoted at $104 before volume negotiation. The team's analysis spreadsheet shows Concept A meeting torque and speed with a 1.4 safety factor on gear tooth bending, and Concept B with a 2.1 factor. A supplier says Concept B could reach $80 at 5,000 units but will not commit in writing. The schedule allows one prototype round.
1. How should the team treat the unwritten $80 price for Concept B in the selection decision?
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An unwritten price is not a requirement-satisfying fact; it is an assumption, and it should be modeled as such.
At $104, Concept B fails the $85 cap by $19 (about 22%). At a promised $80 it passes by $5. The entire cost case for B rests on a verbal discount with no commitment, so the team should run a sensitivity: at what price does B become preferable on total lifecycle cost, not just unit price?
Lifecycle matters here. Brushed motors wear brushes; at 20,000 hours continuous, brush life may be the limiting item for Concept A and could force replacement or a shorter service interval, adding field cost. Concept B's higher tooth safety factor (2.1 vs 1.4) also reduces risk of a gear failure that would be far more expensive than the unit delta.
Recommendation: do not select B on the verbal price. Either obtain a written quote or a volume price schedule, or select A and document the brush-life risk with a test plan. If B is chosen, record the $80 as an unconfirmed assumption with an owner and a date to confirm, and keep A as a fallback. Never let an uncommitted number silently flip a requirement.
2. Concept A meets torque and speed but has a 1.4 safety factor on tooth bending. Is that acceptable? What would you check before deciding?
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1.4 is not automatically acceptable or unacceptable; it depends on what the factor covers and what the failure consequence is.
Check: (1) Is 1.4 applied to the actual peak torque or only nominal? Conveyor starts, jams, and reversals can spike torque well above 12 N·m; if 1.4 is on nominal only, the margin under a jam may be below 1.0. (2) What is the basis of the strength value - material allowables, heat treatment, and whether the gear is through-hardened or case-hardened? (3) Does the factor include load-sharing between planet or spur teeth, misalignment, and dynamic factors, or is it a bare stress ratio? (4) What is the consequence of tooth failure - a stalled conveyor, product damage, or a safety issue?
If 1.4 is a true stress ratio on worst-case torque with realistic material data, it may be fine for a benign failure mode. If it is nominal-only, it is probably inadequate. Recommend a jam-torque measurement or a torque-limiting clutch, and a gear fatigue test to 20,000-hour-equivalent cycles, before accepting A.
3. The team can afford one prototype round. What should that prototype be used to learn, and how does that affect the choice?
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One prototype round is a scarce information budget; spend it on the highest-uncertainty, highest-consequence unknown.
The unknowns differ. For A, the big unknown is brush and gear life at 20,000 hours plus behavior under jam torque. For B, the unknowns are the real unit cost and controller integration, but its mechanical margin is already comfortable.
A prototype cannot run 20,000 hours in the schedule, so use accelerated life testing (elevated speed or load with a validated acceleration model) and a jam-torque test on A. If A's brush life projects short of target or its gears fail under jam torque, A is out regardless of price, and B becomes the choice - which then makes obtaining a written B price the critical path.
Decision logic: prototype A because it carries the disqualifying risk. If A passes, its lower committed price wins. If A fails, negotiate B in writing. This sequences the spend to resolve the question that can actually change the answer.