It Started With a Design Review

I'm a quality compliance manager at maxon-motor. Every day, I review specifications for precision drive systems before they reach our customers—roughly 80 technical proposals and design documents each month. And in Q1 2024, I rejected about 22% of first submissions due to vague component referencing. That's where this story starts.

The design review was for a custom liftgate pump and motor assembly. The engineer had specified "RE 50 motor or equivalent" in the BOM. That phrase—"or equivalent"—immediately caught my attention. I asked him: "What's your equivalent?" He shrugged. "I dunno—something with similar torque."

That's the moment I knew we had a problem. And it's the moment I decided to change how we handle motor selection documentation. (Should mention: this was for a 50,000-unit annual order. The stakes were higher than usual.)

The Problem With 'Or Equivalent'

I have mixed feelings about the phrase "or equivalent." On one hand, it gives procurement flexibility. On the other, it's a recipe for disaster when you're dealing with brushless DC motors or stepper motors that have tight integration requirements. A drop-in replacement that's 2mm longer won't fit the mounting bracket. A BLDC motor with a different winding scheme might overheat the controller.

I learned this in 2022. When I implemented our verification protocol for motor datasheets, we caught 14 specification mismatches in the first six months. Not all were critical, but five would have caused at least a $5,000 rework each. The engineer who designed one assembly had specified a maxon brushless motor based on torque alone, without checking the inertia match to the load. The vendor had already ordered the motor (ugh). We had to redesign the linkage around the wrong motor—an $18,000 project delay.

That failure changed how I think about motor selection documentation. The 12-point checklist I created after that mistake has saved us an estimated $8,000 in potential rework just in the last year.

What I Now Require

For every critical motion component—whether it's a servo motor, stepper motor, or BLDC—I now require the design engineer to specify exactly one approved alternate model, with verified datasheet compatibility. No more "or equivalent." If I see that in a BOM, I send it back. (I really should automate this check in our PLM system.)

The alternate must match on these parameters:

  • Mounting dimensions (within 0.5mm)
  • Supply voltage range
  • Continuous and peak torque
  • Thermal resistance
  • Encoder compatibility

At least, that's been my experience with precision motion applications. For simpler DC motors in a fan application, you might have more leeway. But for servo-driven robotics? You need exact specs.

The Project That Proved Me Right

In June 2024, we were specifying a maxon bldc motor for a new robotic arm joint. The primary choice was the EC 45 flat motor. I required an approved alternate: the EC 60 flat with a different winding. The R&D lead pushed back—said it was unnecessary, that the primary would work, and the alternate added review time. He wasn't wrong about the review time. But I held the line.

Three months later, the primary motor failed a critical stall-torque test at the customer's site. The 60mm alternate fit the housing (we'd verified the mounting pattern), and because we'd already validated its thermal curve, we swapped it in two days. The alternative? A five-week redesign of the arm linkage. That would have delayed product launch and cost us a $42,000 penalty clause. (Thankfully, we avoided that.)

The design manager sent me an email after the swap: "I was wrong. That alternate spec saved our ass. I'm never fighting you on this again."

The Cost-Conscious Engineer's Objection

I've heard the counterargument plenty: "Specifying two approved motors doubles the qualification workload." That's true—for the first project. But once you have a library of approved alternates, the workload drops. On our fourth project using the same motor platform, the qualification took less than 30 minutes. The rework we avoided was worth it.

I don't have exact numbers across all projects, but in our precision motor division, the rule of thumb is: one day of up-front verification saves three to five days of downstream rework. That might vary by application, but for high-volume B2B projects like liftgate motor assemblies, the ratio holds.

5 minutes of verification beats 5 days of correction.

The Takeaway (And the Mic Drop)

If you're a procurement manager or engineer, I'd recommend:

  • Never accept "or equivalent" on a critical motion component unless you've verified the datasheet yourself
  • Build a short list of approved alternates for each product line
  • Document the verification criteria explicitly in your RFP or design spec

This was accurate as of my Q3 2024 audit data. The motor industry changes fast, especially with new brushless technologies, so verify current specifications and pricing before locking in your BOM.

I'd add one more thing: when you specify a maxon-motor, you're paying for consistency. The datasheet values are conservative and verified. That's worth factoring into your risk assessment—not just the unit cost. (Not that I'm biased or anything.)