AC Servo Motors vs. Stepper Motors: How to Choose the Right Lovejoy Coupling Size

I coordinate emergency orders at a power transmission distributor. Last quarter alone, we processed 47 rush orders with a 95% on-time rate. In most of those cases, the motor itself was fine. The part that stopped the plant was a cheap spider, a missing keyway, or the wrong bore. So when someone asks for a Lovejoy-coupling, the first question I ask is: AC servo motor or stepper motor?

The Comparison Framework: AC Servo Motors vs. Stepper Motors

This isn't a contest about which motor is 'better.' It's a decision about how you'll use the shaft, how much speed you need, and what you can afford to maintain. The coupling size you need—whether that's a 5/8 Lovejoy coupling or a larger L-series hub—usually follows from that decision.

We'll compare three dimensions:

• Speed: what really happens above 1,000 RPM
• Torque: how much, for how long
• Coupling selection: why lovejoy coupling sizes are about torque, shaft bore, and a little bit of humility

At the end, I'll give you a scenario-based recommendation—not a 'buy this' lecture. And yes, I'll tell you where I've been wrong myself. This is not about putting one motor on a pedestal. I do not mean that. It's about matching the motor to the load and the coupling to both.

Dimension 1: How Fast Can a Stepper Motor Turn?

Short answer: faster than it should under load. A NEMA 23 stepper can spin 2,000–3,000 RPM with no load. But 'no load' is not a machine. In a real application, available torque drops quickly after a few hundred RPM. Under a typical load, a stepper's useful ceiling is around 600–1,000 RPM. If you run it above that, you don't always get a stall alarm. You get a mushy position loss that shows up later as a scrap part.

An AC servo motor, by comparison, holds its torque rating across a much wider speed band. Most are rated continuously at 2,000 or 3,000 RPM, and many can do 5,000 RPM in short bursts. That's why the speed question matters more than motor frame size. Why does this matter? Because the coupling has to survive the acceleration, not just the average speed.

One caveat: 'How fast can a stepper motor turn?' is not the same as 'how fast should I run it?' I've seen a 2,000 RPM stepper melt a Lovejoy spider insert because the operator pushed the speed past the torque curve. The motor spun, the insert got hot, and the coupling failed. The fix was adding a helical gear reducer to multiply torque and drop the coupling speed. That's the right use of gearing—not a workaround for a weak motor.

Dimension 2: Torque Behavior and Overload

Steppers are brilliant at holding position. At zero speed, a stepper motor provides strong holding torque—sometimes more than a servo of the same frame. That's why steppers dominate simple indexing and positioning tables. The catch is open-loop operation. The drive sends pulses; the motor follows them. If the load resists too much, the motor loses steps and keeps moving as if nothing happened. No error message, no correction.

An AC servo motor is closed-loop. The drive gets feedback from an encoder or resolver and corrects for load changes. If a spike hits the load, the servo can deliver extra torque for a short time, then recover. That doesn't mean the coupling can ignore it. A servo's peak torque can be two to three times its continuous rating, and that peak torque is what sizes the coupling.

I always ask: What torque does the motor produce at stall, at peak, and at maximum speed? Then I look up the lovejoy coupling sizes that cover the peak number with a safety factor. For a 5/8 Lovejoy coupling, I also check whether the hub can be bored to 5/8 without exceeding the max bore. A torque rating is useless if the motor shaft can't fit into the hub.

Dimension 3: Lovejoy Coupling Sizes and the 5/8 Lovejoy Coupling

Here's where the practical mistakes happen. A '5/8 Lovejoy coupling' does not mean the coupling is 5/8 of an inch wide. It means the hub bore is 5/8 inch. That's a common shaft size for NEMA 23 stepper motors and some smaller AC servo motors. But the bore alone doesn't tell you the coupling size. You need the torque rating, the keyway width, the set screw location, and the max bore rating.

Lovejoy coupling sizes—L035, L050, L070, L090, L095, L099, and so on—are based on torque and bore capacity. A motor with a 5/8 shaft might fit a small L-series hub. But if the motor is an AC servo with high peak torque, the same shaft might need a larger coupling because the insert needs the extra load capacity. The hub will still be bored to 5/8. The size is not the coupling's physical diameter; it's the size needed to carry the load without failing.

I had a customer ask me to solve that exact problem once: motor shaft 5/8, hub max bore 1/2. His suggestion was 'just drill it bigger.' No. You don't drill a hub beyond its maximum bore rating. You move up to the next Lovejoy size with a larger max bore.

What About a Helical Gear Reducer?

'Helical gear' is one of those terms that shows up in the same conversations about motor speed. A helical gear reducer changes the speed and torque that the coupling sees. On the low-speed output side, a simple Lovejoy jaw coupling often works fine because torque is higher and speed is lower. On the high-speed input side, you may need a balanced coupling and good shaft alignment.

If your application needs sustained 1,500+ RPM with variable load, I would push you toward an AC servo. If your load is low speed and you need reliable holding torque, a stepper plus a helical gear reducer is simpler and cheaper. Both are legitimate. The problem is choosing one first and then trying to make the coupling fit afterward.

Scenario-Based Recommendations

Go with AC servo motors when:

• You need sustained speed above 1,500 RPM.
• The load changes during the cycle and position feedback matters.
• You're okay paying more upfront to avoid step-loss issues and tuning time.

Go with a stepper motor when:

• You need high holding torque at zero speed.
• The cycle is short, low speed, and low to moderate load.
• You want simple, inexpensive controls and can live with an open loop.

For the coupling, I start with a Lovejoy jaw coupling for both. The L-type spider absorbs shock, replacement inserts are cheap, and the sizes are easy to understand once you know the torque and bore. For larger shafts, an S-Flex or gear coupling is worth comparing. But the key is knowing the motor type, shaft diameter, and keyway before you ask for a '5/8 Lovejoy coupling.'

One more thing: transparency. I've learned to ask 'what is NOT included' before asking the price. In March 2024, a client called at 4 p.m. with a down packaging line. The drawing called out a 5/8 Lovejoy coupling with L095 hubs, and the motor was a 1.5 kW AC servo. Normal lead time for a bored-to-size hub was seven days. We found a local machine shop, paid $140 extra in overnight machining and freight, and delivered the part by 6 a.m. The client's alternative was a $50,000 penalty for missing their production start. Did the coupling itself cost more? No. But the quote would have been misleading if I'd listed only the base part and surprised them with the rush fee later. Per FTC advertising guidelines (ftc.gov, accessed May 2025), pricing claims need to be truthful and not misleading. I hold our quotes to that standard.

Honestly, I'm not sure why some distributors quote overnight shipping before asking for the bore size. My best guess is they're reading from a price list, not a shop floor. I still kick myself for a rush order years ago where I didn't ask for the keyway width. If I'd started a simple checklist earlier, that client wouldn't have lost a full day.

So: get the motor type, get the torque curve, get the shaft and keyway dimensions, and then get the coupling price. That order has saved me more times than I can count. And it will probably save you a Saturday.