I’m the person who orders gearboxes and motors for a mid-sized automation company. I’ve been doing this for five years, and I’ve made enough mistakes to know what to ask before I push “submit.”
Here’s the thing: there’s no single “right” way to buy a Bonfiglioli gearbox or motor. It depends on what you’re doing. If you’re replacing a broken unit, you need different information than if you’re building a new assembly. If you need exact positioning, a VFD-driven induction motor won’t cut it. If you only need adjustable speed, a servo system is overkill.
So I’m going to walk through the three scenarios I see most often, what I’ve learned in each, and how to figure out which one you’re in.
Scenario 1: You Have a Machine, but No Bonfiglioli Gearbox Manual
This is the most common way I started. A machine arrives, or has been on the floor for years, and no one can find the documentation. You type “bonfiglioli gearbox manual pdf” into a search bar and hope.
Good news: Bonfiglioli publishes manuals and datasheets for most product lines. But you need the right model code first. The model code is usually stamped on the nameplate. Take a photo before you clean anything off. If the nameplate is missing or worn, measure:
- Input and output shaft diameters
- Shaft center distances (if possible)
- Mounting foot or face dimensions
- Overall length and width
That sounds basic, but I’ve watched people skip it. One of our vendors sent us a “compatible” gearbox based on a partial model number. The shaft was 5 mm smaller than the original. That doesn’t sound like much until you try to fit a coupling. Not ideal, but workable.
I said “same assembly” to a supplier once. They heard “same mounting position.” Result: the gearbox had the right foot arrangement but the output flange was 90° off. Discovered this when it arrived and the coupling didn’t line up.
The phrase “Bonfiglioli gearbox assembly” usually matters in two ways. First, the assembly code tells you the mounting orientation — where the feet go, where the output shaft points, and where the vent plug has to sit. The same gearbox can be assembled in several positions. If you order the wrong one, the oil fill and breather may be upside down. The gearbox won’t necessarily fail immediately, but it will run at the wrong oil level and may leak.
Second, assembly can refer to the construction of a gearbox with a motor and maybe a backstop. That’s where things get confusing, because the same catalog number can mean different things to different distributors.
Here’s what I do now: I get the manual PDF, find the assembly diagram, and confirm the position code in writing. Then I ask the supplier to send a screenshot of the exact assembly drawing they’re quoting. A thirty-second check has saved me from ordering the wrong configuration more than once.
One thing I’ve learned the hard way: buying a used or third-party reconditioned “Bonfiglioli gearbox” can be fine, but only if the assembly code is verified. “It came off the same machine” isn’t a spec.
Scenario 2: You Need a High Torque Servo Motor (or DC Servo Motors for a Retrofit)
This scenario is different because you’re designing or upgrading motion, not just replacing a failed part. If you need fast starts, stops, or precise positioning, you’re usually looking at a high torque servo motor and a servo gearbox. Bonfiglioli’s planetary servo gearboxes are commonly paired with servo motors in this world.
Before buying a servo motor, I check three things:
- Continuous torque vs. peak torque. The “high torque” in the name doesn’t mean you get it all the time. You need to know both numbers. The motor can do more for a short time, but if you select based only on peak, thermal issues show up later.
- Inertia. If the load inertia is many times the motor inertia, the motor will work, but tuning becomes harder. Not a failure, just painful.
- Feedback. Incremental encoder, absolute encoder, resolver? For a compact PLC setup, absolute is easier; for a simple speed/position update, incremental may be enough.
And what about DC servo motors? We still get asked about them. Most modern systems use AC servos, but there are legacy machines with DC servo motors. If you’re doing a retrofit, don’t assume a DC servo motor is a better match just because it’s cheaper. Replacing an existing DC servo motor requires matching voltage, current, and tachometer/encoder feedback. I once quoted a “drop-in” DC servo motor for a customer and almost missed that the old motor had a separate tachometer feedback, not an integrated encoder. We caught it before ordering, but it took two email threads and a phone call. Per FTC guidelines (ftc.gov), “drop-in” claims need to be substantiated. I take that as: ask the seller to show how the replacement matches the original specs.
Here’s my honest take after managing these purchases: don’t buy DC servo motors for new designs. Used-market DC motors can be a trap. The “savings” from a lightly used motor disappear when you pay for custom cables, connectors, and tuning time. I learned this when I saved $450 on a used servo motor and then spent $700 on a matching connector and encoder cable. Not my best moment.
Look, if you want a high torque servo motor option, pay for the manufacturer’s selection software or ask a local distributor to run a sizing study. I know that sounds like extra cost. But the extra cost is tiny next to a machine that vibrates through every speed node.
Scenario 3: You Just Want to Adjust Speed — How VFD Control Motor Speed Fits In
Not every application needs servo positioning. If you have a conveyor, a mixer, or a pump, you probably just need a variable speed range and controlled acceleration. That’s where a variable frequency drive (VFD) and a standard AC gear motor make sense.
Let me answer the “how VFD control motor speed” part simply: a VFD changes the voltage and frequency supplied to an AC induction motor. Motor speed is based on frequency. Going from 60 Hz to 30 Hz reduces the motor’s synchronous speed roughly in half. The motor, as long as it has enough torque, slows down proportionally.
Wait — torque and frequency aren’t perfectly parallel. At low speeds, motor cooling becomes an issue with a standard enclosed fan-cooled motor, because the fan slows down too. With a gearmotor, this matters if you run below 20–30% of rated speed for long periods. Check the motor’s inverter-duty rating or add forced cooling.
What I tell people:
- If you need adjustable speed with occasional stops and starts and no positioning demand, a VFD plus AC gearmotor is the sensible route.
- If you need high speed AND high torque at low speed (zero-speed torque), a servo motor or a properly sized vector drive with an inverter-duty motor is a safer bet.
- If you need position control — indexing, synchronized motion — you’re back to a servo motor or a motion controller with a closed-loop VFD.
One of our recent conversions used Bonfiglioli gearmotors with VFDs on a bottling line. The old machine had a mechanical variable-speed pulley arrangement. After switching, the operator could enter speed on a panel instead of crawling under the machine with a wrench. Revenue was the same, but maintenance time dropped. There’s something satisfying about that.
The bad part? We almost ordered VFD cables with unnecessary shielding, and the first quote didn’t include a brake resistor for the dynamic stop. The vendor finally said, “what’s NOT included?” and that changed my approach to every quote since. I’ve learned to ask that before “what’s the price.” The vendor who lists all fees upfront — even if the total looks higher — usually costs less in the end.
How to Know Which Scenario You’re In
If you’re reading this because your machine is down and you have a part number, start with scenario 1. Get the manual PDF and verify the assembly code before anything else.
If you’re designing a new machine or upgrading motion control, ask: does the process need positioning or just speed? If positioning, scenario 2. If speed, scenario 3.
If you’re replacing a legacy DC servo motor, treat it as scenario 2. Don’t make the change based on the cheapest motor. Match the feedback, voltage, and torque curve.
One more thing. I don’t have hard data on how many orders fail because of one omitted line-item, but based on the last few years, my sense is it’s higher than most people think. It’s usually not the gearbox. It’s the wrong assembly code, the missing manual page, or the overlooked feedback cable. A little extra verification costs almost nothing compared to the downtime.
And if a supplier won’t send you the assembly drawing before you pay? Take that as a sign. Better to know upfront than after the truck arrives.