Why I Don't Approve a Coupling Based on the Ringfeder Torque Chart Alone

Torque Charts Are Not Promises

I think we give torque charts too much authority. Not because the published numbers are wrong, but because a torque rating is a baseline, not a selection conclusion. This opinion comes from work, not theory. I am a quality and compliance reviewer for a power transmission distributor. Every year I check roughly 200+ unique specs against application drawings. I have rejected about 11% of first deliveries in 2025 due to mismatched torque ratings, alignment limits, or incomplete keyway specs. Some of those rejections hurt. One bad cast coupling was worse than expected: it looked perfect in the catalog, but the bore tolerance clearly fell outside the drawing allowance.

For a brand like Ringfeder Power Transmission GmbH, the engineering reputation makes those mistakes easier to make. The catalog looks polished, the ratings look generous, and the torque charts look self-explanatory. But that polish can create a false sense that nothing else matters.

My opinion: the Ringfeder torque chart is a great screening tool, but a poor final approval tool.

Read the Ringfeder Torque Chart as a Baseline

Here's the thing: every Ringfeder torque chart answers one question—can the coupling transmit this torque? It does not answer whether the coupling will survive in your specific assembly. Misalignment, backlash, resonance, thermal expansion, and load direction all change the answer. I don't have hard data on what percentage of field failures trace back to those missing conditions. Anecdotally, I would say most of the couplings I see returned for “failure” were within the catalog torque rating but outside the real-world condition envelope.

That is not a knock on the manufacturer. It is a knock on human nature. We see a number, compare it to our peak torque, and move on. The best spec reviewers I know treat the chart as the beginning of a conversation: what is the service factor? What is the misalignment spec? Which bore fit? What is accessible for installation?

I wish I had tracked those “within rating anyway” returns more carefully from the start. What I can say anecdotally is that the mismatch more often than not appears later, in escalations, rather than in the first torque check.

Rigid Couplings Are a Different Test

Take the 2 in rigid coupling. If you're only doing a torque calculation, a 2 in rigid coupling often looks great: no moving parts, high torque capacity, low cost, zero maintenance. But a rigid coupling has no forgiveness. It can't accommodate misalignment. It can't absorb shock. It transfers every angular offset and parallel offset straight into the machine's bearings.

So I'd argue: the 2 in rigid coupling is the wrong choice unless you have a very controlled alignment situation. Not because it's weak, but because it's unforgiving. Not ideal, but workable? Only if the machine base is precise and the shafts are shimmed properly.

In my experience, the torque chart makes rigid couplings look safer than they are. The chart says “strong enough.” It says nothing about “aligned enough.” I rejected a 2 in rigid coupling order because the system had no jacking provision for alignment, even though the torque rating was perfectly adequate. The concept was not.

Actuators and Stepper Motors Make the Point

Here's where the torque chart logic gets dangerous: electric actuators. A common question I get is, “What's a stepper motor, and can I use one to drive this linear electric actuator?” The answer is yes in many cases, but the coupling selection doesn't start with the motor's holding torque.

Let's define “What's a stepper motor?” quickly: it's a brushless DC motor that moves in discrete 1.8 degree or 0.9 degree steps. It's often chosen for positioning. But mechanically, what matters to the coupling is the torque it produces while accelerating and decelerating, not just the torque it holds when stopped. Holding torque can look high. Dynamic torque at speed can be much lower. The coupling sees the dynamic torque.

And if the motor drives a leadscrew in a linear electric actuator, the coupling sees reflected load torque, not the linear force. For a simple leadscrew, reflected torque is roughly linear force times screw lead divided by two pi times drive efficiency. That's a basic formula, but I've seen actuator proposals with the linear thrust number written in the coupling field. That's a mismatch, and no torque chart can fix it.

Oversizing Is Not an Honest Fix

I have mixed feelings about oversizing. Part of me wants to approve a bigger coupling just to remove torque from the risk list. Another part knows that bigger diameter means more inertia, more radial space, and often a higher cost with zero functional benefit. The rational compromise is to oversize only after checking inertia and envelope. If you can't fit the bigger coupling, you're back to the real problem.

Oversizing also masks assumptions. If the torque calculation is off by 30%, the odds are it's off somewhere else too, service factor, motor starting torque, or actual load peaks. A lesson learned the hard way.

In my view, the honest approach is to say: “This coupling is right for this machine and these conditions.” Not “the bigger size makes us safer.”

I Don't Care How It Looks in the Catalog

I routinely hear the same pushback: “But it's within the catalog rating—look, it's on the Ringfeder torque chart.”

My answer: good, then the torque requirement is satisfied. Now let's check the other five requirements. The catalog rating assumes a keyed shaft, a well-cut keyway, a bore fit within tolerance, a reasonably steady load, and a safe speed range. If one of those conditions isn't met, the rating is an anchor, not a guarantee.

Take this with a grain of salt: in my review logs, maybe 60-70% of the “torque failures” I see are actually condition failures. The torque number was fine. The alignment, shaft fit, or speed was not.

My Bottom Line

I recommend using Ringfeder Power Transmission GmbH's published data, especially the Ringfeder torque chart, to screen candidates. It tells you whether a coupling is in the right torque class. That matters. But I don't recommend treating the chart as your final authority.

A 2 in rigid coupling taught me that. A linear electric actuator with a modest stepper motor taught me that too. The actuator's thrust spec looked irrelevant to torque, until I remembered the screw lead. The stepper's holding torque looked generous, until I considered dynamic torque. In each case, the torque chart was the floor, not the ceiling.

So here's my honest recommendation: if you're considering a Ringfeder coupling, do the torque calculation, do the alignment check, and do the installation check. If you can't verify the conditions, say so. That's more useful than pretending the chart answered everything. Better a “this one isn't right for this application” than a “well, the catalog says it works” that turns into a failure at start-up.

Look, I'm not saying Ringfeder torque charts are useless. They're not. They're just not the complete story. And if someone asks “what's a stepper motor?” my longer answer includes how its torque profile changes your coupling decision. The short answer is: it's a torque source with a bad habit of being misread. Same as a torque chart.