Ringfeder Torque Specs: The Engineer's Guide to Reading Them Correctly

Ringfeder torque specs are system values, not fixed ratings. The same locking assembly can transmit more torque on a machined steel shaft with clean, dry surfaces than on a worn shaft with a loose fit and a film of grease. The catalog number is where you start, not where you stop.

I work as an applications specialist at a power transmission distributor, and I've handled more than 200 rush orders in the past seven years. In March 2024, 36 hours before a packaging line restart, a client called with a new brushless DC motor that didn't match the existing hub. The motor's peak torque was below the catalog transmittable torque for the Ringfeder locking assembly we planned to use. But the hub had a shoulder that limited the contact length to about 60 percent of the assembly's clamping length, and the usable torque dropped below the motor's peak torque. We sourced a longer hub, had it machined overnight, and the line restarted on time. It was close.

What "Ringfeder torque specs" actually mean

Ringfeder locking assemblies are keyless shaft-hub locking devices. They use tapered rings and clamping screws to create radial pressure. Instead of a keyway, torque is transferred through friction between the shaft, the assembly, and the hub. That's why the torque spec depends on the clamping force and the friction coefficient at the contact surfaces.

According to Ringfeder Power Transmission's published technical documentation (available at ringfeder.com), the rated transmittable torque is based on a specified shaft diameter, a specified tolerance, a specified hub contact length, and a specified tightening torque on the clamping screws. Change any of those variables and the result changes. There is no single torque value for a "40 mm" locking assembly without also knowing those conditions.

Put another way: the spec sheet says this assembly can transmit X Nm only under the exact conditions it assumes. In the real world, you're responsible for recreating those conditions.

The two torque numbers that matter

When people ask me about Ringfeder torque specs, I ask them which number they're looking at. A typical spec sheet includes at least two torque-related values: transmittable torque and tightening torque. They're connected.

• Transmittable torque is the torque the locking assembly can safely transfer from shaft to hub at the specified tightening torque. It's the value you compare against motor torque or load torque.
• Tightening torque is the torque you apply to the clamping screws. That's what creates the clamping force that makes the transmittable torque possible. If the screws are under-torqued, the transmittable torque drops quickly.

In other words, a larger assembly with loose bolts can slip before a smaller assembly that's correctly tightened. The tightening torque is not a maintenance detail—it's the whole system.

Why the catalog value can be misleading

When I first started specifying locking assemblies, I compared the motor's peak torque to the catalog's transmittable torque and thought I was done. Three years later, after a few slip failures and one very expensive rework, I realized the catalog value assumes ideal conditions. The shaft, the hub, the surface finish, and the person with the torque wrench all affect the actual number.

The common culprits are:

• shaft tolerance at the low end of the allowable range
• hardened or coated surfaces that change friction
• a hub that is shorter than the recommended clamping length
• grease, oil, or contamination on the contact surfaces
• incorrect tightening sequence or an uncalibrated torque wrench

Those items aren't small. In my experience, a combination of two or three of them can reduce the effective torque capacity by 30 percent or more. I want to say the derating was around 20 percent in the March 2024 case, but don't quote me on that—the exact number depends on the specific assembly.

How to apply the specs to stepper motors, brushless DC motors, and linear actuators

If you're reading this because you searched "what's a stepper motor" or because you have a linear actuator on the same shaft, here's how the motor type changes the calculation.

What's a stepper motor?

A stepper motor is a brushless, synchronous electric motor that moves in discrete steps, one step for each electrical pulse. It's often used for precise position control. The torque value to check isn't the motor's nominal continuous torque; it's the pull-out torque from the speed-torque curve. If the coupling slips during a fast move, the motor can lose steps and the position is wrong. A backlash-free locking assembly helps, but only if the torque spec actually covers the worst-case load.

Brushless DC motor peak torque

A brushless DC motor has a high torque-to-inertia ratio, so the peak torque during acceleration and reversal can be two to three times the continuous torque. If you size the coupling using only continuous torque, the connection can slip at startup. Check the Ringfeder spec sheet's transmittable torque against the motor's peak torque—including the inertia of the load and any external thrust loads.

To be fair, the continuous torque number is fine if the application is smooth and has no dynamic spikes. But for most servo-style axes, the peak value is the right one.

Linear actuators and reflected torque

Linear actuators are a separate category because the torque at the motor shaft is usually not the torque on the actuator's output. If a rotary motor drives a lead screw, the reflected torque equals the axial load times the lead divided by (2π times the efficiency), plus friction and acceleration. For high-load actuators, the reflected torque can be higher than the motor's rated torque at low speed. Check the locking assembly against the reflected torque, not just the motor nameplate.

A real rush-order example

In March 2024, a packaging plant called on a Sunday afternoon. A gearmotor had failed on a bagging line, and the replacement was a brushless DC motor with a different shaft size. The plant was already down. The locking assembly we wanted to use was in stock, but the hub was too short to provide the full clamping length. The catalog torque looked fine; the derated torque did not. We sourced a longer hub, had it machined overnight, and the line restarted about 36 hours after the call. The extra machining cost about $850. Waiting for a custom adapter would have meant a week of downtime.

This is where total cost of ownership matters. The price of the locking assembly is small compared to the cost of downtime, rework, and lost production. When I compare options, I calculate the total cost, not just the unit price.

The limits of torque specs

Ringfeder torque specs are reliable when you respect the installation conditions. They are not a substitute for checking the actual shaft-hub connection. Published ratings typically don't apply to soft hub materials like aluminum, to hubs with very thin walls, or to shafts with deep scoring or corrosion. If you need frequent disassembly, use a design that allows re-tightening, and follow the recommended tightening sequence each time.

Granted, this adds work at the front end. But it saves time later. The goal is not to find the most impressive torque rating on paper; it's to make sure the connection you install will do the job on day one and after a long service life.

The catalog spec isn't wrong. It's conditional. Check the shaft, check the hub, check the tightening torque, and then use the Ringfeder torque spec with confidence.