Before You Trust the Ringfeder Torque Chart: A Servo Motor and Gear Rack Checklist

If you're sizing a servo motor on a gear rack and somebody just sent you a ringfeder torque chart, stop before you “find” a number in the table. I have a specific reason for saying this. My job is procurement and application support for a small motion-control shop. I've approved a lot of RINGFEDER components over the years, and I've also scrapped enough—roughly $8,000 worth—to make me obsessive about checking the assumptions behind every torque spec.

I'm not a design engineer. I'm the person who has to catch the difference between what's on the datasheet and what's on the machine. This checklist is for engineers, maintenance people, and buyers who need a servo-driven gear rack axis to work on the first day.

When to use this list

Use this when you're doing any of these:

• Sizing a coupling between a servo motor and a gear rack pinion.
• Picking a locking assembly using ringfeder torque specs.
• Trying to decide if a stepper motor can move the same axis.

The process is pretty much the same for an OEM design or a retrofit. You just need an extra safety factor for the unknown loads.

Step 1: Start with servo peak torque, not the continuous torque

Servo motor datasheets usually list continuous torque and peak torque. For a gear rack axis—especially one with quick positioning or an emergency stop—peak torque can be three to five times the continuous value. The ringfeder torque chart usually shows torque capacity under steady operating conditions. If you use continuous torque, you miss the peak. If you use peak torque for every calculation, you over-spec. Both are expensive.

In 2017, I sized a locking assembly for a servo motor that looked fine at 3.8 Nm continuous. The peak torque was 12 Nm for about 200 milliseconds. The assembly held during normal cycles. It slipped on an emergency stop and scored the shaft. The component was “over-qualified” on paper and under-qualified for the actual torque event.

Step 2: Convert the gear rack load to torque at the pinion

Gear racks are linear-to-rotary systems. The motor and coupling do not feel the linear force directly. They feel torque, which is that force times the pinion pitch radius, divided by efficiency. Here's the calculation I still use:

T_req = (F_linear × r_pinion) / η

F_linear is the total force at the rack: cutting force if any, acceleration force, friction. r_pinion is the pitch circle radius. η is the gear rack efficiency, typically 0.85 to 0.95. Then multiply by an application safety factor.

Example: 1,200 N linear force, 20 mm pitch radius, 0.9 efficiency → (1200 × 0.02) / 0.9 = 26.7 Nm. With a 1.5 safety factor, the minimum transmittable torque at the coupling is 40 Nm. That's the number you compare against ringfeder torque specs.

This step matters more on gear racks than on ballscrews because a rack pinion is not self-locking. The torque seen during deceleration is not the same as the average torque during acceleration.

Step 3: Read the ringfeder torque chart as a checklist, not a list of options

Don't just take the highest number. A typical ringfeder torque chart includes the transmittable torque, the tightening torque for the clamping screws, and the recommended bore range. The torque value is tied to a specific bore and contact length. If your hub is shorter than the catalog condition, the rating changes.

Honestly, I'm not sure why some catalog layouts show torque values and axial force values on the same line. My best guess is that they assume you know which one applies to your application. You do, but by then you're in a hurry. Use the torque column for rotary drive applications, and use the lower of the two capacity columns if the chart does not clearly separate them.

Also remember that ringfeder torque specs are static values. A servo motor with a peak pulse can produce temporary torque spikes that exceed a steady-state chart value. Add a dynamic safety factor.

Step 4: Check torsional stiffness before you touch the servo tuning

Torque capacity alone won't tell you how the system will behave. A servo-driven gear rack can oscillate if the coupling is too soft. It can amplify shock if the coupling is too stiff. The ringfeder torque chart is the first document, not the last. Ask for the torsional stiffness of the exact coupling size you selected.

My rule: if I can't find the torsional stiffness in the supplier data, I treat it as a red flag. I don't need a perfect stiffness match; I need a number that lets me predict whether the system will need very low gains or whether it will be stable at normal gains.

Step 5: Before going the stepper route, answer “how fast can a stepper motor turn?”

People ask me this all the time, usually because they're thinking about replacing a servo with a stepper to save money. The short answer is that no-load speed tells you almost nothing. A typical NEMA 23 stepper can spin around 3,000 rpm with no load, but torque drops long before that. At 1,000 rpm, torque may be half of the holding torque. At 2,000 rpm, it can be 15 to 20 percent of holding torque. The useful speed depends on the load, the drive voltage, and the torque-speed curve.

If you're sizing a gear rack with a stepper, compare torque at your required traverse speed. Do not compare the holding torque stamped on the motor frame. Then check that your ringfeder torque chart rating is still valid at that speed. Most torque charts are static capacities. Acceleration peaks and shock loads can reduce the practical transmittable torque.

A nearby shop installed a NEMA 34 stepper because it cost about 40 percent less than a servo package. At the required operating speed, the available torque was around 30 percent of the brochure holding torque. The pinion stalled, the axis lost position, and a camera hit a fixture. That three-hour incident cost more urgent repair time than a servo would have cost in total.

Step 6: Verify shaft and bore tolerances before the purchase order goes out

This is the boring part, and it's the one that has cost me the most money. In 2022, I ordered 12 RINGFEDER locking assemblies based on a torque chart. The torque calculation passed. The bore tolerance looked correct. But the shaft tolerance on the gear rack was about 0.06 mm below the drawing. The assemblies still fit, but they clamped with less than the required surface pressure. Two days later, the axis lost position. The repair cost $890 in machining plus a three-day delay.

Before you place an order:

• Measure the actual shaft diameter. Do not trust the drawing nominal.
• Confirm the keyway dimensions, if any, follow DIN 6885-1.
• Clean and degrease the bore and shaft before assembly.
• Find the screw tightening torque on the ringfeder torque chart and put a torque wrench on the install process.

Step 7: Put the calculation on one page and challenge the cheap quote

Every RINGFEDER-related order in our shop now goes through a one-page checklist. It lists the required torque, the selected catalog torque, the torsional stiffness, and the bore verification. The last line asks: “Does the machine actually need this rating?” If the answer is yes, we stop comparing only price.

I have no problem with comparing prices. But if a cheaper coupling saves $250 and the gear rack stops for one hour, the calculation changes. A $250 savings can turn into a $1,500 problem when you add a rush charge, machine down time, and the embarrassment of a second failure. The value of a component is not its price tag. It's the chance that it still holds torque at 8 p.m. on a Friday.

Things that still surprise me

Never expected a 0.05 mm tolerance issue to cause more pain than the torque calculation. The torque calculation is what everyone worries about, but the bore and surface condition cause the failures that are hard to diagnose.

I also don't have a complete answer for why identical-looking torque specs vary so much between suppliers. My guess is differences in test methods, surface preparation, and safety factors. Take any torque chart with a grain of salt and verify your actual contact conditions.

Final word

If this checklist feels overly detailed, that's okay. The ringfeder torque chart solves the torque part, but only if you bring the right torque value to it. On a servo-driven gear rack, torque is never the only number.

Here's the short version:

1. Use servo peak torque.
2. Convert gear rack force to pinion torque.
3. Read the ringfeder torque chart for your exact bore and hub length.
4. Check torsional stiffness.
5. If someone asks “how fast can a stepper motor turn,” show them a torque-speed curve, not a no-load rpm.
6. Verify shaft tolerances and screw tightening torque.
7. Apply a safety factor for dynamic loads.

Do that before you decide based on price. It's cheaper to do it on paper than to explain why a coupling slipped on a production line.