5 Mistakes I Made Ordering RINGFEDER Couplings (So You Don't Have To)

I've been handling power transmission orders for RINGFEDER for about six years now. In that time, I've personally made (and documented) eight significant mistakes. Total wasted budget? Roughly $4,200. Not including the embarrassment.

This article is the checklist I wish I'd had in 2019. It's for engineers, buyers, or maintenance pros who need to order a RINGFEDER coupling—specifically the 7000 series or a locking assembly—and want to get it right the first time. There are 5 steps here, plus a few 'don't do what I did' notes at the end.

Step 1: Verify the torque rating *before* you match the shaft size

I made this mistake in my first year. I found a shaft size that matched perfectly—both diameters were spot on. Placed the order. When the coupling arrived, it was rated for 350 Nm. The motor it was connecting to? It could spike to 600 Nm under load.

That error cost us $890 in redo plus a 1-week delay. The coupling worked fine for light testing, then failed under real torque. RINGFEDER provides a torque chart (they call it a 'spec sheet') for each series. Don't just look at shaft bores. I've learned to check the max continuous torque first, then see if the shaft fits within that range.

Your check: Pull the torque chart for the specific series. Find the row that matches your driven load. Then see what shaft bores are available in that torque class. Not the other way around.

Step 2: Measure the exact shaft tolerance, not just the nominal diameter

This sounds basic. It's the one that keeps tripping people up.

I once ordered a dozen RINGFEDER locking assemblies for a batch of gear motors. The shaft was spec'd as 30mm. I ordered 30mm bores. They didn't fit. Not even close. When I called the manufacturer of the gear motor, they said the shaft tolerance was 30.05mm to 30.12mm—h6 tolerance class.

The locking assemblies I ordered were for a 30mm shaft expecting an h8 tolerance (29.98mm to 30.00mm). That's a difference of less than a tenth of a millimeter. But that gap meant the locking assembly couldn't grip properly. We had to expedite a different set—$450 wasted plus a 3-day production delay.

Your check: Get the actual measured tolerance of your shaft and hub. RINGFEDER catalogs usually specify the shaft tolerance range they're designed for (often h8 or h9). If your shaft is h6 (tighter), you need a locking assembly designed for that tighter range. Not all 30mm bores are created equal.

Step 3: Specify the exact series number, not 'a RINGFEDER coupling'

I've seen this happen three times in the last two years. Someone sends a PO that says 'RINGFEDER coupling, 25mm bore.' The supplier sends a 6000 series jaw coupling with a clamping hub. What the engineer needed was a 7000 series disc coupling for low backlash. The clamping hub on the 6000 series couldn't handle the reversing loads. Wrong part, wrong application.

The 'RINGFEDER' brand covers a lot of products. You've got shaft-mounted locking assemblies, disc packs, gear couplings, torque limiters. That's like ordering 'a Ford' and expecting a specific F-150 trim level.

Your check: On your PO or requisition form, include the full model number from the catalog. Example: 'RINGFEDER Series 7000, Model 7012-25, with disc pack Type B.' If you don't know the exact model, look at the application drawing. Gear motors often need the 7000 series. Shaft-to-shaft connections on conveyors might use a 6000 series. Don't leave it to the supplier to guess.

Step 4: Verify the application type—torque limiting vs. rigid connection

This one's a bit more nuanced. RINGFEDER makes both torque-limiting couplings (which slip at a set torque) and rigid locking assemblies (which don't slip).

In September 2022, we had a machine that kept shearing keys. The engineer said 'we need a RINGFEDER torque limiter.' I ordered one with a torque rating of 500 Nm. It slipped immediately—the machine's normal running torque was 450 Nm, but it spiked to 600 Nm on startup. The torque limiter was set to 500 Nm, so it slipped every time the machine started.

Turns out what we actually needed was a rigid locking assembly with a slightly higher torque capacity than the motor's peak torque. The locking assembly wouldn't slip at all, but it would transfer the shock to the shaft. We ended up replacing the shaft, too. That whole debacle cost about $1,000 in parts plus a week of downtime.

Your check: Are you trying to protect downstream components from overload? Use a torque limiter. Are you trying to prevent a key from shearing? That's a locking assembly question. Torque limiters are sacrificial; locking assemblies are not. Know which one you actually need.

Step 5: Don't assume 'cogged v-belt' or 'gear motor' are standard specs

This is the mistake that finally made me create our pre-order checklist. I'm talking about the 'what size is LM8LUU linear bearing?' or 'cogged v-belt' type of confusion.

I once tried to order a RINGFEDER coupling for a gear motor. The spec sheet said 'shaft diameter 28mm.' Great. I ordered a coupling for a 28mm shaft. When it arrived, the gear motor shaft was actually 28mm *with a keyway*. The coupling I ordered had no keyway provision—it was designed for a keyless locking assembly. The gear motor had a keyed shaft. The coupling would have worked if I'd also ordered the right locking assembly, but I didn't. I had to return the locking assembly and get a different one with a keyway. That was a $200 mistake plus two days of shipping.

The point: 'gear motor' isn't a specification. Neither is 'cogged v-belt.' You need the exact shaft dimensions, the keyway dimensions (if any), and the exact tolerance. The LM8UU linear bearing question (I hear it a lot) is similarly tricky—the 'LM8UU' is a specific size, but 'LM8LUU' is longer. People get them mixed up constantly.

Your check: If you're ordering for a gear motor, get the motor's datasheet. Not the nameplate, but the actual engineering drawing that shows the shaft extension, keyway, and tolerance. Same for any other component. 'Gear motor' is a category; 'WEG NEMA 56H Frame output shaft 7/8" diameter with 3/16" keyway' is a specification.

Notes & Common Errors (From My Mistakes)

• Torque chart misuse. I see people using max torque ratings for continuous duty. The chart gives static torque limits. For reversing or shock loads, derate by 20-30%. I learned this the hard way.
• Don't trust shaft size alone. Measure it. Even if the drawing says 30mm, micrometers are cheap. That $50 tool saved me $3,000 in rework over the years.
• The spare parts trap. I once ordered a spare disc pack for a 7000 series coupling without checking the part number. The '7000 series' had three different disc pack variants. I got the wrong one. Now I check the specific model number on the coupling's ID tag.
• Rush orders aren't worth it if you skip verification. The 12-point checklist I created after my third mistake has saved us an estimated $8,000 in potential rework. 5 minutes of verification beats 5 days of correction.

Prices as of early 2025; verify current RINGFEDER pricing for your region. This checklist is based on my own experience with RINGFEDER 7000 series couplings and locking assemblies. Your application may differ. Consult the RINGFEDER catalog for your specific series.