The short version: size a KTR coupling in four steps. Work out the rated torque from power and speed, multiply by the operating, starting, temperature and direction factors, pick the family that suits the drive, then check misalignment limits, speed and bore against the catalogue. Skip any step and the coupling either fails early or cannot be fitted.
Every factor and limit below comes from KTR's own coupling selection documentation and the ROTEX operating instructions, so each figure can be checked against the source.
Step 1: work out the torque, then the factors
Start from the machine, not a catalogue page. KTR's formula turns motor power and speed into the rated torque the coupling must carry:
T_N [Nm] = 9550 x P [kW] / n [rpm]
A 200 kW motor at 1500 rpm gives 1273 Nm. That is the beginning of the calculation, because four factors sit between the machine torque and the coupling rating:
T_KN is at least T_N x S_B x S_t x S_R
T_Kmax is at least (T_N + T_S) x S_Z x S_t x S_R
The first line covers steady running. The second covers shock loading, where T_S is the peak torque of the machine. KTR puts the shaft-hub connection outside this scope: its transmittable torque must be reviewed by the customer and is the customer's responsibility.
Two factors catch people out. An S_R of 1.7 nearly doubles the required rating on a reversing drive, which is why an indexing table often needs a larger coupling than its motor suggests. And S_t is not one curve: all-steel lamina couplings hold full capacity to +150 °C, while rubber-element types need an 80 percent allowance by +80 °C.
KTR also splits drives in two. Centrifugal pumps, fans and screw compressors have no periodical torsional vibrations and are sized with the formulas above. Diesel engines, piston compressors, piston pumps and generators do, and need a torsional vibration calculation first.
Step 2: pick the family before the size
KTR builds more than fifteen coupling families, and the choice is driven by what the machine needs rather than by torque alone.
KTR's own descriptions add detail: RADEX-N, RIGIFLEX-N and RIGIFLEX-HP are torsionally rigid, backlash-free, maintenance-free and all-steel, with RADEX-N available single or double-cardanic, while REVOLEX KX-D is fail-safe and axially plug-in.
Read that list backwards and the logic falls out. No backlash tolerance points to the steel lamina and servo families, damping points to an elastomer family, a failure that must stay contained points to a fail-safe design, and widely separated shafts need a double-cardanic or spacer version.
Step 3: check the misalignment it will absorb
A coupling is chosen partly for what it tolerates, not only for what it transmits. KTR's ROTEX instructions state the purpose plainly: it compensates shaft misalignment from manufacturing inaccuracies, thermal expansion and similar effects.
ROTEX publishes displacement limits per size and spider hardness, valid up to rated torque, at 1500 rpm and +30 °C ambient.
Those figures are for the 92 and 98 Shore A spiders. The stiffer 64 Shore D grade tolerates less: on ROTEX 24 it gives 0.15 mm radial at 1500 rpm and 0.8 degrees angular, against 0.22 mm and 0.9 degrees for the softer grades.
Three rules govern the numbers. The limits are single-use: radial and angular offset occurring together must be limited in proportion, with the percentages summing to no more than 100. KTR's illustration is 60 percent radial against 40 percent angular, so a ROTEX 38 showing 0.8 mm radial and 0.8 degrees angular is inside each limit alone and outside them combined. The figures are also tied to rated torque, 1500 rpm and +30 °C, so speed reduces them. Angular offset appears in degrees and in millimetres at the hub.
Step 4: check the speed and the bore
The bore limit is a safety limit. KTR's wording is blunt: the maximum permissible bore diameters must not be exceeded, and if those figures are disregarded the coupling may tear, with rotating particles causing danger to life.
Bores are machined H7 with a keyway to DIN 6885 sheet 1 at JS9, plus a setscrew. Recommended fit pairs follow DIN 748/1: shaft k6 against bore H7 up to 50 mm, shaft m6 above that. A keyway should sit between the cams, and for alternating torsional direction or shock loads KTR raises the keyway tolerance from JS9 to P9.
Speed interacts with Step 3. Each size has a maximum permissible speed in the catalogue, and the permissible radial offset falls as speed rises. Above size 110 no radial figure is published at 3000 rpm at all, so a high-speed application moves down the size range rather than up. ROTEX clamping ring hubs suit fast shafts, rated for circumferential speeds up to 40 m/s.
Worked example
KTR publishes a complete example that shows the two inequalities working together.
The drive is an IEC motor, size 315 L, 200 kW at 1500 rpm, on an 80 mm shaft, coupled to a radial pump on a 75 mm shaft. Shaft ends are 250 mm apart. Conditions: six starts per hour, +65 °C ambient, one torsional direction.
The selected RADEX-N is rated 2400 Nm and 4800 Nm, so it clears both conditions with a sensible margin. Because the shafts are 250 mm apart the coupling must be double-cardanic steel lamina to bridge the gap, so the family decision came from geometry rather than torque. And the shaft-hub connection still has to be verified separately, by the customer.
The same method works on small drives. A screw compressor on a 30 kW motor at 3000 rpm with 38 mm shafts, eight starts per hour and +30 °C ambient gives T_AN of 96 Nm. At a rotary compressor factor of 1.75 the required T_KN is 168 Nm, and a ROTEX 38 with a 92 Shore A spider is rated 190 Nm and accepts a 38 mm bore in the steel hub version. Confirm the catalogue speed limit for that size before ordering, and note the elastomer spider runs -30 °C to +90 °C.
Frequently asked questions
How do I calculate the torque for a KTR coupling?
Use KTR's formula, T_N = 9550 x P / n, with power in kW and speed in rpm. Apply the operating, temperature and direction factors for steady running, and the starting, temperature and direction factors for shock loading.
Which KTR coupling family handles the highest torque?
By the maximum torque figures KTR publishes per family, the largest are GEARex all-steel gear couplings at 2,750,000 Nm and REVOLEX pin and bush couplings at 1,350,000 Nm, both fail-safe designs. Where high torque also needs backlash-free, torsionally rigid behaviour, RADEX-N reaches 300,000 Nm.
Can a KTR coupling compensate radial and angular offset at the same time?
Only in reduced proportion. KTR states the displacement figures may be used one by one, and if radial and angular offsets appear simultaneously they must be limited proportionally, with the percentages summing to no more than 100.
How do I know which bore a KTR coupling accepts?
Check the finish bore range for the size and hub material in the catalogue and never exceed the maximum. ROTEX steel hubs reach 62 mm by size 48, while cast and nodular iron hubs run larger, from 40 mm at size 38 to 200 mm at size 180. The shaft-hub connection must then be verified separately.
Do I need a torsional vibration calculation before selecting a KTR coupling?
It depends on the drive. Centrifugal pumps, fans and screw compressors have no periodical torsional vibrations and can be sized with the operating factors alone. Diesel engines, piston compressors, piston pumps and generators do, and KTR requires a torsional vibration calculation first.
Sourcing KTR couplings
QINUO supplies genuine, brand-new KTR couplings, torque limiters and accessories from its Hong Kong operation. We do not hold a factory appointment for every line we carry, and any supplier claiming one without documents deserves the same scrutiny as a cut-price listing. What we do is run the four steps above against your duty, then quote with the misalignment, speed and bore figures written down.
Start from the brand directory or the KTR brand page. Four entry points cover most enquiries: ROTEX jaw couplings for general drives, ROTEX GS servo couplings for positioning axes, RADEX-N steel laminae couplings for long shaft distances, and REVOLEX pin and bush couplings where a fail-safe element is required. For overload protection rather than transmission, RUFLEX torque limiters are the counterpart to all of them. The full product catalogue covers the rest, and our technical insights explain the neighbouring families. Send motor power, speed, shaft diameters, shaft-end distance and ambient temperature through the contact page and we will work the calculation before anyone quotes a price.
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