HRC, jaw or tyre coupling: how to choose

A flexible coupling joins two shafts and absorbs shock, vibration and small misalignments, so the motor and the driven machine are not left to take them. HRC-type, jaw and tyre couplings are three elastomer designs that buyers often compare. They differ in how the elastomer is loaded, how much misalignment they accept, and what happens if the elastomer fails.

All engineering numbers and formulas below come from manufacturer catalogues. They show typical values and ranges, not a promise for your drive. Confirm every figure in the catalogue of the coupling you intend to buy. Probe's couplings range lists jaw, HRC, gear, tyre, pin bush and resilient couplings. This post compares three of them.

How the three types are built

• HRC-type. A flexible flange coupling with a resilient element. Two cast-iron flanges drive through a rubber element that works in compression. Probe's range grid lists sizes HRC-70 to HRC-280, with taper-bush or through-bore fitting. Two manufacturer catalogues put the top of that size range at 3,150 Nm nominal torque, and Probe's grid shows the same upper figure.
• Jaw. Two hubs with interlocking jaws and an elastomer insert, called a spider, between them. The spider works in compression.
• Tyre. Two flanges joined by a flexible rubber tyre, which is clamped to each flange by a ring.

Probe's range grid describes its jaw coupling as a general-purpose flexible coupling for incidental misalignment.

Choose first by misalignment

Manufacturer catalogues give these maximum values:

• HRC-type: angular misalignment up to 1 degree, parallel offset 0.3 to 0.5 mm and axial movement of +0.2 to +1.7 mm across sizes 70 to 280. Two catalogues agree.
• Tyre: angular misalignment up to 4 degrees, again in two catalogues. One catalogue lists parallel offset of 1.1 to 6.6 mm and end float of 1.3 to 8.2 mm across its size range. Another quotes up to 6 mm and 8 mm.
• Jaw: typically up to 1 degree angular and 0.015 in (about 0.38 mm) parallel, per one manufacturer. Precision jaw designs allow less.

These are maxima. One HRC-type catalogue reduces the allowable misalignment as speed rises, to half the value at 1,501 to 3,000 rpm, and again when more than one misalignment is present. Align as closely as practical.

Probe's FAQ lists the same decision inputs: torque requirements, the degree of angular or parallel shaft misalignment, and the presence of shock loads or vibration.

Then size by torque and service factor

Sizing follows the same steps in most catalogues:

1. Find the running torque. Torque in Nm equals 9550 multiplied by power in kW, divided by speed in rpm. Two independent catalogues print this formula.
2. Apply the service factor. Multiply by the factor for the machine class, the hours of operation and the prime mover. Some makers add temperature and start factors.
3. Choose the size. Pick the coupling whose nominal torque is at least the design torque.
4. Check the rest. Check maximum (peak) torque, bore and keyway, and speed.

Service factors differ by maker, so use the table in your chosen manufacturer's catalogue. As an example, one HRC-type catalogue lists these ranges for electric-motor drives, rising with hours of operation per day:

• Uniform load: 1.00 to 1.25.
• Moderate shock: 1.60 to 2.00.
• Heavy shock: 2.50 to 3.12.

For internal-combustion drives the same catalogue ranges from 1.25 to 4.00, and it multiplies by a further 1.15 for centrifugal compressors. Two catalogues place belt conveyors, centrifugal pumps and blowers in the lightest class and crushers and piston compressors in the heaviest. If your drive is a conveyor or sits in the crusher sector, Probe has pages on conveying systems and solutions and the crusher industry.

Temperature changes the answer. One HRC-type brochure applies temperature factors of 1.2 to 1.8 in its higher ambient bands. A precision jaw manufacturer's example uses 1.4 at 60 °C.

Peak torque needs care. One tyre-coupling catalogue states that its maximum torque figures are short-duration overload ratings, for example direct-on-line starting. They are not continuous ratings.

For diesel engines, piston compressors and piston pumps, two manufacturers say sizing by service factor alone is not enough. Those drives need a torsional vibration analysis.

What happens if the elastomer fails

Manufacturers describe HRC-type and jaw couplings as fail-safe in this sense: if the elastomer fails, the interlocking dogs or jaws keep the drive connected. The catalogues reviewed make no equivalent statement for tyre couplings. Ask the manufacturer what happens if the tyre fails.

Changing the element

One maker says a tyre can be replaced without disturbing the driver or the driven shaft. You loosen the clamping screws, place the new tyre between the flanges and rings, and tighten the screws. Another manufacturer says the HRC-type element can be replaced by moving the coupling halves axially, without disassembling the connected machines. That is a manufacturer claim and needs axial room. For a jaw coupling, ask how the insert is changed on the exact model.

Jaw couplings and backlash

One manufacturer says standard jaw designs are usually not recommended for engine-driven or frequent start-stop-reversing duty, because of backlash. Precision jaw couplings that preload the insert are backlash-free, but they are a different product from a standard jaw coupling.

Probe's range grid describes its tyre coupling as highly elastic and backlash-free, absorbing shock and vibration.

Read the element's temperature rating

Elastomer temperature ratings are manufacturer-specific:

• HRC-type element: -20 to +80 °C in one brochure. Another catalogue lists ranges of -40 to +100 °C and -20 to +80 °C, and does not make clear which applies to which element.
• Nitrile jaw spider: about -40 to +100 °C, per one manufacturer.
• Polyurethane jaw spiders: -30 to +120 °C in one manufacturer's range, but -39 to +71 °C for a 90 Shore A grade from another.

Take the limit from the datasheet of the exact insert, and apply the temperature factor at your operating temperature.

Which standards matter

Manufacturers commonly apply the DIN 740 Part 2 method for selecting torsionally flexible couplings. Several catalogues cite it, and this post does not reproduce its tables. ISO 14691:2008 sets requirements for general-purpose flexible couplings in the petroleum, petrochemical and natural gas industries. It does not define criteria for selecting coupling types, so it matters when a client specifies it.

What to tell us

Probe says its engineering team carries out an application-focused assessment before recommending a coupling. Probe also states that it offers in-house custom bore sizing, keyway broaching and balancing. To get a recommendation, bring:

• the driving machine, its power in kW and its speed in rpm;
• the driven machine and its load type: uniform, moderate shock or heavy shock;
• hours of operation per day, and how often the drive starts, stops or reverses;
• whether the prime mover is an electric motor, a diesel engine or a piston machine;
• shaft diameters, bore and keyway requirements;
• expected angular, parallel and axial misalignment;
• operating temperature and any oil or chemical exposure.

To request a recommendation, use Get a Custom Quote on the power transmission products page. Probe is based in National Industries Park, UAE. For how conveyor duty classes affect the choice, see belt, roller or slat conveyor: choosing the right system.

Sources

• ISO 14691:2008, EVS standards listing.
• DIN 740-2:1986-08, standards listing (listing only; the standard text was not read).
• Coupling data come from manufacturer catalogues and technical articles, which are not named here.