You may not always find the "perfect" coupling for your motion system, but when it comes time to select a coupling that will best suit your needs, keep these 7 tips in mind to help find the best solution possible.
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The inertial mass of the rotating system has a significant impact on the demands put on your coupling. Both the rate of acceleration and rate of deceleration needs to be factored in. Sudden braking of a high-speed or high-inertia system can cause a large torque spike to pass through the coupling resulting in its damage.
While a machine is running it may be heating up and cooling down or operating at an elevated temperature. When this happens, there can actually be some relative movement between the shafts that had not been considered previously. The reason this is important is that not all coupling solutions are capable of axial compliance, and are considered axially rigid. The risk in this situation is the possibility of putting a large axial load onto the bearings of the motor. The same motor loading can be true with thrust loads.
In a motion control system, there can be loading of the motor bearing if a solid coupling is used and transmits axial loads to the motor before engaging thrust bearings.
Parallel offset is a more difficult misalignment to accommodate than angular misalignment. Couplings generally do a good job at .010 inches and below (parallel offset). Once you start hitting .020 inches or more of offset, you're going to have a harder time finding a coupling that's going to fit in a relatively compact package. If you find a coupling that has a large parallel offset capability be sure and check its speed rating. Usually when a coupling can handle a large parallel offset the product is limited in its maximum RPM rating.
Keep in mind if the coupling is operating in a harsh environment and/or corrosive atmosphere. Different materials will need to be considered depending on the type of environment. In a severely corrosive environment, specialty materials such as MP35N or Titanium may be the best option. Another environmental consideration is temperature. Certain alloys lose significant strength at elevated temperatures, or for certain elastomeric type couplings, the temperature limits may be limited. In higher temperature applications Inconel may be the material of choice. If the coupling is used in food or medical environments then the material options might be limited to certain stainless steel or Titanium.
Many applications are relatively easy to solve with most couplings if you’re operating at 5,000-10,000 RPM. In today’s high-speed applications it may be possible to reach 25,000 RPM, and sometimes up into 75,000-80,000 RPM. You’ll need to factor in not just torque capacity but how well balanced is the coupling for that type of speed because if it’s not either a balanced coupling or design that’s symmetrical by nature, which makes it a statically balanced part, you’re going to be running into a vibration situation due to the imbalance.
When looking for your best coupling solution, remember there is no governing body for couplings and how their performance is rated. It could be from calculations or the results of testing by the manufacturer. Make sure that you read the data carefully because one manufacturer may have a torque rating and when you look at the footnotes, their torque rating is for static torque which means it’s the torque that will yield the coupling versus another company that uses a dynamic torque, where the rating based upon a high-cycle environment. We actually de-rate the coupling torque capacity based upon realizing that they’re going to be going through millions and millions of revolutions.
Grooved pipe couplings are some of the most important equipment in fire protection, allowing quick and water-tight connections without pipe threads, flanges, or welding. Instead, installers simply butt the ends of pipes together and seal them with a gasket and clamp.
But there are also two kinds of grooved couplings that serve different purposes. Rigid grooved couplings don’t allow much movement and serve as a standard pipe joint. But for situations where some give is needed—specifically, seismic protection of fire sprinkler systems—flexible grooved couplings are the solution.
This article reviews the basics of grooved pipe couplings and details the differences between a rigid and flexible coupling. We also explain when and how NFPA 13: Standard for the Installation of Fire Sprinkler Systems ( edition) requires using flexible couplings for seismic protection.
Shop our selection of rigid grooved couplings and other fittings for your pipe-joining needs. Looking for flexible couplings? Contact QRFS today for a quote or to place a special order.
Grooved couplings are the fastest way to join pipes. And the technology is simple. Earnest Tribe, the founder of Victaulic, patented the grooved pipe joint in after developing the tech to meet wartime needs. Grooved couplings avoid the need for welding or other joint types. Instead, they use three components: 1) pipe with grooved ends, 2) synthetic rubber gaskets, and 3) c-shaped clamps.
To form a joint, installers butt two pipes with grooves cut in the ends together. A lubricated gasket slips over the pipe ends so that it overlaps the joint. And a c-shaped clamp or housing fits into the grooves of the pipe, compressing and stabilizing the gasket. This video from Victaulic shows how the joints are made:
But as we mentioned, there are two kinds of grooved joints. So, what’s the difference between a rigid and flexible coupling?
From the outside, you can’t look at a grooved coupling and know whether it’s rigid or flexible; they look the same. The difference is on the inside of the joint in the way that the gasket, groove, and clamp fit together. A rigid clamp permits next to no separation, rotation, or angular movement. But a flexible coupling permits some movement and space without compromising the seal or damaging the pipe. NFPA 13 officially describes how much flexibility is allowed:
3.3.83 Flexible Coupling. A listed coupling or fitting that allows axial displacement, rotation, and at least 1 degree of angular movement of the pipe without inducing harm on the pipe. For pipe diameters of 8 in. (200 mm) and larger, the angular movement is permitted to be less than 1 degree but not less than 0.5 degree.
For more Grooved Fittings Manufacturerinformation, please contact us. We will provide professional answers.
Key to this definition is the term “listed.” A testing laboratory such as UL or FM Global must verify that a flexible grooved fitting meets specific flexibility, seal, and strength requirements before it can be used in a fire sprinkler system.
This diagram from Bentley Communities helps illustrate the physical difference between the two types of joints:
Almost all grooved couplings use rigid joints because rigid, unmoving joints are desired for most situations. But flexible couplings, while less common, do play essential roles in seismic protection for fire sprinkler systems that require an earthquake-resistant design.
Some fire sprinkler systems require design measures to protect them from earthquake damage. In our overview of the principles of seismic protection, we explain the two basic techniques for protecting pipes from damage: adding rigidity and adding flexibility. Rigidity, in the form of seismic bracing, ensures that pipe moves with a building in an earthquake instead of shaking violently and breaking loose.
Flexibility, on the other hand, prevents damage to pipes caused by differential movement. When an earthquake’s powerful p-waves shake a building, the whole structure can move—but not always all at the same time. For example, the ground floor moves first in tall buildings, and the top floors lag behind. This wave-like motion that resembles a slinky is called “story displacement.” And the differential movement it causes in various onsite equipment can snap fire sprinkler pipes.
The solution to this problem is flexibility, which designers can achieve with flexible grooved couplings. The modest ‘give’ allowed by these devices can prevent pipes from being damaged in an earthquake.
NFPA 13 requires grooved flexible couplings for pipes that are 2 ½” or larger (Section 18.2.1) in certain buildings that fall into specific design categories (SDC). And because story displacement significantly affects risers, most of these rules involve risers (18.2.3.1):
Flexible couplings are also required at horizontal tie-ins to the riser (18.2.3.2) and the connection of drop-pipes (18.2.4).
Flexible couplings allow flexibility when two structures move independently. But don’t forget that bracing, which ensures pipe moves with a building, is the other core technique of seismic design. Flexible couplings can work against this objective by reducing rigidity. So, to compensate, NFPA 13 says that flexible couplings should only coincide with structural separations (18.2.2). And where they are used outside the explicit instructions, extra sway braces are needed (18.2.3).
In our previous blogs, you can learn more about the principles of sway bracing and how cable sway braces work.
Flexible couplings can also be used to construct seismic separation assemblies. These pipe assemblies are used where horizontal pipe must cross an engineered gap, called a seismic joint, between two structures.
The “structures” can be two independent buildings or two freestanding wings of the same building. When a pipe crosses a gap like this, NFPA 13 requires seismic separation assemblies (18.3.1) to keep it from breaking when the two structures move during an earthquake.
There are two ways to build a seismic separation assembly (18.3.2). One approach uses flexible pipe arranged in a u-bend. The other involves a zig-zag pattern of flexible grooved elbows. Either way, the assembly should accommodate a wide range of motion in three directions: the complete closing of the gap between the structures, a doubling of the gap, or lateral movement equal to the size of the gap.
You need a stiff joint for the vast majority of sprinkler pipe connections. That’s why most grooved pipe joints use rigid couplings. But sometimes, flexibility is necessary for seismic design, and the extra range of motion provided by flexible couplings can prevent earthquake damage.
Be sure to check your local codes, including whether a building falls within a seismic design category (SDC) that requires special equipment to keep fire protection systems safe. A qualified system designer must make this determination. Regardless of what you need, QRFS can provide:
If you have questions or need help finding the part you need, call us at (888) 361- or .