Coupling manufacturers have an “acceptable” misalignment tolerance range that is typically much greater than precision shaft alignment tolerances. Why is that? Because flexible couplings are designed to allow for incidental movement of machines during operation.
Seal manufacturers have very tight tolerances for allowable misalignment. Why? Because of how they seal. Typical mechanical seals have mating faces or surfaces that are separated by a fluid barrier. The fluid could be process fluid or water. The fluid separates the faces and creates the seal. This fluid “ring” acts as both seal face lubricant and physical seal against leakage of process liquids.
I am referring to two faced carbon mechanical seals used extensively in pumping water or non-aggregate applications. These are commonly called Single or Dual Cartridge Seals.
The surface finish of the seal faces is so fine that just the oil from your fingers on the face of one of the carbon rings will ruin the seal. To measure the smoothness of the sealing faces of the carbon rings, the manufacturers use a light reflection/refraction standard.
Using “acceptable” coupling manufactures alignment tolerances vs “acceptable” precision shaft alignment tolerances can cause a number of forces that will transfer down the shafts to the other components of the machines. When misalignment is present, the bending motion of the shafts or eccentricity in the rotation of the shaft centerlines changes the clearance between the seal faces causing uneven pressure within the seal that can allow leakage.
The primary effect is that the sealing faces may begin to contact each other and cause wear. Depending on the severity of the misalignment, this could be catastrophic to your seal and pump. These seals, unless split seals are used, require a virtual pump rebuild to change, not to mention the cost. To illustrate the cost of these failures to our mechanics, we used a benchmark of $1500 per inch of shaft diameter as a replacement cost if one of these seals failed. Not including downtime or labor costs.
Chesterton and John Crane are two manufacturers with great information on seals of all types. I would suggest checking out their resources for additional information.