Monday, February 23, 2015

S-Flex (Sleeve) Couplings: Product Overview

Lovejoy S-Flex Coupling Family Shot
The S-Flex (Sleeve) coupling is a coupling type that Lovejoy has been selling since 1985, and one that has very strong acceptance in the US pump industry. While this coupling has gained acceptance in other applications and in other parts of the world, the US pump industry (and especially the ANSI chemical process pump segment) remain its strongest core market.

Much like Lovejoy's signature jaw coupling (Lovejoy was the patent holder for the straight jaw coupling), the S-Flex is simple three part design.  A standard coupling is comprised of 2 flanges and 1 sleeve.  Unlike a standard jaw coupling, the sleeve of an S-Flex coupling is in-shear between the two flanges, with teeth around its perimeter which mate to corresponding teeth in the flanges (much like a gear coupling). The torque is transmitted through the twisting of the elastomeric sleeve. There are several features to this coupling which translate into tangible benefits to the user:

  • As with other elastomeric couplings, it is maintenance-free and non-lubricated.
  • Since the design is double-engagement, it is very soft radially. It provides very low reactionary loads when misaligned which protects bearings/shafts.
  • The torsionally soft design of an in-shear elastomer helps to damp out most peak overloads and prevent vibratory torque from going back to the driver. The EPDM and Neoprene sleeves will twist as much as 15° at full torque, Hytrel up to 7°.
  • The sleeve has a large open center which allows close positioning of the shafts.
  • The in-shear design allows the coupling to act as a "fuse" to protect the driver and driven from torque spikes or system overloads which might cause more expensive damage than just a replacement sleeve. The torque overload capacity of this coupling is only 3 or 4 times the rated torque (the point at which the sleeve will tear, round-off the teeth, or "pop out"), versus the 6 or 7 times for a jaw coupling.

The S-Flex style of coupling is best suited in the following applications:
  • Where coupling alignment may be hard to maintain over a period of time. The S-Flex is a somewhat forgiving coupling and will not load the bearings due to misalignment. The sleeves won't last as long but the coupling doesn't harm the equipment.
  • Non-piloted pump applications, where the motor and pump are on a base plate but there is no pump mounting bracket involved.
  • Closely coupled shafts.
  • For shafts that are relatively small for the torque loads or the bearings are light duty.

Care should be taken however, that S-Flex is not used under the following conditions:
  • High-inertial loads, especially if they produce variable torque loads.
  • Where overloads/spikes are expected to be greater than 2x nominal ratings.
  • Any reciprocating engine driven equipment. S-Flex does not respond well to torsional vibrations. This applies to reciprocating pumps and compressors as well.
  • Do not select this coupling if it is used at anything less than 25% of its rated torque. The sleeve teeth will wear prematurely due to the rubbing action against the flange if too lightly loaded. This can be a concern particularly with the Hytrel sleeves since they have such high ratings.

In general, the S-Flex coupling will work well on electric motor driven applications with uniform loads such as; centrifugal pumps, blowers and fans, screw compressors, some conveyors, line shafts, and vacuum pumps. It competes primarily with other elastomeric designs such as tire types, jaw couplings, as well as Grid spring couplings for some pump applications.

The major manufacturers of this design all produce their product to be broadly dimensionally interchangeable. However, since the design has quite a few "mating" points with the sleeve/flange teeth, the possibility of imperfect fits from one manufacturer to another is always possible. This is due to the tolerance that is built into each company's initial design criterion (i.e. how tight or loose they want the fit between components to be), and the state of wear of the tooling that produces the sleeves and flanges. Furthermore, rubber quality/performance does vary between manufacturers. As such, Lovejoy recommends that mixing of components from different manufacturers be avoided if at all possible. This also assures that the product warranty is not voided.

To keep learning on the blog, go to:

S-Flex (Sleeve) Couplings: Flange Types
S-Flex (Sleeve) Couplings: Elastomer (Sleeve) Types
S-Flex (Sleeve) Couplings: How To Select a Coupling
S-Flex (Sleeve) Couplings: Failure Analysis

To get Lovejoy S-Flex technical data, part numbers, or download a product catalog, please visit Lovejoy's product page, where you can download catalogs, search for part numbers, and click through to watch installation videos. 

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