Friday, February 27, 2015

S-Flex (Sleeve) Couplings: Failure Analysis

S-Flex Sleeve couplings are subject to many failure modes similar to other coupling types. Starting with the metallic hubs, excessive peak torques can cause keyway bursts... which would be a weak point in the hub and, barring a material defect, the most likely place it would fail.

Moving to the elastomer, similar to jaw couplings, an S-Flex sleeve can be degraded if exposed to environmental conditions it was not designed for (i.e. - excessive heat, cold, or chemical exposure).

Standard S-Flex Sleeve
Standard S-Flex Sleeve
Where failures get particularly interesting/unique for S-Flex couplings is with sleeve failures themselves. The signature failure for an over-torqued S-Flex coupling will be a diagonal tear (clear evidence of excessive wind-up) across the width of the elastomer.  In such a circumstance, a user would likely be well served to upgrade to a stronger elastomer and/or upgrade to a more torque intensive coupling (either a larger size or different design). 

A second failure mode in an S-Flex elastomer is a straight tear that runs parallel to the coupling flanges. Such a tear demonstrates either elastomer fatigue that can be attributed to misalignment, the flanges being too close to the elastomer (with them pressing in on it).

The third and least understood of the elastomer failures are when the teeth of the elastomer wear away (often on one side). At the highest level, it must be understood that S-Flex couplings are designed to accommodate misalignment through the flexing of the elastomer itself and that the tooth profiles of the elastomer are to be snug and engaging with the coupling's metallic flanges (not chattering). 

If the coupling is oversized for the application (or if there is a poor fit between the elastomer and flanges), and there is not enough wind-up to have the elastomer's teeth fully engaged with the steel flange teeth, torque transmission will be localized to the tips of the elastomeric sleeve, leading to premature tooth wear, chatter, and ultimately sleeve failure. If not detected early enough sleeve teeth can wear away completely, often only on one side, with the sleeve spinning freely inside the given flange.

This last scenario (the coupling being oversized for the application) is particularly vexing to most end user and even power transmission veterans because, across many elastomeric coupling types, a common way to address an elastomer failure is to either upsize the coupling or go with a strong elastomer material. Given the wind-up chatter issue... going to a stiffer material or even more oversized coupling will only exacerbate the problem. 

Jaw In-Shear Coupling
Lovejoy's Jaw In-Shear Coupling
So... if you are having rapid tooth wear on a S-Flex sleeve, the  first option you might consider is making sure that you cannot either select a softer material or down size the coupling (while still meeting your system's requirements). An excellent second option, particularly if you can't go down a size or switch to a softer material is to switch over to a Jaw In-Shear coupling. Like an S-Flex the elastomer is in-shear, but because it is using a standard jaw coupling the wind-up and chatter issue is eliminated. Furthermore, the product leverages standard off the shelf Lovejoy jaw coupling hubs (making it an affordable solution) and the design has a radially removable elastomer for fast inspection and change outs if/when required. 

For more information on sleeve coupling failures, consider checking out the MPTA bulletin on it (Sleeved Element Failure Analysis - MPTA C8c-2011), which Lovejoy contributed significantly to... or reach out to a Lovejoy application specialist directly

To keep learning about S-Flex couplings on this blog, go to:

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

Article Shout Out: I would like to specifically thank Michel "Mitch" Bouchard of General Bearing Services in Canada for requesting that we cover this question on the blog. If you would like to submit or request a question be addressed on this blog, please fire us off the question here.

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