Elastomeric Couplings

[sinbj1]

All,

I have an application where we are experiencing premature pump / motor coupling failures. We are using a Hytrel elastomeric toothed, shear, sleeve and flange coupling. The motor is equipped with a VFD that operates in steps starting at 3600rpm then steps down to 2700rpm then to 1800rpm. The motor nameplate Hp is 60 but never gets over 45 during operation.

We had a recent coupling failure where the elastomeric coupling melted. I have read that VFD’s will produce torsional pulses at low speed greater than those at high speeds. I have also read that heat can be generated through the effects of vibration and hysteresis of the elastomeric coupling sleeve.

Is it possible that when the VFD is operating at the lower speed the vibratory pulses are generating enough vibration for hysteresis heat to be an issue?

Any feedback is greatly appreciated.

 

[JJPellin]

I would have a few comments on your problem. I have experienced problems with elastomeric couplings using Hytrel elements. In one particular application, a coupling has lasted for more than 10 years using a Buna element. At a routine maintenance inspection, the coupling element was replaced with Hytrel which is rated to higher temperature. The coupling failed within 3 weeks. If you read the coupling manufacturer’s literature, Hytrel can operate to higher temperatures, but it can only tolerate about ½ as much misalignment. I am not certain that this is relevant to your problem, but I wanted to mention it anyway. I don’t like Hytrel.

Variable speed drives do put out additional frequencies that can affect torsional conditions. With the particular type of coupling you describe, I would find it believable that the torsional pulsations are generating additional heat. I would prefer a different type of coupling. My first choice would be a metallic flexible element (disk-pack) type coupling. This would be much stiffer and could transmit the torsional pulsations to the driven equipment. If an elastomeric coupling was necessary, I would prefer a torus (rubber dough-nut) or a jaw-type with a star pattern element.

Someone with an electrical background may also be able to recommend a change to the VSD to reduce the torsional pulsation. I have no expertise in this area and simply know enough to know that this can be minimized.

Johnny Pellin

 

[ApplicationENG]

i would also recommend a metallic disk pack coupling.

one other consideration is to determine if the coupling is sized properly i.e. HP/RPM. At lower speeds the coupling may be undersized and not rated for that speed of operation. you may need to resize your coupling. Also, check your coupling service factor and see if there is any wiggle room.

 

[flexibox]

Check the coupling selection at 1800rpm. It is probably under rated. Using a metal membrane coupling will also fail if selected incorrectly but will be a better coupling than what you have. There is also no back lash.

I would change to membrane coupling selected on 60 HP @ 1800 rpm with a 1.5 SF

 

[sinbj1]

All. Thanks for the feedback. The coupling vendor will be on-site this week to discuss our options. This was great dialogue and I gained some valuable information.

 

[btrueblood]

Have had similar experience as JJPellin with hytrel elements. We had no luck in trading out the elements for nitrile or Buna, and ended up going to a much larger diameter coupler; this had two positive improvements - the coupler elastomeric elements were larger and thus had lower stress levels, and the coupler housing itself was larger and heavier and had a much larger torsional moment of inertia, which helped to reduce the shaft torsional accelerations and thus lower the angular deflections that the elements needed to absorb.