Friday, February 13, 2015

Coupling Temperature or Chemical Failure? (Site Visitor Question)

Curved Jaw Chemical Failure and New Urethane Spiders
Chemical Failure & New Urethane Spiders
Question: I have a question regarding premature failures of Lovejoy Curved Jaw/SPIDEX urethane spiders (size CJ 90), which are breaking into pieces on multiple pieces of equipment on our offshore drill rig in Western Africa. In each case the coupling is connecting a hydraulic pump, and I don't believe alignment is the issue.

Our weather is hot and humid, and I learned from your company's website that there is a green colored spiders for very humid condition. We have been using the red (high torque) Polyurethane spiders. Is it possible that change to green colored (high humidity) spiders will eliminate the premature failures we have been seeing? Your expert comment on this query is highly appreciated. 

Lovejoy Application Specialist Answer: Thank you for the information and photo showing the spider that failed. You are correct when you state that Lovejoy offers a spider that will withstand a modestly higher temperature, higher torque, and higher humidity.  The green colored spider that is equivalent to the CJ 90 red spider you currently use is 68514472268, CJ 90 SPIDER 64D GREEN. This spider is rated for 39,800 in-lbs of torque, 7% higher temperature rating (230°F, 110°C), and slightly higher humidity due to the shore hardness of the Urethane. I do need to let you know that the green spider, due to the high shore hardness, cannot accommodate as much misalignment as the red spider. Care should be used to ensure the couplings are quite well aligned when using the green spider. So, our answer is ‘yes’, you should be able to use the green spider.

However, I would also like to review your photo. The appearance of the spider in your photo is not what we would expect to see in a spider that has filed due to humidity and heat. The blackened surface appears more like the spider may have been subjected to some sort of chemical attack. When the spider fails due to heat, it loses some of the ability to transmit torque. When enough of the torque capacity is lost, an overload, or overload condition will occur and the legs first crush, then push into the center of the coupling causing a ‘squaring’ effect as seen in the second picture below. 

When I zoom in on your photo, it appears there is some foreign material or chemical on the spider pieces.  I need to ask if this is some sort of oil, possibly synthetic hydraulic oil. Normally Urethane stands up to exposure to most petroleum based hydraulic oils. When exposed to synthetic oils, the spider will break down, sometimes breaking into small pieces as part of the failure process. If you use a green spider and expose it to the same chemical, it will fail much like what is shown in your photo. You may want to take care to prevent this type of exposure if that is what happened. You may even be able to continue using the red spider if it accommodates your torque requirements and if a chemical attack was the actual cause of the failure. 

Customer Reply: The customer confirmed that oil may have been dripping on the coupling spider, and was very grateful for the Lovejoy Application Specialist's feedback. The customer replaced the spider, and communicated to the Specialist that he would be working to ensure the oil in question would not drip on the coupling in the future.  

For More Information on Curved & Straight Jaw Coupling Failure Mode Analysis: Please read Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos).

Do you have another coupling question you would like answered by an expert? Ask away here!

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