Hammered Shut

You know we provide o-rings and precision-molded rubber parts, but did you know you can count on us to help solve product performance challenges?

Here’s an example … 

A company approached us experiencing seemingly random “in-the-field” performance issues when some of their water hammer arrestors (WHA’s) were initially installed.

WHA’s are used in plumbing systems to keep pipes from clanging. Installed near a plumbing fixture, a WHA contains a sealed pressurized piston which absorbs the backflow (pressure) created when a valve is closed, thus eliminating the clanging in your pipes.

Water Hammer Arrestors Stuck In Place

The vast majority of their WHA products performed as expected upon installation.  But they experienced unexplained random issues with some of their WHAs in the field.

When they approached us for assistance, they had already ruled out material compliance, mechanical tolerances, temperature sensitivity, and incorrect field installation.  Additionally, an extensive review of their manufacture lot test data reinforced the fact that every part left their plant in full compliance with all specifications.  So all the usual suspects checked out, and a “root cause” could not be identified.  There had to be some reason, some explanation but what was it?  

During our investigation, the customer explained that the pistons appeared to be frozen or stuck, but with a little manual stimulation (“banging with a hammer”) it would break loose and function properly for the life of the installation. Additionally, we discovered that it was not uncommon for these units to sit on a distributor’s shelf for a year or more before being installed in the field. 

This information led us to suspect adhesion between the rubber o-rings and the copper tubing.  We requested an opportunity to review the design in order to identify the root cause and suggest design enhancements to eliminate the issue.

Product Design Diagnosis

The original design consisted of three (3) o-rings made from an EPDM material compound, seated along the piston.

After close examination we determined the following:

  • When the parts sat on the shelf, the o-rings took a compression set, adhering to the copper tube.
  • The presence of the third o-ring caused additional friction without enhancing the sealing function.
  • Finally, we found that the interior diameter of the tube was not completely concentric, creating additional drag on the piston.

These three factors combined to increase the amount of force required to overcome the coefficient of friction and initiate actuation of the piston at initial installation.  

Our Recommended Sealing Solution

We suggested a two (2) o-ring design made from an internally lubricated NSF approved EPDM material. 

First, we determined that a single o-ring could deliver the sealing requirements for this application. However, to center the piston correctly in the tube and ensure smooth movement within the bore, the application required a second o-ring.  A wiper ring or u-cup is a common choice for this type of seal, but this application needed an o-ring because of the pressurization on both sides of the seal.

Second, we proposed changing to an internally lubricated material to address the “compression set”/adhesion issue occurring when the parts spent extended periods of time sitting on the shelf.  An internally-lubed material contains either tfe or paraffin which is added to the compound mix, while it is being batched prior to the molding process. This lubricating agent is integral to the compound, and significantly diminishes the adherence that can occur between the o-ring and the tubing.

Finally, during our testing, we observed subtle non-concentric conditions within the bore which impacted the consistency of actuation force.  While all mechanical dimensions were found to be within stated tolerances, we believed the design could be improved and therefore recommended that they investigate modifying the copper tube to make the bore more concentric, which they did.

The effect of these recommendations combined to reduce the breakaway coefficient of friction of the original design, eliminating the random piston actuation issues.  In the end, working alongside their engineering team, we helped the customer improve their product, increase the satisfaction of their end users, and we provided a design that required 33% fewer o-rings reducing the cost and assembly time of their design.  

(It is worth noting that we were the only vendor whose redesign went from 3 o-rings to 2 o-rings and identified the non-concentric bore issue as a contributing factor to the design issue.)

The Lowly O’Ring