Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Machine Design - Case Study No. 74: Steam Impingement on Bearing Seal with no Packing Box

Agitator Gearbox Exposed to Vessel Vapors

Packing boxes, or stuffing boxes, are installed for a good reason. Use them. This agitator gearbox has lost its lower bearing several times over the last 10 years. I know, I've replaced them all. It's funny, but Production personnel are a different breed of people. It doesn't matter how many times a maintenance thug tells a Production person to fix the packing, it just doesn't happen. After all, Production people are "out to make a buck," and Maintenance people are "out to spend a buck." At least, that's how Production people see it. I look at this picture and I see that the Production people must be making a whole lot of money because they sure are going to be spending a whole lot of money replacing this gearbox again pretty soon.

Aside from the cynical opening to this month's article, there is value to seeing "a wreak in progress." Just how good do you think that lower bearing seal is on the gearbox? What you don't see in this picture is the dynamics. That steam is coming out of the vessel under pressure and is impinging on the bottom of the gearbox. There is a way to minimize that problem, and it's called a "packing box" or a "stuffing box." There is a stuffing box in this picture and it is located in the flanged nozzle at the top of the tank where the agitator shaft goes through. Several rings of packing go inside and close the clearance around the shaft and presto-change'o there's no more leak! Failure analysis can be truly embarrassing sometimes. Who in the audience can predict the failure of the gearbox lower bearing? Let me see a show of hands.

File this failure analysis under the keywords ... "Just Shoot Me."


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