Rotating seals or lip seals?

Analyze lifecycle costs, use to pick the right application

08/20/2014


Reliability-focused engineers and technicians often read, with great interest, articles on equipment upgrade opportunities. If nothing else, even an older article can prove informative because it allows readers to determine if progress has been made in the intervening years. In this instance an editorial write-up on "Reducing Moisture Contamination in Bearing Lubrication" caught my eye. It dealt with the effects of moisture contamination and the need for bearing protection-subjects of obvious importance.

The article noted that "lip seals seem to be permanently denigrated when they are not used in an optimum configuration. The automotive industry uses cartridge arrangements ('cassettes'), to what appears to be good effect. They are widely used on trucks and buses, which now often have a half million-mile warranty. This must be in excess of 10,000 operating hours, so would be potentially quite adequate for many intermediate duty pumps."

Environmental and energy savings Comparison of elastomeric lip seal (top part of illustration) and an advanced rotating labyrinth seal (lower part).

Of course, the rolling element bearings of many hundreds of millions of electric motors, various process pumps, industrial machines, and hundreds of millions of motor vehicles are successfully protected against lubricant loss and contamination by lip seals. I certainly concur that lip seals have served industry for more than a century in applications where the "lip" (a flexible elastomeric component, top half of Figure 1) received ample lubrication and where the shaft surface velocities were moderate.

A 20 mm (0.78 in.) automobile drive shaft operating at a maximum speed of 2,000 rpm (2,093 mm/s, or 82.4 ips) would represent a rather strenuous application for an automobile. Nevertheless, this velocity is much lower than the 12,250 mm/s, or 482 ips rubbing velocity of a 65 mm (~2.56 in.), 3600 rpm shaft in a centrifugal pump.

A rather universally accepted rule-of-thumb assumes that rubbing wear increases as the cube of the velocity ratio. Therefore, if a well-designed lip seal in an automobile had a life of 1,000,000 miles at 50 mph, this would equate to 20,000 operating hours on a set of lip seals.

In the industrial equipment example and at a surface velocity almost six times greater, the wear life would be diminished by a factor of 200 and lip seals would last 100 hours-a very unattractive choice by any measure.

Lifecycle cost consequences

It is reasonable and defensible, based on industrial experience, to relate two different scenarios for the two completely different bearing housing seals illustrated in Figure 1. A lip seal is depicted on the upper portion of the shaft. Purely for the sake of using a simplified example, we will assume this particular lip seal costs $5; the lower portion shows a modern rotating labyrinth seal and we choose to price it at $100.

Scenario 1: Machinery bearing housing application

To avoid shaft fretting, moisture intrusion, and premature bearing failure (assuming labor and materials to remedy a bearing failure cost $6,000), we replace a $5 lip seal twice a year. If labor (including overhead) is billed at $500 per event, labor and materials would require a combined outlay of $1,010 per year.

Alternatively, and purely for the sake of illustration, we make the decision to replace a $100 modern dynamic O-ring rotating labyrinth seal after just two years of operation. In that case, labor is $250/year and materials cost $50/year. Our total outlay would then be $300 per year. (In actuality, advanced rotating labyrinth seals have an estimated operating life of 10 years.) The payback exceeds 3:1.

Scenario 2: Machinery bearing housing application

This time, assume we use a lip seal and run it to failure. Allowing the lip seal to degrade might cause a bearing failure after perhaps two years of operation. We have "saved" $1,010 and, assuming we do not incur production outage time, the repair still costs the plant $6,000 per event, or $3,000 per year. We seem to lose by pursuing this scenario.

In both scenarios, upgrading to advanced rotating labyrinth seals would be more cost-effective than staying with lip seals. The fact that rotating labyrinth seals, at least in this example, cost 20 times more than rotating labyrinth seals is of no consequence in lifecycle cost analyses. Generally speaking, it's easy to cost-justify superior rotating labyrinth seals for protecting the bearing housings of millions of process pumps, gear boxes, motors, and similar equipment.

Energy savings are possible

As long as a lip seal is operationally effective (i.e., there is little or no elastomer lip wear) and tightness and/or lack of lubrication has not caused degradation of the shaft due to wear (top of Figure 1), it is reasonable to assume that 160 W of frictional energy are consumed by an average lip seal. At $0.10/kWh, this would equate to $140 per year. The frictional energy to be overcome in an equivalently sized and well-designed rotating labyrinth seal (lower half of Figure 1) is probably only a fraction of 160 W.

If, in the aforementioned Scenario 1, precautionary lube oil replacements (oil changes) were performed and a lube oil charge and its environmentally acceptable disposal were factored in, the picture would shift even more in favor of modern dynamic O-ring rotating labyrinth seals. While reasonable people will certainly agree that lip seals have their place in disposable appliances and in machines which, for unspecified reasons, must frequently be dismantled, engineers should always look at the full picture.

While in no way claiming all lip seal applications are past their prime, there are now viable alternatives for an increasingly reliability-focused and energy-conscious user community. Lip seals rarely measure up to the expectation of the majority of intermediate duty pump users. As maintenance persons will know, tight-fitting lip seals can wear a groove into the shaft (Figure 1, top half). However, the modern rotating labyrinth seal upgrade (lower half) will not contact pre-existing wear grooves in the shaft. All things considered, plant engineers may look at the rotating labyrinth option.



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