Oil Sight Glass Plugging Case Study

Challenge
The general recommendation to replace a gearbox drain plug with a device that adds a sight glass to the drain fitting, and to replace the fill plug with a combination desiccant-breather and fill cap device is sound advice. We have, however, had problems in some cases with the piping to the sight glass becoming plugged.

This may give a false indication of oil level. Additionally, the oil in the sight glass may appear clean and pristine contrary to the condition of the oil in the sump. Lastly, the top, open vent may bleed oil if set at the oil level plug due to thermal expansion of the oil during operation.

The oil level plug is usually in the side of the reducer housing. The oil level is at the invert or bottom of the level plug hole. If a vent is installed in the oil level plug it would be positioned horizontally. If the oil level rises slightly, oil will bleed from the vent. If the vent is externally piped from the oil level plug, there is a possibility of oil covering the horizontal opening at the level plug and thus allowing pressure to build up within the housing.

Most reducers have a vent on the top of the housing and this vent should be employed rather than installing a vent at the oil level plug. Figure 1 shows the vent in the top of the housing and a plug at the oil level. This configuration should correct the problem of a vent near the oil level.

Solution
To avoid these problems, we connect the sight glass to the horizontal nipple with an L-port three-way ball or plug valve. One position of the valve connects the oil reservoir drain to the sight glass. The other position drains the sight glass to a container below the sight glass. When opened to the oil reservoir, the oil will flow to the sight glass.

To check the condition of both the piping run and the oil level, the ball valve is repositioned to direct flow to the container. This drains the sight glass. The valve is then repositioned to the oil reservoir allowing oil to flow back to the sight glass for a true reading of the oil level. In some instances, the oil may also be checked for color change and contamination.

The container collects the oil discharged from the sight glass. The sight glass must be vented out the top, of course. The L-port ball valve does not allow the reservoir to drain through the valve by accident.

A tee with a drain plug in the side run is included in the piping to the sight glass from the reservoir to allow for draining the gear box. The side run may be located at any angle from horizontal or below to allow for capture of the oil. The tee drain may also be piped and valved as necessary.

The tee has a straight run, which is piped to the sight glass, the side outlet of the tee or side run is plugged. The oil from the reducer reservoir or sump can be drained by removing the plug in the side outlet of the tee. The side outlet must be positioned horizontally or below the horizontal. If the side outlet is positioned above the horizontal plane, oil will not drain completely.

Oil Analysis Overhaul Procedure Change
Harold J. Matroni, ALCOA Mill Products

After reinstalling a two-stage double reduction gearbox that had been overhauled off-site, the gearbox sump was refilled with the proper ISO 220 EP gear oil required by the original equipment manufacturer (OEM). After a short run-in period, the initial routine oil analysis appeared to be out of specification.

When compared to reference readings of the virgin gear oil used, the levels of calcium, magnesium, phosphorus and zinc were very different. The commercial analysis lab questioned if the proper oil was used when the box was refilled. A second oil change was performed to correct the situation, with the analysis now much closer to reference specifications.

A few months later, another gearbox was removed for overhaul. A supply of the refill oil was staged on-site and clearly labeled for that gearbox to be used after installation. Again, I experienced the same odd readings and questions after a short run-in period. Something was wrong.

Oil samples from drums in storage were sent for analysis to verify that it was not a problem coming from the supplier. The suppliers, lubrication engineers and research lab personnel had conversations to discuss possible explanations of the analysis. What common factors linked these two situations?

Overhaul!
The facility which overhauled both gearboxes was contacted to review its procedures. It stated that after any gearbox was worked on, the unit was flushed to remove any foreign material from the unit. It was then filled with a break-in oil and spin-tested for operational problems. During testing, the unit was piped to a filtered circulation unit for lubrication. Upon completion of testing the unit was drained, all openings plugged, and it was shipped back to the customer with an empty sump.

Analysis of the break-in oil identified the source of the strange readings received from the lab analysis. It was the residual break-in oil remaining in the lube system that was the culprit.

To prevent a repeat of the past, a plan was formulated for future overhauls. It was implemented a short time later, just before another large gearbox was to be overhauled.

Procedural Improvements
A full analysis of the oil in use was completed to determine its condition prior to the scheduled overhaul. With the oil in good shape, it was removed from the gearbox as part of the disassembly routine and pumped into clean, lined drums using a portable filtration unit. The oil was further filtered while recirculating in the drums while the gearbox removal was in process. The drums were then sealed and shipped with the gearbox to the overhaul facility. The overhaul facility used the filtered oil as a flushing and break-in fluid after repairs were completed. Upon reinstallation the sumps were refilled with the new, proper filtered oil per OEM requirements.

Because this routine is now standard procedure, we no longer need to change oil due to cross-contamination conditions caused by residual break-in oils.