Purchasing gear lubricants: be careful when playing the numbers game
Written by John Sander
5. Know Gearbox Construction & Capacity
A gearbox contains various components, including – but not limited to – the case, gears, bearings, shafts and seals. The construction of the gearbox, including its geometric configuration, is often contingent upon how the gearbox is required to transmit power within the given application. As described by Tim Cooper of Lubrizol:
"Today's gearboxes often are smaller and made from new materials … they are getting pushed to produce more power and at the same time be more durable and reliable than before. To meet these increasing demands, today's industrial gear oil must contain high performance additive chemistry …".
You must know the construction of the gearbox and use this information as part of the lubricant selection process. This includes the metallurgies, gear geometries and the cuts of the gears (rough or smooth). Table 2 illustrates the part that gear geometry plays in the lubricant selection process.
The gearbox capacity is a subcategory of construction that merits its own discussion. As noted previously by Cooper, many gearboxes are getting smaller. In a small gearbox, less oil is present. As such, it could run hotter, be sheared more by the gears and be affected more dramatically by contamination. The corollary is that in a large box, the oil may circulate less, run much cooler and last much longer. Although this sounds like an endorsement for larger gearboxes, they are not suitable – or even possible – with some applications.
Many lubricant additives activate at certain temperatures. With that in mind, it is possible that a large gear set might experience elevated wear because the oil never gets hot enough for the additives to activate. On the other hand, the wrong additive system employed in a small, hot gearbox could result in an aggressive gear additive prematurely activating, oxidizing and leaving behind deposits. The oil capacity of the gearbox can have a dramatic effect on the gear oil. As such, capacity must be considered as a part of the selection process. Also, if the goal determined in step 1 was to extend the interval between lubricant drains, the size of the gearbox case is very important. The more lubricant there is in the gear case, the more additive reserves there are to extend the life of the lubricant.
