Ask The Experts

“If an oil with a viscosity grade (ISO VG) of 680 has an oil temperature of 60 degrees C but the journal bearing temperature is 88 degrees C, will it affect the oil condition or can the oil withstand this temperature?”

Temperature affects many things in the realm of machinery reliability. At this elevated temperature, you would expect the viscosity to decrease, the lubricant flm to become thinner, an acceleration in abrasion and scuffng conditions, the oil to age sooner, the additives to deplete quicker, an acceleration of corrosion, and the formation of sludge and varnish. These are only a few of the side effects of running at a higher temperature, but they play a critical role in the overall health of the machine and the lubricant.

The relationship between temperature and chemical reaction rates (oxidation or the chemical aging of the oil) was theorized by Svante Arrhenius around the turn of the 19th century. According to Arrhenius’ rate rule, lubricants will degrade twice as fast for every 10 degrees C the temperature is increased after their base activation temperature has been reached. The statement could also be made that if you were to reduce the temperature by 10 degrees C, you would expect to cut the oxidation rate in half, thus doubling the life of the lubricant.

Viscosity is the single most important property of a lubricant, and everyone knows that a change in temperature leads to a change in viscosity. However, few people realize just how much of an effect it has. In the question above, a temperature difference of 18 degrees C is mentioned. This temperature difference accounts for a drop in viscosity of 71 percent, assuming the lubricant in question is a mineral-based fluid with a viscosity index of 95. This drop in viscosity has a staggering effect on the flm strength and the ability of the lubricant to keep the journal bearing from operating in a boundary (metal-on-metal) condition.

The selection and use of an ISO VG 680 oil in a journal bearing is outside the ordinary. This specifc application would have to be very slow moving and/or very heavily loaded to warrant the use of such a high viscosity. For example, if the shaft were 12 inches in diameter and 12 inches in width, spinning at 900 revolutions per minute and at the proposed operating temperature of 88 degrees C, the required viscosity at this temperature would be 15.8 centistokes (cSt). The ISO VG 680 oil provides 59.38 cSt. This is extremely excessive, and the consequences can result in heat generation and energy consumption because of internal fluid friction.

So elevating the temperature to 88 degrees C will affect the oil condition, but the root cause of the heat may be an incorrect lubricant selection. If this is the case, it is easily addressable.