- Buyer's Guide
Part of my job is to carry out benchmarking surveys at clients’ sites. This survey, referred to as the Lubrication Program Development Phase I, is a bird’s-eye view of the lubrication practices at a site. As consultants, we benchmark the site using a questionnaire of approximately 230 yes/no weighted questions covering 12 different categories. A spider diagram is then generated to show performance in each of the categories.
There are a few things I look for immediately when going on-site, which give me a good idea of where the customer’s lubrication program stands. These include the presence, or lack thereof, of lubrication program performance charts, the setup of the oil storeroom and what grease is being used in fan bearings. The latter is the subject of this article.
Many people make the mistake of using a multiservice grease in their fan bearings and other high-speed bearings. This is not best practice due to the high rotational speed of these bearings. The problem is that the viscosity of the base oil used in a multiservice grease is typically 150 or 220 cSt at 40°C. Generally speaking, for the same load (torque) on the bearing, as the rotational speed increases the viscosity should decrease. The higher viscosity base oil of a multiservice grease could lead to overheating, premature degradation of the grease and possible damage to the bearing.
Let’s look at an example of a fan I’m currently working on: a 15 hp motor, running at 1,750 rpm is driving a fan through a belt drive with a one-to-one reduction. The bearings on the fan have a pitch diameter of 50 mm. Various methods are available for determining the correct viscosity to use, but the one I used calls for a desirable operating-temperature viscosity of 19 cSt. The operating temperature of the bearing was estimated at 60°C. Using a viscosity-reference chart, this approximates to a desirable base-oil viscosity of 50 cSt at 40°C. Moving up to the next grade reaches a required base-oil viscosity of 68 cSt at 40°C. Getting a grease with such a low viscosity base-oil is not an easy task, but 100 cSt (an electric motor grease) as opposed to 150 or 220 cSt (a multiservice grease) remains the better option.
That was the scientific rationalization. Now consider a more common-sense view. A motor driving a fan provides a certain torque at a specific speed that can be quantified in horsepower or kilowatts. Assuming that it is a directly coupled fan or a one-to-one belt drive (a typical application), then the fan absorbs the same torque, minus negligible losses in the belt or coupling, at the same speed. It stands to reason that the fan bearings are carrying a load similar to those in the motor and are probably going to the same size. So why use a different lubricant?
Ideally, the same grease should be used in high-speed bearings and electric motors.
Now this is where problems can occur. In most cases, the grease used in electric motor bearings has a polyurea thickener. If a multiservice grease is being used in fan bearings, it will typically have a lithium-complex thickener. Mixing polyurea and lithium complex-based greases may cause bearing failures because some of these thickeners are incompatible. Therefore, the options are to either introduce a new grease into the lubricant store room or purge the fan bearings and use polyurea-based electric motor grease in the fan bearings.
Purging can be accomplished in two ways: by opening the bearing and cleaning out the old grease (the preferred but often impractical method), or by refilling the bearing with the new grease at frequent intervals until all the old grease is removed. The second method is probably acceptable for slow-rotating bearings, such as conveyor bearings, where the risk of overheating due to the overfilling of the grease cavity is low, but for high-speed bearings, this is risky. My preference is to invest in a new grease compatible with the existing grease in the fan bearings, but one with the lower base oil viscosity. If a multiservice lithium complex-based grease is being used in fan bearings, then one should look for a lithium complex-based electric motor grease.
Communication is the Key
Introducing a change like this requires thorough communication with the lube technicians. Here are a couple more suggestions to ensure the smooth transition of the grease change. Identify the affected components with a lubrication tag clearly indicating the required grease, the amount and the frequency of relubrication and color-code the grease zerks with different-colored caps. Both of these options should be standard procedure. They’re not difficult, and once completed, you’ve made another step along the road of lubrication excellence.