Oil Viscosity in Grease and Importance

Hi, I would have to go with the recommended viscosity the bearing manufacturer suggested. A "one size fits all" approach isnt the best. You can contact the bearing manufacturer to determine which grease to use for your bearings. SKF has a great grease/oil viscosity chart they use with their bearings,....rpms,size,load etc. We have used it here in our paper mill. Brett

Don,

Base oil viscosity is the most important property of a grease. After all it is a lubricant and same rules apply that film thickness is a result of viscosity.

The choice of viscosity is a balance between bearing contact friction and fluid film friction meaning that if the viscosity is too low it will produce frictional heat and excessive wear and if it is too high it will also produce frictional heat, energy losses and shorten lubricant life. At slow peripheral speeds the impact on bearing life and bearing replacement cost exceed the cost for energy losses but at normal to higher speeds the cost of the energy losses can significantly exceed the bearing replacement cost.

Management and procurement always want to simplify and reduce SKU's but the cost of doing so can sometimes be very high. The energy consumption due to power losses could increase beyond your immagination or the bearing life and bearing replacement cost could increase. In addition lubrication interval, labor costs, lubricant consumption and waste handling costs could be affected.

When going through a porfolio harmonization like that you need expert help to ensure that you don't make any critical misstakes that would end up on someone elses budget.

This is the kind of stuff i've been doing for a living for the past 25 yrs.

Cheers,
GG

While I agree 99.9% with GreasGuru, sometimes there is equipment where the grease specification for a piece of equipment seems to be personal preference or assembly of "recommendations" for each component.
I worked with a company where the line came with 14 grease recommendations, 14 cases of different greases (one grease gun). Specs for the individual greases overlapped, or were minimum, where you could certainly go better.


In the end, we simplified with a hi-temp lithium complex EP2, A Bentonite Hi-Temp EP2, and a Food Grade EP2. The lithium complex was already used in the rest of the plant, the bentonite did not really need a gun. They used the gun for the Food Grade.

But as said. Viscosity is your first and most important characteristic. the thickener is just there to like a sponge to hold it.

Always the iconoclast, I'll stake out the position that NLGI grade & base oil viscosity are interdependent and of equal importance. MOV is moot if the grease either can't get where it has to go or won't stay there.

The general rule of thumb is that you can go up or down one ISO grade without too much concern. So, if the equipment is designed for a 220 base oil, you could move up to a 320 or down to a 150, but I'd get the equipment manufacturer in the boat with me if I were going up to a 460.

I don't think moly is going to be your friend here, as nice a girl as she may be. Typically you'll see higher viscosities as the application moves toward high loads, shock loading and lower speeds. Moly helps with all of those stressors. Lower viscosities tend to be preferred with lower loads, less shock loading and higher speeds.

I'm all for consolidation. You may get less than optimum performance from the bearing in terms of overall life & energy consumption, but in some cases you can crater a bearing in one shift when the wrong lubricant is applied. Reducing the number of lubricants in inventory doesn't just save money for the bean-counters, it reduces the chances for mis-application. This is particularly true where a supplier has a winning brand name that they then apply to multiple products, some of which have very different properties. (If the bean-counters don't get us, the marketing guys will!)

BUT, you can't go overboard. I have a vision based on the movie "Repo Man" with two white drums with black block letters - "OIL" & "GREASE". Overconsolidation will likely cause as many problems as too many SKUs, they'll just be different ones. You need to hit the sweet spot. That's a tough assignment for humans, who seem to love the full-scale deflection. If we have a problem of "too much", our solutions frequently look like "not enough".

Right now, you're in the midst of the perfect tension to optimize the situation. You want to use the exact specifed grease for each application, management wants to save money. You need to be flexible where you can but show them where they reach the point of diminishing returns. At a certain point, the cost savings from consolidation will be offset by increased maintenance & energy costs. If you just dig in your heels, you'll get steamrollered; if you work with them, you can get close to the sweet spot.

Good Luck.

Agree with Widmans comments which doesn't really conflict with my statement. Many OEM recommendations are based on personal preference, track record or market availability rahter than technical requirements. Most recommendations are made to simplify lubrication rather than optimize equipment life and cost of ownership.

If you have many very similar products under different brand names those can fairly easilly be consolidated or potentially upgraded to a higher quality with a greater range but when consolidating products with huge difference in base oil viscosity and/or thickener type you have to be a bit more cautious.

The thickener type is not irrelevant. It is the second most important property after viscosity since most of the properties of a grease is a result of its thickener type and content such as the obvious ability to stay in place or sump, sealing ability, washout resistance, shear stability, pumpability, chemical resistance etc. etc. Even EP and AW properties are affected as most thickeners do contribute to those properties and in some cases EP/AW chemistry is attached to the thickener structure enhancing those properties.

Thickeners do contribute to film thickness but there are no standardized methodology to predict or estimate its impact so therefore we rely on base oil viscosity.

MoS2 and other solid lubricants should generally be avoided especially for roller bearing applications since they are hard solid particles with a particle size which is several times the order of a typical lubricant film thickness which can result in indentations and escalating pitting. Most of these additives also bond water which affect rust inhibition.

When you consolidate it is important that the benefits of the consolidation exceed the potential impact on energy efficiency, component life and downtime, labor cost, waste handling cost etc. This is the part which may be tricky to evaluate on your own.

Cheers,
GG

I maintain that thickener type is at least as important as viscosity, more imortant when considering a change. I've overseen a lot of grease compatibility studies. When the mixture turns to soup and the Pen drops like a rock, it's not the two base oils that made that happen.

I'd agree that NLGI grade is equal in importance to base oil vis, but having a compatible thickener type is the ante, it's the price of admission, it's the secret handshake - in many cases the amount of work needed to adequately prep contact surfaces for a grease with an incompatible thickener will knock that grease out oif consideration.

Thanks for all the great comments!

Recently searched on WWW for the different grease products that are on the market with the different thickeners and numerous oil viscosity packages. (I had learned of the grease differences years ago but with locations like this message board it sure helps to bounce thoughts off others with such a vast background!)
Found charts explaining what is compatible with each thickener. Found recommendations for oil viscosity by temperature, speed and application.

As all have mentioned, I'm sure lube mixing as well as using grease with the wrong oil for wrong temperature, speed and application is happening. I will be meeting with those that can help correct this problem soon.

It is my belief that there are many maintenance people not aware that the various thickeners and oil in greases can be so different. I'm sure most would not put an oil with VG 150 in a combustion engine that uses 5W30 but they will put a grease with VG 460 oil into a ball bearing that will not see 120F and should use a VG oil of 115. It is just a matter of awareness.

I appreciate all the information and will enjoy sharing with the others.