Buyers of lubricating grease typically base their decision around three things, says Chuck Coe of Grease Technology Solutions. One is the type of thickener system used-is it lithium, calcium or aluminum soap, polyurea, clay or some other gelling agent? Another need, especially for those seeking longer life or high-temperature performance, may be a synthetic base oil. But the factor ranking highest in buyers minds, he says, is grease consistency: Just how stiff is the stuff inside the cartridge or drum?
Consistency is evaluated in ASTM D217, the cone penetrometer test. In this venerable procedure, dating to 1925, a cone-shaped weight is allowed to drop freely into a sample of grease. After five seconds the depth to which the cone has sunk is checked. For worked penetration, the grease is churned first for a set number of strokes, then measured.
The result appears on grease labels worldwide as the NLGI grade, from 000 to 6. The higher the number, the harder the grease. D217 is fast, intuitive and simple, but has no correlation to actual field performance. This is because penetration measures resistance to deformation, not resistance to flow, explained Coe. There is no industrywide test for measuring grease resistance to flow.
This leads many to put their faith in the NLGI grades, says John Sander of Lubrication Engineers in Wichita, Kansas, but penetration cant predict how a grease will respond to changes in temperature, pressure, bearing speed or other variables. Will the grease soften in the presence of water, or bleed out too much base oil if heated? What about the greases tackiness? How easily can it be pumped? The penetrometer is worthless for such questions, he observed.
We sell at least five or six NLGI No. 2 grade greases, Sander told LubesnGreases, and anyone can see they arent even close to each other. One may be sticky or tacky, another looks runny, another is buttery. One may be intended for low-temperature conditions, and yet another is a clay thickened grease, which has no dropping point and would be a good choice for high temperature applications. Yet the D217 test says theyre all the same thing: NLGI No. 2 grease.
The solution, say Sander and his LE colleague Wade Flemming, is to establish a performance specification that addresses flow, pumpability, bleed, tackiness and other grease properties. This specification could be modeled on SAE Internationals well-known J300 engine oil viscosity classification system, Flemming suggested in June to the annual meeting of the National Lubricating Grease Institute. J300, he reminded the gathering, evaluates all kinds of viscosity-kinematic viscosity, low-temperature viscosity, high-temperature/high-shear viscosity-and then classifies engine oils into various straight (like SAE 30) and multigrade viscosities (SAE 5W-20, 10W-30, 15W-40, etc.) By contrast, penetration doesnt really indicate much beyond whether or not grease will pour out of a container, he said.
A grease specification could cover parameters like speed, temperature, moisture and consistency, the two researchers said. Tests might include the Lincoln ventmeter test for pumpability; the U.S. Steel grease mobility test for flow characteristics; and a grease bleed test. But topping their wish-list is a rheometer. Used in advanced grease laboratories for 30 years but still relatively rare, rheometers measure flow and how it changes with time, temperature and pressure.
Rheometers come in a dizzying array of configurations-cones, spindles, plates, cylinders-and many grease manufacturers opt for a controlled stress model. A very small sample is placed between opposing plates (flat or conical) and pressure applied as one plate rotates or oscillates at the determined speed. The greases resistance to flow can then be graphed against time, temperature and/or yield stress. Unlike D217, the result is an in-depth profile of grease behavior.
If rheometry were standardized and widely accepted, Flemming and Sander believe the cone penetrometer could be retired.
Thats unlikely, said Raj Shah of Koehler Instrument Co. in Bohemia, New York. He noted that grease manufacturers worldwide use penetrometers to check each batchs hardness. In fact, quality control was the devices original purpose, and penetrometers from his company and others still shine in this role today.
Coe agrees. Back in my ExxonMobil days, the Round Hill, Virginia-based consultant related, we had two or three penetrometers on the floor of the grease production plant, and the operators did the tests right there, to check on the quality of their batches and their process controls.
Other industries use penetrometers, too, Shah added. They are used extensively to check batches in many other industries: wax, asphalts, petrolatums, processed foods like chocolate and margarine and fruit preserves, he said. Estee Lauder, which makes lipsticks and so on, has hundreds of penetrometers in service, only they use a needle-not a cone-to penetrate the surface.
In 1998 Shah published Yield Stress Studies on Greases with Peter Whittingstall of rheometer maker TA Instruments, in the NLGI Spokesman journal. So he understands rheometrys promise and pitfalls. The beauty of the rheometer is that you can do so much, but its overly complex to understand and the cost can be 10 times the cost of a penetrometer, he related. Besides, the penetrometer is not static. It can be dynamic. You can take measurements at different temperatures, and you can also do worked penetration, where the material first is worked for 10,000 strokes or 100,000 strokes, and then tested.
No way the penetration test will go away, exclaimed Paul Bessette, head of Triboscience & Engineering, Fall River, Massachusetts. For one thing, this industry moves more slowly than at glacial speed. For another, a penetrometer is fairly inexpensive to buy, while a rheometer can cost upwards of $100,000. For that, you do get a complex look at the greases visco-elastic properties. However, its a fussy test to run, and when youre testing really hard greases, it can be hard to get the pressures needed.
The penetrometer is still a good laboratory or plant instrument, to help maintain the homogeneity of your product from batch to batch to batch, he went on. Its a good quality control tool, and for product R&D its OK as long as you recognize that its extremely limited. The benefits are that its easy to run, almost indestructible, and a reliable tool.
More richly, the rheometer gives you a comprehensive plot of how the grease will behave under shear, and at various temperatures if you want. You can set the shear ramp too, so under low shear a grease sample behaves like a solid, then under high shear it behaves like a liquid. This kind of regime makes it ideal for developing greases for rolling element bearings.
For a small company or small Q.C. lab, the penetrometer may be enough, but it wont get you to the next level, continued Bessette. Thats why last year he bought a high-end rheometer to bolster his companys research efforts. We bought a used TA rheometer, and it was more expensive than a new PDSC and gravimetric analyzer combined, he revealed. The other place youll see these is academia, for high-level research.
What justified the expense? We work with some of the fussiest people on the face of the earth, such as manufacturers of bearings for aircraft and aerospace, Bessette replied. We had to have the tools to show them how the grease behaves, and why. For example, they may demand ultra-clean greases, where you push it through micron-size membranes. Thats a whole different ball game, and you need insights into how the grease will perform under that shearing.
Youd have to agree that researchers can obtain better and more useful information with a rheometer, commented Coe. However, there are three problems: First, its hugely expensive. Second, it requires significant operator knowledge and training. And third, you have to settle on the conditions of the test-temperature, shear, pressure-or you get meaningless results. Rheology is incredibly valuable for product development, where you can settle on your own procedure to get information. But to settle on an accepted procedure to get one number that is widely applicable in all situations? So far its not happening.
The variety of rheometer models and configurations also slows standards-setting, Coe continued. For example, after you squeeze the grease onto the plate, are you going to apply rotational pressure or oscillating? How will you account for edge effects? People can tout the rheometer all they want, and in absolute academic terms, its better. But from the practical perspective, its not there. If we want to measure grease resistance to flow using a rheometer, somebodys going to have to come up with a test, define a good standard set of conditions, and get everyone to agree on it.
Its daunting but not impossible, insisted Sander. The challenge is that there is no ASTM rheometry standard, although weve been talking about it in ASTM D2s Subcommittee G on Grease. Unfortunately, given the speed at which ASTM moves, and absent a regulatory or end-user need, were not making much progress. It would help if we had an OEM come forward with a requirement beyond the NLGI grade number. Then we could do temperature profiles, shear profiles, dynamic viscosities, even certain geometries, all using a rheometer under a set of accepted conditions, to create a procedure.
Industries like asphalt and coatings have established standards based on rheology, points out Timothy Cassell, who is with Anton-Paar USA in Ashland, Virginia, which makes both rheometers and penetrometers. The penetrometer tells us about consistency, or rather resistance to penetration, Cassell said. Rheometry tells you much more about how the material will behave under different pressure and temperatures. If you want to have a good understanding of that products flow, thats what youll get with a rheometer. The penetrometer shows that the product is good; a rheometer shows you why.
Cassell sees rheometry making inroads for quality control, too. Major coatings manufacturers are doing just that. A rheologist can design a template of how the material should behave, like a fingerprint, and the user on the manufacturing floor doesnt need to know why. With training, the operators can just follow a set procedure and see if theyre matching the profile. Anton-Paar has a new, more affordable line of rheometers for this market, the 72 and 92 series, he added. Their torque range is narrower and the body is different, but theyre highly sensitive. One difference is the 72 series has a mechanical bearing, while the 92 has an air bearing so theres no noise in the test.
Cost is a hurdle, Sander acknowledged. For the best, most flexible rheometers, youre looking at $75,000 to $100,000. I asked for that thing in my budget for seven years, and my boss would always say, John, cool doesnt get you an instrument. You have to make the case for its practical use.
He did that by working with an outside lab that owned a rheometer, using it to resolve cases where customers needed help with grease applications or pumping. This led to key insights into both products and manufacturing processes, and prompted the company to acquire its own rheometer.
Even if the grease industry develops a rheometry based specification as Sander and Flemming envision, the penetrometer wont go away, assured Coe. Penetration is a good test, as long as you recognize its shortcomings. Youve got to have a penetration number on your specification because thats what youll manufacture to-it has meaning there.
Bessette seconded that view. No one is really in favor of getting rid of the penetrometer test, and you dont need to do a TA rheometer test on every batch you make. If all youre making is a fifth-wheel grease for a tractor trailer, maybe the rheological profile isnt relevant, he concluded. But generally I think the more you know, the more useful it is.