NLGI Cone Penetration test

For those wondering about the headline image read on to find out more. When it comes to maintaining industrial machinery, selecting the right grease is like finding the right tool for the job—it can make all the difference. But how do you know which grease is best suited for your application? One of the most reliable methods is by understanding the NLGI grading system and the Cone Penetration Test. In this article, we’ll explore what these are, why they matter, and how you can use this knowledge to make informed decisions about grease selection for your equipment.

What Are NLGI Grades?

The National Lubricating Grease Institute (NLGI) grading system is a standardized method used worldwide to classify greases based on their consistency. This system helps in comparing different greases and determining their suitability for various applications.

NLGI grades range from 000 to 6, with each grade representing a different level of grease consistency:

  • 000: Semi-fluid, almost like a thick oil.
  • 0: Very soft grease.
  • 1: Soft, similar to tomato paste.
  • 2: The most common grade, with a consistency similar to peanut butter.
  • 3: Firm, like vegetable shortening.
  • 4 to 6: Very firm, almost like a bar of soap.

Understanding these grades is crucial because the consistency of the grease affects how it behaves under different operating conditions, such as temperature and load.

The Cone Penetration Test: What It Is and Why It Matters

The Cone Penetration Test is the standard method used to determine the NLGI grade of a grease. But what exactly does this test involve, and why is it so important?

The Test Procedure

There are two main methods used to determine grease consistency. The ASTM D217 and its equivalent ISO 2137 determine the method for measuring grease consistency by cone penetration. We will start on what’s called a P0 penetration or basically straight from the tub measurement of consistency and speak about variations on this later.

Cone penetration in action The sharp tip of the cone is lowered so it is just touching the surface of the grease We push start on the instrument and the support preventing the cone dropping is removed by an electromagnet and the cone drops A timer for 5 seconds is started at this point After 5 seconds the support is locked in place so not to move whilst measuring and the distance travelled by the top of the cone is recorded by a very accurate depth gauge

The Cone Penetration Test measures the depth to which a cone of specific dimensions penetrates a sample of grease under controlled conditions. Here’s how it works:

  1. Sample Preparation: A grease sample is placed in a test cup and leveled off to ensure a flat surface.
  2. Temperature Control: The sample is brought to a standard temperature (usually 25°C or 77°F). This is commonly done in a water bath or oven. I often give a second scraping the surface after this step to ensure any expansion is flat right before the test.
  3. Cone Application: A standardized cone, with a specific weight, is allowed to drop onto the surface of the grease for five seconds. If using electronic grease workers the worker is held in place after 5 seconds by a magnetic clasp to stop it penetrating further and to allow depth measurement.
  4. Measurement: The depth (in tenths of a millimeter) to which the cone penetrates the grease is measured. In our system it’s the distance from a fixed point above the cone holder that the cone has dropped we measure.

This depth of penetration is then used to assign the NLGI grade. For example, a penetration of 265-295 tenths of a millimeter corresponds to an NLGI grade of 2.

Working the grease

Grease is essentially a soap and oil. The soap acts like a sponge and soaks up the oil but under pressure or load releases the oil to do lubrication and then sucks it back up again. Imagine squeezing a sponge soaked in water on your kitchen counter. Then place that same dry sponge onto the water. It soon is pulled back up into the sponge as you release the pressure. Hence the grease works by giving a local supply of oil in hard to reach places. Worked releases the oil when loaded and when not loaded sucks it back up again to prevent it escaping. You can find out more about a grease tendency to separate during storage from our conical sieve test article.

Why is this important? Well this means the properties of grease in the tub can differ from when in use. We have covered measuring the unworked grease called P0, but we can work the grease to make it act similar to how it will be in use.

Left picture grease workers left to right baby Daddy and mummy workers also called quarter full and half cone workers Right picture left to right cone ends for full half attached and quarter right

I mentioned a grease worker. So what is that? Well it looks like a glorified potato masher to me. It is used to work the hard grease by squeezing through the pores of the worker (masher) simulating how grease is extruded in surfaces in bearings.

We work grease by taking a sample of the grease and placing into a worker cup and screwing tight We then pull up and down called a stroke or stroke pair

Grease manually is typically worked 60 times, that’s 60 ups and 60 downs. This is called a P60 or worked penetration after 60 strokes up and down. Yes I get there is some double entendres in the terminology too for those of you laughing now.

You can extend this working though and do 10,000 or even 100,000 strokes. You will get big muscles from doing that. My arms were aching after doing a batch of 6 samples in triplicate 60 times on a full cone test.

Although you are welcome to try to do 10000 or 100000 manual grease strokes we have fortunately got an automatic grease worker too that works with a piston action to pump the grease Its a beast weighing in around 100kgs so much so we had to buy a reinforced table to hold it It also doesnt lose count if someone speaks to you whilst doing it Imagine having to start again 99384 strokes in

Now you have worked out what I mean by working a grease – notice the punn, we can talk about the cone sizes.

I did briefly mention full, half and quarter cone workers. In reality very few people do a full cone penetration as to be honest it uses so much grease. It’s basically a full tub to test and it is seldom requested at our lab. Unless you are producing vast quantities of grease it’s not viable and certainly for in use monitoring it’s rare to get anywhere near this amount of grease unless it’s something huge like a wind turbine. Hence today’s pictures are all from a half cone volume of sample we were asked to test.

Full Cone vs. Half and Quarter Cones

In addition to the standard full cone penetration test, there are also half-cone and quarter-cone tests that are sometimes used, especially when dealing with firmer greases or smaller sample sizes, the latter being most common.

  • Full Cone: The gold standard method, where a full-size cone is used to measure the penetration. This is suitable for most greases and provides a standard measurement for NLGI grading. If you want test a brand new product this is the way to do it, but apart from that scenario smaller cones are fine.
  • Half Cone: This uses a cone with half the surface area of the full cone. It’s typically used when the grease is too firm for the full cone to penetrate effectively or when only a small amount of grease is available.
  • Quarter Cone: This method uses an even smaller cone and is particularly useful for very firm greases (NLGI grades 4-6) or when the grease is in limited supply.

These variations in the cone penetration test offer flexibility in testing, ensuring accurate measurements across a wider range of grease consistencies as well as on smaller grease sample volumes which is typically the issue.

Correcting for a cone size.

The first thing I was surprised by when I started learning about cone sizes is they don’t all give the same answer. They are standard sizes but I assumed they would make smaller cones heavier to account for the smaller sizes to allow penetration. Hence for a NLGI 2 I may get an answer of around 70 for a ÂĽ cone, 140 for a ½ cone and 280 for a full cone. You need to know the size to accommodate. This seems too confusing as you only ever see full cone charts.

When we talk about grease methods there is an additional grease method for smaller cone sizes called ASTM D1403 which covers some nice back calculations to do it more precisely. On the whole it’s nominally x4 for a quarter cone and x2 for a half cone, but the exact formulae are a bit more complex. Because I found it a pain typing in the values to convert to full cone I made a little calculator for our lab which as loyal readers of learnoilanalysis you can use too.

NLGI grade calculator

Why It Matters

The NLGI grade, determined by the cone penetration test, is critical in selecting the right grease because it directly correlates to how the grease will behave in real-world conditions. A grease that is too soft might not stay in place, while one that is too firm might not provide adequate lubrication. The test ensures that the grease will perform as expected in the application for which it is intended.

Practical Considerations: Using NLGI Grades in Grease Selection

Now that we’ve covered the basics, how do you apply this knowledge when choosing a grease?

  1. Match the Grade to the Application: Consider the operating environment of your machinery. For example, in high-speed applications, a softer grease (e.g., NLGI 1 or 2) might be more suitable, while a firmer grease (e.g., NLGI 3 or 4) might be better for heavy-load, slow-moving equipment.
  2. Account for Environmental Factors: Temperature, humidity, and exposure to contaminants like water or dust can all influence the performance of a grease. For instance, in extremely cold environments, a softer grease (e.g., NLGI 0) might be necessary to ensure it remains pliable.
  3. Don’t Forget Compatibility: When switching greases, ensure compatibility with the materials of your machinery and the grease already in use. Incompatibility can lead to issues like corrosion or degradation.

Beyond NLGI Grades: Other Important Grease Properties

While the NLGI grade is a key indicator of consistency, it doesn’t tell the whole story. Other properties are also crucial for ensuring you select the best grease for your needs:

  • Dropping Point: The temperature at which the grease becomes fluid enough to drip. This helps you understand the upper temperature limit of the grease.
  • Oxidation Stability: How well the grease resists reacting with oxygen, which can cause it to harden and lose its lubricating properties.
  • Water Resistance: The ability of the grease to repel water, which is vital in wet or humid environments.
  • Mechanical Stability: How well the grease maintains its consistency under mechanical stress.

Conclusion: Empowering Your Grease Selection

By now, you should have a solid understanding of NLGI grades, the Cone Penetration Test (including full, half, and quarter cones), and how to use this knowledge to select the right grease for your equipment. Remember, the goal is to ensure that the grease you choose will perform reliably in your specific application, reducing wear, preventing failures, and ultimately saving you time and money.

Selecting the right grease is not just about picking something off the shelf—it’s about making an informed choice that will keep your machinery running smoothly and efficiently. So next time you’re faced with a grease selection decision, you’ll know exactly what to look for and why it matters.

Happy greasing!