Truck Checks: Grease: A brief introduction
By Don Henry
Grease: A brief introduction
By Don Henry
Grease is usually understood as a shot of black something, sometimes and somewhere. Let's spend some time and look at the many different types of grease, when they are used and where. Their thickeners, their additives and their grades classify greases. Why use grease instead of oil lubricant? There are many reasons. The first is mainly to minimize the leakage of lubricant – grease is easier to seal in. Grease is also used when oil circulation is impractical such as with open gears on an aerial truck's turntable. Grease can perform when there are extreme high loads, where oil lubricants would fail and in sealed-for-life applications.
Types of thickness
Normal grease starts out with base oil, then has thickeners added to it to give it certain characteristics. The thickeners can be metallic soaps, bentonite clays, polyurea or inorganic thickeners. The most common of the metallic soaps are calcium, aluminum, sodium, barium, and lithium. These greases can withstand far higher temperatures than grease made with fats from animals or vegetables.
The greases made from base oils and metallic soaps are called complex greases. These complex greases can withstand temperatures as high as 260 C (500 F). At temperatures higher than that the lubricant will separate from the thickener. Both aluminum and lithium are very good at high temperatures, with the lithium 12-hydroxystearate grease being the most popular. With very good base grease such as above, grease manufacturers will add additives to impart characteristics to the grease. A typical grease formula is 75 to 96 per cent fluid (also called the base oil), four to 20 per cent thickener and zero to eight per cent additive. The base fluid may be a synthetic or conventional mineral oil. As the oil is the lubricant, the properties of the oil are very important when considering the grease selection.
What makes good grease? Extreme pressure additives (EP) and anti-wear additives are the most common additives. Sulphur, zinc, molybdenum disulphide (often called moly), graphite and fluorocarbon powders are some of the additives of choice. The applications of these various greases seem complex, but there are some general guidelines to follow if the information is not available from the manufacturer. In areas of dirt and water such as pins for stabilizing arms, gears from turntables and sliding contact such as steering components, a grease often specified is a molybdenum additive with a thickener of lithium or aluminum, the latter of which is very common. The moly works as an extreme pressure additive, keeping these slow moving but extreme-high-pressure parts from contacting each other.
Not all greases are compatible with other greases. Some greases when mixed react with the other grease thickeners and can lead to bearing failure. Avoid mixing aluminum complex, clay or silica thickeners greases with other thickeners. While these are very good greases, they do not mix well with other thickeners.
It is important that you do not buy your grease from the lowest bidder unless you can ensure that the new grease is compatible with the old grease and that the quality is the same. Viscosity is very important when selecting a grease. Too low a viscosity (too thin) will flow very easily, but not keep the metal parts from rubbing each other. Too thick, and the grease will not flow in between the bearing parts and will form a channel. A grease's measure of this is called penetration or hardness by some manufacturers.
A test called the ASTM D217 is used to measure this hardness. A cone of a certain weight and design is dropped into the grease; how far it penetrates will reflect the grease's hardness.
A grade 000 is very thin grease; a 6 is very thick grease.
This grade number is called an NLGI grade. Grades of 0, 1 or 2 are very common and very often specified. Grades of 0 and 1 are used a lot in outdoor applications and in colder areas. As fire equipment is stored in areas above freezing 32 F (0 C) then an NLGI grade of 2 is very common. Many automatic centralized grease lubrication systems use a grade 1 grease. Naturally, follow the manufacturer's recommendations, but if in doubt as to what grade of grease to use or if you have repeated bearing failures, here is a handy method to calculate the type of grease to use. You first have to calculate the DN factor – this is also called the surface speed of the bearing. DN is calculated by the shaft diameter in millimetres times the revolutions per minute of the bearing:
At speed less than 200,000 use an NLGI grade of 1 or 2;
At speed at 200,000 use an NLGI grade of 3.
At speeds above 200,000, convert the grease system over to an oil bath system, as no grade will work well. At speeds above 200,000 the NLGI grease number 1 or 2 will cause the grease to leave the bearing because of centrifugal force. Here is a sample calculation: a bearing in a fire pump is ball-bearing grease-able, it has a shaft diameter in millimetres of 26 and it turns at 4,200 rpm. Then the DN speed would be 109,200 and an NLGI grease of 1 or 2 would work well.
Now, suppose that the manufacturer has had many bearings failures with a certain bearing, so the solution may seem to be putting a bigger bearing in, let's say a 50mm one at the same speed of 4,200 rpm. The new DN number is now 210,000. If the same grade of grease were used you would have far more bearings fail not fewer.
Common lubrication problems
Let's look at a very common lubrication problem, the bearings on a front wheel on a heavy truck. On most modern trucks these bearings are usually lubricated with oil. By comparison, the front wheel bearings on a rear-wheel-drive car are lubricated by grease. Now at first glance that may seem to be a contradiction. As the load increases shouldn't grease be used and as the speed increases shouldn't oil be used? A heavy truck must have a bigger load on its bearings than a car – look at those heavy loads it carries and would not a larger wheel rotate more slowly and therefore need grease, than a small car wheel to go the same distance?
But you will find the bearing on a truck axle is bigger than a car bearing, a lot bigger, and this has two effects. The first is that there is more bearing to spread this larger load over (in fact there is less of a load on each ball bearing than with the car bearing). This means an oil lubricant can be used. Secondly, this truck bearing is not only larger in size across but also in diameter than the car's bearing. This larger size means its circumference is also larger, much larger, and therefore its speed is faster than the car bearing even though the large truck wheel turns more slowly. This faster speed dictates that an oil would be superior and should be used, as a grease would not be able to lubricate this fast-moving truck bearing.
I know at first this sounds silly, but you do need to grease your brakes. For the brake slack adjusters and brake cam bushings, an NLGI grade 1 is recommended. Do not over-grease the brake cam bushings because grease could get into the brake drums. Be careful and do not over-lubricate a self-adjusting slack adjuster as you could rip the rubber boot and allow water, grit and other contaminants into the slack adjuster.
For U-joint grease lubrication a grade 2 is recommended. For reference, you cannot really over-grease a U-joint. In fact, to complete a proper job, grease must come out of (purge) the seals of all four of the trunnions. If it does not, find out why that trunnion is not getting proper lubrication. It may need to replaced.
The front axle above will also need to be greased. On most models the front axle wheel will need to be lifted slightly off the ground before it can be properly lubricated. Be careful and use a jack rated for the load and a set of axle stands. Ensure the rear brakes are set and chocked.
Tie rods need to be greased until old grease purges from the tie rod boot.
Most high-speed bearings fail because of over-lubrication, not lack of lubrication. A bearing housing is normally only 1/3 full of grease. When a bearing is over-greased it will overheat, the lubricant and the thickener will separate and the lubricant will leak out. This will leave only the thickener and it is not a lubricant in itself. The bearing will fail. The solution may at first seem to grease the bearing more often to keep it from failing. This will just make the problem worse and the bearings will fail more again.
The formula for bearing re-greasing is a bit long to print here but at the end of this article I will give you the address of a great lubrication handbook you should have. For example, a 25mm bearing (that's one inch for the rest of us) turning at 2,500 rpm would need to be re-greased every two months — that's 24 hours per day times 30 days per month times two months or 1,440 hours of continuous operation. How many years would it take to get that on your truck? A maximum of two shots of grease from a standard hand-operated grease gun is more than enough. Even a cheap grease gun can produce pressures as high as 10,000 psi and this high pressure can blow out the seals and allow water and dirt back into the bearing.
Many bearings now come sealed from the factory so that grease cannot be added. The reasons for this are that most people will use the wrong grease, too much grease, mix incompatible greases, or not remove all the dirt from around the grease zerk and will contaminate the new grease. It is very important that you wipe off the grease zerk to ensure it is very clean before applying the new grease. Failure to do so will inject the new grease with the grit that was on top of the zerk. This contaminated grease will work like a very efficient grinding compound and the bearing or component life will be severely shortened.
Reference: For a copy of the STLE Alberta Section Handbook of Lubrication, 2nd Ed, contact Cameron Cox, STLE Alberta secretary, 403-210-4236 or firstname.lastname@example.org .