Let's face it: grease is not a welcome consumable in modern shops. With its historical association with dirty shops run by uneducated "grease monkeys," plus its colour, odour and texture, grease just isn't sexy compared to say, engine...
Let’s face it: grease is not a welcome consumable in modern shops. With its historical association with dirty shops run by uneducated “grease monkeys,” plus its colour, odour and texture, grease just isn’t sexy compared to say, engine management hardware. Basic components like tie rod ends and wheel bearings still need lubrication however, and grease isn’t going away in the foreseeable future.
What is grease? Simply, it is lubricating oil mixed into an emulsion with a soap. The term “soap” doesn’t mean the stuff you use to wash the grease off your hands. It is a chemical term for a metallic salt of a fatty acid, typically a calcium, sodium or lithium compound. It’s the soap emulsifier “base” that gives lithium grease its name … the lithium isn’t an additive to the lubricant, although anti-wear additives are in the mix.
The emulsifier base matters, however, to the temperature and water resistance of the grease, as well as any reactivity to metals or chemicals in the bearings and seals. Tar, graphite and even clay have been used as thickeners, as have other non-soap compounds.
Non-synthetic greases are “thixotropic,” meaning they shear down under pressure to a thinner oil-like viscosity. This happens locally, at the contact point between ball or roller and race, which means that below the greases dropping or melting point, the bulk of the grease in the bearing isn’t lubricating but damming the thinned lube, providing extra lube as bearing temperatures climb and protecting against corrosion and moisture.
Synthetics don’t change state from solid to liquid but “drop” their lube out of the solid state, so you can’t compare regular greases to synthetics by melting or dropping point. Synthetic and conventional-base greases rarely break down in automotive use, but if the dropping point is exceeded, the grease will liquefy and run out of the bearing, with the expected failure following quickly. The heat can come from the bearing itself, high loading and high speed operation, inadequate lubrication, dirt and water in the bearing and a major undiagnosed cause, disk brakes.
Why disk brakes in particular? Compared to drum systems, they’re much more fade resistant, which allows drivers to use them harder, longer and with greater loads. They also have their moving parts exposed to the elements, unlike drum systems, and dirt, salt, rust etc, can seize calipers and drag a brake, greatly increasing heat build-up. Owners will often drive a car with a noticeable pull caused by a dragging brake and may not seek service until the pads go to the backing plates. The brakes get a total replacement but have the wheel bearings been affected? If there are any signs of grease seepage around the hub, a bearing failure may be around the corner.
For light trucks and the few rear-wheel drive cars left in the fleet, you’re likely repacking and inspecting anyway, but for the majority of hub bearings, greasing is not an option. Set up your dial indicator if necessary and vertically rock the disk to check for bearing wear. Don’t be fooled by tie rod or steering system slack and do test using the disk on the hub, not by handling the hub directly … the wheel bearings see the leverage of the entire wheel assembly and will deflect a lot more than you hand pressure on the hub flange.
Can you sell a hub bearing replacement with a brake job? Ideally, yes, but in any case, tell the driver that delaying the brake service has consequences for the wheel bearings … and don’t forget to tell them that front/all-wheel drive hubs are not greaseable like conventional spindles.
Extreme Pressure and NLGI numbers
Extreme pressure greases are used where metal-to-metal contact would occur with conventional lubes in applications like gear drives or heavily loaded bearings. Formulations use phosphorus, sulphur and chlorine and well known additives like graphite and molybdenum disulfide to react with the surface of the metal for extra lubricity. In automotive applications, hypoid gear drives (ring and pinions) running in oil are most common.
Be careful selecting grease from off-brand or unknown suppliers as descriptors like “Meets NLGI #2” simply means that its thickness is equivalent to the National Lubricating Grease Institute’s rating for the thickness of the grease. Ratings vary from 000 (flows like oil) to 6 which is a stiff semi-solid. #2 is the common rating for automotive use because it stays in the bearings in normal use and will flow through pumps, guns and grease nipples with reasonable pressure.
The number ratings are not an indication of quality or suitability for wheel bearing or chassis use. Common lithium-based “white” greases for general open lubrication are often NLGI #2, but won’t adequately lube chassis parts or wheel bearings. Lithium is, however, a good high-temperature base for approved automotive bearing and chassis greases, so don’t go by the grease’s base. The grade to look for is NLGI GC-LB which has good resistance to wear, rust and oxidation, and is compatible with the materials used in automotive seals. Naturally if the vehicle uses a special foreign rating spec, it’s important to use the grease certified to that spec, even if it means ordering in a tube. Wheel bearings are safety items and liability can stick if you use a non-specified lube regardless of its quality.
What happens if you mix greases? Nothing if they’re compatible, but if they’re not, thinning is the usual result. If you change grease composition you should thoroughly clean the old bearing. If you’re replacing brake disks with integral hubs and bearings, don’t assume that they’re pre-lubed correctly … check the spindle for signs of blueing, distortion or cracks. If it fails or vibrates because of the spindle, the bearing will fail too and so will the replacement. If the vehicle has been in an accident, it’s possible that the bump-and-paint guys didn’t check beyond a four-wheel alignment. Fresh bodywork is a sign that the spindles may have been involved, especially the weak rears on smaller front-drive cars. And remember – don’t spin a wheel bearing with compressed air.
There are libraries of technical information about greases and bearing lubrication, but in the bays, grease rarely comes to mind. Can you just grab a gun and go when it comes to grease? Yes, if it’s a quality, certified product from a reliable vendor. But read the data sheet or labels. If the only certification is NLGI #2. it just might be “#2.”