Best Practices for Wheel-end Lubricants

While much thought is given to major automotive lubricants like engine oil and transmission fluid, there are many other components in a vehicle that have specific lubrication requirements. One such component is the hub assembly, where what are known as wheel-end lubricants are commonly used.

Hub assemblies can be oiled or greased, and just like motors and transmissions, the lubricant and lubrication practices will have a considerable impact on their life expectancy and operation. The better you take care of these components, the longer they will last.

How Hub Assemblies Work

The majority of hub assemblies utilize a rolling-element bearing on the spindle to allow the wheel to rotate freely. These typically are tapered element bearings to help support loads in multiple directions. They are often installed in pairs on steering wheels. For wheels that don’t steer, known as drive or idle wheels, you may find traditional ball bearings.

Although these bearings can reduce the amount of friction between the moving parts, the bearings must be lubricated periodically to enable maximum life of the assembly. This raises the question as to which lubricant should be used to lubricate, protect and ensure the bearing will provide trouble-free service for an extended period of time.

Lubricant Selection

Oils have been employed in many hub assemblies with good results. When using an oil bath to lubricate the wheel ends, there is generally less drag on the bearings when compared to grease lubrication. The oil is applied either directly to the hub via a fill port or supplied internally from the differential utilizing a piping mechanism. In both cases, maintaining the correct oil level and selecting the appropriate oil are key to preserving the bearing and ensuring that the hub functions properly.

Bearings that share oil from the differential will commonly be lubricated with gear oil. Even hubs that are splash lubricated in their own individual bath may be lubricated with an approved gear oil. While most people may be familiar with gear oils, the ones used in automotive applications are slightly different than those for industrial applications, which have their own designations and viscosity grade systems.

Gear oils for these applications should meet the specifications of the American Petroleum Institute (API). For drive wheel ends, where the oil is shared between the hub and the axle, this becomes very important, as the oil will be lubricating a gear set as well as a bearing set.

Be sure to consult the original equipment manufacturer’s recommendations to see which gear oil designation is preferred. The most current and widely used is GL-5. This designation is employed with most hypoid gear sets and has standard performance levels listed in the ASTM D7450 specification.

Much of this specification is dedicated to testing the lubricant’s ability to reduce wear on gear components. Other tested parameters include foam stability, rust protection, and thermal and oxidative stability.

The full range of gear oil designations span from GL-1 through GL-6. The only active designations are GL-4 and GL-5. GL-4 is utilized in many manual transmissions and transaxles. These tend to be used more at lower speeds and loads when compared to GL-5. The other designations are considered inactive and usually are not recommended in these applications.

Breaking the mold is the MT-1 designation, which is frequently employed in non-synchronized manual transmissions. These oils are more specialized, and care should be taken to prevent accidentally mixing these with other fluids.

In order for a lubricant to meet these specifications, there must be a healthy dose of additives to fortify the base oil. The most common additives in these types of fluids are antioxidants, viscosity index improvers and extreme-pressure (EP) additives.

Some EP additives contain active sulfur phosphorus, which can be corrosive to softer metals, so always check your equipment manual before selecting and using any oil for your particular application.

In addition to the oil designation, you must also choose the proper oil viscosity to replace or refill the system. The viscosity will be denoted by the SAE viscosity grade and typically will be listed in the same place as the oil designation.

Many automotive applications utilize a 75W-90 viscosity oil in these applications, but it is not uncommon to find some viscosities higher than SAE 140. The higher viscosities are often used for higher loads and higher torque applications or for those experiencing higher temperatures.

Oil Levels

When using oil to lubricate a wheel end, the oil level will be critical to the system’s health and overall performance. If the wheel end has its own individual reservoir, there likely will be a sight glass which can be used to monitor the oil level. Depending on the type of wheel end, the oil level can vary, so be sure to check the sight glass for the “full” mark and not simply look for signs of oil in the glass.

For wheel ends that share a sump with the rest of the axle assembly, you may need to pull the level plug from the differential or gear assembly to verify that the oil is at the appropriate operating level.

Ideally, you should check these levels when the vehicle is on level ground, as the slope of the surface could impact the oil level and give you a false reading. Remember, having too high of an oil level can be just as dangerous and destructive as having an oil level that is too low.

Grease Lubrication

The vast majority of wheel ends are grease lubricated. Unlike oil, grease does a better job of staying in place within the bearing and housing. It also can resist breaking down while in service better than a traditional oil. The downside is that grease can be difficult to replace, and overgreasing is common in many applications.

Passenger-car wheel bearings are typically grease lubricated and sealed for life, which means the lubricant can’t be replaced. Not only must the in-service grease provide the same lubricating properties as oil, but it also must be able to resist the temperatures present in the wheel end to avoid excessive bleeding of oil from the thickener. This is very important in applications that require frequent braking, as the friction in the brakes is often transferred into the wheel bearings.

When selecting a grease for a wheel bearing, start by considering the recommended lubricant or the grease that is already in use within the bearing. Greases are known to have compatibility issues when different types are mixed. This risk is greatly minimized if you can fully replace the grease charge by cleaning out all remnants of the old lubricant.

Some research into the grease’s thickener and base oil can shed light on whether the new grease will be able to mix with the in-service grease. When in doubt or if there are compatibility concerns, try to perform as thorough of a grease changeout as possible.

Automotive greases have performance specifications that are similar to the gear designations of oils. These specifications, which were originally classified by NLGI, follow a testing protocol outlined by ASTM International. The classifications for automotive greases fall into two categories: chassis and wheel bearings.

Some greases that pass both testing protocols may bear the dual-service mark. Chassis greases are classified as “LA” for light duty and “LB” for severe duty. Wheel bearing greases have three classifications: GA, GB and GC. Like the chassis classifications, the main difference is how robust the grease is and how severe the driving conditions are.

They range from mild duty (GA), to moderate duty (GB) and severe duty (GC). The regreasing frequencies follow the same pattern, with more frequent relubrication for GA to less frequent with GC. Most greases employed in automotive applications tend to hold the dual-service mark, which includes LB and GC.

The GA-GC wheel bearing classifications have similar testing performed per the specification, but the protocol varies based on the classification. All wheel bearing classifications are subjected to consistency, dropping point and low-temperature performance tests.

They also all have the same criteria for consistency, which puts them in the NLGI consistency range of 1-3. The most common grease consistency is NLGI 2. For the other two tests, the acceptance limits change according to the classification, with GC requiring higher values in most of the other tests.

Additional testing for corrosion protection, water resistance, leakage, high-temperature life, wear protection and elastomer compatibility is performed for GB and GC greases. The GC greases are tested for extreme-pressure performance as well.

During installation, grease is often packed into these bearings by hand. Not only should grease be packed into the bearing, but some grease should be applied to the housing and on the wheel end’s inside surfaces where the bearing sits. This will help prevent corrosion.

The housing should also have some grease in the cavity surrounding the bearing, but it should not be filled. This space in the cavity will permit grease to be spun out of the bearing as well as provide room for thermal expansion of the lubricant. If the cavity is completely filled, grease churning and excessive heat will result.

Certain wheel ends have grease fittings that enable periodic regreasing of the bearing. If regreasing these with a grease gun, be sure to apply grease slowly and stop if abnormal back-pressure is experienced. Keep in mind that a grease gun can generate enough pressure to blow out seals and create an opening that will allow contaminants to enter and oil to bleed out.

Key Considerations

When lubricated correctly, wheel ends should meet or exceed their estimated design life. The key items to consider include maintaining the oil level, regreasing at the proper frequency, using the right specification for oils and greases, and routinely monitoring the wheel assembly for any signs of leakage.

If the seals are leaking and the lubricant is allowed to be flung out of the housing, it may reach the brakes, which could lead to a malfunction. Also, if lubricant is no longer present, the bearing may heat up and cause a fire. The wheel could even become unattached and result in damage or harm to other motorists.

Therefore, wheel ends should be properly maintained for the performance of the vehicle as well as the safety of the public. Their lubrication doesn’t need to be mysterious and can often be performed during normal maintenance of your vehicle. If you take your vehicle to a repair shop, be sure the principles outlined here are followed so you will be set for your next road trip.

References

ASTM D7450-13 Standard Specification for Performance of Rear Axle Gear Lubricants Intended for API Category GL-5 Service, ASTM International 2018 Annual Book of ASTM Standards

ASTM D4950-14 Standard Classification and Specification for Automotive Service Greases, ASTM International 2018 Annual Book of ASTM Standards

Fitch, J.C., Scott, R., & Leugner, L. (2012). “The Practical Handbook of Machinery Lubrication - Fourth Edition.”

Pirro, D.M. & Wessol, A.A. (2001). “Lubrication Fundamentals - Second Edition, Revised and Expanded.”