Maintenance managers who want to extend the reliability of their vehicles should get to know greases. That’s because, too often, fleets overlook proper wheel bearing lubrication, leading to unnecessary repairs and aggravation.
In fact, inadequate lubrication is a leading cause of wheel bearing failures in trucks, vans and vehicles of all types. The useful life of a roller or ball bearing depends greatly on proper lubrication, which creates a film that:
- Separates the bearing's moving parts.
- Carries away heat.
- Protects surfaces from wear and corrosion.
This is especially true in wheel bearings where dirt, moisture and high and low temperatures are constant challenges to optimal performance.
How Damage Occurs
The damage caused to bearings by inadequate lubrication can be classified into seven categories:
- Underfilling – Grease starvation can cause excessive metal wear. Noise is often an early indicator of a problem.
- Overfilling – Packing too much grease into bearings can cause problems as well. Excess grease tends to churn inside the bearing, generating more heat.
- Wrong specification – If the base oil viscosity is too heavy, the bearing’s rolling elements may have difficulty pushing through the grease and begin to skid. If the viscosity is too light, the thin lubricant film may cause premature wear.
- Incompatibility – If you use an incompatible grease or substitute, the new mixture may soften and leak out of the bearing or become lumpy, discolored and hard.
- Incorrect intervals – Failure to follow maintenance schedules can lead to excessive oxidation in the bearing, causing the lubricant to deteriorate.
- Deterioration – Grease is a precise combination of additives, oil and thickener. Time and temperature can deplete these properties to the point where the grease is not suitable for use.
- Contamination – Even small amounts of dirt and debris can disrupt the lubricant film and cause bearing damage. Water is particularly harmful. As little as one percent water in grease can have a significant impact on bearing life.
Problem Prevention
The good news is that it’s easy to avoid most bearing failures. The key is for technicians to pay closer attention to lubrication practices. Be sure to share this information with them.
To start, if you are unsure about the lubricants you are currently using, work with your bearing supplier to determine the right grease type, grade, amount, additives, supply system and replenishment cycle for the vehicles in your fleet.
Lubricants should be properly engineered for each disc and drum bearing application, based on a combination of factors, including loading, speeds, sealing systems, service conditions and expected life. Particularly where vehicles operate in extreme hot or cold weather, it is prudent to make sure your lubricant is up to the task.
Your supplier can also provide recommendations for properly storing grease and maximizing its shelf life.
Preparation
When servicing bearings, preparation is the key to success. Have your technicians follow these steps to properly pack bearings with grease and avoid problems down the road:
By hand:
- Thoroughly clean hands, or put on new latex gloves.
- Place grease (golf ball sized) into the palm of one hand.
- Using your other hand, push the large end of the bearing cone assembly into the grease.
- Continue pushing grease into the large end, rotating the assembly until grease is forced out evenly around the entire small end.
- Smear excess grease on the outside of the assembly.
Using a mechanical grease packer:
- Thoroughly clean hands, or put on new latex gloves.
- Place the bearing cone assembly, small end down, into the grease packer funnel.
- Plug the bore of the large end of the assembly with the conical retainer.
- Firmly press down on the conical retainer.
- Smear excess grease on the outside of the assembly.
For best results, a best practice is to allow ample space in the housing. This allows excess grease to be thrown from the bearing. Normally, the housing should be one-third to no more than half full of grease during bearing assembly.
Once the lubricant has been applied, it is important to monitor it and the bearing for signs of trouble at regular maintenance intervals.
Diagnose Damage
Bearing damage can vary greatly in appearance, depending on how far the problem has progressed. Always be on the lookout for these warning signs:
- Darkening of the metal caused by excessive heat – Check for discoloration of the bearing’s races and rollers. In mild cases, this is the result of the lubricant staining the bearing surfaces. In severe cases, the metal itself is discolored.
- Scoring or peeling – Look for marks cut into the metal or the metal peeling away. This can be an indication of a serious lubrication-related problem requiring immediate attention.
If a problem is observed, consult a bearing service engineer. In most cases, the bearing will need to be replaced before it can be placed in operation again.
With so much to do, busy fleets can lose track of bearing maintenance. However, a bit more time and attention up front can mean big savings and thousands more service-free miles over the long haul.
