Tech Tip: Bearings

Enhance your employees’ product and industry knowledge with PTDA’s Tech Tips. This library of online tips, hints and techniques may be used to educate new and current employees on power transmission/motion control (PT/MC) products, technologies and concepts and serve as reference and reminder for more experienced staff. Tech Tips are based on the expertise of PTDA member companies and content from PTDA’s Power Transmission Handbook^®, the definitive resource and training tool on PT/MC products.

Visit the full library of Tech Tips for additional products.

What is fretting corrosion?

Fretting corrosion is the result of the microscopic movement or oscillation between two surfaces which then oxidates. This can take on the appearance of rust and is commonly found between the mating surfaces of the shaft and the bearings inner ring or housing ID and bearings outer ring. 

What are the four main types of insert ball bearing shaft locking styles?

1. Eccentric Locking Collar

2. Dual Set Screw

3. Adapter Lock

4. Concentric Lock

  

What causes electric arcing and how can it be prevented?

Electric currents can damage a bearing by burning a pit into the race at the point of contact and can cause grooves along the affected surface. As the current continues to pass through the bearing, the contact points change as the bearing turns. A simple prevention method is to ensure machines are properly ground so that it will route electrical current around the bearings. 

Generally speaking, what are the four major reasons to lubricate bearings? 

1. Reduces friction and wear
2. Dissipates heat
3. Protects surfaces from dust and corrosion
4. Helps seals protect bearings

What are two types of needle rolling bearings? 

There are generally two types of needle bearings. The first, cage separable types, have a bearing retainer or element separator as part of the assembly. This type of bearing is designed to accomodate higher speeds and more precision rolling. The second type of needle bearing assembly is a full complement style. The needle roller elements are positioned in the outer ring so that each needle is up against the other. They are not separated like the cage style element. This style of needle bearing is also referred to as a drawn cup needle bearing. 

 
Above left: caged machined needle bearing. Right: drawn cup needle bearing.

How are static and dynamic load ratings for rolling element bearings measured?  

Static load is a measure of the load a bearing can withstand before a deformation of the rolling element or raceway occurs which will be detrimental to the life of the bearing. Dynamic load is a measure of the load a bearing can withstand for one million revolutions without a decrease in performance. Dynamic load is also sometimes referred to as Basic load.

What are the two main variables you need to identify when selecting a clearance for a rolling element bearing?  

When selecting clearance for a rolling element bearing, it is important to identify two main variables: fit tolerance of the bearing and housing/shaft interface and ambient temperature. All rolling element bearings are designed with some gap (clearance) between the rolling elements and their races. This gap is to allow the rolling elements to perform their function, unimpeded; after the bearing has expanded or contracted when press fit onto a shaft, into a housing, or has changed size due to temperature variations. It is important to always consult the manufacturer.

What do you need to know when deciding between a single row and double row deep groove ball bearing?  

When deciding between a single row and double row, deep groove ball bearing, the only limiters that need to be considered are radial load and physical space requirements.  Double row, deep groove ball bearings carry the same or very close speed and thrust load ratings to single row, deep groove ball bearings.  Double row, deep groove ball bearings do carry significantly increased radial load capacities over the single row versions.

Replacing Bearings in Multiple Bearing Sets

For machine tool spindles, pumps and other applications using multiple bearing sets (pairs, triplex, quads), you will occasionally see only one bearing fail and the other bearings of the set appear to be in good condition.  The question arises if you can re-use those bearings and just replace the one failed bearing.  The answer is always "no" for two reasons.  First, multiple bearing sets are often matched together to provide equal fits and equal load sharing in the application.  Second, and more importantly, the bearings that appear to be good on the outside may indeed have internal damage that is not evident.  These bearings also have used up a certain portion of their life already, so putting them back into service will increase the risk of unplanned downtime.  This always costs more than a new bearing set.

How Can I Correct an Electrical Arcing Problem?

Arcing through bearings causes premature failure by “welding" the rolling elements to the rings. There are several options to correct an electrical arcing problem. The best solution is to insulate the bearing housing or shaft to prevent current flow. If this can not be done, then:

Switch to an insulated bearing. There are two types available: (1) ceramic hybrid bearings where ceramic rolling elements are used along with rings of standard bearing steel, and (2) insulation coated bearings where ceramic material is coated on the bearing's outer ring circumference and faces. Both prevent the flow of current through the bearing and both are much more expensive than standard bearings.

Use conductive grease in place of standard lubricants in the bearings. This solution can make the bearings noisier and the effect will wear off over time.

Replacing a Pillow Block Bearing

When replacing a pillow block bearing, always verify that following dimensions and components are compatible:  1) base to center height, 2) overall width, 3) weight, 4) shaft size, 5) shaft locking device, 6) seal type, 7) lubricated or non-lubricated, 8) duty class of bearing, 9) bearing type, 10) overall height, 11) bolt pattern.

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.