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Needle In The Haystack

Thu, 03/17/2011 - 12:05pm
David Mantey, Editor, PD&D

While it’s difficult to specify the best bearing — each application calls for different performance and cost characteristics — the needle bearing can take many forms.

Needle bearings can often offer major advantages over conventional bushings because they operate with a rolling motion rather than a sliding motion. As a result, friction is reduced, particularly in industrial applications, and the torque required to rotate the shaft is lowered. The design allows for reduced heat buildup and wear and simplified lubrication needs. 

According to Luis Quinteros, product engineer at Designatronics, needle roller bearings should be used in applications that require a high radial load and higher speed than conventional bushings allow. “Needle rollers offer decreased friction when compared to conventional bushings because of the rolling elements; however, they have more friction than regular ball bearings,” he says.

In most needle bearing applications, the bearing doesn’t have an inner race. The shaft that the bearing is mounted to acts as the inner race, so needle bearings provide high-load capacity in a small volume.

“Needle bearings can withstand high radial loading because each needle theoretically offers a line-of-contact versus a ball bearing’s point-of-contact for each ball,” says Doug Ericson, project engineer for Hartford Technologies. “Many needle bearings also represent a very economical bearing solution. The needles have a high degree of precision associated with their manufacturing, but there are well-established, high-speed manufacturing methods, such as centerless grinding, for producing these components at low costs — albeit in large batches.”

Many needle bearing cages/retainers are stamped or molded and many of the outer shells are drawn. When compared to their ball counterparts, needle bearings are produced using relatively less expensive manufacturing processes. Ball bearing manufactures, especially for the inner and outer races, typically have more expensive forging, machining, and grinding associated operations.

According to Quinteros, overall design cost is lowered because less material is needed to accommodate a needle roller bearing of a certain shaft size.

“The weight of the design is decreased, and the number of bearings needed is decreased because of the needle’s load-carrying capabilities,” adds Quinteros. “For example, a piece of equipment might use 10 ball bearings to bear a load that could be born by four needle bearings.”

Take Many Forms

When design engineers specify a needle bearing, the major factors to consider are the physical envelope (outer and inner diameters and length), load requirements, life expectancy, speed, temperature, and operating environment.

While it’s difficult to specify the markets best bearing — because each application calls for different performance and cost characteristics — the needle bearing can take many forms.

  • Retainer plus needle rollers: The retainer is metal or plastic. The shaft acts as the inner race and the housing acts as the outer race.
  • Full complement needle roller bearings: Featuring machined or stamped outer housing and needle rollers, the rollers are retained by grease until they are assembled onto a shaft.
  • Drawn cup needle roller bearings: With a stamped outer shell, cage, and needle rollers, drawn cup bearings typically have good performance in an economical form. The many options include two open ends, one closed end, sealed, and unsealed.

“We had a customer who reduced overall manufacturing cost when they switched from a bushing to a needle bearing, because they were able to use a smaller motor to rotate the shaft,” Ericson recalls.

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