Bearings in motion systems

Bearings are simple machine elements crucial for motion applications. They usually consist of smooth rollers or metal balls and smooth inner and outer surfaces known as races against which rollers or balls roll. These rollers or balls act as the load carrier for the device, allowing it to spin freely.

The primary function of a bearing is to reduce frictional forces between moving parts by giving a surface something to roll on, rather than slide over. The secondary function of a bearing is to transmit loads.

Bearings carry two kinds of load: radial and axial. Radial loads are perpendicular to the shaft while axial loads are parallel to the shaft. Depending on the application, some bearings experience both loads simultaneously. Bearing types abound to satisfy various applications.

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Ball bearings
One of the most common forms of bearings is the ball bearing. As the name implies, ball bearings use balls to provide a low-friction means of motion between two bearing races. Ball bearings are usually inexpensive and when selected properly require little maintenance. Because of these characteristics, ball bearings are some of the most popular of all bearings.
Because the contact area between the balls and races is so small, ball bearings can’t supoert as much load as other bearing types, so are best suited for light to moderate loads. However, their small surface contact also limits the heat generated by friction, which makes ball bearings useful in high-speed applications.

The contact angle between the balls and the raceway also affects the characteristics of a ball bearing. Single-row deep-groove ball bearings primarily handle radial loads with the ball at 0° in relation to the raceway. Designers can increase this angle in certain types of bearings to accommodate axial loads in one direction. Called angular-contact bearings, designers often install these in pairs, because a second bearing adjusted the first counteracts force in the axial direction from radial loads.

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Roller bearings
Possibly the oldest form of bearing, the rollers in roller bearings can be spherical or cylindrical. Common in applications such as conveyor belt rollers. Because of their shape, roller bearings have greater surface contact than ball bearings, so can handle larger loads without deforming. Their shape also allows for a moderate amount of thrust load because the weight is distributed across cylinder line contacts instead of sphere point contacts.

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Needle-roller bearings
When you need to reduce friction between two moving parts but have very limited space to do so, a needle roller bearing may just be what you’re looking for. A needle roller bearing is a roller bearing with rollers whose length is at least four times their diameter. Despite their low cross section, the large surface area of the needle roller bearing allows them to support extremely high radial loads.

They usually consist of a cage, which orients and contains the needle rollers and an outer race, which is sometimes the housing itself. The bearings can often be found in two different arrangements. The first is a radial arrangement, in which the rollers run parallel to the shaft. The second is a thrust arrangement, in which the rollers are placed flat in a radial pattern and run perpendicular to the shaft.

These bearings are often used in automotive applications, such as rocker arm pivots, pumps, compressors and transmissions. The drive shaft of a rear-wheel drive vehicle typically has at least eight needle bearings (four in each U joint) and often more if it is particularly long, or operates on steep slopes.

Thrust ball bearings
Thrust ball bearings are designed for use in applications with primarily axial loads and are capable of handling shaft misalignment. These bearings are also useful in high-speed applications, such as in the aerospace and automotive industries.

Thrust roller bearings
Thrust roller bearings are designed so that the load is transmitted from one raceway to the other, meaning that these bearings can accommodate radial loads. Bearings like these also have a self-aligning capability that makes them immune to shaft deflection and alignment errors.

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Tapered roller bearings
Tapered roller bearings feature tapered inner and outer ring raceways with tapered rollers arranged between them, angled so the surface of the rollers converge at the axis of the bearing. These bearings are unique in that, unlike most bearings that can handle either axial or radial loads, they can handle large amounts of load in both directions.

A single row taper bearing is limited in that it can only take high axial loads from one direction, but if adjusted against a second tapered roller bearing, that axial load is counteracted. This allows the bearings to accept high radial and axial loads from multiple directions.

Tapered roller bearings can accommodate angular misalignment of the inner ring in relation to the outer ring only to a few minutes of arc. As with other roller bearings, tapered roller bearings must be given a minimum load, especially in high-speed applications where the inertial forces and friction can damage effect the rollers and raceway.

One variation: Linear bearings
Linear bearings are a variation on radial bearings. They include a straight array of rollers in a carriage to carry loads on a straight or curved slide or rail. Engineers usually pair linear bearings with a manual crank or motor. Note that linear bearings experience overturning moments of force instead of radial and axial loads.

Another variation: lain Bearings
Plain bearings are the simplest form of bearing available, as they have no moving parts. They are often cylindrical, though the design of the bearing differs depending on the intended motion. Plain bearings are available in three designs: journal, linear and thrust.

Journal bearings support radial motion where a shaft rotates within the bearing. Linear bearings go in applications requiring slide plates, as these bearings permit motion in a linear motion. Plain thrust bearings do the same job as roller bearings, but instead of using cone-shaped rolling elements, use pads in a circle around the cylinder. These pads create wedge-shaped regions of oil inside the bearing between the pads and a rotating disk, which supports applied thrust and eliminates metal-on-metal contact.

Out of all the bearing types available, plain bearings tend to be the least expensive. They can be made from a variety of materials including bronze, graphite and plastics, such as Nylon, PTFE and polyacetal. Improvements in material characteristics have made plastic plain bearings increasingly popular in recent years. Plain bearings of all types, however, are lightweight, compact and can carry a substantial load.

As far as lubrication is concerned, some plain bearings require outside lubrication while others are self-lubricating. Plain bearings made of bronze or polyacetal, for example, contain lubricant within the walls of the bearing, but require outside lubrication to maximize performance. For other plain bearings, the material itself acts as the lubricant. Such is the case with bearings made from PTFE or metalized graphite.

The growing popularity of plain plastic bearings and increasingly stringent industry standards has resulted in more consumers requiring the bearings to meet FDA and RoHS standards. There has even been a call for the bearings to meet the standards of EU directive 10/2011/EC, which also takes the material manufacturing process into account.

Lubrication for rolling-element bearings
Some engineers see lubricants as a straightforward, messy ancillary of the industrial age. However, much like rolling-element bearings themselves, lubrication is an ancient technology that’s highly engineered in modern forms. In fact, engineers have used fluids to reduce friction thousands of years, but the advent of the oil industry in the late 19th century spurred modern bearing lubrication. Today, bearing lubricants serve several functions:

  • Creating a barrier between rolling contact surfaces
  • Creating a barrier between sliding contact surfaces
  • Protecting surfaces from corrosion
  • Sealing against contaminants
  • Providing heat transference (in the case of oil lubricant)

Lubricants take the form of either oil or grease. Oil lubricants are most common in high speed, high-temperature applications that need heat transfer away from working bearing surfaces. Bearing oils are either a natural mineral oil with additives to prevent rust and oxidation or a synthetic oil. In synthetic oils the base is usually polyalphaolefins (PAO), polyalkylene glycols (PAG) and esters. Although similar, synthetic and mineral oils offer different properties and are not interchangeable. Mineral oils are the more common of the two.
The most important characteristic when specifying oil for a bearing is viscosity. Viscosity is a measure of a fluid’s internal friction or resistance to flow. High-viscosity fluids are thicker like honey; low-viscosity fluids are thinner like water. Engineers express fluid resistance to flow in Saybolt Universal Seconds (SUS) and centistokes (mm2/sec, cSt). The difference in viscosity at different temperatures is the viscosity index (VI). An oil’s viscosity is correlative to the film thickness it can create. This thickness is crucial to the separation of the rolling and sliding elements in a bearing. Bearings in some applications use oil, but grease is the lubricant of choice for 80 to 90% of bearings.

Grease consists of about 85% mineral or synthetic oil with thickeners rounding out the rest of the grease volume. The thickeners are usually lithium, calcium or sodium-based metallic soaps. Formulations for higher-temperature applications often include polyurea. The higher viscosity of grease helps contain it within the bearing envelope. The most important considerations when choosing a grease are the base oil viscosity, rust-inhibiting capabilities, operating temperature range and load-carrying capabilities.

Applications
Bearings abound in everyday life and most of the time they go unnoticed. But without them, most motion systems could not function. The ball bearings’ simple design, ability to operate at high speeds and relatively low-maintenance requirements, make them one of the most common roller bearings found in a variety of industrial applications.

For example, deep-groove ball bearings often go in small to medium-sized electric motors because they can accommodate both high speeds and radial and axial loads. Self-aligning ball bearings, on the other hand, are ideal for use in fans. These bearings have two rows of balls with a common raceway in the outer ring. This design allows for angular misalignment while maintaining running accuracy. One caveat: They’re one of the most difficult bearings to install correctly.

Tapered roller bearings are another form of bearing that just about every industry depends on one way or another. They are common in applications that need support for axial and radial loads — for example, in a tire hub where the bearing must withstand radial load from the weight of the vehicle and the axial load experienced while cornering. These bearings are also common in gearboxes where they are generally mounted with a second bearing of the same type in a face-to-face or back-to-back orientation. They provide rigid shaft support, keeping deflection to a minimum. This reduced shaft deflection minimizes gear backlash. Tapered bearings also have the advantage of having less mass but high efficiency, however this does limit their overall speed.

In applications where bearings are mounted vertically, they are typically oriented in a face-to-face setup, while horizontal applications use a back-to-back setup. Some pumps use this design because of shaft deflection concerns.

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