Motion System Design

Tips on choosing plain linear bearings

Need to choose a plain bearing for a linear application? Not sure which one works best in your application? Here’s a quick reference guide to help you narrow your selection

Seventy percent of applications that need a bearing can use either a rolling element or plane bearing. Rolling element bearings are best for half of the remaining 30%, and plane bearings are best for the other half.

To select the optimum linear bearing for that 30% of the application segment though, engineers first must know the application. Other factors to consider include the initial bearing cost as well as the costs for premature field replacement of bearings and shafting. And don’t forget to factor in the cost of downtime.

Familiarize yourself with the materials and bearing types that are able to handle your application’s load. This information is usually available from the manufacturer or independent sources such as The Engineering Guide to Plane Bearings,* and the Plane Bearing Standards Association (PBSA), in Naples, Fla.

Before final bearing selection, it is a good idea to verify manufacturer performance claims through your own testing or through an independent source. An independent lab that specializes in tribology can save time and costs, as they already have the necessary testing equipment. In addition, they can offer reassurance of the validity of the bearing design.

After all of this, the engineer needs only select the right type of linear bearing (bronze, split metal with various overlays, and so on). There are hundreds of choices, but the following will help you narrow your choice.

Ball bearings

Strengths. Ball bearings are the most widely used, readily available linear bearings, although they are not designated as plain bearings. When not excessively preloaded they have very low friction (less than 0.1) and a predictable life span. Because they can be preloaded, they hold precise tolerances (depending on preload), which reduces runout and sloppiness.

Constraints. Depending on the applications, they can be considered noisy. They require lubrication and are susceptible to corrosion, therefore they are not appropriate for such applications as food processing. Also, it is possible to pack them too tightly with grease during application or maintenance, which will increase operating friction. Seals, which are commonly used to keep contaminants from the rolling elements, also increase friction.

Because there is metal-to-metal contact between ball bearings and the shaft, they can fail catastrophically. To avoid shaft replacement, the bearings are often replaced at prescribed intervals regardless of their condition. The life of a ball bearing is predictable, assuming the bearing is applied properly.

Applications. They are best for applications with highly cantilevered loads because of their low coefficient of friction. The amount of supported load is dependent on friction, which dictates the spacing between the bearings.

In machine tool applications that require tight tolerances (±0.0002 in.), ball bearings are the bearing of choice. They are also suited to applications involving hand motion, and are prevalent in guarddoor applications.

Applications where there is dust or sand, such as those in foundries and paper mills, and many applications near welding equipment are inappropriate. Contaminates from these environments can lock up the balls, causing skidding on the shaft and loss of precision. Extreme cases can weld the bearing to the shaft.

Bronze bearings made with solid and powdered metal materials

Strengths. Bronze bearings can handle loads from 25,000 to 40,000 psi in linear, oscillating, or rotary applications. For the most part, they resist corrosion. They are available in many standard sizes. Use of a non-standard size, however, can be more costly than using other types of bearings.

Constraints. It is impossible to get away from metal-to-metal contact in the moving elements. Thus, they require proper lubrication, and even then, the contact between surfaces causes high friction.

In some cases, bronze bearings rely on their natural porosity for lubrication. If the operating temperature is sufficient, oil trapped in the material is drawn out of the pores. If the operating temperature is not reached, or not reached quickly enough, the lubrication is insufficient and the bearing can damage the shaft. If the bearing surface becomes galled through metal-to-metal contact, the pores in the bearing material can close, shutting off lubrication.

Applications. Bronze bearings are best applied where loads are very high. They are not as well suited to linear applications as are other types of bearings because the heat they generate is dissipated over too large an area. Therefore, they can’t get to the temperature they need to adequately release their lubricants.

They also function well in oscillating or rotary applications, such as on pivoting equipment or assembly lines, where they can develop the temperatures they need for proper lubrication.

Split metal-backed bearing with a thin sintered-bronze layer and PTFE/lead overlay

Strengths. With its surface area and the bronze construction, this bearing accommodates loads from 30,000 to 45,000 psi. It is self-lubricating on a limited basis because it has a thin layer of PTFE on its inside diameter, Figure 1. In continuous or repetitive motion applications, it needs external lubrication. In addition to linear applications, it can also be used in oscillating or rotary applications.

Constraints. It has a high coefficient of friction (0.2 to 0.3). Thus, if the layer of the overlay wears away, the metal-tometal contact can cause extensive damage to the shafting. If the bearing is applied in a dirty environment, it requires the use of seals, which can’t be built into the bearing.

These bearings tend to have difficulty holding dimension or repeatable running clearances because their tolerance must be built up to accommodate the split and the press fit.

Another concern is the use of lead in the overlay. Lead helps ease the running friction of the bearing. However, people at every stage, from bearing designers to users of finished machines are concerned about the use of lead. Presently, there is no legislation against lead in bearings. Even so, bearing manufacturers are working towards replacing it.

Applications. These bearings perform well in heavy-load oscillating applications such as train doors and areas that do not require high precision or repetitions. Without positive lubrication, they are not suited to linear applications because the heat doesn’t transfer well between the materials of the bearing. Without shedding excess heat, the overlay can wear out quickly, causing a metal-onmetal condition.

Solid composite (fiber wound)

Strengths. Solid composite describes a number of bearings, most of which are capable of handling loads from 25,000 to 50,000 psi. Certain composite materials are suitable for wet environments because only 0.1% of wall thickness will absorb moisture.

Constraints. Composite bearings typically have a thick wall (0.125 in. and greater) to boost strength, but this thickness can lead to cold flow of the material during use. It also acts as a thermal insulator. If enough heat does not dissipate, thermal expansion can occur. Engineers must build in extra running clearances in their designs. These bearings are frequently used in applications where the temperature is below 250 F .

Composite bearings have a higher coefficient of friction than other similar bearings, 0.2 to 0.3. The materials are fairly hard, so fine particulate matter is not able to embed itself in the bearing surface. Any particles will remain to scratch the bearing surface and shafting. In some cases, these bearings are fabricated from some exotic materials, so their price may make them unsuitable for an application.

Applications. Composite bearings are good for low-speed (under 200 sfpm), high-load applications. The low moisture absorbing materials are excellent for use as marine rudder-post bearings, and in fact they are replacing many br onze bearings in this application.

Composite bearings are not the best choice for high-speed (above 200 sfpm), high-temperature (above 250 F) or highprecision (more than .001 in. ) applications where cold flow and therm al expansion of the materials can affect running clearances.

Solid plastic with and without a metal sleeve (Nylon, UHMW, etc.)

Strengths. Plastic bearings are a lowcost, self-lubricating type of plane bearing suited for many linear applications, as well as oscillating and rotary applications. They resist corrosion and are available in many materials. Most are soft enough that fine particulate material can embed in their bore. Thus, these bearings won’t damage shafting as readily as other bearings.

Constraints. Plastic bearings have a load capacity of less than 3,000 psi. Their cold flow and thermal expansi on characteristics are similar to their composite cousins. As a result, engineers m ust carefully evaluate the upper ends of speed, load, and temperature in an application to build in extra running clearance in the linear solution. This design criteria can decrease the positioning precision of the bearing. Plastic bearings also h ave a tendency to swell in the presence of moisture, which affects running clearance.

Applications. They work well in i appl - cations like drawer guides in furniture, where the number of repetitio us movements, speed and operating temperature are limited. Plastic bearings don’t hold up as well with repetitive moti on and precise applications like automation or packaging.

Metal backed with bonded bearing surface

Strengths. These bearings have a Teflon coating that is permanently bonded at the molecular level to an outer shell of metal. They support loads to 3,000 psi, at temperatures to 500 F because of the inherent properties of the Teflon coating. This coating also classifies this bearing as self-lubricating. Heat can pass through the thin layer of Teflo n and dissipate through the metal bearing shell.

Because of the heat dissipation, these bearings can use tight runni ng clearances, down to 0.0005 in. The inside and outside diameter of the bearing shell is finish ground to a fine level before the liner is bonded in place.

This bearing has no metal-to-metal contact, resists corrosion, and cannot catastrophically fail.

Constraints. The coefficient oc f fri - tion is greater than that of the ball bearing, (0.15 to 0.25).

Applications. These bearings are suited for use in high-repetition and load applications, like those found in gantry systems, automation and packa ging equipment. They are not good for hand-motion applications that require low friction, like door guides, or in machine-too l applications that require high precision.

*Stellar Publishing (800-329-9054) has recently published a new version of The Engineering Guide to Plane Bearings. If it is not available at your local distributors, the publisher will sell the book directly.

Mark Huebner is a specialized products engineer at Pacific Bearing, Rockford, Ill.

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