Mechanics and Manufacturing Methods -- Sleeve Bearings

There are two common types of sleeve bearings. The rigid steel-backed babbit bearing and the self-aligning sintered bearing are shown in Ills. 71a and 3.71b, respectively.

Babbit is a soft alloy of tin and lead that has a low coefficient of friction. It’s backed by a thin steel material to give the bearing rigidity. A wick notch and oil groove are normally formed in the bearing when it’s manufactured.

Rigid bearings are pressed into the end frame and burnished or machined to size.

An oil reservoir surrounds the bearing. If the bearing is a sintered bronze or sintered iron type, the oil wicks through the pores in the bearing by capillary action and pro-vides lubricant to the shaft. In the case of steel-backed babbitt bearings, a wick of some type must be placed in a hole in the bearing so that oil from the reservoir can reach the shaft.

Self-aligning sintered bearings are held against the end frame by a retaining spring. Oil wicks from the oil reservoir through the pores in the material as in the rigid sintered bearings. Ball bearings are used where it’s necessary to limit radial shaft play or where high side loads are expected in the application. These are the most costly bearings. Rigid bearings are used where moderate side loads are expected. Self-aligning bearings are used where light side loads are expected and starting friction has to be minimized.

The self-aligning bearing is the lowest-cost device, but it also carries the lowest load. The rigid sleeve bearing costs somewhat more than the self-aligning bearing, but it can carry a heavier load. Ball bearings can carry a much heavier load, but they also cost 5 to 20 times more than a sleeve bearing.

The most common shaft and bearing system consists of a steel shaft in a sintered bronze bearing. As a general rule, the harder the shaft, the longer the life of the sys-tem.

Generally, Rockwell C scale 35 to 55 is a good range. Stainless steels are accept-able, except that the 300 series should be avoided with bronze bearings because excessive wear will result. When using bronze bearings, the load speed and shaft-to-bearing clearance must be considered. The pressure velocity factor PV must be calculated to ensure that it’s within the rating of the material selected. Bronze is rated at PV =50,000.

PV =w/LD x pi DN/12 (3.7)

[…]

…where P = load, lb/in^2 V = surface velocity of the shaft, ft/min

w = total bearing load, lb

L = bearing length, in

N = shaft speed, rpm

D = bearing ID, in

Additional lubrication is advised in order to reduce the coefficient of friction.

These bearings may close down when installed and may need to be resized to maintain a shaft-to-bearing clearance between 0.0005 and 0.0015 in. The actual tolerance on bearing clearance will depend on the shaft diameter and the type of lubrication used.

Sintered bearings should be resized with a hard, polished sizing pin or burnished roller to avoid closing down the pores that provide the lubrication to the bearing-shaft inter face. There is a tendency to want to size the bearings with a reamer. This practice should be avoided, because the reamer cuts away the material and smears it across the pores. This results in closed pores and starves the shaft of lubrication. Shaft finish should be 16 µin or better to reduce wear and avoid pumping oil out of the bearing.

Lubrication

Lubricants are used in bearing systems to reduce the friction between surfaces sliding relative to each other. In the case of the electric motor, the shaft moves relative to the bearing, riding on a film of lubricant (oil), as shown in Ill. 72.

The velocity of the oil varies across the thickness of the film, as shown in Ill. 73.

Near the stationary part of the bearing, point B, the velocity is zero. At the surface of the shaft, point A, it’s equal to the shaft speed. The lubricant is thought to move in layers across its thickness.

ILL. 71 Sleeve bearings: (a) rigid and (b) self-aligning.

As the shaft rotates, a stress t is developed in the layers of oil. This stress is known as the shearing stress and is measured in pounds per square foot or similar units. It’s equal to the change in velocity ?? of the liquid over its thickness ?? times the dynamic viscosity µ of the oil.

t=µ ?µ=t (3.8)

Viscosity is defined as the property of a fluid which offers resistance to the relative motion of fluid molecules. The kinematic viscosity v is the ratio of dynamic viscosity to fluid density. It’s measured in square feet per second or square centimeters per second (strokes) or say bolt universal seconds.

The viscosity index (VI) is a measure of how greatly the viscosity changes with temperature. Generally speaking, a high index means a small change in viscosity with temperature and a low index means a large change in viscosity with temperature.

It’s evident from this information that the lubricant's viscosity has an effect on the amount of frictional loss in the electric motor.

ILL. 72 Hydrodynamic lubrication.

ILL. 73 Velocity variation of oil layers.

ILL. 74 Oil flow in rigid nonporous sleeve bearing.

ILL. 75 Oil flow in porous sintered sleeve bearing.

In the case of the rigid nonporous sleeve bearings ( Ill. 74), the rotating shaft picks up oil from the felt wick and forces it up the groove in the bearing. As the oil gets to the thin portion of the groove, it’s forced out onto the surface of the bearing and provides a film between the bearing and the shaft.

Finally, we consider porous sleeve bearings ( Ill. 75). In this system, the oil wicks through the small holes in the bearing and covers the shaft. As the shaft rotates, it causes the oil to be forced onto the nonporous surfaces of the bearing, building up a film similar to that of the rigid nonporous sleeve bearing.

Application

The selection of the shaft and bearing were discussed earlier. The selection process is based on loads and speeds. Selection of the lubricant is usually based on two factors, system friction and temperature range.

Friction consists of viscous friction, static friction, and coulomb friction. Viscous friction is a function of the viscosity of the oil and changes with speed. Static friction is a retarding force that tends to prevent motion from starting. Once motion has started, static friction falls to zero. Coulomb friction is a torque force which has a constant amplitude and is not a function of velocity.

In a motor, these frictions translate into torques as follows:

Viscous friction

Tv(t) = B (3.9)

Static friction Ts(t) =±(Fs)?= 0 (3.10)

Coulomb friction Tc(t) = Fc

… where B = viscous friction coefficient

(Fs)?= 0 = static friction

Fc = coulomb friction coefficient

?= angular displacement

The combination of these frictions are present in motor bearing systems, as graphically represented in Ill. 76.

The static friction Ts(t) causes a high torque loss at starting (zero speed).There is a ripple effect as the oil film is formed. Thereafter, the coulomb friction Tc(t), and viscous friction Tv(t) cause the torque to increase in this nonlinear fashion.

Temperature considerations are important when selecting a motor lubricant.

Extreme cold or heat can adversely affect motor performance if the proper lubricant is not selected. It’s common to select a light (low-viscosity) oil for cold-temperature operation and a heavy (high-viscosity) oil for high-temperature operation. If a broad temperature range is necessary, it’s common to select a synthetic oil that has operating characteristics well above and below the desired range.

Life at high temperatures has been limited because of oil degradation. Many potential applications have been abandoned or limited because of this problem. The availability of perfluoropolyether (PFPE) oils has made many of these applications more practical. These oils have been shown to be stable at these higher temperatures.

A few problems have occurred when applying these oils. Most of these problems occur because of interpretation and comparison of the specification sheets for lubricants.

Specification sheets list temperature range and viscosity. The viscosity that is listed is generally the kinematic viscosity in centistokes (cSt).The comparison would lead a person to select a PFPE oil that is likely to be much heavier than necessary.

This will result in higher friction and possibly higher bearing temperatures than with a normal oil. The trick is to predict the dynamic viscosity value of each oil and compare them. This is obtained by multiplying the kinematic viscosity by the density of the oil. When making this calculation, be certain that units are consistent.

Once the oil has been selected, an increase in friction may still be observed. The PFPE oils tend to stick to the shaft and fill the entire void between the shaft and bearing. Bearing systems are designed to provide a hydrodynamic wedge or wave of oil on which the shaft rides, as shown in Ill. 77.

ILL. 76 Frictional damping speed torque curve.

ILL. 77 Shaft riding on wedge of oil.

This leaves an empty space over part of the bearing-shaft interface. When the PFPE oils fill this space, additional viscous friction results, which produces additional torque and heat losses. If this situation occurs, a few ten-thousandths of an inch additional clearance may be necessary between the shaft and bearing, or a bearing with a different porosity may be required. The exact value is best determined by experimentation.

Sleeve-bearing system life may be further extended by adding shaft slingers that throw the oil back into the oil reservoir.

Tbl. 15 and16 list some oils commonly used in sintered bearings.

TBL. 15 Oils Used in Sintered Bearings

Oil | Temperature range | Viscosity at A 40°C (104°F) | Application focus

[Nye synthetic oil 310B Nye synthetic oil 132B Nye synthetic oil 181B Nye synthetic oil 188B Nye synthetic oil 634B* Nye synthetic oil 605* Fluoroether oil 490 Fluoroether oil 491 ]

[-20 to 125°C 5

(-4 to 257°F)

-60 to 120°C 2

(-148 to 248°F)

-40 to 125°C 6

(-40 to 257°F)

-40 to 150°C 3

(-40 to 302°F)

-40 to 150°C 3

(-40 to 302°F)

-40 to 150°C 6

(-40 to 302°F)

-75 to 225°C 9

(-103 to 437°F)

-65 to 250°C 1

(-85 to 482°F) ]

[ 500-560 cSt

20 cSt

60 cSt

35 cSt

35 cSt

60 cSt

90 cSt

150 cSt

]

[ High-viscosity polyalpha olefine (PAO) with additives to reduce friction and wear in sintered iron bearings.

PAO, plastic-compatible, light-viscosity oil for improved low torque start-up

PAO, plastic-compatible, medium-high-viscosity oil; most commonly used viscosity for sin tered bearings

Light-viscosity polyolester-based oil with copper pacifier and anti wear additives for low-torque applications

Light-viscosity polyolester-based oil with copper pacifier and anti wear additives for low-torque applications

Medium-viscosity polyolester oil; excellent lubricity; contains cop per pacifiers and load-bearing additives

Medium viscosity, exceptional lubricity and chemical inertness, low volatility, wide temperature capability

High viscosity, good film strength, exceptional lubricity and chemical inertness, very wide temperature capability.]

* Ester-based oils may adversely affect some plastics, such as acrylonitrile-butadiene-styrene (ABS), poly carbonates, and polyphenylene oxides. If compatibility questions arise, contact the vendor prior to lubricant selection.

TBL. 16 Krytox PFPE Lubricants for Sintered Bearings