MOUNTING AND DISMOUNTING OF ROLLER BEARINGS

MOUNTING AND DISMOUNTING OF ROLLER BEARINGS

The most important thing to remember when mounting or dismounting a roller bearing, of any type, is to apply the mounting or dismounting force to the side face of the ring with the interference fit. Keep this force from passing from one ring to the other through the ball or roller set. This is particularly

important during mounting, since damage can easily occur internally to the bearing. Cleanliness is, of course, extremely important. Not only the bearing but also the shaft housing must be free from chips, burrs, dirt, and moisture.

Bearings should be kept wrapped or covered until the last possible moment. Since most modern

rust preventives used by bearing manufacturers are compatible with petroleum-based lubricants, the

slashing compound is normally not removed. However, there are exceptions to this rule. If oil-mist

lubrication is to be used and the slushing compound has hardened in storage or is blocking lubrication

holes in the bearing rings, it is best to clean the bearing with kerosene or other appropriate

petroleum-based solvent. Obviously, the other exception would be if the slashing compound has

been contaminated with dirt or foreign matter before mounting. It is also permissible and sometimes

desirable to wipe the rust preventive from the bore or outside diameter of the bearing, depending on

which surface will have the tight fit. Before mounting or dismounting a bearing, always take the time

to collect the proper tools and accessories. The use of inappropriate tools is a major cause of bearing

damage. Also remember, never strike a bearing directly with a hammer, sledge, or mallet.

Cold Mountings. All small bearings (4-in bore and smaller) may and sometimes must be mounted cold by simply forcing them on the shaft or into the housing. However, it is important that this force be applied as uniformly as possible around the side face of the bearing and to the ring to be press-fitted. Mounting fixtures should be used. These can be a simple piece of tubing of appropriate size and a flat plate as shown in Fig.3. Do not try to use a drift and hammer, because the bearing will become cocked. Force may be applied to the simple fixture described above by striking the plate with a hammer or by an arbor press, as shown in Fig. 4. It is a good idea to apply a coat of light oil to the bearing seat on the shaft and bore of the bearing itself before forcing on the shaft. It should

be noted that all sealed and shielded ball bearings should be mounted cold in this manner.

Temperature Mountings. The simplest way to mount any open straight-bore bearing, no matter

what size, is to heat the entire bearing and simply push it on its seat and hold in place until it cools

enough to start gripping the shaft. For tight outside-diameter fits, the housing may be heated if practical; if not, the bearing may be cooled by dry ice. However, if the ambient conditions are humid, cooling the bearing introduces the possibility of condensation on the bearing, which will induce corrosion later.

There are several acceptable ways of heating bearings. Some of these are as follows:

1. Hot plate: A bearing is simply laid on an ordinary hot plate until it reaches the approved temperature. The disadvantage of this method is that the temperature is difficult to control. A Tempilstik or pyrometer should be used to make certain the bearing is not overheated.

2. The temperature-controlled oven: This method needs little comment. The bearings should be left

in the oven long enough to heat thoroughly. However, never leave bearings in a hot oven overnight

or over a holiday or weekend.

3. Induction heaters: These are available and can be used to heat bearings for mounting. One of these is shown in Fig.5. It must be remembered that this is a very quick method of heating and that some

method of measuring the ring temperature must be used or the bearing may be damaged. 

A Tempilstik or pyrometer can serve this purpose. Bearings must be demagnetized after using this method.

4. A hot-oil bath: This method may also be used to heat the bearing and, in fact, is the most practical means to heat larger bearings. This method has some drawbacks, since the temperature of the oil is difficult to control and may overheat the bearing or even become a fire hazard. A mixture of soluble oil and water can eliminate both these disadvantages. Make the mixture 10 to 15 percent soluble oil. This solution will boil at approximately 210⁰F, which is hot enough for most bearing fits. The heating solution should be placed in a tank or container which has a grate or screen several inches off the bottom, as shown in Fig. 6. This will allow any contaminants to sink to the bottom and keeps the bearings off the bottom of the container.

As mentioned above, 210⁰F is not enough to mount most bearings. If you are using one of the

other methods of heating or another solution, 250 F maximum will do the bearings no harm.

However, this temperature should not be exceeded for small ball bearings (2-mm bore and smaller).

Larger bearings can be heated somewhat higher than this without harm, but metallurgical damage

will occur at approximately 300⁰F.

Mounting Tapered-Bore Bearings. Tapered-bore bearings can be mounted simply by tightening the

locknut or clamping plate, which will locate it on the shaft until the bearing has been forced up the taper

the proper distance. However, especially for large bearings, this technique will require a good amount

of brute force. There are special techniques that may be used to reduce the amount of force required.

Before reviewing the mounting techniques for tapered-bore roller bearings, we will discuss the

special case of self-aligning ball bearings. The bearing should be put on its tapered seat and the locknut

hand tightened until all looseness is removed between adjacent parts. Then, using a spanner

wrench, not a drift and a hammer, tighten the nut one-eighth turn further. Bend a lock-washer tab

into the nut slot nearest to a washer tab in a tightened direction. At this point, the outer ring should

rotate as well as swivel freely.

Tapered-bore spherical roller bearings can be mounted a bit more scientifically. Since the internal

clearance in a roller bearing is significantly larger than in a ball bearing, this clearance can be

measured with a thickness feeler gauge. As the bearing inner ring is pushed up the tapered seat, the

inner ring expands, thereby reducing the internal clearance. Hence the amount of this reduction is a

direct function of the interference fit between the bore of the bearing and the shaft. Therefore, if we

measure the internal clearance of the bearing unmounted and control the amount the clearance is

reduced during mounting, we control the shaft fit within very close limits. The internal clearance of

a spherical roller bearing is measured as follows:

The bearing is unwrapped and placed on a table so that it can be easily handled. With one hand

grasping the lower portion of the inner ring, oscillate the inner ring and roller set in a circumferential

direction to seat the lower rollers properly in the sphere of the outer ring, on the roller paths of

the inner ring, and against the separate guide ring between the two rows of rollers. Select a gauge

blade of perhaps 0.003- or 0.004-in thickness or less for small bearings. The usable length of the

blade should be somewhat longer than the length of a roller. It should not be equal to or greater than

the width of the bearing. While pushing the top roller against its guiding surface, inset the blade

between two rollers and the outer ring and slide the blade circumferentially toward the roller at the

top of the bearing, as shown in Fig. 7 The blade should pass between the uppermost roller and

the inside of the outer ring. Do this with successively thicker feeler blades until a blade will not pass.

Move it so that it approaches the bite between a roller and the outer ring sphere; then, with one hand

grasping the inner ring as described earlier, slowly roll the uppermost roller under the feeler blade.

With the blade between the uppermost roller and the sphere, attempt to swivel the blade and withdraw

it axially. The swiveling motion helps to center the roller in its proper operating position, and

withdrawing it with the characteristic wiping feel of a line-to-line contact will show that thickness

to be the looseness over that roller. If the blade becomes looser during the swiveling and withdrawing

process, attempt the same procedure with a blade 0.001 in thicker and continue until a blade cannot

be swiveled or withdrawn. The internal clearance over that roller will be the blade that can be

swiveled and withdrawn after a thicker one has jammed.

Repeat this procedure in two or three other locations by resting the bearing on a different spot on

its outside diameter and measuring over different rollers in one row. Either repeat the above procedure

for the other row of rollers or measure each row alternatively in the procedure described above.

Make a note of this unmounted internal clearance.

After the unmounted radial clearance is measured, the bearing is placed on its tapered seat. If the

shaft provides for a locknut, it is then assembled, but the lock washer is left off the shaft at this point.

The locknut should then be tightened against the bearing, pushing it up the taper until the internal

clearance is reduced by the specified amount, as shown in Table1. An impact-type spanner wrench

as shown in Fig. 8 is ideal for tightening the nut

The amount of force required to drive a tapered-bore bearing can be greatly reduced if the shaft is

drilled and grooved as shown in Fig.9 If these fittings are available, attach a hydraulic pump to the connection at the end of the shaft. Drive the bearing on the taper just enough so there is some interference; then build up hydraulic pressure under the bore of the bearing. A pressure of 3000 to 6000 psi will be needed, but with this pressure between the bore of the bearing and the shaft it is possible to float the bearing up the taper with much less torque applied to the lock nut or clamp plate than in a dry mounting.

Another convenient way to mount a tapered-bore bearing is to use a hydraulic nut or mounting

tool, as shown in Fig. 10. This technique also can be adapted to sleeve mountings that are large

enough to be drilled and grooved. Cylindrical and tapered roller bearings with tapered bores are not as common as their spherical counterparts, and the manufacturer will have specific mounting instructions for each application.

                                                  

Dismounting of Bearings. A wide variety of tools are available commercially which are designed

to remove a rolling bearing from its seat without damage. Typical bearing pullers are shown in

Fig. 11. In removal, we should again keep in mind the basic rule to apply force to the ring with the

tight fit. Pullers normally can be applied to bearings so that this rule is observed. However, sometimes

supplementary plates or fixtures may be required. For smaller bearings, an arbor press is equally effective at removing as well as mounting bearings.

Also, techniques such as the one shown in Fig.12 may be used where size permits.

Hydraulic Removal. Where shafts have been designed to apply hydraulic pressure to the fit between shaft and bearing, removal is quite simple. First, the locking device, whatever it is, should

be backed off a distance greater than the axial movement of the mounting; 1/4 in will be sufficient in

virtually every case. Then connect a hydraulic pump to the fitting provided at the end of the shaft, as

shown in Fig.13, and start building up pressure. When pressure becomes great enough to break the

fit, usually about 3000 to 6000 psi, the bearing will literally jump off the taper with a sharp bang.

The retaining device, still being loosely connected, will prevent the bearing from coming off the end

of the shaft. Never completely remove the retaining device.

Hydraulic pressure may be used with straight-bore bearings, but a puller must be used in conjunction

with the hydraulic pump, since there will be no axial component of the hydraulic pressure

to blow the bearing off its seat. See Fig. 13.

Larger sleeve mountings also may be designed to utilize hydraulic pressure for dismounting. If

this feature is available, follow the same procedure as outlined above. However, if the sleeve mounting does not have this feature, other techniques such as shown in Fig. 14, must be used. For withdrawal sleeves, a special nut must be used, as shown in Fig. 15. For large sleeves, a hydraulic nut is desirable for dismounting.

LUBRICATION

The primary purpose of lubrication in a rolling bearing is to separate the contacting surfaces, both

rolling and sliding. This purpose is rarely achieved, and boundary lubrication or partial metal-tometal

contact frequently occurs. By far the most common lubricants are petroleum products in the form of grease or liquid oil. Synthetics are, however, finding more use in high-temperature

applications.

Generally, the machine builder decides whether a bearing will be a grease- or oil-lubricated component and normally will recommend the basic specifications of the required lubricant. However,

because the machine designer cannot foresee all the variable conditions under which the equipment

will operate, some judgment is required on the part of maintenance personnel. Some knowledge of

lubricants is therefore useful.

Oil Lubrication. For oil lubrication, the Annular Bearing Engineers Committee (ABEC) has issued

the following recommendations:

The friction torque in a ball bearing lubricated with oil consists essentially of two components.

One of these is a function of the bearing design and the load imposed on the bearing, and the other

is a function of the viscosity and quantity of the oil and the speed of the bearing.

It has been found that the friction torque in a bearing is lowest with a very small quantity of oil,

just sufficient to form a thin film over the contacting surfaces, and that the friction will increase with

greater quantity and with higher viscosity of the oil. With more oil than just enough to make a film,

the friction torque will also increase with the speed.

The energy loss in a bearing is proportional to the product of torque and speed, and this energy

loss will be dissipated as heat and cause a rise in the temperature of the bearing and its housing. This

temperature rise will be checked by radiation, convection, and conduction of the heat generated to

an extent depending upon the construction of the housing and the influence of the surrounding

atmosphere. The rise in temperature, due to operation of the bearing, will result in a decrease in viscosity of the oil, and therefore, a decrease in friction torque compared with the friction of starting,

but soon a balanced condition will be reached.

With so many factors influencing the friction torque, energy loss, and temperature rise in a bearing

lubricated with oil, it is evidently not possible to give definite recommendations for selection of

oil for all bearing applications, but two general considerations are dominant:

1. The desire to reduce friction to a minimum, which requires a small quantity of oil of low viscosity.

2. The desire to maintain lubrication safely without much regard for friction losses, which results in

using larger quantities of oil and usually of somewhat greater viscosity in order to reduce losses

from evaporation or leakage.

This second condition is most frequently met when bearings have to operate in a wide range of

temperatures. An oil that has the least changes with respect to changes in temperature, that is, an oil

with high viscosity index, should be selected.

In the great majority of applications, pure mineral oils are most satisfactory, but they should, of

course, be free from contamination that may cause wear in the bearing, and they should show high

resistance to oxidation, gumming, and deterioration by evaporation of light distillates, and they must

not cause corrosion of any parts of the bearing during standing or operation.

It is self-evident that for very low starting temperatures an oil must be selected that has sufficiently

low pour-point so that the bearing will not be locked by oil frozen solid. In special applications, various compounded oils may be preferred, and in such cases, the recommendation of the lubricant manufacturer should be obtained.

Grease Lubrication. Where grease lubrication is used, we need to consider a few of the basic physical

and chemical characteristics of the lubricant. Greases are a mixture of lubricating oil and usually

a soap base. The base merely acts to keep the oil in suspension. When moving parts of a bearing

come in contact with the grease, a small quantity of oil will adhere to the bearing surfaces. Oil is

therefore removed from the grease near the rotating parts. Bleeding of the oil from the grease obviously cannot go on indefinitely, so new grease must come in contact with the moving part or a lubrication failure will result.

Many maintenance departments want to use one grease to lubricate all bearings in the plant.

Some lubricant suppliers even advocate this technique. However, it is a risky procedure at best,

since there is no true universal ball and roller bearing grease. A ball bearing is best lubricated with

a fairly stiff grease which will channel. On the National Lubricating Grease Institute (NLGI) code,

greases of the number 2 consistency, or 265 to 295 worked penetration, are normally recommended.

For roller bearings, a grease stiff enough to channel is not desirable, since the full width of the roller

track would soon be starved for lubricant if the grease is not soft enough to slump back into the

bearing when it is pushed aside. This generally means greases in the number 0 or 1 consistency

class with worked-penetration numbers of 355 to 380 for grade 0 and 310 to 340 for a number

1 grease. Whatever the consistency of the grease, it is still the properties of the oil compounded in

the grease that determine if the bearing will be satisfactorily lubricated. All statements and

guidelines outlined above in the discussion of oil lubrication also apply to grease-lubricated

bearings.

Another characteristic of a grease that must be considered is its drop point. This is the temperature

at which the grease passes from a semisolid to a liquid. Typical dropping points are as follows:

              Calcium                       -14 ± 140⁰F

              Sodium                    - 22 ±  176⁰F

              Lithium                     -22 ± 230⁰F

              Bentone                - 22 ±  266⁰F

              Silicone                     -22±  266⁰F

             Calcium complex         -4 ± 266⁰F

           Aluminum complex    -22 ±  230⁰F

The drop point is the characteristic referred to when a grease is advertised as being good up to

400⁰F. Whether it will lubricate a bearing or not is still a function of the viscosity of the lubricating

oil, not of the drop point of the base. In fact, common industrial bearings made of standard through hardened or case-hardened materials have temperature limitations of 200 to 300⁰F depending on the

material and how it was heat-treated. The bearing manufacturer should be consulted for specific

information.

Never mix greases that are incompatible. If two such greases are mixed, the resulting mixture

usually has a softer consistency which will eventually cause failure through leakage. If you don’t

know what type of grease a bearing was lubricated with originally, do not regrease without first

removing the old grease both from the bearing and the surrounding environment.

Generally, bearings are not lubricated until after mounting. The most important reason for this is

cleanness. The later grease is applied, the greater are the chances of avoiding contamination. The

bearing should be lubricated prior to mounting only when pregreasing is the only way to obtain an

even distribution of grease.

The right quantity of grease is as important as the right type of grease. Follow these general rules

for quantity:

A bearing should be filled completely with grease, but free space in the housing should only be

partially filled (between 30 and 50 percent). However, in nonvibrating applications, many lithium

soap greases, also called total-fill greases, can fill up to 90 percent of the free space in the housing

without any risk of a rise in temperature. Thus impurities can be prevented from entering the bearing,

and relubrication intervals can be extended. Bearings that have to operate at high speeds, for example, machine tool spindles, where it is desirable to keep the temperature low, should be lubricated with small quantities of grease.

In vibrating applications, such as wheel hubs, vehicle axle boxes, and vibrators, grease fill should

be no more than 60 percent of the housing.

When relubrication intervals are long, then the housings should be easy to open. If more frequent

lubrication is required, the housing should be fitted with some kind of grease-filling device, preferably a lubrication duct with a nipple.

In the optimal situation, grease can be injected with a grease gun. Some bearings are provided

with grooves and ducts for relubrication; others have to be relubricated from the side.

Only the grease in the bearing should be replaced. The amount of grease, therefore, depends on

the bearing size. If relubrication instructions are available from the original manufacturer, follow

them. If not, or if you suspect the lubrication amount is inadequate, use the following formula to

determine the correct amount

Gq = 0.114× D×B (in ounces)

where

 Gq=grease quantity in ounces

D =bearing outside diameter in inches

B = total bearing width in inches

Selection of Lubricant. Research in elastohydrodynamics (EHD) has contributed greatly to the

knowledge of lubricants, rolling bearings, and how they work. Results of this work have been published in various forms that may be used as a guide in the selection of the correct lubricant.

 Figure 16 is an example of these data in graph form which plots required viscosity of the lubricant in centistrokes at operating temperature as a function of bearing size and speed. The abscissa of the curve

is the bearing size, expressed as mean diameter in millimeters; the diagonal lines are the speed in

rpm; and the ordinate is the required viscosity at the operating temperature of the bearing. It is obvious from an examination of this chart that the larger the bearing and the slower the speed, the higher is the required viscosity. This characteristic of the EHD theory sometimes produces a paradox where the lubricant required may not be realistically used for other reasons. Lubrication experts should be consulted when this situation exists.

When grease lubrication is used, the values obtained from the chart apply to the base oil of the grease. The example shown on the graph indicates that a rolling bearing with a mean diameter of 335 mm, running at 133 rpm, will require a lubricant having 41 centistokes at its operating temperature.

Reasonable estimates of bearing operating temperature can be made ahead of time based on analytical calculations or experience. In most cases, a combination of the two gives the most realistic

estimate. The machine builder and/or the bearing supplier can help in this area and will normally

make lubrication recommendations for new equipment. Some adjustment may have to be made to

the original selections after operating experience is obtained.

In current literature and specification sheets, lubricants are normally grouped into viscosity

grades according to standards established by the ISO. Lubricants are rated from ISO 2 to ISO 1500,

the numbers indicating the mean value of a specified range of viscosity at a temperature of 104⁰F.

Figure 17 plots these lubricants on a temperature-viscosity diagram.

In the selection of a lubricant, it should be kept in mind that the oil temperature in the bearing

may vary 5 to 10⁰F from the temperature of the housing .There are no exact rules for this estimate,

but it is wise to add these degrees if housing temperature is used as a criteria for determining operating level of the bearing.

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