Sunday, January 25, 2009

Angular Contact Thrust Ball Bearing

The angular contact thrust ball bearings shown here were originally designed to support the rotary tables of drilling rigs but are also suitable for other applications where high load carrying capacity, high axial stiffness and low friction torque are important. In contrast to conventional thrust ball bearings, angular contact thrust ball bearings can accommodate radial loads in addition to axial loads and are able to operate at high speeds.

Single direction angular contact thrust ball bearings
Single direction angular contact ball bearings (
fig 1) are able to take up axial loads acting in one direction. They are of separable design, i.e. the washers and ball and cage assembly can be mounted individually. In the main application area for these bearings – rotary tables – two bearings are always adjusted against each other. The second bearing has to carry the weight of the drill when it is stationary and has a higher load load carrying capacity than the first bearing.

fig 1

Double direction angular contact thrust ball bearings
In bearings of the double direction design (
fig 2), the upper bearing with the higher load carrying capacity and the smaller bearing which locates in the opposite direction are combined togther to form a unit. These bearings have low height and can accommodate axial loads acting in both directions as well as moment loads. A single bearing can therefore be used to support rotating machine components in relation to stationary components. These double direction bearings are also of separable design.


fig 2

Other angular contact thrust ball bearings
In addition to the angular contact thrust ball bearings described here, SKF also manufactures single direction bearings – screw support bearings (
fig 3)


– and double direction precision bearings for the work spindles of machine tools (fig 4

and fig 5).



These machine tool bearings are avialable with a contact angle of 60° for high stiffness, or where high speed capability is required, with a contact angle of 40° or 30°. Information about these precision bearings will be supplied on request.

Misalignment
Angular contact thrust ball bearings cannot tolerate any misalignment of the shaft with respect to the housing or any angular misalignment of the support surfaces on the shaft and in the housing.

Cages
Angular contact thrust ball bearings are produced either with a full complement of balls (without cage) or are fitted with machined brass cages (
fig 6) which are centred on the shaft washer or the housing washer


fig 6

Minimum load
When angular contact thrust ball bearings operate at high speeds, the centrifugal forces and gyratory moments acting on the balls may cause them to slide on the raceways. To prevent the damaging consequences of this, for example smearing, the bearing must always be subjected to a given axial load.
The requisite minimum axial load to be applied to single direction angular contact thrust ball bearings can be obtained from

where
Fam =minimum axial load, kN
A= minimum load factor (see product tables)
n = rotational speed, r/min
When starting up at low temperatures or when the lubricant is highly viscous, even greater loads may be required. The weights of the components supported by the bearing, together with the external forces, generally exceed the requisite minimum load. The double direction angular contact ball bearings are preloaded by adjusting the two shaft washers against each other.

Equivalent dynamic bearing load
For single direction angular contact thrust ball bearings
P = Fa when Fa/Fr ≤ e
P = Fa + XFr when Fa/Fr > e
The values of factors e and X will be found in the product tables.

Equivalent static bearing load
For single direction angular contact thrust ball bearings
P0 = Fa + X0Fr
The values of factor X0 will be found in the product tables.

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Needle Roller Bearing

Needle roller bearings are roller bearings with cylindrical rollers which are thin and long in relation to their diameter. ISO uses the definition that the roller length is 2,5 times the roller diameter or more. They are referred to as needle rollers. In spite of their low cross section the bearings have a high load carrying capacity and are thus extremely suitable for bearing arrangements where radial space is limited.
The SKF range of needle roller bearings is extensive and, in addition to customized designs, it comprises the following types:
– Needle roller and cage assemblies


– Drawn cup needle roller bearings
with open ends

and with closed end


– Needle roller bearings
with and without flanges

with

and without inner ring


– Sealed needle roller bearings

with

and without inner ring
– Alignment needle roller bearings

with


and without inner ring

Combined needle roller bearings
*Needle roller/ball bearings
*Needle roller/thrust ball bearings
*Needle roller/cylindrical roller thrust bearings


Combined needle roller bearings consist of a radial needle roller bearing combined with a thrust bearing and are consequently able to take up both radial and axial loads. Combined needle roller bearings provide the means to produce locating bearing arrangements in a minimum of radial space. They are particularly useful where the axial loads are too heavy, speeds too high, or lubrication inadequate for simple thrust washers to be suitable, and other types of locating bearing take up too much room.
Combined needle roller bearings are available in the following designs:
– needle roller/angular contact ball bearings


– needle roller/thrust ball bearings


– needle roller/cylindrical roller thrust bearings



Needle roller thrust bearings Needle roller thrust bearings can support heavy axial loads, are insensitive to shock loads and provide stiff bearing arrangements which require a minimum of axial space. They are single direction bearings and can only accommodate axial loads acting in one direction. Particularly compact bearing arrangements can be made, taking up no more space than a conventional thrust washer, if the faces of adjacent machine components can serve as raceways for a needle roller and cage thrust assembly. For applications where adjacent components cannot serve as raceways, the assemblies can also be combined with washers of various designs.
Because of all the possible combinations, all bearing components must be ordered separately.
–Inner rings for needle roller bearings
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Taper Roller Bearing

Taper roller bearings are produced in many designs and sizes to match their many uses. These can be grouped as follows:
–single row taper roller bearings (
fig 1)


–paired single row taper roller bearings (
fig 2)


–double row taper roller bearings (
fig 3),

and
–four-row taper roller bearings (
fig 4).


These are described in four separate sections under the appropriate headings.
There are also sealed, greased and preadjusted units based on taper roller bearings, such as
– hub bearing units for passenger cars (
fig 5)


– hub bearing units for trucks (
fig 6),

and
– tapered bearing units (
fig 7) for railbound vehicles.


Design featuresTaper roller bearings have tapered inner and outer ring raceways between which tapered rollers are arranged. The projection lines of all the tapered surfaces meet at a common point on the bearing axis. Their design makes taper roller bearings particularly suitable for the accommodation of combined (radial and axial) loads. The axial load carrying capacity of the bearings is largely determined by the contact angle α (
fig 8);

the larger α, the higher the axial load carrying capacity (fig 9)

An indication of the angle size is given by the calculation factor e; the larger the value of e, the larger the contact angle and the greater the suitability of the bearing for carrying axial loads.
Taper roller bearings are generally separable, i.e. the cone, consisting of the inner ring with roller and cage assembly, can be mounted separately from the cup (outer ring).
SKF taper roller bearings have the logarithmic contact profile that provides for optimum stress distribution over the roller/raceway contacts. The special design of the sliding surfaces of the guide flange and large roller ends considerably promotes lubricant film formation in the roller end/flange contacts. The resulting benefits include increased operational reliability and reduced sensitivity to misalignment.
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