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Bearings and Bearing Units for High Temperatures

For bearing arrangements, which have to operate at extreme temperatures in the range 150 to +350 C, or which have to withstand very large temperature differences, e.g. in kiln trucks, furnaces or the conveyor systems of lacquering equipment, normal rolling bearings are unsuitable. SKF has therefore developed high-temperature

  • deep groove ball bearings (fig 1)
  • Y-bearings (fig 2)
  • Y-bearing plummer block units (fig 3)
  • Y-bearing flanged units

to meet the widely differing engineering demands for

  • reduced machine operating costs
  • extended maintenance-free service life
  • high operational reliability

in this wide temperature range, even in a severe environment. The bearings and units for high temperature belonging to the SKF standard assortment are described in the following and listed in the relevant product tables. To special order SKF can produce bearings for extreme low or high temperatures, which are engineered to meet specific needs. If such engineering products are required, the SKF application engineering service should be contacted.

Deep groove ball bearings

SKF deep groove ball bearings for high as well low temperatures correspond in design to the appropriate standard single row deep groove ball bearings. They have no filling slots and are able to accommodate moderate axial loads in addition to radial loads. The characteristic features of these bearings include large radial internal clearance and special cages. The large clearance is four times the C5 clearance and prevents the bearings from seizing even when they are rapidly cooled. All surfaces of the bearings and the shields are manganese phosphated. This provides protection against corrosion and improves running properties.

SKF deep groove ball bearings for high temperatures have a cylindrical bore and are available in five different designs as described in the following.

VA201 design for the most common applicationsBearings of the VA201 design are not sealed and have a pressed steel cage. They are lubricated with a polyalkylene glycol/graphite mixture which can be used at temperatures between 40 and +250 C. At temperatures above +200 C, dry lubrication pertains.

2Z/VA201 design with protective shields

The bearings of the 2Z/VA201 design are of the same design as the VA201 bearings but have protective shields on both sides to prevent the ingress of solid contaminants. Additionally, these bearings have twice the amount of the polyalkylene glycol/graphite mixture as the open VA201 design bearings.

Note Bearings of the 2Z/VA201 design are not recommended for mainly non-rotating applications.

2Z/VA208 design for high demandsBearings of the 2Z/VA208 design have a segmented cage of graphite and can be used at temperatures between 150 and +350 C. The segments separate the balls and also provide the necessary lubrication. The bearings are fitted with two shields, which axially guide the cage segments and prevent the entry of solid contaminants. The minute quantities of graphite powder released by the cage during rotation provide adequate lubrication for the bearing.

An additional advantage of these bearings is that they are environmentally friendly. Even at maximum temperatures, no dangerous gases or vapours are emitted.

2Z/VA228 design for top requirementsThe 2Z/VA228 design bearings are the "top-of-the-range" among SKF products for high temperatures. They are fitted with a "coronet" cage of pure graphite, which opens up additional application areas for these high-temperature bearings. The coronet cage is a unique SKF development and permits operating speeds of up to 100 r/min.

2Z/VA216 design for aggressive environmentsFor bearing arrangements where the environment is particularly aggressive, bearings of the 2Z/VA216 design are recommended. These bearings are charged with a creamy-white multi-purpose grease based on a fluorized polyether oil mixed with PTFE, for operating temperatures from 40 to +230 C. Otherwise the bearings have the same design as 2Z/VA201.

For normal cases, the lubricant fill is between 25 and 35% of the free space in the bearing. On request other filling grades can be supplied.

Y-bearings

SKF Y-bearings for high temperatures correspond in design to the appropriate Y-bearings in the YAR 2-2FW series with grub screws.

The characteristic features of these bearings for extreme temperatures include large radial internal clearance and special cages and shields. As for the high-temperature deep groove ball bearings all surfaces of the Y-bearings are manganese phosphated. This provides protection against corrosion and improves running properties.

SKF Y-bearings for high as well as low temperatures are available in two different designs.

Y-bearings of the VA201 and VA228 designs

Y-bearings of the VA201 (fig 5a) and the VA228 (fig 5b) designs have the same features as the deep groove ball bearing designs having the same V-number identification, with the exception of the bearing clearance, which is only twice the C5 clearance. The Y-bearings are fitted with pressed steel shields and flingers on both sides, which protect the bearings from solid contaminants.

Y-bearing units

Y-bearing units for high temperatures have a grey cast iron housing and are available in three different designs as

  • plummer (pillow) block units (fig 6)
  • flanged units with square housing and four bolt holes (fig 7)
  • flanged units with oval housing and two bolt holes (fig 8)

The Y-bearings incorporated are described in the previous section.

The cast housings of the units are interchangeable with those of the standard Y-bearing units except for a few sizes where some dimensions differ slightly. The housings are zinc coated and yellow chromated to provide better protection against corrosion.

The housings do not have a grease nipple as the incorporated bearings are lubricated for life. The housing bore is coated with a lubricating paste and the tolerances are such that initial misalignment can be accommodated, even at high temperatures.

DimensionsThe boundary dimensions of

  • the deep groove ball bearings are in accordance with ISO 15:1998
  • the Y-bearings are in accordance with ISO 9628:1992
  • the Y- bearing units are in accordance with ISO 3228:1993
  • plummer block units
  • flanged units, square housing
  • flanged units, oval housing.

TolerancesThe deep groove ball bearings and the Y-bearings are produced to Normal tolerances in accordance with

  • ISO 492:2002 (Normal) and
  • ISO 9628:1992 (table 1), respectively.

However, because the bearings have been especially surface treated to protect against corrosion and improve running properties, there may be slight deviations from the standard tolerances. Any such deviations will have no influence on mounting or bearing operation.

The Y-bearings for inch shafts are made to the same tolerances as the corresponding basic bearing for metric shafts.

The tolerances for the height of the shaft axis above the support surface, dimension H1 of the plummer block units are 0/0,25 mm.

Internal clearanceSKF deep groove ball bearings for high temperatures are manufactured with four times the standard C5 clearance.

The Y-bearings and the corresponding Y-bearing units have twice the standard C5 clearance according to ISO 5753:1991.

The clearance limits for the various bearings can be found in table 2 and are valid before mounting under zero measuring load. For the deep groove ball bearings the clearance limits are also given in the product table.

MisalignmentBecause of their large internal clearance, the deep groove ball bearings for high temperatures can tolerate angular misalignments of the outer ring in relation to the inner ring of 20 to 30 minutes of arc. This applies only when the bearings rotate slowly as the rolling conditions in the bearing under such misalignments are unfavourable.

During mounting Y-bearing units are able to compensate for errors of alignment up to 5.

SpeedsSKF deep groove ball bearings as well as Y-bearings for high temperatures in the VA201, VA208 and VA228 design are developed for bearing arrangements where they rotate slowly, i.e. a few revolutions per minute. Experience has shown, however, that it is possible to run bearings for long periods at speeds up to 100 r/min without maintenance. It is recommended to contact the SKF application engineering service if the bearings are to be used at higher speeds.

Design of associated componentsIt is advisable to provide support to the shields of bearings of the 2Z/VA228 and 2Z/VA208 design because of the axial guidance these provide for the graphite cage. Therefore, it is recommended that the housing shoulder or the bore diameter of the spacer sleeve should have a smaller diameter than the outer ring shoulder diameter D2 provided in the product table. If this is not possible a support washer with the appropriate bore diameter should be inserted between the bearing and housing shoulder or spacer sleeve (fig 9).

Selection of bearing size

The requisite size is determined based on the basic static load rating C0, as bearings and bearing units for high temperatures rotate at very slow speeds.

At high temperatures, the load carrying capacity of a bearing is reduced. This is taken into account by multiplying the basic static load rating C0 by a temperature factor fT.

The requisite basic static load rating can be determined using

C0 req = 2 P0/fT

where

C0 req = requisite basic static load rating [kN]

P0 = equivalent static bearing load [kN]

fT = temperature factor (table 3)

The equivalent static bearing load P0 is obtained from

P0 = 0,6 Fr + 0,5 Fa

where

Fr = actual radial bearing load [kN]

Fa = actual axial bearing load [kN]

When calculating P0 the maximum load that can occur should be used and its radial and axial components inserted in the equation above. If P0 < Fr, then P0 = Fr should be used.

For different loads and temperatures the requisite basic static load rating C0 req is shown in table 4. Using the requisite basic static load rating calculated from the above, or taken from table 4, a suitable bearing or Y-bearing unit can be selected from the product tables.

The bearing or Y-bearing unit selected should have a C0 value that is equal to or greater than the requisite value.

MaintenanceSKF bearings and Y-bearing units for high temperatures are lubricated for life and are therefore without any relubrication facility. The open deep groove ball bearings of the VA201 design however should be inspected after some six months of operation. It is sufficient to open the housing or in case of kiln trucks to withdraw the wheel with the bearing from the journal and to remove contaminants using bellows.

If there is no longer a film of dry lubricant on the raceways, indicated by a bright metallic shiny track, the bearing should be re-lubricated using the original black, high-temperature paste, which is a mixture of polyalkylene glycol and graphite.


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This page is still under construction. All bearing information remains the property and copyright of SKF Group.