Rolling bearings in
industrial gearboxes
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1
Industrial gearboxes – overview
2
Bearing types for industrial gearboxes
3
Design of bearing arrangements
4
Dimensioning the bearing arrangement
5
Lubrication and maintenance
6
Recommended fits
7
Mounting and dismounting bearings
8
Application examples
1
2
3
4
5
6
7
8
Rolling bearings in
industrial gearboxes
Foreword
This Handbook is intended to provide the gearbox designer
with the knowledge required to select bearings for gearboxes
and to correctly design gearbox bearing arrangements.
Recom-mendations are given based on experience gained by SKF
during decades of cooperation with gearbox manufacturers the
world over.
General information regarding the selection, calculation,
mounting and maintenance of ball and roller bearings is given
in the SKF General Catalogue. The questions arising from the
use of rolling bearings in industrial gearboxes are dealt with
here. Data from the General Catalogue are only repeated here
when it has been thought necessary for the sake of clarity.
The application examples described comprise proven
gearbox designs from major manufacturers which are worthy
of note.
Grateful thanks are extended to the companies concerned
for the provision of the detailed information about their
prod-ucts and the permission to publish.
1
Industrial gearboxes – overview ... 9Types of gearbox ... 9
Geared transmissions... 10
Demands made on gearboxes ... 14
Selecting the gears ... 14
Designing the casing ... 15
2
Bearing types for industrial gearboxes ... 17Deep groove ball bearings ... 18
Angular contact ball bearings ... 20
Cylindrical roller bearings ... 22
CARB™ roller bearings ... 24
Spherical roller bearings ... 26
Taper roller bearings ... 28
Spherical roller thrust bearings ... 30
3
Design of bearing arrangements... 33Shafts and gear wheels in spur gearboxes ... 33
Shafts in bevel gearboxes ... 44
Shafts in worm gearboxes... 50
Shafts and gear wheels for planetary gearboxes... 56 Made by SKF®stands for excellence. It symbolises
our consistent endeavour to achieve total quality in everything we do. For those who use our products, “Made by SKF” implies three main benefits.
Reliability – thanks to modern, efficient products, based on our worldwide application know-how, optimised materials, forward-looking designs and the most advanced production techniques.
Cost effectiveness – resulting from the favourable ratio between our product quality plus service facilities, and the purchase price of the product.
Market lead – which you can achieve by taking advantage of our products and services. Increased operating time and reduced down-time, as well as improved output and product quality are the key to a successful partnership.
4
Calculation of bearing arrangement ... 65Bearing loads ... 65
Determination of external forces ... 66
Calculation of bearing loads ... 74
Dimensioning the bearing arrangement ... 76
5
Lubrication and maintenance... 91Grease lubrication... 92
Oil lubrication ... 95
Maintenance ... 98
6
Recommended fits ...1037
Mounting and dismounting bearings ... 109Adjustment of angular contact bearings... 109
SKF is an international industrial Group operating in some 130 countries and is world leader in bearings.
The company was founded in 1907 following the invention of the self-align-ing ball bearself-align-ing by Sven Wself-align-ingquist and, after only a few years, SKF began to expand all over the world.
Today, SKF has some 43 000 em-ployees and more than 80 manufactur-ing facilities spread throughout the world. An international sales network includes a large number of sales com-panies and some 20 000 distributors and retailers. Worldwide availability of SKF products is supported by a com-prehensive technical advisory service.
The key to success has been a con-sistent emphasis on maintaining the
highest quality of its products and services. Continuous investment in research and development has also played a vital role, resulting in many examples of epoch-making innovations.
The business of the Group consists of bearings, seals, special steel and a comprehensive range of other high-tech industrial components. The ex-perience gained in these various fields provides SKF with the essential know-ledge and expertise required in order to provide the customers with the most advanced engineering products and efficient service.
The SKF Group
– a worldwide corporation
SKF manufactures ball bearings, roller bearings and plain bearings. The smal-lest are just a few millimetres (a frac-tion of an inch) in diameter, the largest several metres. In order to protect the bearings effectively against the ingress of contamination and the escape of lubricant, SKF also manufactures oil and bearing seals. SKF's subsidiaries CR and RFT S.p.A. are among the world's largest pro-ducers of seals.
but the thinking is green. The latest example is the new factory in Malaysia, where the bearing component cleaning process con-forms to the strictest ecological standards. Instead of trichloroethylene, a water-based cleaning fluid is used in a closed system. The cleaning fluid is recycled in the factory's own treatment plant.
SKF has developed the Channel concept in factories all over the world. This drastically reduces the lead time from raw material to end product as well as work in progress and finished goods in stock. The concept enables faster and smoother information flow, eliminates bottlenecks and bypasses unnecessary steps in production. The Channel team members have the know-ledge and commitment needed to share the responsibility for fulfilling objectives in areas such as quality, delivery time, production flow etc.
The SKF Engineering & Research Centre is situated just outside Utrecht in The Netherlands. In an area of 17 000 square metres (185 000 sq.ft) some 150 scientists, engineers and support staff are engaged in the further improvement of bearing perform-ance. They are developing technologies aimed at achieving better materials, better designs, better lubricants and better seals – together leading to an even better unders-tanding of the operation of a bearing in its application. This is also where the SKF New Life Theory was evolved, enabling the design of bearings which are even more compact and offer even longer operational life.
– overview
Types of gearbox . . . 9 Geared transmission . . . 10 Demands on gearboxes . . . 14 Selecting the gears . . . 14 Designing the casing . . . 15
1
Industrial gearboxes –
overview
Gearboxes are devices for the transmission or
translation of movement. In industry gearboxes
are used to transform the speeds and torques
produced by the prime mover in order that
they are appropriate to the machine which is to
be driven. The speeds and torques required by
the machine are dictated by its use. Prime
movers can generally only meet these
require-ments when combined with gears.
Types of gearbox
Gearboxes are characterised by having at least three members: the power in-put, power take-off and the casing. The casing transmits the support moment to the base.
In contrast, a coupling has only two members: the power input and power Types of gearbox P1 M1 PV n1 P v
take-off. The coupling housing has no part in the flow of force.
The symbols used for power transmission by gearboxes and coup-lings are shown in figs 1 and 2.
Fig 1 Fig 2 Gear Torque M1< M> 2 Rotational speed n1>≤ n2 Coupling Torque M1 = M2 Rotational speed n1≥ n2 (with slip)
Geared transmissions
Geared transmissions are the most commonly used. They transmit power without slip, have high operational re-liability and long life, require little main-tenance and are characterised by the ability to accept overloading, small size and high efficiency.
Spur gears
The spur gear is the most well-known and commonly used design of geared transmission. The dimensioning and manufacture of the gear wheels are the easiest to control. Their kinematic behaviour also forms the basis of plan-etary gears. Spur gears are in rolling contact and, irrespective of tooth type, have parallel axes.
Geared transmissions
The main types of power transmis-sion equipment are shown in the following.
In addition, there are many com-binations, for ex-ample bevel/spur gears, spur gears with belt drive input, or variable traction drives combined with a planetary gear.
Types of gearbox
Fixed ratio transmissions, shift transmission Geared transmissions • Spur gears • Planetary gears • Bevel gears • Worm gears • Hypoid gears • Helical gears Eccentric drives • Cyclo drives • Harmonic drives Traction drives • Belt drives • Chain drives Infinitely variable transmissions Mechanical transmissions • Belt drives • Roller drives • Ratchet gears Hydraulic transmissions • Hydrostatic transmissions • Hydrodyanmic transmissions
● Gear wheels with straight cut teeth (➔ fig a) are simple in design and can be accurately produced. The axial forces generated by in-accuracies and deformations (twisting) are negligible.
● Gear wheels with helical teeth (➔ fig b) run more smoothly and can carry heavier loads than those with straight cut teeth. A more elab-orate bearing arrangement is re-quired because of the axial forces.
● The double helix or herringbone (➔ fig c) allows for large tooth widths and can carry particularly heavy loads. The axial forces cancel each other out. Deviations in the helix angle cause axial vibrations.
3 3 3
● Internal gearing (➔ fig d) has greater load carrying capacity than external because of the favourable osculation, but is more difficult to produce. The bearing arrangement is more complicated. The most fre-quent use is in planetary gears.
Bevel gears
The common characteristic of this type of rolling contact gearing is that the axes of the wheels intersect each other. There are three basic designs categorised by the form of the flank.
● With straight cut teeth (➔ fig a), the mesh begins and ends across the total tooth width. The noise pro-duced considerably limits the use-fulness of straight cut bevel gears.
● Bevel gears with helical teeth (➔ fig b) have straight flanks. The teeth are usually ground and the mesh is gradual. The total over-lap is bigger and the noise behav-iour better than with straight cut teeth.
4
4
3 ● Bevel gears having spirally cut teeth (➔ fig c) with curved flanks have clear advantages in respect of load carrying capacity. Particularly those with ground teeth are quieter than the types described above. For bevel gears which have to transmit high power, the spiral bevel gears are the most frequently used.
4
Spur gear unit
a) straight cut teeth b) helical teeth c) double helix d) internal gearing
1
Geared transmissions Fig 3 Fig 4 a b c dBevel gear unit
a) straight cut teeth b) helical teeth c) spirally cut teeth
Geared transmissions
Worm gears
The worm and wheel axes cross each other at a considerable distance and usually at an angle < 90° (➔ fig ). Worm gears are suitable for large single stage speed reduction. Their operation is quiet and vibration damp-ing. The efficiency is lower than that of competing bevel/spur and planetary gears, because of the higher propor-tion of sliding mopropor-tion. To reduce the friction, the use of synthetic lubricants is favoured.
The most commonly used design is the cylindrical worm paired with a glob-oid wheel (➔ fig a). The cylindrical worm can be hardened and ground which improves load carrying capacity; it is also freely adjustable in the axial direction so that bearing arrangement and mounting can be simplified. Two other designs – globoid worm with spur wheel (➔ fig b) and globoid worm with globoid wheel (➔ fig c) – are also used.
Depending on the flank form, the worm types are classified as follows:
● ZA worm: trapezoidal worm thread in the axial cross section;
● ZN worm: trapezoidal worm thread in the normal cross section;
● ZK worm; trapezoidal tool (in normal cross section);
● ZI worm; evolvent thread in end face cross section;
● ZC worm: concave worm flanks
6 6
6
6
Hypoid gears
The pinion axis is displaced so that the axes of this type of bevel gear do not intersect but are crossed (➔ fig ).
The wheels of hypoid gears are usu-ally spirusu-ally cut. The advantages of this type of gear derive from the larger pin-ion and thus the smaller circumferential force for the same torque, as well as from the axis displacement which often allows the pinion to be supported at both sides so that the bearing arrange-ment is stiffer. The noise behaviour is also improved by the sliding motion in the longitudinal direction of the teeth. However, the additional sliding motion increases the friction, wear and risk of smearing and requires the use of hypoid oils with high additive content.
5
Fig 5
Hypoid gear unit
Fig 6
a b c
Worm gear unit
a) cylindrical worm with globoid wheel b) globoid worm
with spur wheel c) globoid worm
with globoid wheel
H
S P
Z
The ZI and ZC designs are the most popular. The ZI worm can be very ac-curately ground whilst the favourable osculation conditions of the ZC worm (concave worm, convex wheel) bring load carrying advantages.
Planetary gears
From the point of view of the tooth flanks, planetary gears are mostly spur gears. In contrast to the spur gear units so far described, the shafts of which are supported in stationary casings, the planetary gear unit has gear wheels which circulate. They are also referred to as epicyclic gears.
In the simplest design (➔ fig ), which is that most commonly used in industry, the sun wheel drives the plan-etary wheels (when acting as a speed reducer). These are supported in the hollow wheel and drive the planetary carrier from which the power is taken off.
Planetary gears have the following important advantages compared with conventional spur gear units:
● the volume, weight and centrifugal mass are smaller;
● the rolling and sliding velocities in the mesh are lower, so that noise is reduced;
● some of the power is transmitted as coupling power, so that efficiency is higher.
These advantages have led to a continuous increase in the economic importance of planetary gear units in spite of their disadvantages which include more difficult inspection, main-tenance and repairs.
7
Geared transmissions
Simple planetary gear unit (prin-ciple) Z sun wheel P planetary wheel H hollow wheel S planetary carrier
1
Fig 7Demands made on
gearboxes
The most important demands which must be fulfilled are:
● there must be a sufficient safety margin in respect of fatigue and/or requisite life for all components so that the torques and speeds can be reliably transmitted;
● there must be sufficient cooling even under maximum power transmission conditions;
● noise emission should not exceed the permitted limits.
In addition to these demands, special requirements in respect of operation and design are dictated by the various applications. Some examples:
● radial and/or axial forces on the in-put and outin-put shafts, e.g. for ex-truders;
● external forces on the casing, e.g. in mining;
● heavy impacts, torque peaks, e.g. when driven by single cylinder com-bustion engines or when driving bucket excavators;
● vibrations, e.g. in wire drawing;
● extreme environmental influences in respect of temperature, dirt, dust, water, e.g. in arctic or tropical open cast mining and in continuous cast-ing plant;
● seals subjected to pressure, e.g. in submerged gearboxes of dredgers or in mixing equipment in the chemi-cal industry;
● reversing operation, e.g. for rolling mills;
● return stop, e.g. for conveyors;
● operation with little or no clearance and torsional stiffness, e.g. for posi-tioning antennae and for robots;
● precision, e.g. for printing presses;
● lubrication with non-flammable lub-ricants, e.g. in mining;
● minimum maintenance, e.g. in wind power plant;
● arrangement, e.g. slip-on gears for converters;
● accessibility of measuring points to monitor lubrication, temperature, vibrations or torque, e.g. for large plastic extruders.
Selecting the gears
To avoid either under or over-dimen-sioning a gear unit the load and the load carrying capacity of the gear must be able to be determined as accurately and reliably as possible. The size is correctly chosen when a comparison of the load spectrum and the load carrying capacity gives the desired service life. The determination of the load spectrum is a time-consuming and costly exercise calling for con-siderable measurements. Therefore, dimensioning is usually based on the rated torque of the driven machine, i.e. the operating torque for the most arduous work conditions. For a rolling mill, for example, this is the maximum continuous rolling torque (not the initial entry). The actual loads are higher because of additional external forces, produced by accelerations and vibra-tions, for example. When calculating the load carrying capacity of the gear wheels, these additional loads are considered by an application factor KA according to DIN 3990.
One standard work on the subject lists the following criteria for evaluating the load carrying capacity of gear whe-els:
● resistance to pitting (tooth flank fatigue),
● root strength (tooth fracture from fatigue),
● resistance to scuffing (hot tooth flank welding),
● wear strength (slow wear of tooth flanks),
● “grey spot” resistance (fatigue from micro pores on the tooth flanks, and
● lubricant film formation.
The load carrying capacity which is used as the basis for dimensioning gear wheels is determined in rig tests under standard conditions (partly stand-ardised: FZG test to DIN 51 354). Demands made on gearboxes/Selecting the gears
Designing the casing
The following functions have a de-cisive influence on the design of the casing:
● forces and supporting moments must be taken up and transmiitted at the same time as the position of the gear wheels and the form of the bearing seatings must be accurately maintained;
● there must be adequate heat removal;
● noise radiation must be at a min-imum;
● gear wheels and bearings must be protected against contamination by foreign matter;
● lubricant loss must be prevented.
The increase in load carrying capacity of gear wheels and rolling bearings resulting from design improvements, improved materials and enhanced quality has enabled gearboxes to be downsized or uprated. The higher specific loads, frictional losses and in-creased noise resulting from this trend mean that the casings must be more stable so as to keep deformations to a minimum, but also that they should have a sufficiently large surface to pre-vent inadmissible heating and prema-ture lubricant ageing, and should be properly designed with respect to mini-mising noise so as not to exceed the noise emission limits.
Designing tha casing
gearboxes
Deep groove ball bearings . 18 Angular contact ball
bearings . . . 20 Cylindrical roller bearings . . 22 CARB™ roller bearings . . . . 24 Spherical roller bearings . . . 26 Taper roller bearings . . . 28 Spherical roller thrust
For the support of the shafts and gear wheels of
industrial gearboxes, rolling bearings are used
almost exclusively. The exceptions are in some
specialised areas, such as turbo drives, where
hydrodynamic plain bearings are used.
There are many good reasons for this dominance of rolling bearings:
● good location with minimum radial and axial play enables optimum meshing to be achieved;
● high specific load carrying capacity with low friction;
● wide range of internationally stand-ardised products produced in high volumes at reasonable prices and having good availability;
● can be calculated using reliable load carrying capacity values;
● little design work for the user;
● simple arrangement;
● axially compact so that short and stiff shafts can be used;
● normal tolerances and surface fin-ishes for shaft and housing seatings;
● less sensitive to misalignment than plain bearings;
● ability of radial bearings to accept axial loads;
● not influenced by direction of load or rotation;
Almost all bearing types are used in industrial gearboxes and almost all the available sizes. In the majority of appli-cations, standard “catalogue” bearings can be used; any variants with respect to clearance or cage design are also generally common, so that the com-prehensive range of SKF “catalogue” bearings for general engineering appli-cations covers the needs of gearboxes very well and enables the designer to make an optimum selection. The most important bearing types for gearboxes are described in more detail in the following.
Bearing types for
industrial gearboxes
Deep groove ball bearings
Deep groove ball
bearings
Deep groove ball bearings are the most popular of all bearing types and this also applies for gearboxes. The most important characteristics which make them so popular are
● they are able to carry radial loads as well as axial loads acting in both directions;
● they are suitable for high and very high speed operation as their friction is low;
● they have practically no tendency to smear, i.e. cold welding when the balls are accelerated;
● they run quietly, particularly if they are lightly preloaded by axial force;
● they are robust in operation and require little maintenance;
● they are favourably priced.
The dominant role for deep groove ball bearings is where shafts have to be located axially and loads are relatively light. This is the case in
● spur gear units (drive shaft and hol-low take-off shaft),
● multi-ratio gear units (switching spur gear wheels),
● geared motors
● worm gear units (worm wheels),
● planetary gears (drive shaft, planet-ary carrier) and
● coupling shafts.
These improvements also bring ad-vantages when the bearings are used in gearboxes. In particular the reduced sensitivity to misalignment means that there is no reduction in bearing life under the slight misalignments of up to approximately 3 minutes of arc which are normally encountered. The im-proved surfaces reduce friction lead-ing to lower runnlead-ing temperatures so that lubrication conditions are im-proved and bearing life extended.
2
Benefits offered
by SKF In recent years SKF has made a number of improvements to deep groove ball bearings which have resulted in further performance enhancements. The more import-ant include
● optimised raceway geometry and finish, reducing friction, run-ning noise and sensitivity to misalignment;
Angular contact ball
bearings
The raceways of these bearings are arranged at an angle to the bearing axis (contact angle), so that they are able to carry heavier axial loads than deep groove ball bearings. Sliding movements of the balls are superim-posed on their rolling motion, so that the single row bearings require ac-curate adjustment or a minimum axial load to function properly.
Angular contact ball bearings are available in the following designs:
● single row, single direction angular contact ball bearings,
● double row, double direction and paired single row angular contact ball bearings and
● four-point contact ball bearings, i.e. single row, double direction ball bearings.
Single direction implies that axial loads acting in one direction only can be accommodated, whereas double direction bearings (and paired single direction bearings, depending on the arrangement) can take axial loads acting in both directions.
The single and double row angular contact ball bearings are preferred as locating bearings for worm shafts. Four-point contact ball bearings are used primarily as thrust bearings in high speed spur gear units, where the outer ring is radially free.
The improvements made by SKF to single and double row angular contact ball bearings, e.g. reinfor-cement of the ball set (single row – BE design, double row – A and E designs) to give higher load carry-ing capacity means that worm gear units can transmit more power and, at the same time, the reduc-tion in fricreduc-tion means that bearing temperature can be lowered. The reduced tolerances for axial clear-ance and for dimensional and run-ning accuracy which are standard for SKF single row angular contact ball bearings for paired mounting of the CB design, because of the improved location and reduced running noise, are advantageous in low-noise worm gear units such as those required for lifts and escalators.
2
Angular contact ball bearingsBenefits offered by SKF
Cylindrical roller bearings
Cylindrical roller
bearings
The special properties of cylindrical roller bearings make them a popular choice for gearboxes and include:
● high radial load carrying capacity;
● low friction – the lowest of any roller bearing under purely radial load;
● suitable for a wide range of operating speeds, including very high speeds, as the cage has the correct combina-tion of roller guidance, strength and sliding friction properties;
● ability to accommodate moderate axial loads, when they are simulta-neously under radial load, via the slid-ing surfaces of the roller
end/flange contact, although the inc-reased
friction means that lubrication and cooling must be adapted to the conditions;
● the ease with which lateral displace-ment can take place within the bear-ing makes them ideal as non-locat-ing bearnon-locat-ings;
● proven good performance under external radial accelerations;
● most designs are separable so that mounting and dismounting are simple.
These characteristics make cylindrical roller bearings ideal for the following applications:
● as the non-locating bearings of all high-performance units; the NU design with its flangeless inner ring is perhaps the most used, but also the NJ, NJG and NCF find applica-tion; the rings of these bearings need only be axially located at one side, and by mounting the rings with relative axial displacement the bear-ings can accommodate lateral displacement in both directions.
● in spur gear units, even where com-bined radial and axial loads are
pro-Practically all improvements made to cylindrical roller bearings by SKF could be considered as tailored to gearbox needs, so that they make an appreciable contribution to in-creased performance. The main characteristics are
● the reinforced roller complements and “opened” flanges of the EC design give increased radial and axial load carrying capacity;
● the logarithmic roller profile en-sures an optimum stress distribu-tion over the whole roller length so that edge stresses are avoid-ed even under heavy loads and the permissible misalignments;
● the refined raceway micro-geo-metry reduces friction and im-proves lubricant film formation;
● newly developed cages ensure proper bearing function over the increased performance range; the standard polyamide cages (designation suffix P) of small bearings have low friction, are elastic and have good sliding properties;
the steel window-type cages (designation suffix J) which are standard for medium-sized bear-ings and can also be fitted to the smaller sizes (to special order) withstand high temperatures and also medium to strong vib-rations;
the machined brass cages (for gearbox bearings preferably out-er ring centred and in two parts, designation suffix MA, or in one piece, suffix MP or ML) are stan-dard for large bearings and can be fitted to other sizes to special order; they can tolerate high speeds and are resistant to vib-rations and accelevib-rations.
2
Benefits offered by SKF
CARB™ roller bearings
CARB is a completely new type of bearing: a Compact Aligning Roller Bearing. This single row roller bearing, developed by SKF, is characterised by a combination of properties which make it interesting for a multitude of applications:
● the ability to compensate for angular misalignments or initial errors of alignment typical of spherical roller bearings;
● the ability to take up axial displace-ments in the bearing itself typical of cylindrical roller bearings;
● the low cross section typical of needle roller bearings;
● the high radial load carrying capacity imparted by long sphered rollers;
● the low friction obtained from optim-ally matched raceway profiles;
● the quietness of operation.
Because of its many advantages, the CARB makes an ideal non-locating bearing. The points in favour of its use in industrial gearboxes include, in addi-tion its compact design and high radial load carrying capacity even when misaligned, the potential for downsiz-ing or increasdownsiz-ing operational reliability or the power rating. The CARB is par-ticularly suitable for the bearing arrange-ments of
● heavily loaded shafts in spur gearboxes,
● pinion shafts in bevel gearboxes, and
● planetary gears.
Two versions of CARB are available: a bearing with cage and a full comple-ment bearing.
SKF has introduced a completely new roller bearing, the CARB. It is the only bearing available which combines the advantages of three different bearing types without, at the same time, incorporating their disadvantages. For gearbox ap-plications, these advantages trans-late into the following opportunit-ies for enhanced performance.
● Up to 30 % higher load carrying capacity at the bearing position combined with small radial space requirements
● The low cross section allows downsizing or increased per-formance
● Compensation for errors of po-sition and also form of bearing seatings in housings thus allow-ing machinallow-ing costs to be reduced
● Both bearing rings can be mounted with an interference fit so that there will be no wear in the bore and no additional axial loads under conditions of axial displacement
● Quiet running and little vibration
Benefits offered by SKF
2
CARB™ roller bearingsSpherical roller bearings
The self-aligning capability (also in operation) of spherical roller bearings makes their use advantageous where shaft bending occurs or where there are errors of alignment between shaft and housing (casing). They are there-fore used in all cases where misalign-ment of the bearing rings would pro-duce inadmissible edge stresses if rigid bearings were used. Additional import-ant characteristics make the spherical roller bearing a reliable “all-rounder” for gearbox applications. These include
● the high radial load carrying capacity and the ability to accommodate axial loads acting in both directions;
● the wide range of dimension series and very wide range of sizes
● even very large sizes.
The many successful development refinements and the improved charac-teristics resulting from them explain the popularity of spherical roller bear-ings for gearboxes (particularly in spur, bevel and planetary gear units).
The design and functional charac-teristics substantiate the leading position of SKF spherical roller bearings:
● long, symmetrical rollers give very high load carrying capacity;
● the “floating” guide ring between the rows of rollers ensures that the rollers are properly guided (without “wobble”) into the load-ed zone and, in cases where axial loads predominate, that the load is correctly carried by the rollers and symmetrically distrib-uted over the roller length;
● the special form and optimum surface finish of the raceways minimise friction and operating temperature enabling high speed operation;
● the latest development – the E design – has even higher load carrying capacity as the bearing section is more efficiently ex-ploited;
● the position of the guide ring above the pitch diameter in the E design favours lubricant film formation between the rollers and guide ring;
● all SKF spherical roller bearings are fitted with robust metallic cages which perform well even under arduous conditions.
2
Spherical roller bearingsBenefits offered by SKF
Taper roller bearings
The tapered form of the raceways makes these bearings eminently suit-able for combined radial and axial loads. There is a choice of contact angles so that the appropriate bearing for the particular combination of radial and axial loads can be found. The necessity for functional reasons to use two bearings adjusted against each other enables the force distribution on the rollers to be controlled so that maxi-mum life can be obtained at the same time as the stiffness and guidance of gear shafts can be optimised. The main gearbox applications are
● spur gear units with helical teeth,
● bevel and bevel/spur units and
● worm gear units.
As taper roller bearings can support very heavy loads, they are always used when the load carrying capacity of other bearings for combined load conditions (deep groove and angular contact ball bearings) is inadequate.
Because the raceways are at an angle to the bearing axis, an internal axial force is produced when the bear-ing is radially loaded, which acts on the housing via the outer ring and can deform it. With larger units (from approximately 90 mm shaft diameter) and specifically high performance requirements, the casing walls are often not sufficiently stiff, so that the use of double row or paired single row taper roller bearings (or spherical roller bearings) is recommended, because the internal axial forces cancel out each other and the casing walls will not be deformed.
Paired single row taper roller bear-ings in a face-to-face arrangement (designation suffix DF) are always used when the preset axial play can be ex-ploited and when adjustment during mounting is to be avoided.
SKF taper roller bearings have a number of advantages which make them suitable for industrial gear-boxes. These include
● the ideal form and optimum finish of the roller end/guide flange contact enable hydrody-namic lubrication to be achieved and mixed lubrication conditions avoided, so that the critical run-ning-in process normally re-quired when commissioning a gearbox is not needed;
● the logarithmic raceway profiles guarantee optimum stress dis-tribution over the whole roller length and prevent edge stresses;
● the improved surface topography of the raceways enhances lubric-ant film formation and reduces bearing noise.
2
Taper roller bearingsBenefits offered by SKF
machine, e.g. in extruder gearing and water turbine gearboxes. The bearings are used successfully as thrust bear-ings for the pinion and worm shafts of large and very heavily loaded bevel and worm gear units.
Spherical roller thrust
bearings
The special feature of these bearings is their self-aligning capability. This means that their full load carrying capacity can be utilised, in contrast to the very stiff cylindrical roller thrust bearings, even when the bearing washers are slightly out of alignment with each other. The even distribution of load is still maintained when there are small angular misalignments of the seating surfaces. Such misalignments would considerably shorten the life of cylindrical roller thrust bearings.
Spherical roller thrust bearings are used in gearboxes, particularly where axial forces are produced by the driven Spherical roller thrust bearings
SKF spherical roller thrust bearings have particularly low friction thanks to the special roller end/flange contact geometry. Benefits offered by SKF Marine gearbox with spherical roller bearings, cylindrical roller bearings, four-point contact ball bearings and spherical roller thrust bearings
arrangements
Shafts and gear wheels in spur gearboxes . . . 33 Shafts in bevel gearboxes . . 44 Shafts in worm gearboxes . 50 Shafts and gear wheels for planetary gearboxes . . . 56
It is quite possible that several different bearing
types are used in one gearbox, and where
com-bined gear units are concerned, there are
sev-eral types of gearing. A stepwise approach is
therefore appropriate when selecting bearings,
taking each shaft in turn so that the different
conditions for the individual shafts and gear
wheels can be fully considered. The bearing
arrangements described in the following are
well proven and the conditions specific to a
certain shaft are covered. A presentation of the
most commonly used bearing series facilitates
the initial selection.
Design of bearing
arrangements
Shafts and gear wheels
in spur gearboxes
Spur gearboxes are generally used to reduce speed. There are three main types which differ in the way they are
The drive from the prime mover is via a coupling or a belt. The drive is transmitted to the driven machine via a coupling, a quill shaft connection or via a pinion.
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Shafts and gear wheels in spur gearboxeslems of high angular accelerations when starting without load as well as operation without load at maximum speed in order to prevent bearing dam-age caused by the rolling elements sliding on the raceways. There is a danger of this occurring when loads are suddenly applied. The temperature differences and the associated thermal expansions in the radial and axial directions are high for input shafts, as the speed related large power loss and relatively small masses as well as the relatively small surface of the pinion shaft mean that there is insufficient heat removal. The distance between bearings is dictated by the casing and the low torque often means that slim shafts are used. This means that shaft
bending and bearing misalignment must be taken into account, particularly if a belt drive is used.
Two deep groove ball bearings arranged for cross location (➔ fig ) provide a cost-favourable bearing ar-rangement for moderate power require-ments. Deep groove ball bearings are suitable for high-speed operation. Be-cause of the low friction, small quanti-ties of oil are adequate for lubrication and cooling so that the collected oil splashed by the gear wheels dipping into the oil bath is generally sufficient.
In order to prevent axial clamping of the bearings being caused by thermal expansion of the shaft there should be sufficient axial clearance between the outer ring and the cover.
For shaft diameters of up to approx-imately 90 mm, two taper roller bear-ings arranged face-to-face (➔ fig ) are advantageous both from technical and cost considerations. The taper roller bearings are adjusted against each other via the cover so that they will have zero clearance when at the operating temperature or, for reasons of stiffness, they may have a slight pre-load. When determining the initial axial clearance it is necessary not only to consider the temperature differential between shaft and casing but also the deformation of the shaft and, above all, the casing. The casings of larger units are often not stiff enough with respect to the axial forces (tooth force + in-ternal axial forces in the bearings). In such cases bearing adjustment is
dif-2 1
Bearing arrange-ment for an input shaft with two cross-located deep groove ball bearings
Bearing arrange-ment for an input shaft with two cylindrical roller bearings Bearing
arrange-ment for an input shaft with two taper roller bear-ings arranged face-to-face
ficult and shaft guidance is not suffici-ent-ly accurate. The taper roller bear-ing arrangement shown is, therefore, not always suitable.
Cylindrical roller bearings (➔ fig ) have a high radial stiffness and guide the shaft very accurately without having to be adjusted as taper roller bearings. Axial forces are transmitted via the flanges and roller ends. Because this causes more frictional heat, lubrication and cooling must be particularly good.
In order to prevent axial clamping of the bearings when thermal expansion of the shaft takes place, there should be adequate axial play between the flanges.
The classical locating/non-locating arrangement (➔ fig ) is more com-plicated from a design point of view than the cross-located arrangements described above, as the inner and outer rings must be axially located at both sides. However, it has advant-ages with regard to dimensioning as the axial force is always taken up by a given bearing – in this case the spher-ical roller bearing – irrespective of the direction of the load. Additionally, displacement of the non-locating bear-ing is always assured so that there is no risk of axial clamping occurring when the shaft expands.
Two NU-design cylindrical roller bear-ings as radial bearbear-ings together with a four-point contact ball bearing as the thrust bearing (➔ fig ) have proved suitable for very high-speed operation (up to n × dm≈ 1 000 000). For such
5 4
3
high-speed operation the bearings must have
● machined brass cages, centred in the outer ring,
● increased internal clearance: C3 for the cylindrical roller bearings and C4 for the four-point contact ball bearing, and
● seatings having increased accuracy of form and position (IT4/2).
At high circumferential speeds the bearings will reject normal oil supplies. Therefore, it is necessary to inject oil at high speed (v ≈ 15 m/s) into the gap between cage and inner ring. Oil drainage facilities should be provided at the injection side of the bearings.
3
Shafts and gear wheels in spur gearboxesBearing arrange-ment for an input shaft with two cylindrical roller bearings as the radial bearings and a four-point contact ball bear-ing as the thrust bearing Classic locating/non-locating bearing arrangement with a spherical roller bearing and a cylindrical roller bearing
other bearing types or arrangements which are less unfavourable in respect of casing deformation.
In comparison with input shafts, the axial loading of cylindrical roller bearings used to support intermediate shafts (➔ fig ) is less critical. The axial forces at the gears act in oppos-ite directions and cancel each other out, at least partially, so that the axial load on the bearings is light. Also the speeds are lower so that frictional losses deriving from the axial load remain small.
The high radial load carrying capa-city of the cylindrical roller bearings is an advantage as the intermediate shaft bearings are heavily loaded. The choice between caged or full complement cylindrical roller bearings is determined primarily by the factors load, speed, lubrication conditions, friction and cost.
Compared with the input shaft, there is only a small temperature gradient between the intermediate shaft and the casing. This makes it possible to use spherical roller bearings in a cross-located arrangement as shown in fig which is simple in design and therefore cost-favourable.
There is a wide range of spherical roller bearings available, particularly for medium and large shaft diameters, and there is also a choice of several cross sections for each diameter. It is thus possible to easily find bearings which can support the heavy loads acting on the intermediate shaft but
8
7
Intermediate shafts
Intermediate shafts are the most heav-ily loaded as they are subjected to the forces from two gear meshes. The speeds are moderate. The axial forces on pinion and wheel oppose each other when the direction of the teeth is the same so that they partially balance each other. There are no additional external forces but vibrations may be transmitted from the input or output shafts. As there is no torque acting at the shaft ends, reasonably small dia-meters can be used enabling a rela-tively large bearing section to be util-ised for the accommodation of the high radial forces. Design limits for the bearing outside diameter are set by the distance between input and output shafts.
When using taper roller bearings (➔ fig ) it should be remembered that axial forces are produced even though the load is purely radial. This may lead to axial deformation of the casing. These deformations occur in the central, less stiff region of the casing because of the position of the intermediate shaft, and are larger than for the input shaft. They lead to a change in position of the shaft and can therefore cause inadmissibly high mis-alignment of the bearings and the mesh.
Experience shows that the casing deformations occurring in smaller units with shaft diameters up to 90 mm are generally within acceptable limits. For larger units it is necessary to resort to
6
Bearing arrange-ment for an inter-mediate shaft with two taper roller bearings arranged face-to-face
Bearing arrange-ment for an inter-mediate shaft with two cylindrical roller bearings
which have outside diameters within the limits set by the distance between the shafts.
A locating/non-locating bearing arrangement as per fig with a spherical roller bearing at the locating side and a CARB as the non-locating bearing offers the possibility of reduc-ing the cross section of the non-locat-ing bearnon-locat-ing arrangement, because of the high load carrying capacity of the CARB, so that the available space can be better exploited. In many applica-tions there is a risk that the bearing seating in the housing will be ”ham-mered out” so that an intermediate sleeve must be incorporated. By using a CARB bearing this is no longer a problem as the outer ring is mounted with an interference fit in the housing, so that a sleeve is not needed.
9
3
Bearing arrange-ment for an inter-mediate shaft with two spherical roller bearings
Bearing arrange-ment for an inter-mediate shaft with one spherical roller bearing (locating) and one CARB (non-locating bearing)
Shafts and gear wheels in spur gearboxes
(EHD) lubrication, i.e. the formation of a separating lubricant film between rolling elements and raceways, cannot be achieved. Operating bearings under conditions of mixed friction or boundary lubrication will result in wear and shorter bearing life. Besides rota-tional speed, operating temperature and lubricant viscosity are the most important factors determining EHD lubrication.
There is a limit to how high the viscosity of the oil can be because consideration must be paid to the high-speed gears and bearings in the unit. Therefore, a cooling of the gear-box in the region where the low-speed bearings of the drive shaft are situated is often the most effective means of increasing bearing life. Suitable ad-ditives in the oil can also contribute to a reduction in wear.
Other factors influencing drive shaft bearings depend on the gearbox design:
● In stationary, base-mounted gear-boxes, depending on the type of power take-off, it is necessary to consider the forces of the coupling, the propeller shaft, a pinion or of the directly coupled driven machine (e.g. extruders).
Bearing arrange-ment for an output shaft with two spherical roller bearings Locating/non-locating bearing arrangement for an intermediate shaft with two matched single row taper roller bearings and one cylindrical roller bearing
The locating/non-locating arrange-ment shown in fig can carry very heavy radial as well as axial loads. Two matched single row taper roller bearings (DF execution) are used for the locating arrangement. In contrast to the cross-located bearing arrange-ments shown in figs 2 and 6, the inter-nal axial forces of the taper roller bear-ings compensate each other within the bearing pair and do not deform the casing. The intermediate ring supplied with the bearing pair ensures that there is a minimum axial clearance within the bearings. This is adequate for temper-ature differentials between shaft and casing of up to 20 °C. To avoid deform-ation of the thin-walled inner ring as the cover screws are tightened, the length of the centring surface (spigot) of the cover should be chosen to give a preload of approximately 0,01 mm.
Drive (output) shafts
The conditions for the drive shafts are characterised by high torques and low speeds. The torque calls for a large shaft diameter so that the requisite load carrying capacity can be obtained even when using bearings with low cross sections. There are potential problems with lubrication of the rolling contacts if, because of the low speeds, elastohydrodynamic
10
● The bearings in cartridge-type gear-boxes are subjected to the reac-tionary forces of the torque support. Additional forces may also be pro-duced as a result of casing deforma-tion.
● The casings of flanged gearboxes are bolted to the driven machine. The shafts are generally rigidly coupled so that the double support of the output shaft becomes a mul-tiple support in practice. Centring errors of the coupled components produce additional forces in the bearings so that narrower tolerances for the centring should ensure the accuracy of alignment of the bearing arrangement.
The arrangement with spherical roller bearings (➔ fig ) is especially suit-able for applications where rough operation, external additional forces, misalignments and shock loads place heightened demands on the bearings. Axial shock loads are taken up by the less sensitive raceways in the absence of flanges on the rings.
For cartidge-type gearboxes, the relatively large diameters of the hollow shaft mean that bearings having low cross section are suitable. Fig shows a well-proven bearing arrange-ment incorporating full complearrange-ment
12 11
cylindrical roller bearings of series NCF 29 V. For lighter loads but with similar diameters, deep groove ball bearings of series 619 can be used in the same arrangement. For heavier loads as well as larger deformations, but still with the same diameters and arrangement, spherical roller bearings of series 239 are appropriate. Deep groove ball and spherical roller bear-ings have cages and are thus less susceptible to wear when inade-quately lubricated than full comple-ment bearings.
Intermediate gear wheels
An internal bearing arrangement is most suitable for intermediate gears as it takes up the least space. Internal bearing arrangements are character-ised by rotating outer rings. Therefore, there is rotating outer ring load and stationary inner ring load. This means that the outer rings should have inter-ference fits and the seatings should be very accurately machined in order to keep the rotating inaccuracies – which cause increased friction and additional forces on the bearing cage – to a mini-mum.With opposing meshes the circum-ferential forces are added, so that high radial load carrying capacity is re-quired. The axial forces from the
3
Bearing arrange-ment for an output shaft of a cart-ridge-type unit with full comple-ment cylindrical roller bearings of series NCF 29 V
Bearing arrange-ment for an inter-mediate gear wheel with two cylindrical roller bearings of the NJ design
the inner ring) oil should be supplied at the side. To prevent the supplied oil from being rejected by the bearing, the seal gap at the supply side should not exceed 1 mm.
Shifting gear wheels
For reasons of space these gear whe-els are supported internally in a similar manner to the intermediate gears. The torque is transmitted in the engaged condition so that the bearings are sub-jected to the tooth forces. The inner and outer rings rotate but the relative speed is zero. Both rings have rotating load but the rolling elements do not roll. The continuous changes in load under these stationary conditions cause micro-sliding to take place at the rolling element/raceway contacts. As there is no relative rotation of the rings, a ”washboarding” type of wear will be produced in the raceways. This wear can be reduced by using highly viscous lubricating oil containing anti-wear additives.
Where the wheels have helical teeth, the axial force produces a tilting moment and consequently a rotating tilting motion which leads to axial move-ment in the rolling elemove-ment/raceway contacts. This increases wear. Ball bearings, adjusted to zero clearance, behave favourably as the balls can helical teeth oppose each other and
partially cancel each other producing a tilting moment on the bearing which can cause misalignment.
Two cylindrical roller bearings of the NJ design provide the requisite high radial load carrying capacity in a re-stricted space as shown in fig . The design of the associated components of the arrangement is simple. The bearing arrangement of helical intermediate gear wheels must be checked for angu-lar misalignment. An unfavourable com-bination of wheel diameter, pitch and distance between bearings can produce inadmissible values of misalignment. An extended support width (distance between bearing pressure centres) can be achieved using, for example, angular contact ball bearings.
Taper roller bearings in a back-to-back arrangement (➔ fig ) also inc-rease the support width as well as reducing the influence of the tilitng moment on the misalignment if they are adjusted to zero clearance, or a light preload.
Straight cut gear wheels may be supported by a single spherical roller bearing (➔ fig ). The intermediate gear wheels are thus free to align so that a good mesh is achieved.
In order to be able to use standard bearings (without lubrication holes in
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14 13
Shafts and gear wheels in spur gearboxes
Bearing arrange-ment for an inter-mediate gear wheel with two taper roller bear-ings arranged back-to-back
Bearing arrange-ment for an inter-mediate gear wheel with a single spherical roller bearing
also roll in the axial direction and be-cause the movement is reduced by the clearance-free adjustment. Wear is always load-dependent so that bear-ings under low specific loads wear less. The washboarding effect is also less prominent as engagement always takes place at new positions so that the wear is evenly spread over the raceway.
For the support of shifting wheels, deep groove ball bearings have proved to give good performance (➔ fig ). Bearings with increased radial internal clearance (C3) are used. The clear-ance-free adjustment via the inner rings produces a contact angle in the bear-ings of approximately 15°, so that the support width of the bearings is ex-tended. This reduces movement in the relatively stationary bearings under rotating load and thus reduces wear. In addition, the clearance-free back-to-back arrangement improves guidance of the wheel.
Lubrication of the bearings from the outside is difficult as all components of the arrangement – shaft, bearings and wheel – rotate and because the bear-ings are partly covered e.g. by the coupling. The most reliable method is to supply oil internally through the shaft.
16
3
Shafts and gear wheels in spur gearboxesBearing arrange-ment for shifting gear wheel with two deep groove ball bearings
each particular bearing position must be considered. To make the situation clearer in Tables to , the text has been kept as short as possible.
4 2
Demands on the bearings
Modern spur gears generally have hardened gear wheels with ground teeth. It is then possible to obtain high performance with relatively little friction and low noise. A prerequisite for this is the use of high-performance bearings, which should have the properties listed in Table .In addition to these general require-ments with respect to ball and roller bearings for high-performance gear-boxes, other demands deriving from the specific operating conditions at
1
Shafts and gear wheels in spur gearboxes
Demand Required bearing design feature
High load carrying capacity Optimised rolling element size and number. Logarithmic roller/raceway contact.
Good lubricant film formation through low friction and low raceway surface roughness.
High stiffness Optimised rolling element size and number. Logarithmic roller/raceway contact.
High dimensional and running accuracy Particularly the inner ring running accuracy should preferably be to tolerance class P6 or better. Low friction Low friction in roller end/flange contact for taper and
cylindrical roller bearings.
Low friction in roller/raceway contact. Lightweight precision cage.
Low raceway surface roughness. Low running noise High precision of all bearing components.
Specific operating conditions Requirements of bearings/steps to guarantee performance
High speed and thus high friction Use low-friction bearings. and high operating temperature Avoid over-dimensioning.
Ensure lubricant supply when starting up cold.
Provide good cooling.
Large temperature differential when Check required bearing internal clearance; if necessary starting up (slim input shaft heats up select bearings with C3 clearance.
more quickly than the better cooled Ensure axial displacement at non-locating bearing position. solid casing)
Vibration from drive; imbalance Use bearings with stable cages, e.g. cylindrical roller bearings forces with steel window-type cages or outer ring centred machined
cages, or spherical roller bearings with steel window-type cages. Idling under light load Check minimum load. Avoid over-dimensioning.
Use bearings with small roller masses where possible. Do not use full complement cylindrical roller bearings. Choose bearing types less susceptible to smearing, e.g. spherical and taper roller bearings.
Demands on rolling bearings for spur gears
Demands on input shaft bearings
Table 1
Specific Requirements of bearings/steps operating conditions to guarantee performance
Heavy radial loads Use bearings with high load carrying capacity.
Low to Check lubricant film formation. If necessary increase viscosity or moderate speeds improve cooling. Use lubricants with wear-reducing additives.
Bearing selection
The following check list will be found useful when selecting bearings in order not to forget any important factors.
● Adjusted basic rating life
● Axial load carrying capacity when the flanges of cylindrical roller bear-ings are under load
● Friction
● Stiffness
● Misalignment
● Sufficient play to prevent inadmiss-ible clamping when temperature differentials are large
● Minimum load
● Static safety under peak loads
A preliminary choice can be made from the bearing series shown in Table 5.
Demands on inter-mediate shaft bearings Demands on output shaft bearings Bearing selection
3
Shafts and gear wheels in spur gearboxesTable 3
Specific Requirements of bearings/steps operating conditions to guarantee performance
Very low speeds When lubricant film formation inadequate, i.e. a viscosity ratio (actual to required viscosity) κ < 1, use lubricants with suitable EP additives.
Whenκ < 0,5 bearings with cages (not full complement bearings) must be used. Whenκ < 0,1 reduce the specific bearing load;
aim for s0> 10.
Shock loads from power Use robust, self-aligning, spherical roller bearings. take-off;deformations
Operating conditions Bearing series normally used
Input shaft Intermediate Output Intermediate Shifting
shaft shaft gears gears
Light loads 62 63 619 60 618/C3 63 NJ 2 EC 160 62 619/C3 60 Moderate loads NJ 2 EC NJ 22 EC NCF 29 V NJ 2 EC 160/C3 320 X 322 239 CC 320 X 60/C3 222 E(CC) 222 E(CC) Table 4 Table 5
Shafts in bevel
gearboxes
Bevel gears are generally speed reduction gears. The high-speed drive shaft is termed the pinion shaft and the slow-speed driven shaft carries the larger bevel gear wheel.
The pinion shaft is driven by the motor via a coupling, a spur gear or a belt drive. The power take-off is either via a coupling or with bevel/spur gears via a pinion.
Pinion shafts
The pinion is generally supported in an overhung arrangement. In a few cases the pinion is supported between the bearings but it is difficult to design in a bearing with sufficiently high load car-rying capacity at the head. The over-hung arrangement offers more space.
Two taper roller bearings in a back-to-back arrangement as shown in fig offer a cost-favourable and axi-ally as well as radiaxi-ally stiff arrangement for small to medium diameter shafts (d < 90 mm). The bearings are adjusted using a shim between the shaft shoul-der and the inner ring of the bearing at the input side. The adjustment is deter-mined to give zero clearance when the bearings are in operation and warm or, if required for stiffness reasons, a slight axial prelod. When determining the initial axial clearance the tempera-ture differential between shaft and casing must be considered as well as the deformations of shaft and casing.
17
Oil should be supplied between the two bearings. A baffle plate ensures that both bearings are reliably supplied with lubricant. The oil drain at the cover side reduces the amount of lubricant reaching the seal.
Bearing arrange-ment for a bevel pinion shaft with two taper roller bearings arranged back-to-back
For larger shafts, the requisite load carrying capacity can be achieved using a locating/non-locating bearing arrange-ment as shown in fig . The locating arrangement is at the drive side and consists of two matched single row taper roller bearings (DF execution). The intermediate ring which is supplied with the bearing pair ensures that a minimum axial clearance remains when the bearings are mounted which can cope with temperature differentials be-tween shaft and casing of up to 20 °C. For greater temperature differentials such as may occur, for example, in operation when ambient temperatures are very low, paired bearings with larger axial clearance are required (special execution). In order not to deform the thin-walled intermediate ring when tightening the cover screws, the length of the centring flange (spigot) on the cover should be such that a pre-load corresponding to approximately 0,01 mm is obtained.
The matched taper roller bearings operate as a double row bearing. As the axial load from the pinion domin-ates, one of the two bearings – de-pending on the direction of the load – is completely unloaded. Experience shows that this is not a disadvantage when there is little vibration.
The non-locating bearing adjacent to the bevel pinion may be either a spher-ical roller bearing, a cylindrspher-ical roller bearing or a CARB.
18
For one-piece casings, spherical rol-ler bearings offer mounting advantages and they are also relatively insensitive to smearing when loads vary consider-ably and there are long periods of idling. If cylindrical roller bearings are used, the requisite axial displacement can always take place in the bearing itself so that the outer ring can have an interference fit in the housing, and radial guidance is enhanced. The same is true of CARB (➔ fig ). At this position the bearing will not only enable the axial displacements to be easily accommodated, it will also accept the angular misalignments caused by the off-centre point of action of the tooth forces with no reduction in life.
Oil should be supplied to the two taper roller bearings between the outer rings. Experience shows that for small and medium-sized gears (up to approx-imately d = 150 mm) the non-locating bearing can be adequately lubricated by the oil returning from the locating bearings. For larger gears, however, it is necessary to arrange for a separate oil supply to the non-locating bearing. For spherical roller bearings, the oil should be supplied via the lubrication groove and holes in the outer ring for the best results.
19
Bearing arrange-ment for a bevel pinion shaft with two matched single row taper roller bearings arranged face-to-face (locating position) and one spherical roller bearing (non-locating position) Bearing arrange-ment for a bevel pinion shaft with two single row taper roller bear-ings arranged back-to-back (locating) and one CARB (non-locating bearing)