The conditions for the planetary wheels are characterised by heavy radial load from the forces of two meshes as well as by the infuence of radial accelera- tions and the mass inertia forces result- ing from these. Bearings having high radial load carrying capacity are need- ed, and their cages should be able to endure the mass forces.
An internal bearing arrangement is suitable for the planetary wheels as it takes up the least space. This means rotating load for the outer ring and point load on the inner ring. Thus, the outer rings must have interference fits and the seatings must be accurately machined in order to keep the rotating inaccuracy – which leads to increased
friction in the bearings and additional forces on the cages – to be kept as small as possible.
The specifically heavy radial loads, the rotating outer rings, and not least, the mass inertia forces cause high fric- tion. Therefore special demands are placed on the lubrication and cooling of the planetary wheel bearings.
The least space is taken up by a needle roller and cage assembly as shown in fig . This very cost-favour- able bearing arrangement is very suitable for small units (up to approx- imately 50 mm between shafts) as well as for light loads or short periods of operation as, for example, with small lifting gear.
The pins and bores of the planetary wheels serve as bearing raceways. Recommendations regarding raceway hardness and design are given in the section ”Recommended fits” (➔ page 106). The planetary wheel is axially guided by thrust washers. These are secured on the planetary carrier so that they cannot turn.
The bearing arrangement shown in fig with two cylindrical roller bear- ings of the NJ design offers the advant- ages of very high radial load carrying capacity and very high accuracy as well as high rupture strength in respect of the cage forces if window-type cages are used.
The planetary wheel is guided axi- ally by the flanges of the cylindrical rol- ler bearings. To prevent the bearings from being axially clamped, the inter- mediate ring on the pin should be at least 1 mm wider than the retaining ring in the bore of the wheel.
Even though the two cylindrical roller bearings are virtually immediately ad- jacent to each other, it is not necessary to resort to special bearings for paired mounting (DR execution). Modern manufacturing methods mean that standard bearings differ only slightly in their cross section (bore and outside diameters+ internal clearance) from each other. When using two bearings per wheel the deviation will, at the most, cause a slight angular misalign- ment which is largely compensated for by deformation, so that any effect on the mesh or the load carrying capacity of the bearings is negligible.
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To achieve the maximum load carry- ing capacity in the limited space, the bearing outer rings can be dispensed with, as shown in fig . Cylindrical roller bearings of the RN design are used. The wheel is guided axially by the flange rings and the inner ring flanges. The dimensions of the rings are not standardised and should be agreed with the bearing manufacturer. Recommendations regarding design of the raceways in the wheel bore will be found in the section “Recommended fits” (➔ page 106).
Another way to increase load carry- ing capacity is to use full complement cylindrical roller bearings as shown in fig . In this case, a special double row bearing without outer ring is used.
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The design provides very high load carrying capacity in a small space. However, full complement cylindrical roller bearings cause more friction and are susceptible to wear. They are not suitable for high normal accelerations. Therefore, this bearing arrangement is more appropriate for short-term opera- tion, also with heavy load shocks, rather than for constant operation. A typical application area is that of mobile gear units.
The use of a spherical roller bearing to support a planetary wheel, as shown in fig , allows the wheel to adjust to the mesh. When the planetary carriers deform, so that the overhung pins become misaligned, the mesh is im- proved by the use of a self-aligning
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Bearing arrange- ment for a plan- etary wheel with two cylindrical roller bearings
Bearing arrange- ment for a plan- etary wheel with two cylindrical roller bearings without outer ring
Bearing arrange- ment for a plane- tary wheel with one spherical roller bearing Bearing arrange-
ment for a plan- etary wheel with a double row full complement cylindrical roller bearing without
Planetary wheel bearing arrange- ment with one CARB
bearing arrangement, when compared to a rigid bearing arrangement incorp- orating more than one bearing. The advantage of this self-alignment can also be exploited at high speeds and correspondingly small tooth forces, as there is not much deformation in the tooth contact and the mesh will be good. The easy adjustment of the mesh is also an advantage when the wheels are wide. The smaller theoretical load carrying capacity of the single spher- ical roller bearing as compared with rigid arrangements where two or more bearings are used is partly compens- ated for by the even distribution of load over the two rows of rollers.
Because of its exceptionally high load carrying capacity compared with other roller bearings and its low cross section, the CARB is eminently suit- able for planetary gear bearing ar- rangements (➔ fig ). Its insensitivity to angular misalignment is particularly important for correct meshing in this case. The planetary wheel can align itself so that even meshing is obtained across the whole tooth width. The favourable distributon of the tooth forces thus obtained has a positive influence on the life of the gearbox.
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