3.2 Elastic Net
3.3.3 Coordinate Descent
To achieve equal power splitting in planetary gear units, it is possible to avoid the need for an additional bearing support for the planetary carrier if the following conditions apply:
● the planetary carrier is not subjected to load from the output shaft or the torque support;
● the weight of the planetary carrier is negligible.
The planetary carrier centres itself under load via the planetary wheel meshes.
Fig shows a gearbox where the planetary carrier of the high-speed stage is not supported by bearings. It
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centres itself via the planetary wheels in the hollow shaft (housing) and on the supported sun wheel. This multiple centring is only possible if manufactur-ing precision is adequate.
Planetary gearbox of cartridge type with two deep groove ball bear-ings supporting the planetary carrier
Shafts and gear wheels for planetary gearboxes
The casing is supported by the plan-etary carrier of the slow-speed stage.
The two deep groove ball bearings in a cross-located arrangement are under load from the restoring force of the torque support and from the weight of the gearbox. The resultant bearing forces are generally small and the rotational speed low so that the load carrying capacity of deep groove ball bearings is usually sufficient.
The planetary carrier with take-off shaft shown in fig is supported by two full complement cylindrical roller bearings. This arrangement enables additional forces from the power take-off to be accommodated.
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3
Shafts and gear wheels for planetary gearboxes
Bearing arrange-ment for planetary carrier with two full complement cylindrical roller bearings
Demands on the rolling bearings
The special requirements placed on the bearings for planetary gearboxes are derived from the particular condi-tions pertaining at the various bearing positions. A brief summary is given in Tables to .12 14
Bearing selection
The following list may be found useful to check that the chosen bearings satisfy the demands.
● Adjusted rating life
● Permissible radial acceleration
● Permissible speed
● Friction and cooling
● Adequate bearing play to prevent inadmissible preload at the maxi-mum operating temperature (sun wheel) or under interference fits (planetary wheel)
Demands on sun wheel bearings
Demands on planetary wheel bearings
Shafts and gear wheels for planetary gearboxes
Specific operating conditions Requirements of bearings/steps to guarantee performance
Light loads; idling Use of deep groove ball bearings preferred to avoid over-dimensioning.
Requirements for clearance-free Adjust deep groove ball bearings axially by springs.
operation and quiet running
Large temperature differentials Particularly where casings are solid and/or well cooled use when starting up (slim sun wheel shaft deep groove ball bearings with internal clearance to C3.
heats up more quickly than the casing which is better cooled)
Table 12
Specific operating conditions Requirements of bearings/steps to guarantee performance
Heavy radial loads Use roller bearings with high load carrying capacity. If lubricant film formation is also inadequate, corresponding to a viscosity ratio (actual to required) of κ < 1 use lubricants with suitable EP additives.
Whenκ < 0,5 only use bearings with cages (not full complement bearings).
Whenκ < 0,1 reduce the specific bearing load; aim for s0> 10.
Radial accelerations resulting from move- Check cage stresses by calculating mass inertia forces.
ment of the planetary wheels around the Pay consideration to mass inertia forces of planetary wheel axis of rotation of the sun wheel when calculating bearing life.
Increased friction caused by mass Ensure adequate lubricant supply and cooling.
intertia forces and rotating outer Use heat-stable lubricants.
rings (rotating inaccuracy) For gearboxes which continuously, or frequently (high frequency of use) operate at high tempeatures (> 80 °C) and which should also have long service life (> 20 000 hours) bearings with metallic cages should be used.
Deformation of planetary wheel by For thin-walled planetary wheels (wall thickness < 3 × modulus) two meshes on opposite sides take into account the influence of the tension band load distribution
on the loaded zone of the bearing (FEM calculation).
Table 13
Operating Bearing series normally used
Planetary wheels Sun wheels Planetary carriers
Low radial accelerations or NJ 23 ECP 60, 62, 63 618, 619
short operation periods NCF 30 V NCF 18 V, NCF 29 V
NJG 23 VH 239 CC
230 CC 232 CC 223 E(CC)
Moderate radial accelerations NJ 3 ECMA 60, 62, 63 618, 619
and continuous operation NJ 23 ECMA NCF 18 V, NCF 29 V
230 CC 239 CC
232 CC 223 E(CC)
High radial accelerations NJ 2 ECML 60, 62, 63 618, 619
NJ 3 ECML NCF 18 V, NCF 29 V
NJ 23 ECML 239 CC
223 CCJA/VA405
Specific operating conditions Requirements of bearings/steps to guarantee performance
Very slow speeds with additional Use preferably bearings with small cross section.
loads from the drive When κ < 1 use lubricants with suitable EP additives.
Whenκ < 0,5 only use bearings with cages (not full complement bearings).
Whenκ < 0,1 reduce the specific bearing load; aim for s0> 10.
● Deformation of planetary wheel when wall thickness small; influence on the load distribution in the bearing
● Static load safety in respect of load shocks
A preliminary bearing selection can be made by referring to the most fre-quently used bearing series listed in Table 15.
Bearing selection Demands on planetary carrier bearings
3
Shafts and gear wheels for planetary gearboxes
Table 15 Table 14
arrangements
Bearing loads . . . 65 Determination of external forces . . . 66 Calculation of bearing
loads . . . 74 Dimensioning the bearing arrangement . . . 76
Life calculation . . . 76 Static safety factor . . . 79 Axial load carrying
capacity . . . 79 Minimum load . . . 80 Normal acceleration and cage load carrying capacity 80 Friction and cooling . . . 81 Permissible speeds . . . 82 Internal clearance
and preload . . . 83 Adjustment values for
single row angular
contact bearings . . . 85