1、05FTM18Planet Pac: Increasing Epicyclic PowerDensity and Performance ThroughIntegrationby: D.R. Lucas, The Timken CorporationTECHNICAL PAPERAmerican Gear Manufacturers AssociationPlanet Pac: Increasing Epicyclic Power Density andPerformance through IntegrationDouglas R. Lucas, The Timken Corporation
2、The statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractEpicyclical gear systems are typically equipped with straddle-mounted planetary idlers and are supported bypins on t
3、he input and output sides of a carrier. These carriers can be either one-piece or two-piece carrierdesigns. Traditionally many of the higher power rated epicyclic gear systems use cylindrical roller bearings tosupport the planetary gears. This paper will demonstrate that using a preloaded taper roll
4、er bearing in anintegrated package should be the preferred choice for this application to increase the bearing capacity, powerdensity, and fatigue life performance. Based on DIN281-4 calculations, this patented 1, fully integratedsolution allows for calculated bearing fatigue lives to be 5 times gre
5、ater than a non-integrated solution andmore than 1.5 times greater than a semi-integrated solution, without changing the planet gear envelope.Copyright 2005American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2005ISBN: 1-55589-866-11 Planet Pac:
6、Increasing Epicyclic Power Density And Performance through Integration Douglas R. Lucas: Principal Application Engineer The Timken Corporation Introduction - Design Challenges of an Epicyclic Gear System An epicyclical gearing system is particularly well suited for achieving a high reduction ratio i
7、n a relatively small, power dense package. A typical straddle mounted planetary idler utilizing cylindrical roller bearings (CRB) is shown in Figure 1. Because of the power density, epicyclic systems have become a popular choice for designers. They have consequently been incorporated into countless
8、types of machines including automobile transmissions, off-highway equipment final drives, wind turbine gearboxes, and cement mill crusher drives, to name a few. As with any type of power transmission system, an engineer is faced with many analytical challenges during the design phase to ensure that
9、a highly reliable power train is achieved. In the case of an epicyclical gearing system, this challenge is made particularly difficult due to the complex interaction of revolving and rotating components as they transmit power. Figure 1 - Conventional Planetary Idler But, for just about all equipment
10、 types, economics dictate the need for increased power density and improved reliability. A Planet Pac can be used to further utilize an existing design and provide increased reliability, while minimizing the expense of design changes. On the other hand, power density also allows for a design to be m
11、ade smaller, thus reducing cost and weight. What Is Planet Pac? Traditionally high power planetary gear sets are composed of a gear, two outer races, two inner races, two rows of rolling elements, two cages, and a carrier pin. Some applications may also include spacers or snap rings. Most times the
12、gear lives are calculated using a 99% reliability or L1fatigue life; whereas the bearings that support the gears are calculated using a 90% reliability, an L10fatigue life. Some gearbox manufacturers have implemented semi-integrated bearings into their planetary systems. The semi-integrated solution
13、, as shown in Figure 2, involves integrating the bearing outer races into the gear. There are design guidelines that define recommended gear rim thicknesses as a ratio of the gear module, i.e. three times the gear module. By making this change, the bearing pitch diameters and roller diameters can be
14、 increased, thus increasing the bearing capacity and resulting fatigue performance. Figure 2 - Semi-Integrated Planet Pac 2 Substantial improvements can be made by taking advantage of modern bearing technology and further integration of components to achieve an extremely power dense design. This int
15、egration, as shown in Figure 3, may include full integration of the gears with the bearing outer races and full integration of the carrier pin with the bearing inner races. A full complement of rollers is used in combination with a proprietary coating, ES300, to prevent metal adhesion from roller to
16、 roller. This advancement is the Planet Pac. This specific design requires a two-piece carrier design that must be bolted together for assembly of the planetary system. This design eliminates two outer races, two inner races, a spacer, and four surfaces interfaces. Eliminating the interfaces will el
17、iminate potential precessing of the races, fretting corrosion, and loss of bearing setting which then leads to premature bearing failure. There is an additional rib ring that is required to control the bearing setting and unitize the package product. Additional features like force oil lubrication ca
18、n be added to the package. Figure 3 Two-Piece Carrier, Fully-Integrated Planet Pac Some gearbox manufacturers may use a one-piece carrier design. This carrier design uses similar carrier plates as the two-piece carrier design, however they are not separable. The gears must be slid between the two fi
19、xed carrier plates. A pin is then located through the bearing inner races to hold the bearings and gears in place. In this case the Planet Pac, as shown in Figure 4, cannot utilize the inner races integrated into the pin. However, the inner races can be integrated together, a rib ring added for sett
20、ing, and a full complement of rollers with ES300 coating can be used to increase the power density of the bearing assembly. Figure 4 One-Piece Carrier, Integrated Planet Pac Integration allows for increased bearing capacity by adding additional rolling elements to the bearing, increasing the size of
21、 the rolling element, and/or increasing the bearing pitch diameter. This novel approach to design and construction provides increased opportunity to add power density to an epicyclical gear drive in the axial and radial directions. By integrating the inner races into the shaft, the shaft sections wi
22、ll increase under the bearing rolling elements, resulting in decreased beam stresses. The pin diameter into the carrier plate can be maintained at the same size and the bending and shear stresses will not increase. However the integration will introduce a square corner at the pin-carrier interface.
23、This will require a stress concentration factor analysis, per the ANSI/AGMA 6001-D97 2 and if necessary, a pin diameter adjustment. As mentioned previously, both of these designs utilize a detachable large rib ring on the inner race, as shown to the right of the right row of rollers (see Figure 3 an
24、d Figure 4). For tapered roller bearings (TRB) this rib ring would be used to very accurately define the bearing setting at the bearing factory. This eliminates the need for the epicyclic gearbox manufacturer to preload the bearings, one reason that many gearbox manufacturers use CRB or spherical ro
25、ller bearings (SRB). A combination of press fitting and welding may be used to ensure adequate holding force and durability at loads significantly in excess of the maximum applied loads. The ES300 is a proprietary diamond-like coating (DLC) that may be used to prevent adhesive wear in a full complem
26、ent bearing. Testing has shown that this coating is beneficial when used in a full 3 complement bearing. Additionally, during conditions where there is an interruption in lubrication supply, the bearings have been made more forgiving because the coatings have been effective at eliminating adhesive w
27、ear between the rollers and the races. Preloaded Tapered Roller Bearing Preferred Bearing Choice for the Planet Pac Although the Planet Pac could be manufactured utilizing the CRB, needle roller bearings, or other bearing types, the preloaded TRB is deemed to be the bearing of choice for a Planet Pa
28、c. It can be shown analytically that a preloaded tapered roller bearing offers advantages over other bearing types that possess larger amounts of radial clearance, especially when the bearing is required to operate with misaligned gear contacts. This has been demonstrated in a paper written by Flama
29、ng and Clement 3. One key to increased power density is improved load distributions in the bearings and gear. It was shown in this paper that the tapered roller bearing is the preferred solution to an equivalent CRB design. The TRB design resulted in lower stresses and more than doubles the expected
30、 fatigue life because of the TRBs wider support distance, preload, self-centering, and self-adjusting effects. This paper does not focus on the details of showing that the TRB is the best bearing, as this was already shown by the Flamang and Clement paper. It merely builds on what others have alread
31、y demonstrated. Comparisons of Various Levels of Integration A comparison of various levels of integration was performed to show the power density that can be incrementally obtained by increased integration. It is assumed that the carrier is a split, two-piece carrier. The material used for the bear
32、ings was assumed to be constant and an ISO 281 material factor equal to 1.1 was applied. The preload force in the bearings was constant, regardless of the bearing ratings. The speed and bearing cleanliness, ISO -/15/12, were also constant throughout the analysis. Finally, it was assumed that the pla
33、netary gear envelope could not be any larger than the non-integrated design. The gear loads that were used in the analysis were defined by a gearbox manufacturer. It is well known that offset loading will create an uneven load distribution on the bearings. For this analysis, the resultant gear loads
34、 assumed that the loading was not centered. Figure 5 shows that the loads were applied in with a “cross-over“ effect which induces an over-turning moment on the bearings. This loading condition has been found to be the most aggressive loading to increase the loads on the bearing and reduce the predi
35、cted bearing fatigue lives. The torsional wind up of the planetary carrier creates misalignment of the gear contact and the planetary bearing axis. While lead correction on the gear face helps correct the effect on the gear contact, it does not correct the misalignment on the bearing. Likewise, whil
36、e profiling the gear face helps minimize edge loading at the tooth contact, the contact itself drifts from side to side depending on the gear manufacturers tolerances and process controls. A modest off-center gear mesh condition can redistribute loading unequally to the bearing rows, significantly r
37、educing the life of the heavier loaded row. Unfortunately, the misalignment values and contact drifts discussed here are not usually communicated to or accounted for by the bearing supplier. Through closer interaction between the bearing and gear drive manufacturers, a more thorough analysis of the
38、gear and bearing system can account for these phenomena and others like “gear wrap“. Figure 5 - Planet with Overturning Moment The first step was to evaluate a non-integrated design. This analysis was performed using an ISO tapered roller bearing (32248). This is really the base line to which all ot
39、her designs are compared. This is the basic design as most manufacturers employ in their gearbox designs. 4 As shown in Figure 6, the outer races are pressed in to the gear. For wind turbine applications, the gear rim thickness was equated to 3 modules per the AGMA6006-A03 4. Figure 6 Non-Integrated
40、 Cross-Section The next step was to integrate the bearing outer races into the bore of the planet gear, as shown in Figure 7. The integration was performed to maintain the rim thickness, as in the non-integrated design. Since the inner race surface is tapered, the mean cup race diameter was used for
41、 calculating the rim thickness. This allows for the roller and bearing pitch diameters and the roller lengths to increase. All of this increases the bearing rating, power density, and predicted L10life. Figure 7 Semi-Integrated Cross-Section The final step was to integrate the bearing inner races in
42、to the planet shaft (see Figure 8). For this, the bearing mean outer race diameter did not increase. This step introduces a full complement of rollers with ES300 coatings, even larger roller diameters, and similar roller lengths compared to the semi-integrated design. The bearing pitch diameter is s
43、lightly larger than the non-integrated design, but is smaller than the semi-integrated design. Figure 8 Fully-Integrated Cross-Section A comparison of the non-integrated and the semi-integrated designs can be seen in Figure 9. The non-integrated solution is shaded and the semi-integrated design is o
44、utlined. It is easy to see that the semi-integrated design will be capable of achieving an increased fatigue life. Figure 9 Comparison of Non-Integrated and Semi-Integrated Cross-Sections A comparison of the fully-integrated and the semi-integrated designs can be seen in Figure 10. The semi-integrat
45、ed solution is shaded and the fully-integrated design is outlined. You can see that the angle of the inner race was reduced, the roller diameter was increased and the length was kept relatively constant. 5 Figure 10 Comparison of Semi-Integrated and Fully-Integrated Cross-Sections Table 1 below show
46、s a ratio comparison of the bearing ratings, bearing pitch diameters, and the mean outer race diameters for the three incremental designs. The purpose of this table is to show how the bearing ratings will increase with increased integration. Simultaneously the bearing pitch diameters increase, thoug
47、h the outer race diameter may not. ISO C1 Rating Pitch Dia. Mean Cup Dia.Non-Integrated 1.00 1.00 1.00Semi-Integrated 1.26 1.08 1.07Fully-Integrated 1.52 1.03 1.06Table 1: Bearing Size Ratios The bearing designs were compared by several life analysis methods, which included a basic (non-adjusted) IS
48、O L10life calculation, the DIN 281-4 stress adjusted L10life 5, and the DIN 281-4 fully adjusted L10life. In these comparisons the kappa viscosity ratio, , and the contaminations factor, c, were nearly identical because the same ISO bearing cleanliness and lubrication was used. Because the bearings
49、were all of similar size, the assumed surface finishes were similar, per the DIN 281-4. As seen in Table 2, the basic catalog life and DIN281-4 stress adjusted lives increase proportionately the same. When all of the adjustments are applied, there is a significant increase in bearing life by partially integrating the bearing; however the maximum potential of the bearings is not reached without going to a fully-integrated solution. ISO Basic CatalogDIN 281-4 (Stress Adjusted)DIN 281-4 L10 (Fully Adjusted)Non-Integrated 1.00 1.00 1.00Semi-Integrated 2.
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