1、99FTM13 Failures of Bevel-Helical Gear Units on Traveling Bridge Cranes by: J.M. Escanaverino, ISPJAE American Gear Manufacturers Association I I TECHNICAL PAPER COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicesa Failures of Bevel-Helical Gear Units on
2、Traveling Bridge Cranes Jose Martinez Escanaverino, ISP JAE The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract Many bevel-gear units are employed in traveling drive
3、s of large industrial bridge cranes all over the world, as they form compact packages with couplings, brakes and electric motors other gear units do not allow for. In a number of drives, frequent failures of the bevel gear pose a difficult problem for maintenance, and have raised unclosed discussion
4、s about the necessary service or application factor. According to some standards, these drives require gear unit service factors as high as3.0. This paper focuses on the origin of the troubles, with an insight on the dynamics of the gear train. Attention is also given, in such failures, to the influ
5、ence of variable-speed controllers of electric motors. The theoretical analysis is backed by real industrial case studies, taken from the authors recent experiences. Copyright O 1999 American Gear Manufacturers Association 1500 kng Street, Suite 201 Alexandria, Virginia, 22314 October. 1999 ISBN: 1-
6、55589-750-9 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesFAILURES OF BNEL-HELICAL GEAR UNITS ON TRAVEUNG BRIDGE CRANES Jose Martinez Escanaverino, Professor Mechanical Engineering Faculty, ISPJAE Havana 19390, Cuba. INTRODUCTION Bridge cranes are a
7、mong the most useful machines in many branches of modern industry. Using standard hooks or other specialized clamping devices, they can lift, transport, discharge, and stack a variety of loads. Gear technology progress has been always influential to advances in bridge crane design, allowing lighter
8、and more productive cranes. Many bevel-helical gear units are employed in traveling drives of big industrial bridge cranes, as they form compact packages with couplings, brakes and electric motors other gear units do not allow for. 3 /2 /6 I O 1 5 7 Figure 1: Typical bridge crane traveling drive A s
9、ketch of a typical individual wheel traveling drive is shown in Figure 1, where position 1 is the electric prime mover, and position 2 is the gear unit, usually with a hollow low-speed shaft 3. The gear unit is mounted on a floating base 4, common with the 0- prime mover. All the aggregate pivots on
10、 the low- speed shaft and is vertically fixed, at the other end, to the crane framework by means of elastic blocks 5. The flexible coupling between prime mover main shaft and gear unit high-speed shaft usually combines with a drum brake 6. In a number of cranes, frequent failures of travel bevel gea
11、rs pose a difficult problem for maintenance, as a source of downtime. Besides, undosed discussions have raised about the necessary service or application factor to avoid such failures. Recommendations found in prestigious sources give application factor values from as low as 1.1 to as high as 3.0. I
12、n many gear unit catalogs, crane traveling drive selection refers to the manufacturer, giving no other guidance to crane designers or plant maintenance engineers. This paper focuses on the origin of troubles with standard general purpose bevel-helical gear units used in traveling drives of medium an
13、d large size bridge cranes, according to the authors theoretical research and practical experience. NATURE OF THE FAILURES Usually, failures of crane traveling drives are of a catastrophic character, with sudden fracture of one or several teeth in the bevel gear, ordinaily the high-speed stage in th
14、e gear unit. The above mentioned failures are very difficult to anticipate, because time between failures 2 behaves chaotically. Sometimes the gear works well during a relatively long period, in the order of several weeks, and sometimes the gear breaks down after a few minutes of work. I COPYRIGHT A
15、merican Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesSuch an irregular pattern of failure is usually associated with mechanical resonance. But even a detailed analysis of the bevel gear vibration behavior in crane bridge traveling gear units generally shows no resonan
16、ce at all in the gear mesh. This fact may be highly misleading to an engineering researcher trying to find the origin of the above mentioned troubles. However, the bevel gear mesh is not the sole elastoinertial system related to the high-speed stage of the gear unit. Most important, according to our
17、 findings, is the elastoinertial system comprised of gear units high-speed shaft and half-coupling, including the brake drum, Figure 2. /Haff-owpling with brake drum igure 2: Elastoinertial system comprised o high-speed shaft and half-coupling For the sake of brevity, the elastoinertial system const
18、ituted by gear units high-speed shaft and half- coupling with brake drum is referred to from now on as the shats/coupling system. In fact, almost all the torsional elastic compliance cs of the shaft/coupling system belongs to the shaft, due to the much bigger diameter and shorter axial length of the
19、 half-coupling with its attached brake drum. Therefore, it can be easily shown that .S = .vh +hc .$h (1) Where c, is shafts elastic compliance. chr is half-couplings elastic compliance. All elastic compliances in (1) and after are given in rad/(Nm), according to the International System of units, SI
20、. On the other hand, almost ail the moment of inertia 1, of the shaft/coupling system relative to its O rotational axis belongs to the half-coupling with its attached brake drum. This is due to the very small diameter of the shaft compared with the brake drum. Therefore, it can be easily shown that
21、I, = I, CJ, Proper frequency in (3) and after is qiven in Hz, e according to the Intemahona1 System ofunits, SI. The resistive torque at gear units high-speed pinion, Figure 2, has a pulsation with a frequency equal to the mesh freguency of the high-speed gear, given by the relation (4) Where n, is
22、prime movers rotational frequency. z, is high speed pinions number of teeth. Both mesh and rotational frequencies in (4) and after are given in Hz, according to the International System of units, SI. Meanwhile, the number of teeth is considered non-dimensional. Under the excitation of the pulsating
23、pinion torque, the shafl/coupling system develops torsional vibrations, superimposed to the otherwise smooth velocity profile of gear unit high-speed shaft. The severity of such torsional vibrations is higher when the mesh frequency of the bevel gear approaches the,proper frequency of the shaWcoupli
24、ng system. 2 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesThe degree of mutual approach of the above mentioned frequencies governing the vibratory process can be quantified by the tuning hctor 0 7 fz Az =- I- (51 JI:. The tuning factor in (5) and af
25、ter is a non- dimensional quantity, as long as both frequencies are given in the same units. According to widely recognized practice 3, 61, a elastoinertial system is in state of resonance if As the traveling drive operates under variable speed, the tuning factor of the shaft/coupling system sweeps
26、a range of values. According to the commands given by crane operator, the tuning factor stays stochastically in one or another value during a certain time. If enough time is spent under condition (6), the amplitude of the pulsating torque can reach high values. Such high values can develop low cycle
27、 volumetric fatigue damage on the teeth of the bevel gear, leading to its quick fracture. The overload imposed by the resonance effect can be expressed 2, 3, 51 by means of an application factor * The values of the application factor KA under the assumption (8) are plotted as a function of the tunin
28、g factor in Figure 3. It is interesting to note that when the tuning factor equals unity, the application factor reaches its maximum value KA,ur =3.14 (9) That is, the torque transmitted by the bevel pair of the gear unit can reach a value more than triple the nominal; enough to fracture its teeth i
29、f no ample strength has been left. 3.50 , i 3,OO 2,50 2,oo 1,50 1 ,o0 0.50 ! 0.00 0,50 1,OO 1,50 2,OO Nz 1 . KA = (7) Figure 3: Application factor for bevel gear 1- 1 In conformity with this result, the service factor of 3.0 according to AGMA 6010-F97 i appears adequate even when resonance is presen
30、t. Therefore, to dimension a bridge traveling gear unit Where p is the factor of viscous damping. for a bulletproof quick design, or for an emergency overhaul, a service factor with the value of 3.0 could be used, obviously at a cost. The factor of viscous damping in (7) and after is given in (N.m)/
31、Hz, according to the International System of units, SI. The application factor, as is well known, is a non-dimensional maanitude. - ON THE INFLUENCE OF MOTOR CONTROLLERS There is a moderate degree of viscous damping due to oil film in the gear mesh and rolling bearings, and Many of the failures in h
32、igh-speed bevel gears of the internal friction of the elastomeric element in the crane bridge traveling drives appeared after the coupling. Therefore, it is assumed critical damping in advent of the solid-state variable speed controllers the shaft/coupling system. Such condition is the limit for the
33、 electric prime movers. These controllers yield between light and heavy viscous damping. Critical an almost constant toque at the prime mover main damping is present when relation (8) holds. shaft during the starting period, suppressing the e 3 COPYRIGHT American Gear Manufacturers Association, Inc.
34、Licensed by Information Handling Servicesstrong saw-tooth torque ripple characteristic of the prime movers under the older magnetic controllers. Apparently, a number of crane designers shortly after being acquainted with the new high-technology motor controllers, begin to discard the old hyddynamic
35、dutches (the so-called hydraulic couplings) from the traveling drives of new design as an unnecessary piece of hardware. Consequently, the connection between prime mover and gear unit was effected by means of a simple flexible coupling, normally combined with a drum brake. However, no flexible coupl
36、ing has the strong viscous damping characteristic of hydrodynamic clutches that do not allow for high values of resonance loads. Let the increase in viscous damping of the elastoinertial system due to the introduction of a hydrodynamic coupling be estimated conservatively as twofold. Then, Figure 4
37、shows the maximum value of the corresponding application factor to be under 1.7. 1,80 , i I 1,60 1,40 1,20 1 ,o0 0,80 I 0,60 0,40 I 0,oo 9 0,OO 0.50 1,OO 1,50 2,OO Nz Figure 4: Application factor for bevel gear with a hydrodynamic clutch This result can justify the service factors from 1.5 to 2.0 gi
38、ven by certain manufacturers for bridge travel gear units, presupposing the use of hydraulic clutches. However, many times very similar values are recommended without any other necessary condition. Therefore, to avoid unexpected problems, it is suggested that the substitution of hydraulic clutches b
39、y ordinary flexible couplings in drives with modern motor controllers should be undertaken only after a dynamic analysis of the shaftjcoupling system. SUGGESnONS TO MANUFACTURERS Wisely, AGMA Standards point to the application engineer as responsible of an overall system design that avoids operation
40、 at resonance. Nevertheless, the author feels that gear unit manufacturers can also take some basic measures to avoid a near resonance operation of its speed reducers when equipped with standard drum brakes typical of crane traveling bridge drives. There are two complementary characteristics in a no
41、n resonant-prone gear unit for crane travel drives: 1. Minimum elastic compliance of the pinion shaft. 2. Minimum mesh frequency of the bevel gear. Both Characteristics tend to decrease the value of the application factor as given by (7). A shorter and oversize diameter high-speed shaft, allowed by
42、an improved bearing design, seems to be a practical way to attain the first characteristic. A high-speed pinion with a smaller number of teeth, allowed by a special design of the bevel gear, seems a way to achieve the second characteristic. SOLUTIONS FOR EXISTING SYSTEMS On the user side, the instal
43、lation of new gear units with a 3.0 service factor may be too costly, and beyond the possibiiities of existing travel drive systems without a major and irrational overhaul. Three complementary modifications can be done in an existing crane travel drive to improve its resonance behavior: 1. Minimize
44、the compliance of the pinion shaft. 2. Minimize the mesh frequency of the bevel gear. 3. Minimize the moment of inertia of the high- speed shaft half-coupling. All three modifications tend to decrease the value of the application factor as-given by (7). The first and second modifications can be achi
45、eved by the same ways suggested in the former section, at only a fraction of the cost of a new gear unit. To minimize the moment of inertia of the shaftjcoupling system, a simple solution is to invert the flexible coupling, as shown in Figure 5. This way, the high-speed shaft of the gear unit receiv
46、es the smaller half-coupling 2, with a minimum - O moment of inertia, as it lack the brake drum 1. 4 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesI- I /I I / R Inverted I I dl Figure 5: Inversion of flexible coupling A PRACnCAL EXAMPLE A big industr
47、ial enterprise in South America has two special-purpose traveling bridge cranes, each with a total mass of 165 t, working around-the-clock in a pit-furnace building. Bridge traveling drives for individual motoring wheels in every crane were composed as follows: 1. A slip ring AC induction electric m
48、otor with a nominal power of 25 kW at a rotational frequency of 19 Hz (1 140 min-), under an operating regime S3 25%. 2. A solid-state electronic controller for the electric motor, which regulates speed and torque through stator tension and rotor resistance, 3. A bevel-helical three stage gear unit,
49、 with a nominal ratio of 1:71, and a main stage center distance of 200 mm. 4. A flexible coupling between motor and gear unit combined with a drum brake. Gear unit half- coupling carried the brake drum. Crane designers selected these gear units using a service factor of 1.5, as proposed in the technical catalog of the manufacturer. Just a few weeks after plant start-up, began serious troubles with broken teeth in the high-speed stages of the gear units. Time between failures ranged stochastically from 15 days to 15 minut
