AGMA-05FTM19-2005.pdf
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1、05FTM19 The Application of Very Large, Weld Fabricated, Carburized Hardened however, the real loading spectrum is not usually obvious and is seldom simple. Actual torque measurements have shown that the torque transmitted by each gear mesh is not well repre- sented by the torque equivalent of the mo
2、tor name- platepowerandspeed.Becauseoftheveryhighin- ertiaintheentiresystem andthe dynamic conditions that occur as the rolls bite into the steel slab and proceeds to change both its shape and structure,Figure6,thetorquethatisappliedtoeach gear mesh in the system is different and varies sig- nifican
3、tly as the bar progresses through the rolling process. Figure 6. The Basic Steel Rolling Process One would, of course, expect a difference in torque onthemotortomaingearboxandmaintomillpinion gearboxes due to the ratio of the main gear drive. This is true; however, strain gage measurements, such as
4、those shown in Figure 7, show that the dif- ferenceintorqueisoftennotproportionaltothegear ratio alone. This plot shows torque measured on various shafts in the system as a function of time. Figure 7. Torque Measured on Various Shafts in a Steel Rolling Mill (strain gage measurements) Note that the
5、torque variation is particularly high during entry of the bar into the rolls and then it settles down to a more stable “variation” pattern fi- nally dropping off as the bar exits the mill. Motor power, as measured by motor current, also varies but to a far lesser extent as the system inertia insulat
6、es” the motor somewhat from these very wide load variations. Because of this, measure- mentsofmotorpowerormotorcurrentaregenerally poor indicators of the torque applied to the gear mesh and very often under-predict the gear mesh loading.Thiscan leadto prematuregear failureand bearing life problems.
7、Considering these factors, from this data plot, it should be obvious that the torque measured is not just a function of the motor power andthe gearratio but also of the system dynamics. Note that the torque experiences significant variation as the bar entersandexitsthe mill.Even duringthe actualroll
8、- ing process the torque alternates significantly in many cases. While we are dealing with the main gear drive here, itisalsointerestingtonotethatthetorquecarriedby Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licensee=IHS Employees/1111111001, User=Wing,
9、 Bernie Not for Resale, 04/18/2007 11:11:17 MDTNo reproduction or networking permitted without license from IHS -,-,- 4 the upper and lower spindles (and thus the torque absorbed by the top and bottom rolls) is not equal. Our design requirement for this torque split is a 60/40 split to either shaft.
10、 This split represents the usual expected “worst” case but we have observed splits as high as 70/30 to these two shafts. This variation of torque is often referred to as the Torque Amplification Factor (TAF). The TAF is best understood by considering the simplified torque load schematic shown in Fig
11、ure 8. Figure 8. Simplified Torque Schematic Inordertoproperlydesignthegearandbearingsys- tem, it is necessary to have a reasonable knowl- edgeoftheTAFforanysystem.TheTAFcanbecal- culated by creating a dynamic model of the entire millsystemoritcanbemeasuredthroughtheuseof strain gages applied to the
12、 various shafts. It can alsobeestimatedbasedonexperiencebutsuches- timates are fraught with the possibility or error as each system is quite different. In any case, the TAF must be properly addressed. The discussion of loading is especially important in our context be- cause, unlike most gear applic
13、ations, the design of thegearingitselfcansignificantlyimpacttheloading that the gear set will be required to transmit. Many, older mill gears were manufactured with relatively lightwebsandstiffenersandequallythinrimsunder the gear teeth, as the failed gear in Figure 9 shows. Note that the gear shown
14、 in Figure 9 is a cast gear but it is also representative of the configuration of many weld fabricated mill gears as well. The teeth on such gears were often hobbed or, sometimes, shaper cut to AGMA Quality levels in the range of 6 to 8. At these quality levels, the com- binedpitchlinerunoutofthegea
15、randitsmatingpin- ionissuchthatthebacklashinthegearsetwasusu- ally quite high, even considering the relatively coarse diametral pitch typical of mill gears (pitch is generally in the range of 0.75 to about 2.0). Typical backlash for these gear sets would be in the range of 0.060 to over 0.100 inch,
16、depending on quality and pitch. When the bar enters the mill and begins its journey between the rolls, as is clear from Fig- ures 7 and 8, impact loading is applied to the entire system. This loading is fed back through the gear system. Backlash in the gear system exacerbates the effect of the impac
17、t loading thus minimizing backlash is a desirable. In order to allow the back- lash to be minimized, the gear quality level must be increased. A typical set of ATG mill gears, of a size similar to the set shown in Figure 5 would have a backlash in the range of 0.025 to 0.040 inch. This lower backlas
18、h greatly reduces the TAF thus the gear tooth design itself can reduce the applied loading. Figure 9. Failed Roughing Mill Gear In addition to providing dramatically improved strength and stiffness (discussed in more detail be- low), the greater overall mass, thicker gear rims, heavier webs the cost
19、per minuteof unexpected down time is correctly stated!). Figure 15. Scored Finishing Mill Gear In the case of these gears, scoring occurred very shortlyafterstartup.Thegearsetwasreplacedwith a virtually identical set which also scored in a very similarmannerinaveryshortperiodoftime.Careful investiga
20、tion of the system indicated that misalign- ment was at workand certainlyexacerbated thesit- uation.Unfortunately,becauseoftheoverall construction of the gearbox, deflections within the housing made it very difficult to maintain an accu- rate alignment of the gears set under all load condi- tions. T
21、he most effective solution would be to re- place the gearbox in its entirety with a stiffer, higher load capacity unit. After considering all options, however, the client concludedthatreplacingtheentiregearboxwasthe least desirable solution. This being the case, it was necessary to improve the load
22、capacity of the gear set, especially its scoring capability, so that it could survive these less than ideal operating conditions. As a starting point,we calculatedthe basicstrength and durability load capacity of the gear set to pro- vide a basis for redesign to improve capacity. The results of this
23、 analysis, Figure 16, indicated thatthe Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 11:11:17 MDTNo reproduction or networking permitted without license from IHS -,-,- 7 basic
24、 strength and durability service factors for the OEM design (configuration 1 in Figure 16) were, at the actual operating conditions, rather low. Figure 16. Service Factors for Finishing Mill Gear System at Normal Operating Conditions In a steel mill application, we would generally rec- ommend at lea
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