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Hot and cold strip steel mills manufacture steel by reducing a large slab of steel into a long thin sheet. This process is achieved by passing the material through many large rolls, flattening it a little more with each pass. (Fig. 1). 
Fig. 1 - Typical mill layout
As with any manufacturing business, more throughput results in more dollars. The best way for the steel mills to increase their throughput was to implement more aggressive roll reductions (flattening more at each pass). This upgrade demanded more power from the driving equipment and more vertical movement of the rolls, requiring the spindles to handle higher torque loads and misalignments. Lack of attention to the spindles during this modification resulted in many failures and an increase in maintenance and downtime costs, offsetting the increase achieved in production.
To help address this situation, Ameridrives® International has developed a new generation of advanced crowned gearing and its mating sleeve for longer life spindles (patent pending).
With this intensive "Advanced Gear Design" analysis, Ameridrives® International has optimized the geometry of the existing Fully-Crowned Gear Tooth (Fig. 2) to increase capacities by 20 to 300% greater than the existing conventionally designed tooth. This analysis was developed through years of finite element analysis, strain gage testing, dynamic testing and field testimonials from users.
Fig. 2 - Fully-Crowned Gear Tooth
The Basics of the "Advanced Gear Design"
The main concept of the "Advanced Gear Design" is optimization of the tooth geometry to obtain a higher percent of teeth in contact at the coupling operating conditions.
Misalignment Angle is the only factor given consideration in the percent of teeth calculation of a conventional gear coupling analysis.
Theoretically, there are only two teeth in contact when misalignment (Fig. 3 - α) is present and no load is applied. The remainder have a gap between each tooth set.
Fig. 3 - Tooth spacing at misalignment
In calculating the percent of teeth in contact, the Advanced Gear Design method includes the effects of misalignment angle, applied load, flank curvature and hub and sleeve set stiffness. This improved method produces substantially more accurate results when compared to a conventional analysis that only considers the misalignment angle (Fig. 4).
Fig. 4 - Percent of teeth in contact comparison
When compared to a conventionally designed gear, the physical appearance of the Advanced Gear has a norrower face width, but is a larger tooth with a more radical flank curvature and fewer teeth per gear mesh (Fig 5 and 6).
Fig. 5 - Gear mesh designed using the conventional method
Fig. 6 - Optimized gear mesh using the Advanced Gear Design method
Figures 7, 8 and 9 show the results of using Pro-Engineer"s™ 3-D modeling system and NISA Finite Element Analysis (FEA) software.
Fig. 7 - Single tooth 3-D FEA model
Fig. 8 - 360°, 2-D FEA model at 0° misalignment
The bending stress and percent of teeth in contact (Fig. 9) are calculated for the operating conditions.
Fig. 9 - 360°, 2-D FEA model at 2° misalignment
An optimization process consisting of flank curvature, diametral pitch, face width and sleeve barrel was performed on both actual and test configurations. The results were outstanding, decreasing the stress (Fig. 10) and increasing both load carrying capabilities and coupling life by magnitudes in the same diameter gear spindle. The correlation of stresses, percent of teeth in contact and heat generation characteristics were all excellent.
Fig. 10 - Graph showing tooth bending stress as a function of applied torque
Testimonials
In conjunction with the in-house testing, selected high maintenance, short life mill applications were used for field-testing.
In one case, a large steel manufacturer began having spindle failures on the roughing stands of their 84" hot strip mill after operational changes where made to increase the overall output of the mill. The spindle gearing began failing in as little as three to four months. Ameridrives® provided prototype Advanced Gear Design spindles to increase the life and allow the mill to produce at the new desired output. Six months after installation of the prototypes, the spindles were removed for inspection and the gears were in near perfect condition. Since that time, failures have been eliminated and the spindle gears on this mill have lasted in excess of two years (Fig. 11).
Fig. 11 - Graph showing Spindle life of the original and Advanced Gear Designs on various roughing and finishing stands on a hot strip mill
In another application, a high speed cold mill pinion stand with 21 inch diameter gear spindles was producing spalling and temperature failures, destroying the spindles in weeks. Ameridrives® provided them with new spindles with the Advanced Gear Design and improved the life well beyond the expected six month maintenance schedule.
Ameridrives® has applied the new design to all mill applications from 3½ inch leveler spindles, 21 inch high speed cold mill spindles to 38 inch hot mill spindles with significant life increases for all applications.
Inherent Design Features
A mill spindle designed using the Advanced Gear Design method also exhibits some additional operational advantages.
Reduced Vibration Characteristics-
The amount of backlash required to achieve the misalignment angle is much less than a conventionally designed gear mesh. This results in less vibration on the system, providing smoother roll operation, lower shock loads and a more stable system. This improves product quality with less chatter and enables cold mills to increase output by running at a higher speed.
Lower Temperature-
Actual field measurements have demonstrated cooler operating temperatures at the gear mesh. This is attributed to having more teeth in contact, reducing hot spots in the gear mesh and a tooth geometry that produces more of a rolling motion than a sliding action. This extends lubricant and spindle life.
Lower Cost-
Producing mill spindles with smaller diameters for given load conditions results in an overall cost savings for the initial product beyond that acquired from the lower maintenance and downtime savings.
Conclusion
The Advanced Gear Design Analysis enables Ameridrives® to optimize mill spindles for the specific application"s operating conditions. The high angle, largely loaded steel mills benefit with less maintenance, lower cost, increased productivity and minimal failures.
Summary
This article discusses the methods and advantages of the Mill Spindle Advanced Gear Design. There are many other factors not discussed, such as material, lubrication and tooth distortion from hardening processes that effect the operation and life of mill spindles and gear type couplings in general. However, due to the increased accuracy and inherent features of this design method, Ameridrives® International is currently incorporating its use on any high angle gear type coupling it produces.(end)
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