Gear  Mill Introduction

Cement Plant Mill and Kiln

Cement Plant Mill and Kiln

A ring gear and pinion set in a cement plant has two possible applications. The first of which is the heart of any cement plant, the kiln. The kiln requires a rotational speed that typically is operated at speeds between 1.0 rpm and 5.0 rpm. The second cement application for the ring gear is the rotation of the horizontal grinding mills. The grinding can be raw lime stone, finish clinker, or coal pulverization. The rotational mill speeds of the grinding mills can typically vary between 25 rpm and 12 rpm at the mill. For years ring gear manufactures have used AGMA gear rating standards 321.05 and its updated version, 6004, for the design of these unique gearing applications. These two standards are application specific to ring gears and have stood the trials of time after years of successful design. However, these years of experience have provided new insights to materials, quality, and application loading that enabled updating of these design practices. The new standard does use many of the same principles that were used in the past standards. The purpose of this paper is to highlight the changes of the standards and evaluate how the changes will affect the ring gear user and the design/selection of a ring gear for a cement plant. It is not within the scope of this paper to detail rating formulas and calculations. An evaluation of ten kiln gear sets and five mills is performed. These sample set ring gears are representative of the cement industry. The ring gear sample set evaluation rates each gear set to AGMA 321.05 and compares the ratings to the new standards AGMA 6014. The evaluation of ring gear ratings will allow the user to assess the possible torque increase permissible by the new design standard. The new design standard was found to increase the sample set torque by 9.8%. The ring gear sample set evaluation is then taken a step further, with a redesigned ring gear for the same application. A pricing comparison is made between the updated and the previous AGMA 321.05 design. The new AGMA 6014 design standard was found to reduce the average cost of the gear set by 5.2%. In addition to design and rating changes, the new standard now incorporates several annexes for informational purposes which address installation and alignment, lubrication, and operation and maintenance. The cement plant ring gear user will find this information very useful and germane to ring gear care and operation.

Data Requirements for Gear Design

In order to design an optimal gear set for a given application, the gear designer must have the endpoint defined. If the following basic information is not supplied, it becomes difficult if not impossible to design the gear set to ensure it matches the actual installation conditions and the desired performance. The following items are required:

  • Motor Power and Input Speed
  • Desired Gear Ratio (optional)
  • Resultant Output Speed
  • Single or Dual Pinion Drive
  • Inching Drive
  • Mounting Arrangement of Gear to Mill
  • Trunion or Roller Support of Mill
  • Service Factor

The motor power is typically based on the application requirements for the rotating equipment. However, motor speed can be deceiving to some designers, depending on the type of motor selected. For most applications, the user has many choices of motor types including, but not limited to: wound rotor motors, synchronous or non synchronous motors, and, direct current drives. For a specific output speed of the kiln or mill that is required for a given production output, a three percent speed variation can cause a non optimal selection of the gear and an inefficient use of the variable speed drive if so equipped. Depending on capacity or space requirements, it may be optimal to specify a dual pinion drive arrangement where two motors are used to drive the ring gear. For dual pinion applications, an electrical load sharing device and or clutch may be needed to ensure that the load is spit 50 – 50 between the two pinions. Annex B of AGMA 6014 is an informative section of the design standard that guides the ring gear user through additional considerations when selecting a drive for a ring gear application.

Maintenance needs or process requirements may dictate the presence of an inching or Sunday drive to enable low speed rotation of the kiln or mill. Typically this is designed at 0.1 rpm mill speed. The inching system is designed to supply at least 120% of the full load demand power to ensure startup. Care must be taken on dual drive installations in that one can only mount a mechanical inching system on one of the pinions. The entire drive train must be sized to ensure that, in inching mode, the gearing, shafts, and couplings can carry the full inching torque. For dual pinion applications the inching drive load must be designed for twice the main drive torque. The method of attachment is also critical since space considerations can dictate the final size of the gear around the structure.

Sufficient clearance must be maintained between the mill shell and the gear rim to enable assembly and rigidity. A new requirement present for designs to the AGMA 6014 rating standard is the information regarding the support of the mill. Trunion supported mills enable better alignment of the gear with the mating pinion resulting in higher load capacity. The misalignment inherent in roller supported applications such as kilns requires a more conservative evaluation of the load distribution factor in the gear rating practice. And finally the desired service factor of the applications. Kilns, mills and high horsepower designs all have different service factor requirements and the application type needs to be shared with the designer to enable sufficient capacity of the final gear set design. Recommended service factor requirements are detailed in Annex I ofAGMA6014.

Background to Gear Rating Practice

When gears are designed, their power capacity is divided into two values. These values are then compared to the motor characteristics and are commonly referred to as services factors ratings. The first is based on contact stress and is called the pitting or durability rating of the set. This gives an indication of the set’s ability to resist the formation of small pits in the tooth flanks. It is a function of the geometry, material, hardness, speed and the interaction of both the pinion and gear in mesh. The second rating is termed the bending or strength rating of the set. It models the tooth as a cantilever beam and computes its resistance to tooth breakage. It is also a function of geometry, material, hardness and speed. The bending rating does not account for cracking through the rim of the gear. Failures due to lack of lubricant or structure failure are not covered by the standard.

The Old Standard AGMA 321

The industry had used AGMA 321 Design Practice for Helical and Herringbone Gears for Cylindrical Grinding Mills. Kilns and Dryers. It was first approved for use in October 1943. Various iterations occurred with the last major re-write in 1968 when the standard was updated to use the formulations of AGMA 211 Surface Durability (Pitting) of Helical and Herringbone Gear Teeth and AGMA 221 Rating the Strength of Helical and Herringbone Gear Teeth. The last editorial corrections were issued in March 1970.

The major influencing factors in the gear rating formulas were assigned specific values based on the size and experience of the industry with this type of gearing. Two dynamic factors, Cv for pitting and Kv bending were used, but both were a function of the pitch line velocity of the set. Load distribution factor, Cr was a function of face width only, covering the range of 2 inches to 40 inches (50 to 1016 mm) with modification factors to adjust its value when teeth were hardened after completion. The allowable contact stress number, Sac was reduced by the standard; however no metallurgical properties other than hardness were discussed. The hardness ratio factor, CH was expanded from the basis in other gear rating standards to cover a ratio range of 1:1 to 20:1. The allowable bending stress number. Sat was also reduced by the standard but it also remained only a function of hardness. Service factors ranged from 1.0 for kiln and dryer service up to 1.5 to 1.65 for grinding mill service.

Brief reviews of the formulas disclose that a large number of today’s gear rating factors are not present. The influence of tooth attribute accuracy, i.e. lead, pitch and profile, was missing and the impact of tooth modifications to improve load sharing was ignored.

Despite these omissions, gears designed to this standard experienced 20 years of service life. Both end users and original equipment manufacturers appreciated the conservatism built into the standard to account for installation challenges and lubrication issues for these critical service applications.

A new standard for rating mill and kiln gears was released in 1988. AGMA 6004 included two thirds of the major changes that were made to AGMA 218, the then new practice for rating spur and helical gears, namely the dynamic factor and the load distribution factor that were modified for mill gearing use. The changes to the Life factor were not included in this document. This crucial difference was to lead to limited acceptance of the new mill gear rating standard due to large calculated ratings increases that may not have actually occurred in practice.

Faced with the issue that AGMA 6004 was not widely used, the AGMA Mill Gearing committee began work on a new rating practice in November 2001. They felt that the basis of the new standard should be an increase in AGMA 321 ratings of 10 to 15 percent over the 1970’s rating practice. This was based on application experience by committee members due to improvements in materials and tooth accuracy. They also desired to incorporate the current trends in gear rating practice as illustrated by AGMA 2001, the updated version of AGMA 218 for spur and helical gears, namely dynamic factor, load distribution factor, and stress cycle factor. In addition, the document was to give additional guidance on material acceptance criteria, lubrication, and installation issues.

New Standard AGMA 6014

The new standard was a departure in format from the previous AGMA 321 document but not in intent. The committee worked in the framework of new thinking in gear design but built in a safety net of comparing the results to past practice. A rating of a gear set is made up of many factors, considerations, and effects. The results of these interactions are difficult to predict given the diverse nature of the resulting gear sets. As such the committee used a set of sample gear sets to continuously compare the output of the new document to the AGMA 321 rating practice. In this way, the surprises that were experienced on AGMA 6004 were avoided.

Dynamic factor KV was an early target. One of the challenges in AGMA 218 as well as AGMA 6004 was the significant increase in ratings caused by the use of high quality (AGMA Av = 7, Qv = 10) gearing. A change from A9 / Q8 to A7 / Q10 in tooth quality attributes leads to a 12% increase in Kv rating for a sample gear set. The new standard reduces this to 5% increase. The dynamic factor effect was compressed to more closely reflect actual installation performance as a function of tooth attributes.

The second factor undergoing adjustment was the load distribution factor Km. In this case the change was less dramatic.

Changes in the standard include setting the lead correction factor Cmc to 0.95 for modified leads in place of the AGMA 2001 value of 0.80. The pinion proportion factor Cpf was extrapolated to 50 inch (1270 mm) ace width using the same equations. The mesh alignment factor was limited to two choices. The first was roller supported mills and kilns duplicating the open gearing curve of AGMA 2001. The second was bearing supported mills and kilns duplicating the commercial curve of AGMA 2001. In addition the mesh alignment correction factor Ce was set to 0.80 to reflect that at every installation the gearing alignment is adjusted at assembly.

The allowable contact stresses were increased to grade 2 levels for through hardened materials as defined in AGMA 2001. However, a different set of requirements were established for these large cast and fabricated gears. A lower allowable grade M1 was added to accommodate gears made without material certifications or testing. Note that M1 grade allowable stress numbers do not match those values in AGMA 2001 for grade 1 materials as well as surface hardened pinions for both grades M1 and M2.

Impact on sizing for new applications

To see the impact of AGMA 6014 we looked at ten kiln gear sets both single and dual pinion drives, and five mill drive gear sets single pinion drives only. Tables 1 and 2 detail the size and power range reviewed. The data sets illustrate a direct correlation between the power delivered and the over all size of the girth gear, key factors being the face width an outside diameter (OD) of the gear.

For purposes of this paper the following rules were used in the 6014 design case. For Kilns the face overlap ratio is limited to 1.0. Thus as face width was reduced the maximum helix angle increases to maintain this overlap ratio. The maximum helix angle was limited to 11 degrees 30 minutes to limit gear thrust forces within typical permissible levels. Then hardness was reduced to achieve the specified service factor either by meeting the existing set in the case of the spur or 1.0 /1.75 pitting and bending service factors. For mills, the face overlap ratio is limited to 1.1. No upper limit was set for helix angle. Tables 3 and 4 list the significant changes allowed by these rules. Additional changes where made to material hardness of both the gear and pinion when needed to produce a gear set rating as close to the allowable as possible.

For the Mill gear data sets evaluated, the only design change necessary for the 6014 selection was a reduction in face width of the ring gear set. Table 4 shows the face width reduction for each data set of the Mill gears evaluated and shows a range from 13% to 3% reduction in face width. This face width reduction translates directly into a cost savings in the amount of material needed to manufacture the gear set designed to AGMA 6014.

The Kiln gear data sets had a much greater variation in the face width reduction ranging from a low of 3% to a high of 30%. However, if you remove the high and low values as outliers in the data set, the anticipated reduction in face width are found in the band between 10% and 20% reduction of face width. Figure 1 below is a scatter plot representation of the % face width reduction for the mill gear data set with the outliers removed.

The gear set selling price of the new AGMA 6014 gear design reflecting the face width reduction was compared to the original AGMA 321 gear set selling price. The selling price % change is shown for each data set, and is then averaged by application and drive type in Table 5. There is a direct correlation between the power and the reduction of selling price. As the power of the application increases the cost savings decreases. Based on the gear sets analyzed for this study, the average reduction in selling price for a single pinion kiln gear set is 9.0%. The average reduction in selling price of a dual pinion kiln gear set is 7.4%, and the average reduction for the mill gear set is 3.9%. AGMA 6014 rating standard clearly allows for a more compact and cost effective gear design.

The cost savings obtainable for an AGMA 6014 ring gear set compared to the AGMA 321 ring gear design is clearly evident from the data presented. However, existing applications can not accommodate structural gear design changes without significant costs and modifications to guards, foundations and other equipment. In these applications it may be more beneficial for the user to understand the permissible load increase that AGMA 6014 can offer for the same gear design. This increased load capacity might be used to allow the user to increase production throughput, change rotational speeds, or increase motor horsepower. Table 6 below compares the service factor (SF) increase obtained for the data set evaluated in this study. The single pinion kiln gears sets have an average increase in (SF) of nearly 20%. The dual pinion kiln gear sets have an average (SF) increase of 7.5%, and the mill gear sets have an average gear set rating increase of 6.4%.

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