44530377 Introduction Open Gear Drives

Introduction to Open Gear Drives

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Introduction – Open Gear Drives 12.08.2005/RHTHolcim Group Support

Objective

To provide an overview on “open gear drives”, their advantages compared to other drive concepts, technical details of geometry and different designs.

Further details can be obtained from our follow-up training material “Inspection of Open Gear Drives”.

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Index

Arrangement of Drive Elements

Fabrication of Girth Gears

Girth Gear Profile

Gear Geometry and Tooth Contact

Girth Gear Mounting on Kilns and Mills

Pinion Mounting with Tensioning Elements

Tiltable Pinion

Gear Module and Intermeshing

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IntroductionLarge gear drives have been used in plant engineering for many decades.

Usually, single or double-pinion drives are installed in kilns or mills.

Their main advantage compared to other types of drives, for example central drives and ring motors, which are more complicated in design, is the lower investment cost.

A main disadvantage compared to enclosed gears is the fact that gear rim and pinion(s) are supported by separate bearings, whichresults in difficulties when mounting and aligning the drive components.

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Arrangement of Drive Elements

Possible arrangement of drive elements for ball mills in single and double-pinion drives

Single-pinion drive:

up to 4500 KW

Double-pinion drive:

up to 2 x 3500 KW

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Fabrication of the Girth Gear

Gear teeth quality of gear rims is 8 to 11, in most cases 9 to 10, and of pinions 7 to 9 in accordance with DIN 3960.

Gear teeth ratio is normally between 6 and 15.

Material of toothed section of gears is typically made from 34 or 42 CrMo 4 with approx 220 to 280 HB.

Pinions are made of steel 30 CrNiMo 8, hardened to approx. 320 HB.

Modern technology also allows the production of ground pinions made of 17 CrNiMo 6, casehardened to approx. 60 HRC.

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Girth Gear Profile

The gear drives used in tube mills and rotary kilns are usually very sturdy in design and operate at low speeds.

The peripheral speed in mills is normally 6 to 10m/s and in kilns 0.3 to 3 m/s.

The main characteristics are:

High output torqueLarge center distanceMedium to large modulesTooth width up to approx. 1,200mm,

depending on the driveDesign:V – shape profile only for castingsT – shape profile for welded girth gears (nowadays with increased thickness of bandage instead of additional rips.

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Gear Geometry and Tooth Traces

Tooth Contact:

Gear drives can be helical or spur toothed. Helical gear drives have the advantage of gradual flank intermeshing, a higher contact ratio, softer teeth contact and lower running noise. However, they are more difficult to align.

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Advantage of bridge/spring plate design:

Heat shields can be installed between bridge/spring plates and girth gear to reduce radiation temperature and to avoid overheating of girth gear.

Alignment of girth gear after installation is much easier compared to tangential plate design.

In case of girth gear reversal: tangential plate pin holes on girth gear must be re-machined and oversized pins must be installed (due to wear on old pins and holes)

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Pinion Mounting with Tensioning Element

Pinions are manufactured as single part with a shaft, e.g. from a forging blank or as two individual parts. The pinion then is mounted on the shaft by means of feather keys or tensioning elements.

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Gear Drive with Tiltable Pinion

Many efforts were made to solve the problem of insufficient load distribution across the face width.

Such designs are based on the assumption that the pinion has to adapt to the movements of the gear rim.

Advantage of tiltable pinions:

Compensation of alignment errors

Optimum load distribution under any condition

Very favorable dynamic load ratios

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Tiltable Pinion

The pinion is supported by a universal ball joint and is tiltable so that it can follow any tilting movements of the gear rim. The torque is transmitted by means of a toothed coupling located between the pinion and the shaft.

The unit is equipped with a fluid grease for long-term lubrication; re-lubrication is only required occasionally.

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Tiltable Pinion – Cost Comparison

Replacement of standard pinion with tiltable pinion:

approx plus 20 - 30% costs

Replacement of complete girth gear including tiltable pinion:

approx similar price range

Note: Similar price range is justified with a reduced overall weight. Due to the improved contact ratio of tiltable pinions, the wide of girth gear and pinion can be reduced (considering identical load requirements) - leading to a reduced overall weight of pinion and gear. Higher machining and manufacturing costs will be compensated by the reduced material weight savings.

Holcim is recommending tiltable pinions for all mill and kiln girth gear drives (dimensional limits must be considered).

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Gear Module and Intermeshing

Modules:

Typical modules for mills are up to 30mm (25.4 mm) and for kilns up to 45mm.

The objective is to get an increased number of teeth with a lower module without having to increase the size of the drive, thus improving the intermeshing conditions and the load-carrying capacity.

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Tooth Shape Modification

Since recent years more and more larger case-hardened and ground pinions are manufactured. Advantages:

substantially improved load-carrying capacity,

better quality of the ground teeth good surface quality pinions can be made less wide

resulting in better operational reliability. Such a pinion design, however, is only possible when correcting the flanks in the longitudinal direction by tip and root relief.

44530377 Introduction Open Gear Drives

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