Failures of ?Gearbox and Drives?

It is a pain area for the plant management team when the gear drive fails and it is more difficult to arrive at the correct cause of failure. S Sengupta & A Ray Chowdhury from Sprat Consultancy elaborates on some of the common causes of failures, also suggests remedial measures.

The idea of putting pen to paper regarding gear drives seems to be a daunting task. One is apprehensive as to where to begin and to what degree to write is a nerve racking proposition as majority of the readers are qualified, sound practical engineers who are associated with industrial plants such as cement, power, metals, etc. Experience confirms that a meaningful insight on the subject requires around three working days and if fourth day could be added by way of site visit with discussions on practical problems, works out a ?win-win? situation for all.

An endeavour is however being made to jot down some thoughts that may serve as pre curser from selection to use of drives. The order of narration is not sacrosanct and not all encompassing. It is just a brief write up on few attributes hoping it will provoke the mind of concerned personnel be it users or project/technical personnel. Use of gear drives in a system does not imply just operational; it encompasses what happens within a drive train and requirements to achieve desired performance life hence design life.

In many instances over the past 38 years, we have come across failures in drives caused by lack of insight or foresight or lack of correct data or its understanding during selection of drive. A common failure, but not frequent, is lack of perception of what all is to be handled &/or power required to drive the system.

An example that readily comes to mind is in a greenfield cement plant around early 1980s: the external consultant confirmed motor power, application, operating hours per day, etc. and wanted a drive with a safety factor (SF) of 2.5. This particular gear drive was 1 amongst 65 others. It was the only one that was prone to frequent failure to the tune of once every three months. Review of actual operating data confirmed actual power consumed was 40 per cent higher than confirmed during project design & planning leading to premature failures.

Another instance of premature failures observed in a cement plant in Western India of a twin drive bucket elevator where input drive was through fluid coupling. After a year of satisfactory operation failures commenced with regularity in one of the gear drives in the arrangement. As they were imported gearboxes not much hue and cry was raised initially. Replacements from two indigenous producers also failed in the same manner and frequency leading to a pantomime at the plant. Analysis of the drive arrangement confirmed power consumed by individual drives differed by over six per cent. In such a scenario, failure was inevitable and plant further confirmed that after a year one drive motor had burnt out requiring rewinding. Rewinding is common accepted process but what is equally important is that in a twin drive synchronisation of input power is of utmost importance of within two per cent variance. Since synchronise of input power no further failures occurred over a decade.

There are many other instances of failures we often recollect as observed over the last four decades. In all instances failures have taken place for:

• Incomplete or inadequate clarity of specification at initial stage.
• Lack of appreciation of specification, which is more dangerous.
• Hypothesis by OEM of likely operational parameters viz a viz specification thus incorrect supply.
• Augmenting capacity after year/s of use and not sharing data with supplier or supplier not appreciating information conveyed which must be well defined.

In short whatever are the circumstances in life (we consider gear drives also a form of life) it takes two to speak the truth to form an understanding and thus realisation. In the field of machine dynamics the same applies; dialogue between user and supplier must be continual and without inhibition or prejudice. In other words partnership is required with frank exchanges, irrespective of how insignificant the information may appear, to eliminate misgivings consequently failures.

The more this realisation dawns on all in a B2B scenario and quicker the better for all concerned resulting in reliability of operations. Failures are phenomena that cannot be totally ruled out even with best intentions of user and supplier. Any failure, irrespective of its occurrence, within or beyond the warranty period or after extended period of use is relevant at all times towards better and improved designs unless failure occurs beyond design life of rolling elements. This information should be shared with factual details unambiguously.

It is common for most designers to design critical drives, irrespective of type/size &/or application considering a life of 100,000 hours for gears/pinions and around 60,000 hours for bearings. Indirectly, to a large extent, bearing life sets the set point for case hardened and ground gears/pinions although theoretically it has an infinite life.

The question therefore arises why premature failures occur within warranty period or shortly thereafter. One of the primary reasons for failure beyond warranty period is governed by the quality of lubricating oil being used. Often quality is misunderstood with viscosity grade. Quality per ?say? has no relation to viscosity grade; it refers to the cleanliness of the oil.

• Lubricating oil needs to be maintained clean and the desired level is NAS6 for industrial application other than turbine drives. This value of NAS6 also applies to wind mill drives and speed increasers as opposed to high speed drives. The cleanliness value of NAS6 does not readily register with users and to some extent with suppliers of gear drives. To put it mildly, check oil directly from a sealed barrel supplied by OEM for its NAS value and you will invariably find it anywhere around NAS10 or worse. Do not assume it happens only with indigenous supplies as it is far from the truth. Checks conducted with top brand sealed oil drums, indigenous or imported, confirm this is normal and common.
• The onus thus lies with users to appreciate why oil cleaning is required and how does it improve the performance as well as life of the gear drive. It is safe to conclude, which concurs with our observation, that organisation which maintains lubricating oils health is less prone to premature failures. They invariably enhance the life of their drives by any where up to 30% higher than others for same drive conditions. This phenomena can be observed in an organisation &/or plant to plant operations but sadly data and findings are rarely pooled.
• Another disturbing fact is often lubricating oil is procured on price consideration only and neglect issues such as scuffing, scoring, wet-ability etc properties.
• Cost differential between normal mineral oil containing higher levels of sulphur and phosphorous in relation to vacuum distilled mineral oils is around 75-80 per cent more but the usable life of oil, if cleanliness maintained around NAS6, will justify the extra cost as life will be minimum double of normal mineral oil. A cement plant in Eastern India has continually achieved life of three times that of normal mineral oils there by not only resulting in huge savings to the organisation by way of less oil consumption and frequent shutdowns for changing oil.
• Do note, normal mineral oils with higher levels of sulphur and phosphorous have an greater affinity to absorb moisture from the atmosphere leading to formation of sulphuric & phosphoric acids; both are very harmful towards life of bearings, seals and last but not least internal preservative paints applied to gearbox housing walls adding to further contamination.
• A question we need to ask our self, as buyers we seek guarantee and warranty at the drop of a hat then why not for lubricants used?
• Another cause of failure beyond warranty period is the upkeep of breathers, seals, etc. along with external surface of the gear drive. Often it is neglected resulting in breathers getting choked &/or become an ingress point for dirt when drive is stopped. As a result we have oil seal leakages and oil contamination leading to premature failures. Such instances are quite common in conveyor drives of cement grinding section or packing plant, coal handling conveyors, etc. An excuse we at times come across for not maintaining minimal level of cleanliness is, it is not a critical drive! The same excuse is also conveyed when the gearbox is covered with dust. What fails to be appreciated by the user is damage is taking place to investments and it can has a cascading effect.
• There are numerous other instances of failures beyond warranty period but this is nether the forum or place to address these issues.
• Failures during warranty period can be generally summed up under following heads as trends prevailing in gear design are to raise power levels till it does not result in a failure while decreasing volumes thus weight leading to increasing problems of heat dissipation:

• Faulty or inadequacy of design
• Incorrect selection & use of materials for manufacture
• Incorrect selection of bearing
• Manufacturing errors
• Heat treatment errors
• Assembly errors
• Fluctuating or incorrectly defined operating parameters
• Variants from original specification supplied &/or contaminates
• Use of improper or incorrect quality of lubricant

Very rarely only one of the above mentioned causes account for failure to gear drives thus understanding and assessing gear damage requires in-depth knowledge of:

• Gear contact patterns
• Gear tooth failure types and probable causes
• Bearing failure types with probable reasons
• Lubricating oils
• Oil flow within the gear drives be it splash or forced lubrication, etc.

It is not feasible to go through all these aspects in depth through this short article but to create awareness towards minimising risks of premature failures. We as such recommend use of following documents as a starting point to improve performance of gear drives thus overall operations of a plant. The documents relate to what needs to be communicated to the prospective seller and what in return you must get from them without fail.

Info. to be given By gearbox manufacturer

1.With offer for critical drives:
Design calculation in details for safety wrt wear & strength confirming material grade, etc.

2.Along with general arrangement (GA) drawing after placement of order:

• GA drawing for all gear units, unless otherwise agreed upon, that gives full details of all manufactured part numbers and full nomenclature of proprietary parts including prefix and suffix, if any.
• Number of teeth of each pinion and gear to facilitate vibration analysis. ?Spare parts list that can be correlated with GA drawing & the part number.
• Approximate weight of gearbox.
• Direction of rotation of input and output shafts.
• GD? value of critical drives.
• In case of pressure lubrication system water and oil flow rates with pressure range. Should also specify water and oil temperature gradient envisaged between inlet and outlet.
• In case of cooling coil water flow rate and temperature gradient envisaged between inlet and outlet.
• Details of interlocking, if any required to be ensured.

Note:

• Your requirements of above data should be incorporated in your tender or enquiry or most major manufacturers will refuse to comply with the request at a later date.
• Data of number of teeth will not only facilitate vibration analysis personnel but may facilitate in rationalising spares inventory if similar gearboxes are available in the plant or if same series gearboxes are installed of sizes that are just smaller or bigger than that on order.
• Information to be given By a client

Following information are required to be furnished along with enquiry to finalise drive:

1. Prime mover – confirm type with full details like kW, rpm, Hz, type etc:

• Motor
• Turbine
• I.C. engine

2. Input coupling – specify which:

• Pin bush type flexible
• Geared coupling
• Fluid coupling
• Bibby coupling
• Tyre coupling
• Any other than that mentioned above?

Note:

• If coupling is not in the scope of gearbox supplier then its type, make, bore with tolerance of half to be mounted on gearbox are to be furnished.
• Coupling in scope of gearbox OEM then confirm motor shaft diameter & tolerance.

3. Input through belt pulley drive – confirm following:

• Pitch circle diameters of pulleys?
• Direction of rotation of input shaft looking towards it?
• Type of pulleys?

Note:
Provide sketch showing disposition of pulleys with respect to gearbox with dimensions in vertical and horizontal plane.

4. Type of gear drive:

• Configuration of gearbox required i.e. helical, bevel/helical, RH, LH etc.
• Operating hours per day?
• Minimum and maximum ambient temperature where it is installed?
• Place of installation i.e. open space, small confined area or large workshop?
• Environmental condition e.g. normal, dusty, etc.

Note:
Mention if any other speciality is required in the drive.

5. Output coupling – specify which:

• Pin bush type flexible
• Geared coupling
• Any other than that mentioned above?

Note:

• If coupling is not in the scope of gearbox supplier then its type, make, bore with tolerance of half to be mounted on gearbox are to be furnished.
• Coupling in scope of gearbox OEM; confirm machine shaft diameter & tolerance.

6. Output through Sprocket Drive:

• Pitch circle dia of sprockets?
• Direction of rotation of output shaft looking towards it?
• Maximum pull of chain?

Note:
Provide sketch showing disposition of sprockets with respect to gearbox with dimensions in vertical and horizontal plane.

7. Driven machine details:

• Cement mill, coal mill, sugar mill, belt conveyor, kiln, etc.
• Confirm if it is twin drive, etc.
• If possible specify OEM details of equipment manufacturer.