Repairing Large Girth Gear Drives in Full Operation
- When it comes to repairing the damaged tooth flanks of large girth gear drives or preventing damage to a newly installed or turned pinion, repair lubrication provides a number of advantages over mechanical processing.
- Even the operation of seriously damaged gear drives can be improved through repair lubrication so that replacement can often be postponed for several years. For operators, the girth gear drive can remain in operation and production can continue.
Not too long ago, tooth flank damage in large girth gear drives was repaired exclusively by applying mechanical treatments and costly repair procedures. High tooth flank qualities could not be obtained as demands regarding the life span and operation of a girth gear drive prevailed. In turn, costs from the loss of production and the man power required to repair the damage were very high.
The decisive advantage of repair lubrication over mechanical processing is that the girth gear drive remains fully operable and that high loads even increase the success of repair lubrication. Compared to several days of standstill as in case of mechanical processing, repair lubrication only requires short girth gear drive downtimes for control and documentation purposes during the repair lubrication process.
Principle and Effect
Repair lubricants wear off material. They contain specially developed compounds that are highly active and effective. These compounds subject metallic surfaces to mechanical, chemical and corrosive wear. Thus a small amount of lubricant can remove just the right amount of flank material – corresponding to the customer’s repair needs and technical feasibility – in a short period of time. The material is worn off evenly at the contact points. The ability to repair through material removal is limited by the thickness of the hardened tooth flank surface layer. There is only minor influence on the hardened surface layer itself. Surface wear and hence damage repair can be controlled by the amount, application duration and reaction time of the lubricant in each application cycle, and by the repair time as a whole. By applying controlled wear, the tooth flank profile is evened across the entire tooth width and height so that it matches its mating gear.
Scuffings, scratchings and scorings are very easy to repair with this technique. Plastic material deformation can be repaired up to a certain extent. When pittings are encountered, the progress of damage and the formation of new pitting are prevented by eroding material at the overloaded flank parts, which leads to a more even load distribution over the tooth flanks and hence a reduction of specific tooth flank peak loads and a higher rolling endurance. Repair lubricants can also prevent transfer damages from an older girth gear on a new pinion flank.
In mill drives, successful repair lubrication can be achieved in a relatively short time. Compared to kiln drives, the speed of rotation is higher and thus tooth contacts are more frequent. The repair procedure is also accelerated by higher sliding speeds. Depending on the damage, repair lubrication takes one to two working days for mill drives. Kiln drives require higher amounts of repair lubricants due to lower rotation and less frequent tooth contacts. In this case, the process of repair lubrication takes about two to three working days.
Limitations of repair lubrication
Repair by controlled wear is limited by the degree and type of damage. Material removal of a few hundredths to several tenths of millimeters can be achieved. Generally, “only” a few hundredths millimeters are removed, but this can lead to significant operational improvement.
It is not recommended to use repair lubrication for the repair of burrs at the tip and the side of the teeth, material elevation, wear steps at the tooth root, sharp edges and deep pittings. These types of damage have to be repaired mechanically by milling (end-milling cutter, spherical cutter) or grinding (cutting-off wheel for material steps or abrasive buffs for flank treatment).
How the operator is involved
For the preparation of repair lubrication, it is vital to identify the causes for the damage and to eliminate them. Depending on the type and degree of the tooth flank damage, mechanical treatment (grinding, milling) might be required prior to repair lubrication.
To document the condition of the tooth flanks and the contact ratio unambiguously before and after repair lubrication, photographs are taken and silicone impressions of some representative tooth flanks are made. It is important to always document the changes of the same tooth flanks. In doing so, the operator obtains an illustration of the actual condition of the flanks and how it has changed.
Repair lubrication is affected under operating conditions, thus permanent operation is normally ensured, although short interruptions needed for inspection by the service engineer have to be feasible. Despite the damage, a contact ratio of 60 percent or more should be achieved prior to the repair lubrication process because otherwise the gear drive will have to be readjusted.
While the lubrication system runs on “continuous operation” with a running-in lubricant, the repair lubricant is applied by the use of manual spray equipment, which is connected to a pneumatic system. The manual spray equipment is used to apply the repair lubricant in larger quantities to the bearing pinion flanks and the gear rim so that the lubricant is distributed evenly over the tooth flanks. The process of material removal begins. To correct elevations on the tooth flank profile afterwards, the repair lubricant is applied in smaller quantities.
All in all …
Due to the complexity of the repair process and the involved danger of inadvertently causing damage, repair lubrication should only be carried out by a trained and experienced application engineer. Nevertheless, if implemented correctly, repair lubrication is a very interesting alternative to mechanical treatment. When it comes to repair issues, repair lubrication should always be the first choice.
Operators of large girth gear drives can avoid costly production downtimes as the repairing process can be implemented in full operation. In addition, the repair lubricant on tooth flank surfaces improves the girth gear’s operational performance and prevents new flank damage. Replacements can often be postponed for years.