Proper lubrication makes gear maintenance easier
Mill gears often have a design life greater than 20 years, but mines rarely run perfectly. Misalignment, contamination, lube system failures, overloading, or failures in the mill drive-train can result in premature wear of girth gears that result in high vibrations. When gears are damaged, the operator is left with a few costly options:
- Continue to run the gear with the risk of catastrophic failure
- Grind the damaged areas to relieve stress risers
- Turn the current or install a new pinion.
Catastrophic failure can bring a mill down for months. Lead times on new gear sets can reach 6-12 months and the time and equipment required for installing a new gear set is considerable. Also, mills that are allowed to run under high vibrations cause risk to the surrounding equipment and foundation.
Mechanical treatment to the damaged areas can prolong the life and reliability of the gear, but should be carried out by a trained engineer. Grinding requires significant downtime to remove the gear cover and grind each tooth flank and while grinding may alleviate stress risers, it may not lower vibration levels significantly.
Using the unused side of the pinion or installing a new pinion also requires significant downtime. Once the new surface is in operation, there is still a chance of transferring the damage from the bull gear to the new pinion surface.
Repair lubricants can be used as a stand-alone treatment to alleviate stress risers in damaged tooth flanks, prevent surface asperities from welding after grinding, or help mate a new pinion surface to a damaged bull gear. The advantage of using the Klüber Lubrication line of service lubricants is that the girth gear drive remains fully operable.
Compared to several days of standstill, as in the case of mechanical treatment or gear replacement, repair lubrication only requires short downtimes for control and documentation. In fact, repair lubrication is even more successful with higher loads.
Klüber Lubrication repair lubricant, Klüberfluid D-F 1 Ultra, contains specially developed compounds that subject the metallic surfaces to chemical and corrosive wear in the contact points. Klüberfluid D-F 1 Ultra is manually applied by a service engineer while the running-in compound, Klüberfluid B-F 2 Ultra, is applied through the lubrication system. It contains a unique additive package that encourages mixed friction, but prevents surface asperities from welding.
The combination of Klüberfluid B-F 2 Ultra and Klüberfluid D-F 1 Ultra can repair damages such as scuffing, scratches, plastic deformation, and pittings by eroding material at the overloaded areas. This leads to an improved contact pattern which reduces the specific tooth flank peak loads. Due to the minimally controlled wear, repair lubricants can also be used to prevent transfer damages from an older girth gear on a new pinion surface.
Recently, a zinc concentrator near of Montreal, Quebec, contacted Klüber Lubrication for a consultation on a ball mill experiencing significant vibrations. After welding a large crack in the shell, the mill shell became distorted, which caused the axial and radial runouts to fall outside of the recommended values. They had a mill on hand, but due to operational demands and a foundation crack, they were unable to schedule downtime long enough to install the new gear set for 10-12 months. The condition of the mill had become so bad that they decided to reverse the direction of the single pinion drive to continue running, but were still experiencing high vibrations.
An engineer inspected the gear and discovered that the vibration levels were approaching unacceptable levels according to ISO 10816-1. The edge-to-edge temperature differential was over 15°C and the contact pattern was less than 50 percent. The condition of the tooth flank could not be determined due to the over application of an asphaltic based open-gear lubricant preventing the visual inspection of the tooth flanks.
Klüber Lubrication recommended a repair procedure using Klüberfluid B-F 2 Ultra and Klüberfluid D-F 1 Ultra. A member of the our team performed the repair procedure over a 24-hour period and continues to monitor the situation.
After the repair, the operational open-gear lubricant was changed to Klüberfluid C-F 3 Ultra. This lubricant is transparent on the gear surface, which allows the tooth flank condition to be monitored visually. It also offers improved wear protection while reducing the amount of lubricant needed.
The repair resulted in an overall tooth flank temperature reduction of 12°C and the edge-to-edge differential was reduced from 15.1°C to 1.9°C, indicating a contact ratio >90%. The vibration levels were reduced up to 48% and brought back within acceptable limits according to ISO 10816-1.
All in all, lubricants and services for open gear drives can result in significant cost savings in terms of production up time, lubricant consumption, and cost to repair damaged gears.
Justin Koozer, Klüber Lubrication NA, Global Competence Centre, Mining.