In the quest to increase production, efficiency, and reliability, the limiting factor for improvement is often believed to be the mechanical drive system for the driven equipment, such as pumps, grinding mills and conveyors. This opinion, though, is no longer a valid reason to remain complacent with current production figures. The mechanical drive system, consisting of couplings, a gearbox, and a gear set, can be uprated and updated to increase power density and, in turn, increase production.

The quality of mechanical drive system components has evolved and improved over the years. For example, the gearing from a girth gear manufactured in the 1950s will not have the advantages of computerized design and production or improved material hardness, as does its counterpart from the 1990s.

The advances in drive system design, manufacturing capabilities, and materials technology allow for existing components to be replaced with more durable and efficient equipment with significantly higher power densities. Implementing these technological advances into a vintage mechanical drive design will result in a more efficient, durable system that can meet increased production demands.

Technological improvements Some of the greatest advancements in drive system components have occurred with the girth gear set, consisting of a gear and pinion. Material technology, manufacturing techniques, and rating standards have all changed, which now provide uprating and upgrading opportunities to facilities with vintage mechanical drive system designs.

First, material technology for steel castings has been refined, resulting in techniques that have drastically improved quality, integrity, and component hardness. In the 1950s, state-of-the-art material technology could only produce gear blanks with a hardness of 180 HB (typical) or 225 HB (maximum) and pinions with a hardness of 265 HB or 285 HB. Now, with the availability of high-hardness girth gears and through-hardened or carburized pinions, durability and strength rating increases can be realized with the installation of such components. When installed as a replacement for an older drive system, durability rating increases exceeding 100%, and strength rating increases exceeding 50%, can be achieved using the latest technology gears and pinions.

Second, manufacturing methods have improved component quality. Tooth accuracy of pinions and gears is significantly more accurate, due in major part to modern-day machinery used in the manufacturing process. The gear tooth quality that 1950s' (or earlier) vintage gear cutting equipment could produce is in the low range of American Gear Manufacturers Association (AGMA) standard 6 for gears and AGMA 8 for pinions, compared with today's available quality levels of AGMA 10 for gears, and AGMA 12 for pinions.

Last, the equipment rating standards developed by AGMA have changed. In an attempt to quantify difficulties that arose in the manufacture of such large gears, rating methods differed for open girth gears and enclosed gears. Originally, AGMA reduced the rating of large gearing, such as girth gears. Now, due to improved materials and manufacturing methods, rating standards more accurately model the actual performance of all gears.

Uprating, updating options Depending upon the required increase in production and the budget in which to make physical improvements, there are different uprating and upgrading approaches to achieve the necessary output level: replace pinions; recut girth gears; replace girth gears; uprate the gear drive and couplings; or a combination of the following options.

1. Replace pinion in girth gear set Taking into consideration the improvements in materials, manufacturing, and rating standards, a higher-hardness pinion can be installed to increase the power density and production level of a vintage girth gear set.

AGMA rating practices allow a rating increase by increasing only the hardness of the pinion. The replacement pinion can be either through-hardened or carburized, depending upon the rating increase desired.

It is important to note that the rating increase from replacing the pinion assumes the girth gear is in like-new, as-manufactured condition. This is typically not the case and, therefore, the full rating increase may not be realized. An exact value for adjusting the rating of the girth gear set, due to gear wear or damage, cannot be assessed without a thorough inspection of the gear.

When replacing a girth gear set's pinion, tooth modifications can be performed. Grinding the new pinion's teeth to specific modifications will increase load-carrying capacity and operating contact, and extend service life (see Figure 1, page 37).

For a typical application, such as a grinding mill, a significant production increase may be possible by increasing the number of pinion teeth. This results in a lower total gear ratio, which increases the mill's speed and, in turn, production ability. The exact speed increase depends on the original number of pinion teeth. For example, increasing the number of pinion teeth from 19 to 20 will increase the mill's speed 5.3%. A 4.5% to 6.25% increase in speed is possible when adding one additional pinion tooth. Keep in mind, an increase in speed will put the driven equipment closer to a system's critical speed. The original equipment manufacturer (OEM) should be consulted when making this change.

Increasing the number of pinion teeth, though, may require that the girth gear set's center distance also be increased. Allowance for this is usually available in the pillow block foundation bolt holes. If not, the bolt holes can be slotted to accommodate the increased center distance.

When implementing any system upgrade that will increase speed, remember that the motor must be capable of providing the extra power required to drive the system at the increased speed. This is typically not a problem, as most systems draw less than full motor power. If a facility is operating at motor nameplate power, though, adding additional cooling to the motor can usually increase the motor power. When faced with this situation, the motor manufacturer should be consulted for a proper recommendation.

2. Replace pinion and recut girth gear Recutting the teeth of a girth gear restores the original tooth form and makes it possible to take full advantage of a new pinion (see Photo 1, page 37). In addition, the gear structure is completely inspected during the recutting process and any defects are repaired. This provides the structural integrity required to transmit the increased torque.

Defects in the gear structure are identified by non-destructive testing (NDT) methods, such as magnetic particle and ultrasonic inspection, and weld repaired (see Photo 2, page 38). If required, weld repairs are made before the girth gear is completely stress relieved and/or heat-treated to ensure proper integration of the repair with the base metal (see Photo 3, below). All surfaces of the gear are machined to ensure dimensional accuracy and that geometric tolerances meet or exceed the original design.

A complete design review is undertaken to validate all aspects of the gear and pinion design and manufacture. This updates the design with current design methodologies and rating practices.

The new tooth surfaces also yield increased efficiency benefits. A recut gear operating with a new pinion restores the gearing to 99% or greater efficiency (see Photo 4). This can translate into significant operating cost savings. For example, a 1% increase in efficiency for a 1,341-hp mill will result in savings of $3,500 per year.

The cost of refurbishing a girth gear will vary, depending upon the amount of repair that is required. Typically, refurbishing an existing girth gear is 30% to 50% of the cost of a new gear. The exact final cost depends heavily on the amount of weld repair required to the gear structure.

3. Replace pinion and girth gear A third uprating option is replacing both the pinion and girth gear with new counterparts. This allows the use of the latest design methodologies and rating practices. Also, advantage can be taken of the many improvements in manufacturing and materials technology. The rating increase achieved by updating to a modern drive system design is dramatic.

4. Replacing gear drives and couplings Uprating the girth gear and pinion is useless if the main gear drive and/or couplings cannot transmit the increased torque. In some situations, these components will need to be uprated or replaced with new counterparts.

Similar to the girth gear uprating, the gear drive can be replaced with a higher hardness gearing. For a gearbox with through-hardened gearing, the typical uprate achievable using only a carburized pinion is 15%. Replacing both the pinion and gear with carburized elements will result in an uprate of as much as 50%.

At the same time the gearing is replaced, new bearings employing the latest material and manufacturing technology should be installed. Bearing manufacturers have released E-type spherical roller bearings that have significantly more load-carrying capacity than a similarly sized standard bearing. The typical uprate using E-type bearings in place of standard bearings is 15%.

Rating practices for bearings also have evolved to include adjustment factors for lubrication, cleanliness, and load zone. Using these factors can either increase or decrease the calculated life of a bearing. The result is a better understanding of the actual operating life the bearing can be expected to achieve.

New seal design technology, such as Taconite, Magnum, or non-contact, also can be installed during a gear drive upgrade to provide better leak protection, reduced operating temperatures, and longer seal life. New seals with higher allowable operating temperatures, such as Viton seals, are additional technological improvements that can upgrade and uprate a vintage drive system.

Shaft couplings have experienced a similar uprate over the last 30 years. New materials and manufacturing processes have increased shaft coupling power density 70% for grid-type couplings, compared to its vintage counterpart. The rating of gear-type couplings have increased at least 55%, size for size. In essence, a dimensionally interchangeable coupling with a much higher rating can be used, or a smaller coupling with the same rating can be installed, to significantly reduce costs.

Summary There are many ways to increase the power density of existing mechanical drive systems, in order to meet increased production demands. Due to the improvement of materials, manufacturing techniques, design methodologies, and rating practices over the past 40 years, the mechanical drive system is no longer a limiting factor in achieving higher production goals.

Depending upon the uprate and upgrade necessary, and the budget in which to get it accomplished, several options are available. When deciding which one is most appropriate, consult with an OEM for expert advice.