Rebuild, upgrade, or replace that gear drive?
Cost is only one of many factors that enter into the decision of what to do when a gear drive fails or shows signs of damage from premature wear. The wide range of choices available means that, for the most part, there are few obsolete gearboxes. It is increasingly important to expand what was once an either/or decision to "rebuild it or replace it" by considering other options, including: On ...
- Exact replacements may not meet current operating demands.
- It is often economical to rebuild expensive gearboxes..
- Gearbox rebuilders should have adequate test facilities.
Cost is only one of many factors that enter into the decision of what to do when a gear drive fails or shows signs of damage from premature wear. The wide range of choices available means that, for the most part, there are few obsolete gearboxes.
It is increasingly important to expand what was once an either/or decision to “rebuild it or replace it” by considering other options, including:
- Repairing the gear drive, which involves minor work such as replacement of bearings and seals
- Rebuilding or remanufacturing by inspecting and reworking or replacing gearing and other major components
- Upgrading, including improvements that increase performance to prevent premature failures or meet higher operating demands
- Scrap the old drive and order a replacement. This approach is often the least expensive and most expedient option if the damage is significant and a replacement gear drive is readily available.
On older equipment, available gear drives may no longer be exact replacements and changes in operating conditions may require a different approach.
Analyze the application
A common rule of thumb, known as the 60% rule, is a starting point in deciding whether to repair or replace a gear reducer. The rule states that if a repair costs more than 60% of new unit price, the old unit should be scrapped and a new one purchased (Fig. 1). This consideration is only a starting point because there are many other factors to consider.
Fig. 1. Disassembly and inspection are the first steps in determining the extent of work a gear drive needs.
For example, the 60% rule does not consider the cause of the malfunction. If the damaged gear set was overloaded, an exact replacement does not permanently solve the problem. If a replacement is no longer available, the cost of re-engineering the equipment to accommodate a new gear drive may be more than that of a remanufactured or upgraded unit.
If the original gear set was improperly selected or applied, a simple replacement does not last long. Gear sets are often selected based on motor horsepower and speed, but there may be many other factors to consider.
In one case, the lubrication system on the gear drive of a turbine generator set had no means of adjusting the oil pressure to the gear set. While the gear mesh was sprayed at the required 20 psi to provide proper lubrication, the turbine itself could stand no more than 5-psi lube pressure. When the system pressure was turned down, the gear mesh was starved for oil, resulting in premature gear wear. Simply replacing the gear set would have only perpetuated this problem.
It is seldom economical to go beyond simply replacing small, fractional horsepower gear drives. Replacement units are generally available off the shelf at a cost of less than all but the smallest repair. The main exception is when a replacement is no longer available and adapting a different gear drive to the equipment is not cost effective or practical.
As the size of the gear drive increases, the decision becomes more critical. The higher cost of the gearbox and gearing components often makes it more attractive to remanufacture than to invest in a new gearbox, especially if the old one can be upgraded and improved at the same time.
The first thought when a gear reducer fails is to turn to the original manufacturer. This approach works well when the unit is an off-the-shelf product or an existing design that is still in production. At other times, a repair specialist may be a better choice, both to provide improvements and adaptations not otherwise available, and to expedite delivery.
Unlike a manufacturer that has to break into a production run, a company that specializes in gear service can quickly produce an emergency part for a given machine. This responsiveness can help minimize downtime costs.
Evaluating the application at this stage takes advantage of 20/20 hindsight that experience provides. When an original product was developed, the designer may have had limited knowledge about the operating conditions. Catalog selections are usually made from a range of incremental sizes and ratios, which may not fit the application precisely.
Normally, this approach poses no problem, but a company specializing in rebuilding gear drives has the advantage of knowing details of the exact application, operating conditions, and failure modes. This information makes it easier for the company to tailor the rebuilt or upgraded gear drive more closely to the specific needs of the application.
Fig. 2. Testing is an important step in verifying and documenting the performance of remanufactured or upgraded gear drives.
When selecting a supplier to perform any rebuilding or upgrading, check out the company’s capabilities across the spectrum of services that might be required (Fig. 2). Opening a failed gearbox can be the start of a journey into the unknown. Be sure to get an unbiased opinion on the best course of action.
For example, a company with limited capabilities to evaluate and remanufacture gearsets may not be able to carry the process far enough to get to the root cause of the failure. All work should meet or exceed OEM specifications and American Gear Manufacturers Association standards.
Just as important is the company’s ability to perform appropriate tests after the work is finished. While a spin test should be standard procedure to verify that the basic mechanism and lubrication system work properly, a supplier should be able to provide such services as dynamic load tests, sound and vibration monitoring, bearing temperature monitoring, oil flows and pressure, and similar verifications when required (Fig. 3).
Fig. 3. Typical report provides assurance of quality and acts as a benchmark for monitoring and evaluating future performance.
Typically, repairs focus on replacing normal wear parts, such as bearings and seals, and possibly reconditioning shafts for better bearing and seal fits. Sometimes, a certain amount of gear repair is also performed, such as reconstructive grinding to remove minor flaws. This action increases backlash slightly, but if loads are fairly constant and in only one direction, it should not cause a problem.
Casehardened gears that have been running in one direction and on one flank for their entire lives can sometimes be turned around to run on the other flank. This change requires little or no modification.
Through-hardened gears may need to be turned down slightly and recut to remove damaged areas, yet maintain the original shaft centers. In this case, new oversize pinions are needed to mesh with the smaller diameter recut gears. In most cases, the change in diameter can be accommodated without increasing the number of teeth in the pinion. The result is a minimal change in ratio from the original design.
Remanufacturing goes beyond simple rebuilding to provide a like-new or better-than-new gearbox that fits the existing foundation without the need for costly or lengthy modifications (Fig. 4). This option is practical when a replacement would require a long delivery time and the need to limit downtime is important.
Fig. 4. Remanufacturing incorporates machinig major parts in the existing case to provide a like-new or better-than-new gearbox that fits the existing foundation.
Reusing the original case helps keep costs in line and, depending on the extent of work needed, remanufacturing can be less costly than a new unit.
The size of gear drive designs has gotten considerably smaller due to factors such as improved metallurgy and heat-treating. Since a new gear drive often cannot be bolted directly in place of the original, remanufacturing allows the original case to be reinstalled in its original location without costly re-engineering.
Remanufacturing typically includes making new gears, although existing shafts may be used if they are in good condition. Relative costs and lead times of new gear sets vs other alternatives should be considered (see table). In many cases, there is an inverse relationship between lead times and costs; shorter lead times can usually be obtained at a higher cost. Sometimes, a rebuilder may change the gear tooth configuration slightly to match existing tooling, but a reputable supplier never sacrifices gear capacity in the process.
Fig. 5. The housing should be checked and reworked if needed, including milling the parting line and reboring.
In remanufacturing, the housing is checked and reworked if needed, including milling the parting line and reboring the housing (Fig. 5). Sometimes, this rework can correct misalignment due to wear or manufacturing errors and prolong the life of the remanufactured gear drive.
Key components of the gear drive are repaired or replaced as needed. For large diameter gears, if there is enough material remaining under the teeth on the rim of the gear, it may be feasible to turn the original casting or fabrication down and shrink a new, forged rim in place. After the rim is shrink fitted and staked in place, new gear teeth can be cut, saving much of the cost and lead time associated with procuring a large cast or fabricated blank.
Even where the cost of remanufacturing approaches or exceeds that of a new gear drive, there may be good reasons to make the investment if it can be upgraded at the same time.
Gears are often upgraded in quality and rating by using casehardened and ground gears instead of through-hardened gears, allowing them to handle 50—75% greater loads. This upgrade is usually done because operating conditions or performance requirements have changed, resulting in an increased load on the gear drive.
Other components, such as shafting, may also be upgraded to handle added capacity or provide better service under specific conditions. While some original shafts may have been made of medium carbon steel, a company performing an upgrade generally uses alloy steel shafts.
For one user, a large gearbox was modified so it could be mounted on either the top or bottom surface. This approach is often used to allow right hand or left hand mounting, or to accommodate a normal asymmetrical wear pattern by flipping the housing over and transferring the wear to the opposite side of the gear teeth.
An area ripe for upgrading is adding accessories to an existing gearbox. Features such as an improved lubrication system, extra temperature sensors, and better filtration can be added while the unit is in the shop. These options can help extend its life at a nominal cost.
Replacement may include replicating a unit that is no longer available. Custom gear drive rebuilders can make drop-in replacements for obsolete models that are beyond repair, especially if the housing is damaged, cracked, or seriously worn. A replacement can save re-engineering and revising the adjacent power train components.
The same holds true for gear drives that are only available from overseas sources, with long, uncertain delivery schedules. Where downtime costs are high, a replacement that bolts in place can be made while the old unit is still in operation, minimizing the downtime needed for replacement.
A variation sometimes used to shorten downtime for critical applications, where imminent gearbox failure is anticipated, is to make a set of sacrificial gears with a shorter life but a lower cost. For example, the gears might be through-hardened instead of carburized and ground. These lower cost gears are then installed to keep the equipment running during the lead time needed to manufacture the higher grade gears, which could be installed during the next scheduled maintenance downtime.
One of the keys to obtaining a satisfactory rebuilt or upgraded gear drive is communication. Whatever the decision making and authorization process, it is essential to convey as much accurate information as possible to the people who will be working on the unit. Often, the people who work on the equipment are the only ones who really know the history of what happened to it.
Putting the person on the shop floor in touch with the rebuilding company as part of the process ensures the flow of firsthand information necessary to provide an effective solution.
— Edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org
The author is available to answer questions about gear drives. He can be reached at 504-524-2363.
Alternatives to new replacement gears
|Alternative||% of new cost||Typical lead time|
|New gear set||100||5—8 wk|
|Reverse symmetrical gears||5—15||1—2 days|
|Reconstructive grinding||25||2—3 days|
|Manufacturing a new pinion and remanufacturing a bull gear|
|Reduction & recutting||60||4—5 wk|