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Web Exclusive: Proper planning simplifies gear replacement
July 2, 2009By Rick Lynch and Nick Derus
Sooner or later, all gear drives will wear out. Whether it’s 60 years or six years after they were installed, the time will come when they have to be replaced. Proper planning and a well thought out replacement strategy can ensure continued smooth operation.
When a gear drive needs to be replaced, it can be frustrating if an exact replacement isn’t readily available. Because the number of U.S.-based gear-drive manufacturers has dwindled over the last 20 years, there may be no direct replacement or spare parts inventory available for much of their installed base. With ever more frequent design changes, some drives only a few years old have become obsolete. This can leave a user no choice but to search used equipment lists or buy a new and different drive. When the substitute equipment arrives, it may not fit properly or be as well suited for the intended use as the original drive.
Here are several important considerations when replacing a gear drive:
• How does the replacement fit the envelope of the existing drive? The most obvious concern is the base-to-centerline distance. If the old drive measured 10 in. from base to the shaft centerline, and the replacement measures 11 in., this is obviously a big problem – especially if the drive is mounted on a concrete foundation. If the drive being replaced is very old, it may not be an issue, since drives have gotten smaller over each generation, but it can be a deal-breaker if the drive being replaced is of a newer generation. Other important dimensional concerns include space considerations and interference with connected equipment or structures. Also, consider what must be done with the motor to accommodate the new drive.
• How does the replacement fit the connected/connecting equipment? Changes in solid-shaft diameters will affect couplings, sheaves, sprockets and other mounted equipment. Changes in bores of hollow-shaft-mounted drives may make installation difficult or impossible. In some cases, these changes can be accommodated by changing bushings or re-boring. In others, a new hub, bushing or sprocket may be required. If such modifications become too expensive, it may be preferable to replace the drive with one having a special shaft to suit the application, if it is available.
• Are there any foreseeable mechanical issues? Any time a drive is replaced, application parameters should be confirmed. Older drives were designed conservatively, so users could increase power over time without causing any problems. In some cases, motor horsepower and loads were increased by 50 to 100 percent. However, newer drives are developed with less conservative design practices and will not last long if misapplied or overloaded.
Ratios are another major concern, since it is unlikely a new drive will have exactly the same gear ratio as the drive it replaces. In most applications, “close” is good enough, but in some cases, a change in ratio will be detrimental. These may include certain pump applications, dual drives and metering conveyors, among others. In these cases, it may be necessary to request a special ratio that is the same or near that of the old drive.
Relative shaft rotations also may be an issue. If the old drive is a double-reduction gearbox and the replacement is a triple-reduction gearbox, the extra reduction and change in rotation can cause insurmountable problems. This is more of a concern where the driver is an engine with a fixed direction of rotation. On electric motors, the leads can be switched to change the direction of motor shaft rotation. Relative rotation can also be an issue if there is some element in the drive train that must rotate in a specific direction, such as some variable-fill fluid couplings and clutches.
Getting the best replacement drive
With all of these considerations, how can you get the best, easiest-to-install replacement drive? Here are some suggestions:
• First, work with a supplier who has the experience and attention to detail to handle the undertaking. This is not a job to shop around to the lowest bidder.
• Work with a gear-drive manufacturer that has a good library of competitive drive information. Knowing critical dimensional and rating information of the drive to be replaced is extremely important, and the manufacturer’s competitive knowledge can help offset the lack of detailed user records.
• Next, make sure the bidder is given all of the information regarding the application. Original paperwork and drawings are certainly a plus, but if they are not available, get full information from the existing drive nameplate, then verify that it reflects current operation. Make sure the bidder includes a site visit to see the application.
• Ask for a written quotation of what is being proposed and insist that it include full information on what will be required in the interchange. Will it require moving the motor, ordering a new coupling, sheave or sprocket, re-drilling the foundation or other changes?
• Finally, make every attempt to simplify the change-out. Ask the bidder to provide a “drop-in” with an adapter base or bedplate to suit the existing foundation, new coupling hub, etc. What could be easier than taking the new drive off the truck and bolting it in place of the original drive?
Rick Lynch is marketing manager and Nick Derus is applications engineer for Rexnord Industries, Milwaukee.
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