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Let’s face it, people make mistakes – and some mistakes can be quite expensive. Mistakes made in a gearbox rebuild, for example, can cost a plant hundreds of thousands of dollars due to unplanned downtime and even workplace injuries resulting from a bad rebuild. Have you ever taken the time to audit your in-plant or outsourced rebuild facility? Do you require acceptance testing of the components that have been rebuilt to verify they are service ready?

Many rebuild facilities claim they have skilled technicians, quality programs and testing capabilities. But do they really? Realistically, you should not have to just take someone’s word that the rebuild of the equipment was done to the original equipment manufacturer’s specifications. There should be data from a performance test to prove this. If you are not getting such data, then you should be asking why.

Some things to consider with gearbox rebuilds are:

  • What are gearboxes and how can they stop production;
  • How do gearboxes fail;
  • How to prevent these failures;
  • How to test for failures and understand the data.

What Gearboxes Do and How They Can Stop Production

Gearboxes are used to decrease the speed produced from a motor, increase speed from a motor, translate mechanical horsepower to torque {t (lb.-in) = (HPX63025)/RPM} and increase torque {output torque= input torque X gear ratio}. Think of a diesel engine running at 900 rpm and coupled to a speed increaser gearbox with a 1:2 ratio. Now, you have an output speed of 1,800 rpm to run your electric generator.

Gearboxes are used in tandem with electric and hydraulic motors to reach speed and torque requirements. Let’s say you have a gearbox turning a fan that ventilates the restrooms at your office. The gearbox stops working, so the fan stops working. That may be unpleasant and you may not win a workplace popularity contest, but it will not really cost your facility much in production loss. Now, let’s imagine that gearbox is on a conveyor that moves your finished product to the shipping area. That failed gearbox not only causes you to miss your quota but also delays shipment, resulting in customers losing faith in your ability to deliver a quality product.

A gearbox failure also could be costly in downtime. A failed gear reducer on a paper machine can result in production losses costing $70,000 per hour. This can add up quickly, but knowing how this could happen is important in avoiding such catastrophic events.

How Gearboxes Fail

Numerous forces can cause gearboxes to fail. Failure mode and effects analysis (FMEA) is beyond the scope of this article, but here are some basics for those not familiar with gearboxes. Gearbox parts include the housing, bearings, seals and gears. If any of these parts are not handled properly, stored properly, or assembled properly, they can cause the gearbox to fail prematurely.

Bearings are typically the weakest part of a gearbox and can cause a gearbox to fail very quickly. The list of ways a bearing can fail is quite long and always indicates human or latent failure as the root cause of failure. In most gearbox failures, lubrication failure is the result of human error, whether the lubricant is insufficient, inadequate, or unable to perform. Figure 1 shows a lubrication passage that is blocked by a gasket. This failure is the result of human error during the manufacturing of the gasket and assembly of the gearbox.

With so many chances for gearboxes to fail, what can you do? Auditing a rebuild facility is your first line of defense against gearbox failure. Whether your gearboxes are rebuilt at your own site or off-site at a rebuild facility, you can and should audit the facility to ensure you are getting the quality service you require to have a reliable operation.

As previously mentioned, failures are often caused by human error. Even the greatest and most experienced gearbox mechanic will eventually make a mistake. These human errors can be the result of inexperience, lack of training, improper tools, uncalibrated tools, stress at home, stress at work, etc. Your audit should attempt to include all areas where failures can occur. This is to ensure that shop management covers the potential of gearbox failure with a consistent and unbiased operation that includes fail-safes to ensure a quality rebuild and prevent latent failures.

Preventing Gearbox Failure

Anyone who provides a service will always claim to provide a quality product. Policies and procedures are most vital in preventing latent failures, which is the first step in the failure process. It is latent failures that allow human error to occur or go unchecked. Auditing that process will determine if quality can be achieved consistently. General audit items include:

  • Storage;
  • Handling tools;
  • Teardown;
  • Inspection area;
  • Cleaning area;
  • Assembly process;
  • Policies and procedures;
  • Certifications and authorizations.

How can you ensure the rebuild facility follows these policies and procedures in a way that you can track and measure? The most basic quality assurance documentation is a quality control document. This document contains all the steps that should be performed during the rebuild process. It should be signed by two technicians involved in the rebuild process, with one of them a member of management. The purpose of the two signatures is to ensure the work was double-checked.

Figure 2 shows a flange sealant that is .026 inches thick. This is caused by the housing’s fasteners being improperly torqued, putting the bearing bores and input assembly out of tolerance. A two-person inspection procedure is more likely to catch such costly mistakes.

Gearbox Acceptance Testing

The next level of quality assurance is acceptance testing. As the name implies, acceptance testing is any test that measures the performance of an asset, tool, or machine before it is put into service. The obvious question is: Why not test every single gearbox? An acceptance test costs money, including the labor to set up and tear down the motors, sheaves and bolster plate, and the required training and expertise to perform the test.

Acceptance tests are performed at various levels to achieve different results, as shown in Table 1.

If no acceptance test is performed, then any failure that would otherwise be found and addressed in the shop will not be identified until the gearbox is placed in service. Asset managers should review acceptance test results before the gearbox ever leaves the rebuild facility. If the gearbox is not performing to the proper standards, it can be addressed without any additional delays.

The gear wear in Figure 3 is the result of a bearing failure. The bearing, which had an improper fit to the shaft and in the housing, is likely to be the root cause. Acceptance testing with thermal imagery and vibration analysis would catch this failure.

Bolstering Your Uptime

Auditing rebuild facilities is your opportunity to ensure all latent failures are covered by appropriate policies and procedures. Those policies and procedures should prevent or identify any failures caused by human error. Requiring an acceptance test on a rebuilt gearbox verifies policy and procedure adherence

Acceptance testing may add to the cost of the rebuild, so it should be the responsibility of the asset manager to determine if it is required or not.

Implementing a program to audit your rebuild facility and test the finished product will help identify failures before service and reduce unplanned downtime.

Dillon Gully

Dillon Gully worked as a Service Technician for three years at Motion Industries’ Gearbox Rebuild Facility in Pensacola, Florida. His roles included vibration analysis, failure analysis, thermography, and video scoping of stored and dynamic assets. www.motionindustries.com

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