Production goals are ever changing and usually in an upward direction. For example, production management may dictate that belt conveyors run faster and with an increase in loads. Sometimes the production managers who set the goals don't realize how wide ranging the ramifications of those decisions can be on the plant engineers and maintenance people responsible for designing and maintaining the machinery driving production. Since production changes affect the entire drive system, what's a plant engineer to do?
Design and maintenance teams
Any upgrade to a process has two distinct phases that include design and maintenance evaluations.
The first thing to do is create a team from the plant engineering and maintenance staff to design and implement the drive system upgrade. The design phase, usually the responsibility of plant engineering, determines the power and mechanical needs of the upgraded drive train system. System designers must develop an overall scheme for powering the new higher loads.
Subsequently, the maintenance department must undertake a careful and thorough review of the current drive system components that may be used in the upgraded system. It is essential that these two phases be done in tandem. Doing one without the other is asking for trouble and probable downtime later when the upgraded system is asked to drive the new loads.
Drive upgrade design considerations
Many drive systems, such as paper machines or conveyor belts, may have several drive "stations" along the production line. However, for simplicity and clarity, let's focus on upgrading one drive station on the production line. The principles and practices described here apply to every drive unit.
After determining the new production line's overall power requirements, plant engineers must qualify the drive train's motor and gear drive rpm, horsepower, torque, and thermal rating requirements. Questions to ask include:
- Is a higher horsepower motor needed?
- Does the motor rpm need to change?
- Will the existing motor do the job?
- Do the gear drive and coupling horsepower, output torque, and mechanical and thermal ratings meet the new system's requirements?
You must take into account horsepower, torque, and rpm in qualifying the design phase of the upgrade. After determining the new loads on the driven equipment, the best thing to do next is to call a gear drive manufacturer's representative to analyze whether the existing power transmission components rate both mechanically and thermally and can be used in the upgrade.
It is not enough to look only at the gear drive's nameplate to determine if it will meet the new mechanical requirements. For example, a common mistake involves the Service Factors. A Service Factor of 1.5 on the nameplate does not mean that the drive can handle a 50 percent increase in horsepower (assuming it was running at a SF of 1.0). The upper range of the Service Factor number indicates the drive's ability to handle momentary peaks, not continuous duty operations.
Another common mistake is assuming that changing one factor in the equation won't affect the rating requirements of other componenst in the drive system. For example, changing the motor rpm affects the power rating requirements of the gear drive. It's also not true, as some people assume, that an upgrade designed to slow the system will result in easier operations for the gear drive. A slower low speed shaft increases the drive's output torque and the drive system may then face a high torque requirement.
Qualifying the system for thermal rating is as important as doing so for rpm, horsepower, and torque. The upgrade may cause a change in the drive system that alters the thermal requirements. The old thermal ratings may be insufficient to handle the requirements of the upgraded drive system. The upgrade may require larger drives or auxiliary cooling devices such as fans or another type of heat exchanger. A metering oil pan on the drive may be needed. It is more cost effective to determine these requirements in the qualifying stage. If you don't learning of them after system startup and discovering the need to plumb water, run electrical lines for cooling devices, or disassemble the drive to add an oil pan is the only option.
One of several scenarios is likely following design phase findings. New equipment is required because the old equipment does not rate, some machinery may rate and can be used, or perhaps the old equipment rates and can be configured to power the new loads.
Maintenance qualification of those components that checked out okay in the design phase is the second step in the upgrade process. However, just because the "old" gear drive and couplings are rated appropriately for the new system does not mean that you can hook up a motor to the gear drive and call the upgrade complete. You must carefully inspect the drive components to make sure they are in condition to assume the new loading. If maintenance discovers problems, then you can make repairs or order replacement parts before system start up.
A complete inspection of the gear drive bearings is one of the most important maintenance steps in the qualifying process. The gear drive's operational history dictates the condition of the bearings.
Inspect the bearings and gear drive housing bearing bores. Worn bearing bores can lead to premature bearing failure that may result in a catastrophic gear drive failure.
It doesn't take an expert to determine that a bearing is in bad shape. First, remove the drive housing cover and inspect the bearing cage for wear or cracks. Next, inspect the bearings, turn the rollers, and make sure there is no surface distress. Visual inspection of the rollers and races may reveal shallow holes in the surface (pitting), scuff marks, peeling of metal, spalling, or scoring.