For a technician to understand where to place the balance weights on an engine, the vibration/balancing system must know the engine speed and the phase of vibration.

Throughout the aviation industry, jet engine vibration is an everyday concern. Maintenance, repair, and overhaul (MRO) crews worldwide are tasked with monitoring engine vibration to ensure flight safety and efficient service.

Overall gas turbine engine vibration, however, is the summation of contributions from various moving parts within the engine. To correlate vibration magnitude with specific engine components, maintenance engineers rely on vibration analysis and trim balancing tools.

Vibration analysis detects discrepancies in rotational machine dynamics while trim balancing is used to reduce vibration amplitudes of gas turbine shafts. Together, they help engineers ascertain and correct individual sources of vibration within an engine.

Vibration analysis, balancing

MTI offers the PBS-4100 as two different models. A lightweight system (PBS-4100+) is used on the ground for installed engines. A test cell version (PBS-4100R+) is intended for use in a production or overhaul facility. The principle of operation is identical for both systems.

A modern turbine engine will typically contain two or three concentric shafts with compressors, fans, and turbines. These shafts – referred to as spools – are aerodynamically coupled, so each spool turns at a rate variable to its fellow spool. Consequently, each spool contains a speed pickup or tachometer so rotational velocity and the spool rotational angle can be known.

In addition, vibration sensors are affixed to one or more positions on the engine case, measuring the magnitude of physical shaking. These built-in tachometer and vibration sensors are intended to provide a means for measuring speed and vibration as the engine operates. Reliable signal detection, however, is no trivial task as the signals are extremely noisy. Complicating matters further, different engines feature different signal types.

The PBS-4100 turbine vibration analyzer/balancing system gives users the ability to check the engine’s vibration amplitude and balance that engine if necessary. It does this with a series of on-board digitizers designed to measure each spool’s rotational speed and magnitude of vibration. Embedded logic assesses each engine spool’s 12 o’clock position to understand where an imbalance might be located.

A series of configurable tracking filters correlates vibration to the spools. These specialized computer algorithms measure the rotational speed of a given spool and then filter the vibration content outside a narrow band of interest. The narrow band of interest is the characteristic frequency of vibration around each spool’s rotational speed.

By using the tracking filter for each speed, the contribution of vibration of each spool can be separated. As vibration varies with engine speed, measurement data is stored and presented in vibration-versus-speed trend plots.

MTI’s TSC-4800A signal conditioner generates useful output signals from many types of inputs.

Vibration surveys

In testing an engine, the operator will execute a vibration survey on the turbine engine – a slow cycling of engine speed from idle to maximum, then back down to idle again. As this occurs, the vibration analyzer will measure the vibration contribution from each of the spools and chart its findings on a series of plots. The overall vibration is also plotted.

The operator is warned if pre-defined limits of vibration are exceeded. By the end of the survey, the vibration profile is summarized for comparison against original equipment manufacturer (OEM) recommendations. If the vibration of any given spool exceeds an allowable limit, it is possible to add offset weights to bring the spool into balance, similar to the addition of lead weights by an auto mechanic onto an imbalanced tire.

Because the vibration magnitude and the angular position are known on the spool, a solution may be calculated with a variety of techniques using the algorithms within the vibration analyzer. Such a solution would add one or more precisely defined weights onto designated locations/angular positions on the spool.

Signal conditioning

For some aircraft engine MRO situations, vibration data and balancing are not necessary; only the tachometer signal conditioning function is needed. Therefore, a signal conditioning unit fills the bill. The TSC-4800A uses the same tachometer conditioning technology in the PBS-4100, and can accommodate all types of engine speed signals.

With the signal conditioning unit, users can test engines with a long-tooth or short-tooth embedded N1 signal; engines with older high-voltage tachometer generators; or engines with offset tooth design.The signal conditioning unit can be configured to condition as many as three individual speed signals. Channels A, B, and C can be assigned to a different engine speed signal, and each channel can be individually controlled and programmed to condition different speed signals. Because many engines have primary and secondary speed signals, the signal conditioning unit also offers multiple input sources for each channel.

The PBS-4100+ pinpoints engine problems to eliminate avoidable engine removals.

Streamlining tests

Early analysis of turbine vibration can identify problems quickly, saving the time and cost of engine removal. Implementing such troubleshooting techniques, however, can prove difficult. Mechanical constraints under the engine cowls and the complex design of aircraft wiring harnesses can make the installation of accelerometers, charge amplifiers, and cables a time consuming and error-prone task.

A portable vibration analysis and engine trim balance instrument, such as MTI’s PBS-4100+, connects directly to the engine’s built-in sensors to read necessary signals. Coupled with aircraft-specific accessory kits, the unit simplifies jet engine balancing and vibration testing.

As with the larger test cell version, the portable instrument employs a series of on-board digitizers and configurable tracking filters. With these, the operator executes a vibration survey on the turbine engine. The survey is a slow cycling of engine speed from idle to maximum, then back down to idle again. As this occurs, the unit measures the vibration from each spool and plots its findings, including overall vibration.

MTI Instruments Inc.
www.mtiinstruments.com

Isaac Abbott is lead engineer at MTI Instruments. He can be reached at iabbott@mtiinstruments.com or 518.218.2561.