Balancing the myriad rotating parts in a modern commercial aero engine during assembly or reassembly is a tricky business — but it is an absolutely vital one. Depending on the rotor in question, fans, compressor stages, turbine stages, spools and gear shafts in commercial turbine engines may rotate at maximum rates of anywhere from 3,400 rpm to 25,000 rpm. Even the tiniest imbalance between a component’s actual axis of rotation and the axis round which it was designed to rotate can cause severe vibration, performance degradation and damage to the entire engine.
In the initial balancing of a jet engine rotor, there are essentially two ways to correct an imbalance. In some rotor designs, there are designated areas where material can be removed from the rotor to correct the imbalance. The other option is for the operator to add weight. Weights can be either bolted or riveted on the rotor.
While dozens of companies worldwide make balancing machines, only a handful — among them CEMB, MTI Instruments, Schenck Rotec, Hofmann Group and American Hofmann — make balancing machines and analysers commonly used for aerospace applications, according to George Allen, vice president of balancing products and services for Vibration Solutions North (VSN) and chairman of engineering trade association SAE International’s committee on balancing.
Marco Biffi, commercial director of CEMB’s Industrial division, says that in jet engines balancing is performed for each individual rotor, in order to reduce as far as possible the vibration on each component and so minimise vibration of the whole assembly. Firstly, special software is used to optimally position the blades on a fan or compressor hub, taking into account each blade’s weight and centre of gravity, in order to minimise the mechanical intervention necessary to reduce the imbalance. CEMB uses special multi-axis component software with an analysis which splits the total imbalance of an entire assembly into as many as 12 planes. This allows the balancing of each individual stage in multiple axes, not just two.
Since, for most compressor and turbine stages, blades are individually mounted on a hub, the technicians balancing each stage of a new-build or re-built engine must take into account the seating of every individual blade. Mountings are usually machined to an accuracy of within about one-hundredth of an inch (but are often machined less accurately), whereas nowadays the balance tolerance between a turbine stage’s design axis of rotation and its actual axis of rotation can be as little as one one-hundred-thousandth of an inch.
To achieve the remarkable operating efficiencies they are designed to produce, the latest generation of commercial aero engines, along with the new engines now being tested or still being designed, require rotor balance tolerances which were unheard of a decade or so ago. “I think the biggest challenge the gas turbine/turboprop industry has with balance is everything they are trying to do requires tighter and tighter balance tolerances,” says Allen. “The engine spins faster, it’s lighter and the balancing tolerances are orders of magnitude tighter than for traditional engines.”
Another complication is that, because manufacturers are designing the rotating parts of their engines to be lighter and to spin faster, the shafts and rotors of such engines may actually be designed to bend when at rest and become straight when rotating at speed. This, together with the fact that blades in modern engines feature complex shapes in three dimensions to maximise the airflow they produce, increases the complexity of the balancing task and requires application of advanced dynamic calibration and analysis techniques.
Balancing tolerances get tighter
Nowadays, the required balance tolerance for each individual engine stage is often to within much less than one ten-thousandth of an inch of its design axis. Sizeable rotors in the newest engines are now requiring balance tolerances as low as one one-hundred-thousandth of an inch. For a 100lb rotor, this equates to a balance tolerance of 0.16 ounce-inches (the ounce-inch being the standard unit of moment weight used by the balancing industry).
However, traditional tooling used to fixture rotating parts to balancing machines — even tooling made by the balancing machine manufacturers themselves — is often not capable of fixturing jet engine rotors accurately enough during repeated runs ever to allow the technician to obtain repeatable measurements and analysis from the balancing machine.
Accordingly, balancing is often a matter of the technician performing enough balancing-machine runs (20 or 30, say) to obtain one which provides a measurement within an acceptable balance tolerance. However, this measurement may not actually be correct, because the tooling might not have positioned the rotor accurately enough on the balancing machine. “Small amounts of balance can make big differences in the axis [of rotation],” says Allen. “Small errors quickly add up to big assembly errors.”
Balancing has to occur not only upon initial assembly of an engine but also every time the engine is rebuilt after maintenance, repair and overhaul (MRO) work. “Balancing represents a huge portion of the time [involved] in the overall assembly of an engine,” says Allen. He says that, once, the general manager of one turbofan engine which was being developed for production told him that 80 per cent of the entire development delay bottleneck the engine had incurred — a matter of weeks — resulted from the time the company needed to balance the engine’s components adequately.
Allen’s company, VSN - a subsidiary of R&D engineering and manufacturing firm Moscow Mills - is a Vermont-based entity which specialises in rotor balancing technologies and has recently opened an advanced rotor balancing facility for jet engines, power-generation turbines, automotive parts and other complex equipment. It has come up with a solution — recognised by companies as large as Pratt & Whitney (P&W), which uses VSN for balancing training and as an approved calibration source for balancing tooling — to the problem of ensuring repeatability during balance-machine analysis and measurement.
VSN has developed a patent-pending technology — based on kinematics concepts and known as 'Kin-Dex' — which it puts to use in the tooling it designs to fixture rotating parts to industrial balancing machines. The new tooling technology positions the part on the balancing machine so accurately that every run in a series provides an unprecedentedly high degree of repeatability of measurement. VSN says that by allowing extremely accurate, repeatable analysis and measurement, the company’s fixturing tooling slashes the time it takes an operator to complete a rotor balancing operation.
When companies are trying to balance individual rotating components for jet engines, one might see a balancing-machine operator perform five days’ worth of runs on, for example, a low-pressure turbine stage but still not be able to obtain the required balance tolerance. When this happens, says Allen, it usually means the operator is close to reaching the required tolerance but the tooling being used to fixture the rotor to the balancing machine can’t offer the required degree of positional repeatability to produce a reliable measurement. At times like these, “You’re working a slot machine, to all intents and purposes,” he says.
In one case in which VSN became involved, a company found it was taking anywhere from one to more than three shifts to perform the initial indexing of the balancing machine in order to balance a 300lb rotor which required a balancing tolerance in the order of 60 one-millionths of an inch. When VSN introduced 'Kin-Dex' tooling to the process, it reduced the time required to index the balancing machine to 11 minutes. “Balancing time improves with repeatability — if you have repeatability, by default you cut the balancing time down,” says Allen. Rather than taking 20 or more runs to balance a jet engine rotor, operators using 'Kin-Dex' tooling can cut the balancing task down to three or four runs, he says.
Theoretically, in building up an engine module, if each individual stage is balanced to the extremely fine tolerance required then the module itself should be balanced. However, says Allen, in practice many individual stages will have a tiny residual imbalance, even though each stage has achieved a balance within the required tolerance. These residual imbalances cascade from stage to stage and are magnified, with the result that the completed engine module is not adequately balanced. At that point the module has to be trim balanced.
Dave Hicks, PBS system product line manager for MTI Instruments, which makes test cell balancing systems and the aviation industry’s best-known field balancing system, the 'PBS-4100 Plus', says module balancing is generally performed at low rotation speeds: 600 or 900 rpm. “At these speeds, the engine modules are being balanced for mass-unbalance and dynamic considerations are generally not addressed,” he says.
Once each module is balanced, the engine is assembled from the individually balanced modules. The entire engine must then be trim balanced, and tested for performance and vibration before leaving the manufacturing or MRO facility. The engine is balanced in the test cell, using balancing analysis equipment built into the cell.
Hicks says that when engine balancing in the test cell is performed in order to reduce vibration levels, the balancing effort looks at vibration levels throughout the engine’s operating speed range. Most engine manufacturers recommend measuring for balance at five to seven different RPM speeds. “When performing this balancing, the goal is to minimise vibrations caused by residual mass-unbalance, as well as dynamic effects as the engine goes through its operating speed range,” he says.
According to Hicks, in most modern engines the vibration response is relatively linear — that is, the engine shows similar vibration characteristics at rotation speeds throughout its operating speed range, rather than peaking substantially at just one or two RPM speeds. In linear-response engines, manufacturers recommend the best average solution to higher-than-desired vibration.
Test cell balancing of engines can generally be performed in less than an hour, Hicks says. Vibration testing is simply one of the tests performed while the engine is in the test cell for a series of tests, such as checking for leaks, fuel consumption, exhaust gas temperature (EGT) margin measurement and performance testing. Balancing can be accomplished with the data acquired from one 90-second acceleration run. The collected data is used in conjunction with influence coefficients stored in the balancing equipment’s computer to determine the required weight (and where it should be placed), which will reduce vibration to acceptable levels.
In measuring an engine’s vibration in the test cell, MTI Instruments’ systems first measure the amount of vibration and then are able to identify the engine module or part which is causing the vibration problem. If part of the engine core or an accessory is causing the vibration, then the organisation trying to balance the engine may not be able simply to balance the vibration out. In such cases, or in cases where the engine’s overall vibration is strong, the company will elect to tear down the engine and rebuild it.
Pencil weights and blade mapping
Allen says that if an engine is vibrating a little more than the manufacturer or MRO shop would like, the vibration is usually damped by performing a trim balance exercise on its fan — mainly because, in an assembled engine, the fan is the only part of the engine to which technicians can easily obtain access.
Trim balancing the fan involves adding small weights — known as “pencil weights” — to the fan hub or the base of the fan spinner cone (the CFM56 family has this arrangement). The cone has from 30 to 40 locations round its base into which these small weights can be screwed flush. In the CFM56, the range of pencil weights available varies from 2g to 30g.
The weights are applied according to a standard table produced by the engine manufacturer. This table indicates where, and in what combination, pencil weights should be applied for a given phase angle measurement. The phase angle, which indicates at which point vibration occurs during the rotation of the engine’s fan, is determined by the time between successive electrical signals produced for the balancing machine by the engine. The engine produces one electrical signal per fan rotation. Earlier engines did not produce these once-per-revolution signals and special equipment is needed to create them, according to Hicks.
Earlier engines also require “a significant amount of special tooling to facilitate the removal and installation of trim balance weights,” he says. “To add to the complications, the balance-weight positions were typically located inside the N1 shaft or on a flange on the fan hub,” and so older engines required front-end disassembly as well as special tooling.
Many of the latest engines feature locations for trim balancing weights not only on their fans, but also in their turbine modules. While turbine modules are difficult to access during line maintenance, Hicks says providing locations for balancing weights in two different modules of the engine offers better flexibility for the balancing equipment’s algorithms, allowing them to control residual vibration levels better throughout the engine’s operating speed range.
However, an engine manufacturer will only allow so much trim balance weight to be added to a given engine model. For each engine type, the manufacturer indicates a “maximum allowable weight”. If balancing equipment analysis indicates that a weight in excess of the allowable amount is required to damp engine vibration sufficiently, the equipment issues a warning. Too high a trim weight generally means the engine has a bad “blade mapping,” Hicks says.
Although an engine’s fan blades should all weigh virtually the same, some blades (particularly hollow blade designs) weigh a few grams more than others. Over time some fan blades may also wear more than others, for instance as a result of the engine ingesting foreign object debris. After a while, the fan may develop a mass imbalance which cannot be corrected sufficiently with the application of trim weights. At this point, the fan is removed and its blades taken off. Each blade is re-weighed and the operator or MRO uses software provided by the original equipment manufacturer (OEM) to calculate an updated blade mapping that indicates which particular blade should be mounted at which location on the fan hub. It may be necessary to replace some blades.
Allen says that sometimes engine manufacturers come to VSN because they are seeing too high a proportion of new-builds of a particular engine model that need trim balancing. In such cases, they may enlist VSN’s help to obtain better balance tolerances at the individual rotor level before assembling rotors into a module. Additionally, “We’ve got MROs now telling us, ‘We have no idea how we’re going to balance these new engines with these kinds of tolerances’,” he says.
At the macro level, engines which are not balanced perfectly can cause enough vibration in a commercial aircraft that passengers experience a rough ride and can become alarmed. Again, engine vibration results in a loss of performance – since the energy causing the vibration would, in a balanced engine, go into producing thrust – and produce wear and tear on parts within the engine, as well as on the engine pylon and possibly also on airframe structures.
This is where field balancing comes in. The task usually takes place on-wing and according to Biffi, is typically used for engine fans, blowers, impellers, electrical motors and other equipment rather than rotors inside the engine. (To correct an imbalance in an interior rotor you need to have free access to it, which is not usually possible outside the MRO shop.) Generally, says Hicks, new engines or engines which have just left the overhaul test cell do not require much balancing when mounted on an aircraft. However, some airlines like to perform a balance check straight after installing an engine, in an attempt to reduce vibration levels further.
Biffi says the field balancing process is not complicated. Depending on the balancing equipment used, it involves a first spin without test weights, a second one with a test weight on one axis and a third, final spin with a test weight on the other axis. (For a single-axis balancing the third step is not required.) Since the balancing equipment does not "know" the pertinent dimensional or calibration values for the rotor, this is a self-calibration procedure: the machine obtains calibration values from analysing the spins with the test weights applied. After calibration values are obtained, the test weights are removed and the machine can calculate the rotor’s true imbalance.
Field balancing may be even simpler with the portable 'PBS-4100 Plus'. According to Hicks, it involves connecting the balancing equipment to the engine, making one or more engine runs to measure the nature of the vibration, installing the calculated trim balancing weights and performing a final engine run to ensure that the engine’s vibration levels have been reduced successfully.
This task takes from one to three hours depending on the aircraft type, the engine model and local requirements for testing. Connecting field balancing equipment to most modern commercial aircraft is a very easy task, according to Hicks. It involves running a cable from the aircraft’s vibration measuring unit to the balancing equipment. (Boeing calls the units “aircraft vibration monitors”, while Airbus calls them “engine vibration monitoring units”.)
Hicks says installing trim weights in an engine is a relatively quick and simple task. More time is usually expended waiting for the engine to cool down after being run than in actually installing the weights. Most of the time required for vibration trim balancing is usually taken up in actually getting the aircraft to and from its designated engine run-up location on the airfield.
Flying on a commercial aircraft today usually means a quiet, smooth and trouble-free ride; the high-technology balancing act performed by engine manufacturers, MROs, airlines and the specialist companies which make balancing equipment and tooling have a great deal to do with that.