Evaluation of Isotropic SUperfinishing on a Bell Helicopter Model 427-Bull Gear

Evaluation of Isotropic Superfinishing on a Bell Helicopter Model 427 Main Rotor Gearbox


By: Ryan Ehinger and Charles Kilmain Bell Helicopter Textron, Inc. Fort Worth Texas

The surface finish of a gear tooth is a critical factor in the wear, durability, noise generation, and efficiency of modern helicopter transmissions. Currently, optimized grinding and honing processes are used to improve the surface finish of both gear teeth and roller bearing races. With the promise of reduced manufacturing cost, reduced scrap, reduced noise, and improved gearbox efficiency, a new surface refinement process known as isotropic superfinishing is making its way into the aerospace industry. Isotropic superfinishing (ISF) is able to produce 2-4 μin Ra surface finishes using a combination of vibratory me­dia and surface conversion chemicals without modifying the profiles of aerospace quality ground gear teeth. This paper will outline NRTC-CRI funded testing of a Bell Model 427 main rotor gearbox with ISF processed gears. Testing performed includes thermal efficiency, gear tooth bending fatigue, acoustic, and extreme conditions.


The surface finish of a gear tooth, such as that shown in Fig. 1, is a critical aspect in the operation of today’s rotorcraft transmissions.

Tooth scoring, pitting, noise generation, and operational efficiency are all characteristics that are affected by the finish of a gear tooth. For that reason, the surface finish of gear teeth has always been important in gearbox design.

The idea of superfinishing the surface of a metallic compo- nent for improved operational attributes and improved ap- pearance makes great sense in many industries. In fact, processes to improve the surface of a material to a near mirror finish are nothing new and have been used to polish everything from dining utensils to wrenches and hammers. In the aerospace industry, however, where tolerances on gear teeth are measured in millionths of an inch, extreme care must be taken to control processes that refine the finish of a gear tooth, in order to maintain dimensional tolerances. One widely used process to improve the finish of a gear tooth from 16 μin Ra to 8 μin Ra (0.4064 μm to 0.2032 μm) is honing.1

This process has been used to achieve the improved gear tooth surface finishes necessary for efficient operation in the high-torque, low-speed operation of a rotor- craft transmission. Honing has its own issues, requiring setup for each gear component, and often negatively impacts the gear tooth profile while attempting to improve surface finishes on the flank and in the root of the gear tooth. Within the last decade, however, a superfinishing technology known as isotropic superfinishing (ISF) by REM Chemicals, Inc. has been adapted and applied to aerospace quality gears to achieve near mirror finishes on gear tooth surfaces at a reduced cost compared to honing and a reduced scrap rate (Ref. 1).


Fig. 1. 427 ISF processed input pinion.

Fig. 1. 427 ISF processed input pinion.

This paper provides an evaluation of the processing and ef- fects of an improved gear tooth surface finish on the opera- tion of a Bell Helicopter Model 427 main rotor gearbox (MRGB). With superfinished gears, this gearbox was tested for thermal efficiency, gear tooth bending fatigue, extreme conditions operation, and acoustics. Testing such as the thermal efficiency testing and acoustic testing provides data that can be directly compared to gearboxes operating with standard finish production gears, while the gear tooth bending fatigue test and extreme conditions testing provide validation that the improved surface finish of the gear teeth and the chemical processing steps used were not detrimental to the integrity of the gears strength or operation.


Testing was performed on a Bell Helicopter Model 427 main rotor gearbox. The Model 427 and 429 aircraft are shown in Fig. 2, while a 3-D model of the 429 main rotor gearbox, with similar configuration to the 427 gearbox, is shown in Fig. 3.

This gearbox has two engine inputs and is rated to provide a maximum continuous operating power of 800 hp (596 kW), with 400 hp (298 kW) per input, at 6,000 rpm (Ref. 2). Speed is reduced from 6,000 rpm to 395 rpm through two stages of reduction, including a spiral bevel set and a helical bull gear. Lubrication used in the gearbox conforms to DOD-PRF-85734 and is provided by an integral oil pump at a nominal pressure of 55 psi (379 kP). This oil is provided to the gear meshes using oil jets spraying a continuous flow of oil on both the in- and out-of-mesh sides of the gear mesh. A section of the gearbox is shown in Fig. 4, illustrating the reduction stages and the use of spiral bevel gears (input), helical gears (main rotor output), and spur gears (tail rotor output). All gears consist of Carpenter Pyrowear EX-53 gear material.


The M427 development test stand, as shown in Fig. 5, is a mechanical regenerative and absorption type test stand using a 500 hp (372.8 kW) electric drive motor to provide the rotational speed and makeup power for stand and specimen losses. The regenerative torque loop consists of a series of test stand gearboxes and drive shafts that make a closed mechanical circuit. The test stand slave gearbox is designed such that independent torque loads may be imposed on each of the two inputs of the test specimen gearbox. Rotational actuators (ROTACS) adjust the torque within these two re- generative loops. The tail rotor output torque load is applied using a dynamometer. Mast lift and bending loads can be applied at the rotor hub end of the mast by hydraulic cylinders; however, these were not used in this testing.

The various test parameters indicated in the test plan were set and maintained with the use of instrumented and cali- brated transducers located on the test stand. All raw data was acquired by a computer-controlled data acquisition system, which served to transform the data into engineering units, provide a readout to the test stand operator, and store the data for future analysis.

Fig. 2. Bell Helicopter Model 427

Fig. 2. Bell Helicopter Model 427

Fig. 2. Bell Helicopter Model 427 (top) and Model 429 (below).

Fig. 2. Bell Helicopter Model 429

Fig. 3. Bell Helicopter Model 429 main rotor gearbox (gear configuration same as 427).

Fig. 3. Bell Helicopter Model 429 main rotor gearbox (gear configuration same as 427).


Fig. 4. Bell Helicopter Model 427 cross section.

Fig. 4. Bell Helicopter Model 427 cross section.

Fig. 5. Front and side views of the 427 MRGB test stand.

Fig. 5. Front and side views of the 427 MRGB test stand.

Test parameters are monitored using a combination of commercial instruments, such as flow meters and pressure transducers, and devices constructed by Bell Helicopter Textron Inc., such as load cells, torque transducers, and thermocouples.

The torque levels at which the test specimen gearboxes are operated are measured using strain-gage-instrumented shafts at various places on the test stands.

The Model 427 main rotor mast was instrumented with strain gages to read mast torque installed at mast stations 11.6 and 24.9 inches from the top of the mast.

Instrumentation was provided to monitor and record the following test parameters:

  1. Test cell ambient air temperature 2–4 feet (1 ± 0.3 m) from the gearbox.
  2. Transmission oil-in temperature.
  3. Transmission oil-out temperature.
  4. Oil-in flow rate.
  5. Time of day (local standard).
  6. Test time.
  7. Speed at each input.
  8. Torque at each input.
  9. Main rotor mast torque.
  10. Torque at tail rotor drive output.
  11. Transmission oil pressure.
  1. Chip detector indications.
  2. Vibration levels at both inputs, the tail rotor output, and the mast output using accelerometers in both the axial and radial directions.

The allowable tolerance on the recorded data was as follows:

Torque: ±2% of the full-scale value Speed: ±2% of the full-scale value Temperature:±4°F (±2.22°C)
Pressure: ± 2 psig (±13.7 kP)
Time: ± 30 seconds

Acquisition of all data was under the control of an automatic computer-controlled data system. Bell-developed software programs were used to acquire raw data from each of the active channels once per second. This data was used in real time to automatically shut down the test stand if preset limits were exceeded, to avoid damage to the test specimen. All active data channels were recorded by the satellite computer every second. To verify the integrity of the transmission gears during testing, inspections were performed at regular intervals via multiple inspection ports using a borescope to identify gear tooth damage or improper contact patterns.


Gears used for the testing had gone through all production gear finishing operations, including black oxide treatment, and had been run previously in the 427 MRGB for the qualification of bearings under a previous and unrelated test. These gears were not tested at elevated loads or extreme conditions prior to superfinishing. Additionally, no honing operations were performed on these gears as part of their production manufacturing processes.

All gear processing, including any required process development and tooling fabrication, was performed by REM Chemicals, Inc., located in Brenham, Texas. Prior to ISF processing, these gears were chemically stripped of black oxide and dimensionally inspected at Bell for adherence to Bell drive system design specifications and to provide a baseline to quantify the dimensional changes due to the ISF process. Dimensional inspection of the gear teeth included tooth profiles, circular tooth thickness, spacing, and surface finishes, while inspection of the roller bearing integral raceways included surface finish, roundness, and diameter. Magnetic particle inspection was also performed to validate gear integrity prior to fatigue testing. While one benefit of the ISF process is the ability to process entire gears for an improved surface finish, nearly all of the components in this test were partially masked prior to processing. This masking was performed in order to eliminate ISF processing on integral clutch races, ball bearing journals, and threads where it was determined that processing was not necessary and might require further process development outside of the scope of this testing.

The surface finish improvements on the gear teeth as a result of the ISF processing are shown in Table 1. These results indicate that previous operation of the gears in a test envi- ronment had served to “run-in” the gear sets for an improved tooth surface finish prior to processing. This run-in indicates that the surface finish improved beyond the blueprint specifications for the gear teeth during gear operation, which is something that occurs at a lesser degree with ISF processed gears due to the increased gear mesh lambda ratios (Ref. 3). Despite the previous run-in, ISF refinement further improved the finish from an average of 7.51 μin to 3.61 μin Ra or an average of 39%. This finish of 3.61 μin Ra represents a finish that would be difficult or impossible to achieve using typical honing operations while maintaining proper gear geometry but was easily achieved using the ISF process. The most impressive improvement, 89%, in gear tooth surface finish is illustrated in the before and after surface finish traces shown in Fig. 6.

Review of all dimensional inspection data indicated that no significant changes were made to the gear tooth profiles or integral bearing raceways. Significant changes are those that would take the gear out of tolerance when dimensionally inspected. While as much as 0.0002 inch (0.00508 mm) was removed from each ISF processed surface, the material re- moval was distributed across the gear teeth and raceways so that parts remained within tolerance. To illustrate and quan- tify the change in the tooth geometry, a spiral bevel gear tooth was mapped using a Klingelnberg gear inspection ma- chine and the resulting before and after tooth surface maps are shown in Fig. 7. Note that while the geometry is shown to have changed due to the ISF processing, these changes reflect both an improvement and degradation in the gear tooth geometry and the change is not significant enough to take the gear tooth out of tolerance.

Gear surface finishes before and after ISF processing


Prior to testing, analysis was performed on the gear tooth meshes to analytically quantify the change in lamba (λ) ratio before and after the ISF processing. The lamba ratio is a measurement of the level and severity of the tooth to tooth interaction in the gear mesh. This lambda ratio is calculated using the method shown in Equation 1, where the lube film thickness is based on many factors, including the torque at the gear mesh, speed, temperature, and oil viscosity (Ref. 4). The composite roughness average is simply a sum of the roughness of the two gear teeth that are in contact in the mesh.


If λ < 1 then the thickness of the lubricant film is not more than the height of the surface asperities on the mating gear teeth, so metal-to-metal interaction leading to friction and noise will occur in the gear mesh. If 1<λ<2 then the thickness of the lubricant film is sufficient to provide some separation of the gear teeth; however, metal to metal contact is still possible between each gear tooth’s highest asperities. The goal in the optimization of gear meshes is to achieve a ratio of λ > 2, where full metal-to-metal separation is achieved, and even the highest asperities of the gear teeth are separated by a film of oil. This is known as elastohydrodynamic lubrication (Ref. 4)