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.