1. Background

A rear axle is the final stage in the drivetrain of a vehicle, and is responsible for transforming the engine power into useful propulsive force. A major component of the rear axle is the ring and pinion gearset. Typically ring and pinion gears are made by machining, hardening and sometimes shot peening. Since the hardening step introduces distortion, the gears are either lapped and maintained as a matched set, or ground to the final geometry to obtain acceptable noise levels. Since both the lapping and grinding process leaves a rather rough surface, a break-in or run-in process is recommended. Break-in is an attempt to smooth the contact surfaces of the gears through controlled or limited metal-to-metal contact. The roughness of the contact surfaces is reduced during this process until a lower and relatively stable surface roughness is reached. Even when the break-in procedures are followed and a lower surface roughness is achieved, irreversible metallurgical and lubricant damage often occurs since break-in always results in stress raisers, metal debris and an extreme temperature spike, which ultimately can lead to the premature failure of the rear axle.

Winkelmann et al. reported a number of studies that document the improvement in rear axle efficiency by using chemically accelerated vibratory finishing, henceforward referred to as Superfinishing [1]. In another paper, Winkelmann et al. showed that chemically accelerated vibratory finishing can successfully superfinish ring and pinion gearsets to a low 0.1 μm Ra without affecting the gear geometry such that the contact pattern and NVH were acceptable [2]. This technology was used commercially to solve premature wear and noise problems of problematic bus and recreational vehicle ring and pinion gearsets, and is currently used on production pick-up truck gears. Superfinishing using vibratory machines, however, is not easily automated, and requires a finishing time between 30 – 60 minutes depending on the initial roughness and the desired surface finish. Therefore, a much faster process is desired for high-volume manufacturing operations.

This paper discusses a recent development in the Superfinishing of hypoid ring and pinion gearsets in considerably shorter cycles using Rapid ISF®. A novel fixturing method is also introduced to help make this new method fully automatable.

2. Introduction

Only a very brief description of conventional chemically accelerated vibratory finishing will be presented here since it is discussed in detail elsewhere [3], [4], [5]. The process is carried out in vibratory bowls or tubs. A proprietary active chemistry is used in the vibratory machine in conjunction with high density, non-abrasive ceramic media. When introduced into the

machine, this active chemistry produces a stable, soft conversion coating on the surface of the metal gears being processed. The rubbing motion across the gears developed by the machine and media effectively wipes the conversion coating off the asperity peaks of the gears surfaces, but leaves the valleys untouched. (No finishing occurs where media is unable to contact or rub.) The conversion coating is continually re-formed and rubbed off during this stage producing a surface smoothing mechanism. This process is continued in the vibratory machine until the surfaces of the gears are free of asperities.

The rate at which a gear is superfinished is a function of both the rate of formation of the conversion coating and the rate of mechanical removal of the conversion coating by the media rubbing. The rubbing energy must be sufficient to remove the conversion coating. In conventional chemically accelerated vibratory finishing, the rate of superfinishing is limited by the rate of rubbing and in some cases by the rubbing energy. If rubbing energy is insufficient, the superfinishing process will stop resulting in a very heavy tenacious conversion coating.
Consequently, there are two approaches for increasing the rate of finishing. (1) Select a machine that provides a higher rate of rubbing than a conventional vibratory machine. (2) Select a machine that has a higher rubbing energy. Fortunately, the drag finishing machine has both of these features. Rapid ISF® (referred to as Superfinishing from here on) combines traditional high energy drag finishing equipment and a significantly more aggressive active chemistry than used with conventional vibratory finishing. A description of the drag finishing machine is given below by Chmielewski [6].

“The concept of drag finishing goes back centuries to when farmers first pulled plows through their fields. Although the farmers’ intention was not to deburr or polish their plows, dragging the plow through the abrasive soil did just that.
The initial thesis work of drag finishing’s developer focused on the need for a machine that would increase performance levels over conventional mass-finishing equipment. The “think- tank” design people knew that pressure and velocity were responsible for the work of mass finishing; therefore, drag finishing was the natural result.

It is the magnitude of these two elements that provides drag finishing an advantage. Subsequent experience taught the developer that these two elements, in concert with properly prescribed media and compounds, allow drag finishing to achieve performance levels as high as 40 times faster than conventional levels.

Drag finishing uses a circular bowl containing stationary, loose finishing media and a circular rotating turret above the bowl with multiple rotating part-fixture stations. (Ten stations are average.) Fixture stations rotate about their own axes, similar to a planetary system.
Parts are attached singularly or in groups to part-fixture stations around the perimeter and under the main turret. The rotating turret is then lowered so the parts are submerged and “dragged” through the stationary loose media.

Programmed sequences such as rpm, depth and rotational direction provide mechanical repeatability that assures uniformity cycle to cycle. By combining the right media and compound with the physical action of the drag finisher, you can mechanize processes that previously only could be done manually. Refined surfaces as low as one to two Ra (Roughness average) are consistently achievable.”

3. Process Description
Equipment
The machine used in the testing is a Rosler Mini-Drag Finisher. See Figure 1 for a 3D image of the machine. The spindles are fastened to the machine and can be rotated around the axis of the spindle. As the spindles are rotated around their axis, they are also dragged in a larger circle around the circumference of the bowl containing the media. It is this motion that imparts the force of the media rubbing the part and makes it possible to use a faster accelerating chemistry than can be used in conventional vibratory finishing (i.e., circular vibrator).

Figure 1: 3D image of a drag finisher showing the key components.
The specifications of the ROSLER SSA-L Series Mini Drag Finisher are given in Table 1.

Operation Programmable with full automation capabilities

Table 1: The technical specifications of the ROSLER SSA-L Series Mini Drag Finisher.

Description of the Magnetic Fixture

The ring gear may, or may not have bolt holes, and this makes it difficult to mount to the spindle of the machine. The pinion gear is similarly problematic to mount to the spindle. The

problem was overcome by using magnetic fixturing [7]. A plastic fixture sized to fit the dimension of the particular rear axle gears is fastened to a steel shaft as shown in Figures 2 and 3.

Figure 2: The small circles are the location of supermagnets placed around the larger plastic cylinder to hold the ring gear in place. Similarly, the small circles on the smaller cylindrical section are the location of supermagnets to hold the pinion to the fixture.

Figure 3: The plastic magnetic fixture shown above and attached to the spindle.

A number of cylindrical supermagnets are embedded around the larger and smaller cylindrical sections of the fixture. The number of magnets and their strength are adjusted to firmly secure the ring gear and pinion gear to the plastic fixture during the drag finishing process, but also not too strong such that the ring and pinion can be easily removed at the end of the process.

The ring gear and pinion gear are held to the fixture by the magnetic force as shown in Figure 6, thereby allowing one gear and one pinion to be processed simultaneously on each spindle.

Figure 4: The ring gear and pinion gear are held to the magnetic fixture.
The shaft is then secured to spindle of the drag finishing machine as shown in Figure 5.

Figure 5: The spindle is attached to the drag finisher with the ring gear and pinion gear held by the magnetic force to the plastic fixture.