Fig. 1. Vertical load orientation for gears[1]

We have two questions for you today.

1. For gear wheels, which orientation is better for minimum distortion: a horizontal one (i.e. one above another with spacers in between) OR a vertical orientation (i.e. side by side of each, suspended on a horizontal bar)?

2. For pinion, which position is better: vertical (i.e. side-by-side in a vertical way) OR horizontal (i.e. one above another in horizontal way)?

Fig. 2. Horizontal load orientation for gears[1]

Most of the heat treaters in the U.S. are suspending gear wheels on a horizontal bar when possible. In other words, they are in a vertical position. Smaller gears (up to 5 mm) in particular work best this way. Larger gears (where the weight can cause them to sag) might have to lie flat on a tray. One, if not the biggest, factor that affects the distortion of gears is how they are placed and supported in the furnace by the fixturing.

Any heat-treatment process that requires heating the part to above the transformation temperature causes the part to lose most of the strength that it possesses at room temperature. A part subjected to extended times at elevated temperature, as in carburizing, experiences high-temperature creep and resulting plastic deformation due to its own weight unless properly fixtured and supported. Single parts are better than stacked parts. If parts have to be stacked, support is important between the layers, especially for thin-walled parts.

Also important is making sure that the parts are freely exposed to both the heat-transfer medium and the quenching medium. In some instances, fixturing is impractical. In the case of a heavy precision gear, its size may dictate that it be laid directly onto a grid, and if that grid is warped you can expect the gear to attempt to conform to the tray shape during carburizing. Even if the gear is later press quenched, this distortion can be difficult if not impossible to correct. (If you can get the gear flat again, this still will have affected the precision gear geometry.)

Furnace trays are often overlooked when trying to find the causes of distortion. As a general rule, if you can see light between a straight edge and the tray, you should not use that tray. Some heat treaters grind the trays flat periodically when there are no other fixturing alternatives.

Many of the companies who use gas pressure quench technology (as opposed to oil) are finding, however, that their systems work best when the gears are mounted horizontally (Fig. 2).

Pinions should always be hung vertically without allowing the bottom to touch the fixture. These are normally hung behind the gear teeth face so the stem hangs down. Anything other than this will not work well and create significant distortion.