Gearheads push performance envelope

May 1, 2000
New right angle gearheads promise increased capabilities for step and servo applications. A key to this improved performance is an advanced design face gear

A new series of right angle gearheads for step and servo applications addresses the needs of both design and manufacturing engineers. The design engineer generally wants low backlash, plus high torsional stiffness and torque capacity. The manufacturing engineer, on the other hand, looks for easy mounting, quiet operation, and low maintenance.

Developed by Thomson Micron LLC, Ronkonkoma, N.Y., the new gearheads feature an advanced design face gear that meshes with a standard involute spur gear pinion, Figure 1, replacing a set of conventional spiral bevel gears.

Other design and manufacturing advances help boost performance as well. Though company engineers are continuing to perform tests to quantify the degree of improvement in certain areas, here are the basic gearhead capabilities.

Performance characteristics

Lets first look at how the gearheads improve performance in three areas: backlash, torsional stiffness, and torque capacity.

Backlash. To obtain the desired backlash between mating gears for a specific application, the face gear assembly requires adjusting only one axis: the longitudinal axis of the face gear as it moves into mesh with the pinion. Depending on the model, installers make this adjustment in one of two ways: by a threaded nut behind the face gear on its shaft, or a pair of nuts, one behind the gear and one in front. In either case, tightening the nut moves the face gear along its shaft into tighter (or looser) mesh with the pinion.

This simple adjustment eliminates the measuring and shimming errors associated with bevel gears, and enables consistent setting of backlash at the factory. In a typical example (2-in. pitch diameter gear), the face gear reduces backlash from 7 arcmin to 4 arcmin compared to a spiral bevel gear.

Torsional stiffness. Critical positioning applications require the least possible amount of lost motion of the gearhead output shaft compared to the input shaft. Reducing this lost motion means limiting the amount of backlash, and increasing the torsional stiffness of gearhead components so that they deflect less.

The gear teeth contribute a significant, but small part of the total stiffness and deflection. With the face gear, each tooth has a high effective pressure angle (about 34 deg), which means that it is wider at its base, Figure 2. Being wider makes the tooth stiffer so it deflects less under bending loads.

Torque capacity. Gearhead torque capacity depends largely on gear tooth pitting resistance and bending strength, which are in turn influenced by hardness, quality (accuracy), and geometry.

Gear teeth in the new units are carburized to obtain a case-hardened surface that resists pitting, and a tough core that accommodates high bending stress. With AGMA Grade 1 steel, for example, carburizing reportedly achieves at least 25% higher bending strength than nitriding.

To achieve high quality (AGMA class 12), the company finishes the gear tooth surface with a proprietary process after heat treatment.

Tooth geometry is the third element in achieving high torque capacity. Engineers report that the shape of the face gear teeth causes a high contact ratio (multiple teeth in mesh at any instant) when mating with the pinion. Under this condition, multiple teeth share the load, which boosts load or torque capacity.

Installation and operation

Now we’ll see how the new gearheads improve operating characteristics, such as mounting, noise, and maintenance.

Mounting. With many conventional gearheads, the pinion must be mounted on the motor shaft, then interfaced and carefully aligned with the gearhead through concentric pilot holes in motor and gearhead flanges.

The new gearhead offers the Redi- Mount system, which simplifies this process by incorporating the pinion in the gearhead. A split hub on the end of the pinion shaft positions the motor shaft concentrically with the pinion. Then the installer tightens the hub screws and the bolts that connect motor and gearbox flanges. One hub accommodates several different motor shaft diameters.

Noise. The new gearheads limit gear noise by reducing transmission error and variations in mesh stiffness. First, the proprietary finishing method mentioned earlier reduces transmission error by achieving a tooth accuracy level that meets AGMA quality class 12.

Further, because the face gear has an increased tooth contact ratio, this higher ratio reduces the mesh stiffness variation. In addition, the gear teeth are crowned to accommodate minor deviations in gear alignment. This eliminates edge contact so the gears can mesh smoothly and quietly.

Maintenance. The gearheads feature precision finished gears, a stainless steel output shaft, lifetime lubrication, and in some cases heavy duty bearings, to minimize maintenance requirements.

Basic offerings

Four series of the new gearheads were introduced at the Design Engineering Show in March 1997. These units offer ratios of 1:1, 2:1, 3:1, 4:1, and 5:1. Units with planetary output stages boost the ratios up to 500:1. Peak torque of these models ranges up to 31,000 lbin.

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