Steve Buffamonte
Application Engineer
Bayside Motion Group
Port Washington, N.Y.

Many machine designers believe direct drive is the simplest and, therefore, ideal way to design servosystems. But eliminating gears, belts, and pulleys — a tempting approach — often hampers performance and raises costs. Precision gearheads, on the other hand, can optimize servomotor performance while reducing costs. Gearheads allow engineers to select smaller servos that turn at higher speeds and consume less power.

Gearheads mount directly to motor shafts in both in-line and right-angle configurations. They typically convert high-speed, low-torque rotary motion to low-speed, hightorque output. Although gearheads have been a fundamental power-transmission component for decades, engineers now want smaller, more accurate gearheads to take full advantage of today’s smaller, more precise servomotors.

RIGHT ANGLES FOR TIGHT SPACES
Gearheads often drive long mechanisms, such as material-feed systems that move lengths of wire, wood, or metal. Using in-line gearheads would extend the footprint of these machines. For space-hungry applications, right-angle gearheads are a common choice. Mounting right-angle drives vertically, for instance, lets them hang beneath the machines they run. Even horizontal mounting saves space because gearheads and motors can sit behind machines, out of the way of workers.

Bevel gears are commonly used in right-angle drives and are a good option when precise motion is necessary. All bevel gears are conically shaped to allow 90° shaft intersections, but the teeth can be cut straight or spirally. Straight bevel gears typically have contact ratios of about 1.4. Their full-line contact between mating teeth produces more vibration and noise than spiral bevel gears. Spiral-bevel gear teeth gradually engage and disengage, providing contact ratios of 2.0 to 3.0 with high precision and little noise. Spiral-bevel gears’ higher contact ratios let them handle 20 to 30% greater loads than straight-bevel gears. And spiral-bevel teeth mesh with a rolling action that not only increases precision but also reduces friction. This yields operating efficiencies that exceed 90%.

MOUNTING CONSIDERATIONS
As smaller servos push gearheads to higher speeds, vibrations become more likely. Misalignment between motors and gearboxes, which often happens during installation, is a common source of vibration. Mounting traditional motor-gearboxes requires several precise connections. Technicians must attach the motor output shaft to a pinion gear that slips into a set of planetary gears in the end of the gearbox. An adapter plate joins the motor and gearbox. This series of connections and components creates tolerance stack-up and often produces misalignment.

The pinion is integral to smooth operation because it must align exactly with the motor shaft and gearbox. Until recently, technicians always mounted pinions in the field when they connected motors to gearboxes. Unfortunately this often led to vibrating machinery. Some engineers have considered building gearheads directly into servomotors to solve this problem. But there’s one major drawback to this solution: when one component fails, both must be replaced.

A more practical solution is to install pinions into the gearheads during manufacturing. Gearheads with factory-installed pinions are simpler to mount to servomotors than gearheads with field-installed pinions. Technicians just insert the motor shaft into the collar extending from the gearhead’s rear housing, tighten the clamp with a wrench, and bolt the motor to the gearhead.

Factory-installed pinions ensure the gearheads run smoothly. The pinions are balanced before mounting, which lets them spin at high speeds without wobbling. The pinions also produce less friction, wear, noise, and vibration than field-installed pinions.

However, this design requires a floating bearing, developed by Bayside, that supports the shaft with the pinion on one end. A collar on the pinion shaft’s other end mounts to the motor shaft. The self-aligning floating bearing holds the pinion in place until mounting, at which time a pair of bearings in the motor support the coupled shaft. The self-aligning feature of the floating bearing lets the motor bearings support the shaft after installation.

The pinion and floating bearing help seal the unit during operation. The pinion, for example, rests in a blind hole and seals the rear of the gearhead. Sealed gearheads not only keep out dirt, they keep lubricants in the housing. This lets designers use light greases and semifluid lubes instead of heavy grease.

SHOW ME THE MONEY
Gearheads not only make systems run smoother, they also save money. Although adding a component seems like it would raise costs, this is not always true. Precision gearheads work with smaller servos and related components, such as drives, power amplifiers, and cables, thus keeping expenses low.

Gearheads also reduce operating costs. Power companies typically charge customers based on consumed current. Smaller servosystems draw fewer amps, thereby reducing operating costs. Power savings are greatest when applications demand low speed and high torque because direct-drive servos need to be considerably larger than servos coupled to gearheads.

In high-speed/low-torque applications, however, direct-drive servos can be fairly small and still do the job without a gearhead. In these cases servo/gearhead combinations may not make as much sense because costs will be about the same. Of course gearheads still improve efficiency and, in the long run, decrease operating costs because even slightly smaller servos use less power.

The best advice is to consider gearheads on a case-by-case basis, weighing precision gearheads against initial and ongoing expenses. Engineers should first check speed and torque requirements. Applications with high speed and low torque, for example, may be better suited to direct-drive systems. Applications requiring low speed and high torque, on the other hand, almost always require gearheads.

BEVEL-GEAR BASICS

Many applications, such as robotics and injection molding, often require right angles between input and output shafts. When precise motion is also critical, bevel gears are the power-transmission device of choice. Bevel-gear teeth look similar to spur-gear teeth except the tooth surfaces of bevel gears form a cone. Straight and spiral are the most common types of bevel gears. Although not bevel gears, on-center face gears are sometimes used in right-angle drives.

Straightbevel gears are the simplest form of bevels. Their straight teeth produce instantaneous line contact when they mate. Straightbevel gears provide moderate torque transmission but don’t run as smooth or quietly as spiral-bevel gears because the teeth engage with full-line contact.

Spiral-bevel gears have curved oblique teeth. The spiral angle of curvature with respect to the gear axis produces teeth with substantial overlap. This lets teeth engage gradually and keeps at least two teeth in contact at all times. Compared to straight-bevel gears, spiral bevels have lower tooth loading. This is because their tooth profile’s increased radius of curvature distributes the load over a larger area. This also lowers noise and smoothes operation. Spiral-bevel gears can turn up to eight times faster than straight bevels.

Face gears have straight tooth surfaces, but their axes lie in planes perpendicular to shaft axes. Teeth on face gears should, theoretically, mate with full line contact because of their configuration. But because this would lead to frequent misalignment, engineers design face-gear teeth to mate with instantaneous point contact. This results in low load capacities and typically limits the gears to motion-transmission applications. They are, however, inexpensive and easy to manufacture compared to bevel gears.

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