Gearing up for machine tools

July 1, 2000
Confused about how to select gearheads for machine tool servo applications? Here are some tips on what to look for in a gearhead, plus a couple of application examples

Many machine tool applications call for servomotors to obtain smooth motion and accurate positioning, along with precision speed reducers, or gearheads, to provide reduced speed and increased torque. Some applications may dictate using step motors, but these are not considered here.

In deciding which type of gearhead to use with a servomotor, the general approach is to first determine the performance criteria for the application, as well as the servomotor specifications, then match these criteria with a suitable gearhead, using manufacturer’s catalog data and sizing calculations. Knowing the characteristics of different gearhead types will help in the search.

What to look for

The basic characteristics of gearheads that affect drive performance are:
• Size and configuration (inline or right angle).
• Accuracy and backlash.
• Output torque and speed.
• Efficiency.
• Environmental capabilities (sealing, noise, and vibration).
• Cost.

The specific nature of each servomotor application places different levels of importance on each of these characteristics.

Size and configuration refer to the ability to meet space constraints and to transmit torque either in the same direction as the motor or at a right angle. This requires either an inline or a right-angle gearhead, respectively. Both the type of gearhead and the type of gears it contains will affect characteristics such as smoothness of operation and efficiency.

Many inline gearheads used in machine tools have a planetary gear configuration, which contains either spur or helical gears. Such units typically offer speed reduction ratios from 3:1 to 100:1 (with multiple gear stages at the upper end of the range).

Right-angle gearheads contain either bevel gears or worm gears. Bevel gears can be used alone to produce speed reduction ratios up to about 10:1. However, they are usually combined with planetary gears to achieve a wider range of ratios (up to 100:1). Worm gears typically operate at high ratios (30:1 through 100:1), and low output speeds (30 to 60 rpm).

Accuracy and backlash affect both the position of the driven machine tool component, and its smoothness of operation, as in contour machining. Two factors determine the overall accuracy of a gearset: the AGMA quality classification of the gear teeth (see box, “AGMA standards define accuracy”), and the clearance between the teeth of the meshing gears (backlash).

Though the AGMA classification defines the accuracy of the individual gear teeth, assembled gear sets also contain backlash (clearance between gear teeth), which depends on tooth profile, center distance of the assembled gears, and accumulated tolerances. Both spur and helical gears can be produced to the same AGMA class (typically class 12). However, spur planetary configurations typically have 7 to 15 arc-min backlash, whereas helical planetary units typically have 3 to 10 arc-min.

Output torque and speed of a gearhead are dictated by the driven machine component. This requires sizing the gearhead based on the servomotor selection and the speed reduction ratio required to achieve the torque and speed objectives at the driven machine component. Other sizing criteria include:

• Duty cycle.
• Radial and shock loads.
• Distance between driven component and gearhead mounting flange.
• Direction of rotation (one or two way).
• Motion profile (acceleration, deceleration, constant velocity).

Sizing methods vary with different gearhead manufacturers depending on application conditions. Therefore, consult the specific manufacturer for its recommended sizing method.

Helical gears provide up to 30% more torque than spur gears of comparable size. Helical planetary gears also provide higher stiffness because of more teeth in contact, which means that the gear teeth deflect less under load, ensuring more accurate radial positioning of the output shaft.

Efficiency depends on the lost energy as a gearhead transmits torque from the motor to a driven machine component. At least part of this lost energy results from friction, which generates heat that can adversely affect machine tool accuracy, particularly as the heat level varies throughout the day.

Inline configurations of planetary gear units offer high efficiency — over 90% — with the values for spur and helical versions differing only slightly. In right-angle configurations, bevel gear efficiency is comparable to that of spur and helical gears. Worm gears are less efficient (sometimes much less) because they experience sliding friction similar to that of a leadscrew.

Environmental capabilities refer to gearhead noise and vibration, plus sealing to prevent ingress of debris and leaking of lubrication. Sealing depends mostly on product design and has little to do with the gear configuration. However, both noise and vibration depend on the gear configuration. Vibration is particularly important in metal removal operations, especially contouring. In such cases, gear vibrations can be transmitted to the workholding device and adversely affect the surface finish of the machined part.

For inline gearheads, helical gears provide smoother operation, with less noise and vibration than spur gears. In right-angle configurations, worm gears provide exceptionally quite and smooth operation, compared to bevel gears. They are well suited for applications that require low speed and high stiffness.

Cost often depends more on the manufacturer and product design rather than the gear configuration. Some of the characteristics described earlier can be tailored to optimize certain benefits, but you must balanced such benefits against the cost.

Application examples

Having reviewed how gearhead characteristics affect performance, let’s put these in perspective with two machine tool applications: one involving an auxiliary operation (tool changing) and the other a machining operation. Each of these applications places a different level of importance on the gearhead characteristics. Both applications are at Giddings & Lewis, Fond du Lac, Wis., builder of machining centers of the type shown in Figure 1.

Tool changer. In developing a tool magazine drive, Giddings & Lewis designer Chuck Kis needed a gearhead to mount on a hydraulic motor. This motorgearhead combination indexes a 140- tool, vertically oriented chain-driven carousel where each tool weighs up to 70 lb, Figure 2. A hydraulic motor was selected as a low-cost alternative to a servomotor, with hydraulic power readily available. The tool loading is unevenly distributed, which requires a high speedreduction ratio, high efficiency, and torque up to 13,000 lb-in.

The carousel positions each tool in front of a tool loader. After initial positioning within +3 mm, the chain drive must be able to move up or down to center the tool within the grasp of a tapered holder. This requires a gearhead that can backdrive when motor torque is removed or reduced. Otherwise, if the gearhead locks-up, the resultant force could damage the chain. The designers ruled out worm gears because they generally don’t allow backdriving.

To ensure that the hydraulic motor operates at its optimum speed, it was necessary to use a ratio of 90:1. Designers accomplished this with a parallel-shaft, triple-reduction planetary gearhead containing helical and spur gear stages, both of which are capable of backdriving. According to Mr. Kis, “Though accuracy is a factor, we were most concerned with the high ratio and the ability to back drive.”

Workholding table. A five-axis machining center for an aircraft manufacturer required a gearhead mounted on a 3,000-rpm servomotor to drive a workholding table during machining, Figure 3. “Accuracy and backlash were most critical,” according to Mr. Kis, the tool designer. The traveling column machine was designed to perform five axes of contouring and handle parts up to 71-in. diam by 65-in. length. Because this is a machining operation, vibration is also a primary consideration.

Most workholding tables incorporate a built-in worm-gear drive, which provides smooth operation and good accuracy. In this case, however, even a 180:1 ratio worm gear set could not achieve the speed and torque required at the table (5.55 rpm and 167,400 lb-in.). Additionally, the machine configuration dictated that the motor be mounted to the wormgear drive at a right angle. To accomplish the task, a 3:1 ratio right-angle bevel gearbox was mounted on the built-in worm gear unit, providing the necessary 540:1 ratio.

During machining, the table (and gearhead) rotates up to 120 deg about a horizontal axis, Figure 1. To prevent the gearhead lubricant from leaking during this rotation, designers selected a unit with special lubricant fillers and double lip seals at the shafts.

For other machine tool applications, consider the impact of gearhead characteristics on the particular process. For example, grinding applications usually require exceptionally smooth motion that does not impact surface finish. Additionally, where close tolerance machining is required, machine tools are very sensitive to heat generation. Depending on the gearhead mounting and location, heat transfer can affect part accuracy.

Alan Feinstein is director of R&D, Bayside Motion Group, Port Washington, N.Y.

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