Do we still need gear motors?

In this installment of The Answer to Your Problems, CONTROL DESIGN readers weigh in on whether you can get the torque you need from gearhead servo motors without way over-specifying the motor size.

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W

e think we have the right idea using gearhead servo motors--the most important advantage being increased torque as the trade-off for less speed. However, it appears that the more conventional servo motors and controls are getting stronger all the time, providing considerably more torque than they were capable of when we specified gear motors seven years ago. Can we now get the torque we need without way over-specifying the motor size?

From November 2004 Control Design

ANSWERS:


Gearhead Servos Have the Edge
Servo motor gearheads offer far more benefits than just increasing torque via transmission ratio. First, they increase the speed, so the servo motor operates in a standard speed range, thus saving current, and cost, on the servo drive side. Second, gearheads match inertia. The load inertia is reduced by the square of the transmission ratio. By reducing the load inertia (e.g., by the factor of 100=10² with a ratio of 10), the motor has a far easier job to accelerate the load. This is especially important for highly dynamic applications.

In a world, where innovation in the machinery market is tightly connected to higher throughput speeds and increased product precision, dynamics in motion becomes more and more important. Increasing the size of the motor to eliminate the gearhead often points the wrong way—bigger motors offer less dynamics because the rotor inertia increases by a power of four with the diameter of the motor. Bigger is not always better. On top of that, servo drives for direct-drive applications need to be able to handle a large inertia mismatch. Only a few servo drives available on the market offer functions like "inertia feed forward," which guarantee optimum motion performance despite a high inertia mismatch between load and motor.

Last, but not least, it comes down to the price-performance ratio, which applies to a solution with or without gearhead. We find a lot of our customers use direct-drive technology for servo motors with a power less than 1.5 hp. Looking at prices of precision gearheads in that lower torque range, it becomes obvious that a direct-drive solution is by far the better value—smaller precision gearheads very often cost more than the servo motor itself. By upsizing the motor, you spend 30-40% more on the motor, while still being able to use the same drive. In contrast, gearheads look like a bargain in motors from 10 hp and up. Increasing the size of the motor could easily result in a multiplication of the motor price depending on the transmission ratio, not to mention that a larger drive generally becomes necessary as well.

 

Markus Sandhoefner, Sales Manager, B&R Industrial Automation Corp., Roswell, Ga.

Gears Are Good
The list of the greatest inventions of man must include the wheel and the lever. Another could be the gear, essentially a combination of the first two. The advantages of the gear enable one to trade rotational distance for torque, similar to a pulley arrangement, but in totally rotary format.

A gear motor combines the electric motor and the gear assembly to make the aforementioned tradeoff. Within the efficiency limits of the gear train, one can accomplish a better inertia match, a smaller size envelope, and in many cases, a lower cost. If speed is not required, a non-reciprocating axis moving a large mass will likely be better suited for the gear motor. There certainly are large motors to handle such tasks, but costs can go up exponentially for large direct-drive motors and dealing with the associated resonance due to inertial mismatch may be a daunting task. Anytime the inertia ratio from the load to the motor is in excess of 5:1, care is usually required to maintain control.

This doesn’t mean I suggest only using 1:1 ratios. There is a trade-off that must be weighed. Can you afford to waste half of your power accelerating the motor? Can you live with the limited velocity loop bandwidth caused by having a larger motor? Is a motor of the size you require available? Now if this load were 2,500 times the motor inertia, matching, of course, would be impossible. Given the advantage that the reflected inertia of the gearbox is the square of gear ratio, 50:1 would give a reflected ratio of 1:1.

Lee Stephens, Systems Engineer, Danaher Motion, Radford, Va.

Go Direct
This is not as simple as yes or no. The actual component selection will be driven by the requirements of the application where they are used. Users have new flexibility to increase performance, decrease size and/or reduce cost for most applications.

Obviously, motor selection must be based on the torque, speed and duty cycle requirements of the application. There are three primary choices:

  1. Servo motors with integral gearheads
  2. Servo motors applied without speed reduction
  3. Direct-Drive servo motors


Gearing is still the most common method to increase torque. The primary reason is cost. Adding a gearhead may be less expensive than increasing the motor size to meet a torque requirement.

There are several disadvantages to adding gearing to servo motors. Gears wear over time, so life expectancy has to be considered. Wear also will affect accuracy during its service life in most applications. Gearing requires regular maintenance. All gearing has backlash, which can reduce life expectancy in applications with load reversals. Backlash also will define the minimum resolution of movement in an application. Most quality servo gearing can provide 3 arc-minutes resolution with up to 1 arc-minute achievable with special handling. This resolution will increase as gearing wears. In high-performance applications where servo gains are set high, gearing backlash can become a source for resonance and instability.

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