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10 Advantages of Geared Motors

Wednesday - 23/09/2020 11:00
Dolin offers many pre-assembled geared motors for quality and consistency. In addition, gearheads from our industry partners increase the torque range of our motors further to accomodate for more applications.
10 Advantages of Geared Motors
High Torque Output
 
When a geared motor is used, the motor's torque is multiplied by the gear ratio and gear efficiency. A geared motor will output more running and holding torque than an ungeared, standard motor. Along with higher torque output and strengthened rigidity, many related benefits can be realized as I will discuss below. It all starts with torque.
 
Geared motor torque = motor torque x gear ratio x gear efficiency
 
Here we show a chart that depicts the relationship between torque and gear ratio. As gear ratio increases, so does torque. For most gearheads, there is a limit on maximum permissible torque, so the line flattens out when a certain gear ratio is exceeded.
 
For certain gearheads, such as hypoid or FR hollow shaft flat type, there is no limit on permissible torque, so the line doesn't flatten out.
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Torque ranges of different types of motors are listed by frame size. This chart below is shown on every product overview page under the "Product Lineup" tab. As you can see, for a 30 W brushless motor, the rated torque is only 13.6 oz-in. However, the torque can increase up to 53 lb-in with a 200:1 parallel shaft gearhead.
 
Speed Reduction
 
Gearheads are also referred to as "speed reducers". Gear ratios are sometimes referred to as "speed reduction ratios". While speed controllers take care of speed reduction nowadays, gearheads were used to reduce the speed of a motor in the ancient past. When a geared motor is used, the motor's speed is divided by the gear ratio. For example, an 1800 RPM AC synchronous motor with a 5:1 gearhead will run at 360 RPM.
 
Speed of gearhead shaft = motor shaft RPM / gear ratio
 
We typically show both the torque and speed for each gear ratio in our literature. At gear ratios of 120:1 to 360:1, the maximum permissible torque is reached. Therefore, this motor should not drive anything that requires 141 lb-in (16 Nm) of torque or more. Exceeding this specification may degrade the life of the product. Another thing know is that for AC motors, the variation in inertial load can cause the speed to decrease slightly from the designated RPM. This is different than other motors.
 
High Resolution
 
When I started supporting motion control applications a long time ago, some customers actually used gearing to increase their stepper motor resolution. From what I remember, I believe they wanted to avoid purchasing a bipolar microstepping driver, which was more expensive at the time.
 
Geared motor resolution = motor resolution (steps/rev) x gear ratio
 
For example, a standard 200 steps/rev stepper motor with a 5:1 gear ratio will require 1,000 steps to rotate one revolution at the gearhead output shaft. The fine resolution allows the motor to step at a smaller increment for fine positioning applications. Microstepping will take care of most high resolution requirements these days.
 
Drive Large Inertial Loads
 
When a geared stepper motor or servo motor is used, the inertial load that can be rotated increases by the square of the gear ratio in comparison with an ungeared motor of the same size. Bascially, you can drive an exponentially bigger inertial load with geared motors.
 
When using different types of motors, pay attention to how the inertial load is treated. We show some examples below.
 
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For AC motors, we follow this formula below. When gear ratios are less than 3:1~50:1, we multiply the rotor inertia by the gear ratio squared. For gear ratios of 60:1 or higher, we only multiply the rotor inertia by 2500 as there is a limit.
 
For stepper motors, it gets a little more complicated and requires some calculation. In addition, open-loop and closed-loop stepper motors can handle varying amounts of load inertia. The reason is because of something called the "inertia ratio". An inertia ratio is a reference given by a motor manufacturer to quickly determine if the inertial load can be driven by the motor.
 
For ungeared motors
 
Calculated load inertia / rotor inertia = inertia ratio
 
or
 
Max load inertia that can be driven = rotor inertia x max permissible inertia ratio
 
For geared motors
 
Calculated load inertia / rotor inertia x gear ratio² = inertia ratio
 
or
 
Max load inertia that can be driven by steppers = rotor inertia x gear ratio² x max permissible inertia ratio
 
The maximum permissible load inertia for stepper motors is calculated by the formula above. For every stepper motor (or servo motor), it is recommended that the inertia of the load stays under this value. If this value is exceeded, then issues such as missed steps may occur.
 
The recommended maximum permissible inertia ratio for open-loop stepper motors is 10:1 while it increases to 30:1 for closed-loop stepper motors. For servo motors as a comparison, the maximum permissible inertia ratio is 50:1 by auto-tuning and up to 100:1 with manual tuning.
 
Shorter Positioning Time
 
For large inertial loads, the use of a geared motor will achieve a shorter positioning time than a standard type motor. In the graphs below, we compare the fastest positioning time between an ungeared closed-loop stepper motor and a geared closed-loop stepper motorat 5:1 inertia ratio and 30:1 inertia ratio.
 
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As you can see, at 5:1 inertia ratio, the geared motor takes longer than the standard type. However, at 30:1 inertia ratio, the geared motor can position faster until a certain point (540°).
 
Downsize
 
Since a gearhead can increase a motor's torque dramatically, a smaller geared motor can output equivalent torque as a bigger, ungeared motor. In some cases, geared motors can offer downsizing benefits to machine design.
 
Improved Damping Characteristics
 
If the inertial load is large or acceleration/decleration rate is high, a geared motor can increase damping effects more effectively. This ensures more stable operation compared to an ungeared motor; especially during starting and stopping. Geared motors are ideal for applications where a large inertial load, such as an index table, must be driven.
 
Increased Axial and Radial Load
 
Both axial (thrust) and radial (overhung) loads are also increased due to the enhanced strength and rigidity of a geared motor. For example, axial load and radial load for a 25W KII Series AC induction motor increase to 101 lbs and 22 lbs respectively.
rvgearmotor
As a comparison for a round-shaft, ungeared motor, the permissible axial load (thrust load) is only half of its mass, and the permissible radial load (overhung load) is less than 1/4 when compared with the geared motor.
 
Increased Rigidity
 
Geared motors have increased rigidity and are resistant to torsional forces. Therefore, compared to standard motors, geared motors are less subject to issues caused by load torque fluctuations. High stability and position accuracy can be expected even when there are fluctuations in load. For example, the increased gear friction of a geared motor provides more holding torque for a security camera to stay in position even with strong winds.
 
Surface Installation of Load
 
As an additional benefit, harmonic geared motors offer mounting holes right on the rotating surface of the gearhead for easy, direct installation of rotary tables or other types of load. This saves time and energy by eliminating unnecessary parts and processes and also improves reliability in some cases.
 
Gears 'R Us
 
In recent years, Dolin has expanded its own geared motor lineup as well as products from our industry partners such. Dolin we offer a massive gearhead selection for AC motors, brushless motors, and stepper motors. A great advantage of working with a manufacturer that provides both the gearhead and the motor is that all torque values for each gear ratio is already calculated, and that products have already been tested together.

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