Do you know why we still use today gearboxes while servo motors becoming stronger and more advanced? The gearboxes of Apex Dynamics are used in many cases in combination with a servo motor, for example because they have low backlash and are able to deal with high torque. But still we did not give an answer to the previous question: why! In this article we discuss the operation of a servo drive and translate this through to the gearboxes.
What is a servo-drive?
Note that the preload torque fluctuates due to manufacturing tolerances and lead variation, so manufacturers will either provide a range of allowable values (for example, 0.04 to 0.17 Nm), or they will indicate an allowable percentage variation from a nominal preload torque value (for example, 0.10 Nm, ±40%). If you see a servo with a torque of 35kg-cm, what does that mean? Typically, when you see a value like 35 kg printed on a servo motor, what they are referring to is the stall torque, which, in this case, is 35 kg-cm. Stall torque is the torque load that causes a servo motor to “stall” or stop rotating. Now, we can calculate the reduction we would need to achieve the necessary torque of 3.05 Nm. Gear Reduction: We have now chosen a gear reduction of 26:1, which means we can calculate the exact load our elevator motor should encounter. 1 = Gearbox Input Power (kW) S = Safety Coefficient (-) T 2 = Gearbox Output Torque (Nm) η = Gearbox Efficiency (-) µ = Coefficient of Friction (-) π = 3.1459 ν t b The values given in the load table are based on uniform, smooth servo-operation. Since, in practice, the applications are.
The prefix servo comes from the Latin servus which means slave or servant. Technically translated tis means to follow or execute a command. A servo motor follows the (complex) task given to him.
For industrial applications, servo motors are used where a drive-system has to be accurate or highly dynamic. The feedback to the motor is done through a resolver (analogue sensor of rotation) or encoder (digital sensor of rotation). A servo motor is controlled by a servo amplifier, possibly with a shaft controller.
The rotation frequency of the actuator is given back by the resolver or encoder. This is capable in addition to the rotational speed, also to determine the position of the rotor and the direction of rotation. The servo amplifier compares the set rotational frequency with the measured rotational frequency. Now the servo amplifier can drive the actuator to the desired values.
Difference between a stepper motor and servo motor
There is a misunderstanding that a stepper motor is a kind of servo motor. This is not true; a stepper motor does not give any feedback ! Feedback is typical for servo motors, exactly what “servus” stands for.
The various servo motors
There are roughly two types of servomotors (actuators), operating on direct current (DC servo motor) or alternating current (AC Servo Motor). The table below is showing the main features:
| Motor | Advantages | Disadvantages |
| DC Servomotor | for very large capacities | Uses brushes |
| Precursor of servo technology | DC Power supply | |
| Expensive motors | ||
| Maintenance sensitive | ||
| AC Servomotor | Low inertia | Not worth mentioning |
| High speed possible | ||
| Fast torque | ||
| High termal load | ||
| Compact housing | ||
| Low maintenance |
Interesting, AC Servo Motors have no significant disadvantages more! So, why would you use a gearbox?
This has the following reasons:
- If you want a very low speed, possibly in combination with a higher torque.
- If you need high torque. (Actuators with high torque are available, but they are exponentially more expensive compared to the smaller servomotors)
- Inertia matching, in order to prevent that the load determines the behaviour of the motor.
- In order to absorb the high radial or axial forces of the application.
- If you want to go “around the corner” otherwise the servomotor sticks out the application. Gearboxes are available in right-angled versions.
Why a gearbox is essential when using a servomotor
Almost every robot as we know it is equipped with servo technology, which enables them to move faster, more accurately and without interruption. Gearboxes are needed to translate the high speed of the motor in a controlled movement of the arm, for example to pick a product. Without a gearbox, the mass and velocity of this arm will demand the behaviour of motor and it will literally overshoot the target.
What are the criteria using a gearbox for a servomotor?
- Low backlash during the life time!
It makes no sense to build a high-precision motor with a high backlash gearbox. The play is indicated in arcminutes, where one arc minute is 1/60th of a degree. One arcminute is approximately 25 mm deviation at a radius of 100 meters! - High repeatability accuracy.
Better end product - Low inertia!
The motors are able to regulate very dynamic. This is extinguished if he has trouble getting the gearbox speed changed. An oil tanker has a high inertia and is difficult to change direction, a Jet-ski has a low mass inertia and is suitable for fast movements. - High efficiency!
Servo Technology is not cheap. With a high efficiency gearbox, the motor, cables and amplifier remain relatively small which means the amount of investment remains interesting. - High torques!
Servo motors are relatively strong and this torque is multiplied by the ratio of the gear. Therefore, the gearbox is able to handle high torque. Taking account of the radial load. - Low noise!
Often servo technology is used wherever people work, such as operators. A low noise level is very reassuring, or it is even a health and safety obligation. - Suitable to withstand high torque emergency stops
- Low break-away torque
- Maintenance free throughout its lifetime
- Easy and flexible installation
A servo drive = A servo motor with a gearbox.
The gearboxes of Apex Dynamics meet all the above criteria and are thus fully suitable for use with a wide variety of servo motors. Our customers and partners already know that even more is possible. The market knows Apex Dynamics is running harder, which means we guarantee proper delivery, excellent service and high quality. Do ask yourself what gearbox you choose next time?
Belt driven linear systems are common in applications that require long travel and high speed, such as gantry robots and material handling and transport. The motors of choice for these systems are often servo motors, for their ability to accurately control position, speed, and torque.
Sizing and selecting the servo motor requires determining both the continuous and intermittent drive torques required for the application. The continuous torque is calculated by taking the root mean square of all the torque requirements throughout the application — torque required for acceleration, torque for constant velocity, and torque for deceleration. In most applications, the maximum (intermittent) torque occurs during acceleration.
To determine the root mean square (continuous) torque, we first calculate the torque values required during each phase of the move profile.
Torque required for constant velocity
For a belt drive system, the motor torque required during constant velocity is simply the total axial force (Fa) on the belt multiplied by the radius (r1) of the drive pulley.
Tc = torque required during constant velocity (Nm)
Fa = total axial force (N)
r1 = radius of drive pulley (mm)
η = efficiency of belt drive system
Notice that the efficiency (η) of the belt drive system is included in the torque equation. This efficiency accounts for losses such as friction between the belt and pulleys. Also note that we’ve assumed the drive and idler (driven) pulleys have the same radius, which is often the case for belt driven linear motion systems.
Unlike screw drives, which often encounter axial forces due to external operations such as pressing or drilling, belt drives aren’t designed to withstand external axial forces. So the total axial force for a belt drive system consists only of the force required to move the load, which is the weight (m*g) of the load (both the external load and the belt) multiplied by the coefficient of friction (μ) of the guide supporting the load.
m = mass of moved load (external load plus belt) (kg)
g = gravity (m/s2)
μ = coefficient of friction of guide
Torque required for acceleration
The acceleration phase of the move profile is typically the period when maximum torque is required from the motor, and this torque value, Ta, is often taken as the intermittent torque.
The torque required during acceleration includes the torque required at constant speed plus the torque required to accelerate the load.
Ta = total torque required during acceleration (Nm)
Tacc = torque required due to acceleration (Nm)
The torque due to acceleration is found by multiplying the total inertia of the system (Jt) by the angular acceleration (α).
Jt = total inertia of the system (kgm2)
a = angular acceleration (rad/s2)
The total system inertia includes the inertia of the motor (because the motor has to overcome its own inertia), coupling, pulleys, and load.
Jm = inertia of motor (provided by manufacturer) (kgm2)
Jc = inertia of coupling (provided by manufacturer) (kgm2)
Jp1 = inertia of drive pulley (provided by manufacturer, or calculate) (kgm2)
Jp2 = inertia of idler pulley (provide by manufacturer, or calculate) (kgm2)
Jl = inertia of load (kgm2)
Although we assumed above that the drive and idler pulleys have the same radius, their inertias may be slightly different, since the drive pulley is toothed and, therefore, has a slightly larger radius and higher mass than the idler pulley.
The inertia values of the motor, coupling, and pulleys are typically specified by their respective manufacturers. However, the inertia of the load must be calculated. Remember that the load includes the mass of both the external load and the belt, since the motor has to generate enough torque to overcome the inertia of the belt.
ml = mass of external load (kg)
mb = mass of belt (kg)
r1 = radius of drive pulley (mm)
For the angular acceleration, we assume that the system is accelerating from zero to some maximum velocity, with N being the maximum angular velocity and t being the time to accelerate.
N = maximum angular velocity (rpm)
t = time for acceleration (s)
If the system is accelerating from a non-zero velocity, then the equation would simply incorporate the change in velocity (ΔN) divided by the time over which the velocity increase occurred (Δt).
Torque required for deceleration
The motor drive torque required for deceleration is equal to the torque at constant velocity minus the torque due to acceleration.
Td = torque required during deceleration (Nm)
Continuous torque
Now that we know the motor drive torques required during acceleration, constant velocity, and deceleration, we can take the root mean square of these values to determine the continuous torque required by the motor.
TRMS = root mean square (continuous) torque (Nm)
ta = time for acceleration (s)
tc = time for constant velocity (s)
td = time for deceleration (s)
Servo Gearbox Torque Calculator
ttotal = total time for move (including any idle time between moves) (s)
Feature image credit: Rollon S.p.A.