Note: Many of the generalities that are made for brush-type motors can be applied to brushless motors. However, as brushless motors are electronically commutated and are powered by driver electronics, the following sections dealing with brush commutator systems and torque-speed curves do not apply. There is a separate section for brushless motors here.
Q: Can your motors be driven at something other than their nominal voltage? A. Yes. In fact, if you can design your device to run the motor slower (lower than nominal voltage) this is a very good thing. Running at lower voltages (and therefore lower speeds) means less brush bounce and less brush/commutator wear for brush type motors, lower current consumption, and longer motor life. On the other hand, if size restrictions and performance requirements demand additional torque and/or speed, overdriving the motor is possible. You must, however, be willing to sacrifice product lifetime if you overdrive the motor.
Q: Why are the gearheads not correctly matched to the motor from a torque standpoint? A: If the maximum continuous torque produced by the motor through the gearhead is considered for each gearhead ratio, many ratios would far exceed the gearhead torque rating. If we were to design each gearhead to withstand the full torque produced by the combination, the gearhead's internal gears would have to be modified dramatically (larger face width, larger pitch diameter, different material, etc.). All these would contribute to a much larger and more expensive product that defeats the intent of having the "power and performance in the smallest package."
Q: Are all of your gearheads reversible? A: Yes.
Q: How long do your motors and gearheads last? A: This varies according to each application. Factors such as operating environment, duty cycle, input power, and how the motor or gear motor is coupled to the load all directly affect product life. Mechanical design factors of the overall mechanism, such as running the motor into hard stops or back-driving the gearheads, affect product lifetime. Generally speaking, brush type motors can run for several thousands of hours when run at nominal conditions. If long lifetime is one of your design criteria, you should consider using brushless motors. These motors are typically limited in their life only by ball bearing wear. If you have detailed questions on this point, it would be best to call one of our Application Engineers, toll-free from the USA or Canada, at 1-800-807-9166.
Q: What is the maximum continuous current the motor can be exposed to? A: This can be calculated from the specifications shown on the motor data sheet. Here's how: Maximum rotor temperature - Ambient temperature = Allowable temperature rise Allowable temperature rise divided by thermal resistances (add up rotor-to-case and case-to-ambient) = Continuous power that can be dissipated in Watts. Set this power = to the current squared x armature resistance. P = I x I x R Solve for I There are many more examples of how to do motor applications in the Web pages called "General Information and Tutorials."
Q: What's the difference between a "servo" motor and a regular motor? A: The term "servo" implies that there is a feedback loop which adjusts one or more operating parameters of the motor such as motion, velocity, position, and/or torque. Servomotors are used in closed loop systems where accuracy and repeatability are important. "Regular" motors (without feedback) are run "open loop" where positional accuracy is not an important factor. You can read all about these in our Programmable Motion Control Handbook (500k).
Q: Can you run MicroMo® brand motors and gear-motors on batteries? A: Yes. In fact, the System Faulhaber® coil - the basis for all modern coreless motors - is designed so that there is no moving iron in the rotor. Only the copper coil turns (around a stationary magnet system). This makes the rotor very low in inertia and able to run at very low current levels. Such a design is optimal for battery operation.
Q: Can you assemble a motor I want with a specific gearhead I want? What about adding cables and terminations? A: Yes. MicroMo has a Class 100,000 clean room that is used for motor and gearhead assembly, cable making, custom circuit board assembly, special soldering operations, and other value-added processes. If you have a special requirement, call toll-free 1-800-807-9166 for additional capabilities information and help with your design.
Q: What factors affect motor noise? A: Generally speaking our motors are designed to be inherently quiet. This is done through material selection, proprietary design techniques, and controlling assembly processes. However, in your application you have additional factors to consider such as how the motor is mounted in your device, the speed and load at which you operate the motor, and what type of bearing system you use. We can give you some ideas of how to minimize noise, but ultimately you should test any motor product in your device before you finalize your selection. Call an Applications Engineer toll-free at 1-800-807-9166 if you would like to discuss your application in detail.
Q: Can you send me a speed-torque curve for one of your motors? A: Yes. We can do that.
Q: I notice there are two shaft ends on this motor. Can I get only one? A: In most cases yes. You can select almost all of our motors (both brush, brushless, and stepping) with either a single output shaft or a thru- (double) shaft. Call one of our Sales Associates toll-free at 1-800-807-9166 if you want specific information on pricing and product availability.
Q: Is it possible to have a flat on this shaft? A: Yes. You can specify where you want a flat on a shaft. You can also specify if you want a hole through the shaft, a pinion or pulley attached to the shaft, or a special sized shaft. For some motor types, thru- (hollow) shafts are also offered.
Q: Can I get feedback devices and other components put on our motor? A: Yes. MicroMo® and Engel® brand products are designed to accommodate a large variety of supplemental devices. Some of these are spur, planetary, worm, right angle, and hybrid gearbheads; power-on and power-off brakes; DC tachometers; optical and magnetic encoders; and resolvers. Call Applications Engineering toll-free at 1-800-807-9166 for more detailed information or to review your design.
Q: Are your motors CE Marked? A: Some of our motors and most of our controllers are CE Marked. It is important to note, however, that the CE Marking of motors not intended for the retail market is unnecessary. The ultimate responsibility for the CE Marking lays with the builder of the final product.
Q: Can your motors withstand industrial environments? A: This depends on how you define industrial environments. MicroMo® and all our other DC motor lines (Minimotor®, Faulhaber®, and Engel®) are used widely in industrial environments such as automated factories, automotive assembly plants, machining operations, semiconductor processing facilities, and nuclear power plants. They can also withstand extreme environments such as autoclaves, the vacuum of outer space, and use in sewer and oil pipe inspection robots. In some cases, special modifications must be done to make sure the motor stands up. If you have a specific application you would like to submit for comment, please call one of our Applications Engineers toll-free at 1-800-807-9166.
Q: Can your motors be used as generators? A: Yes. Most Permanent Magnet brush type DC motors can be used as generators. However, they put out only small voltages. We also have tachometers and motor-tachs which are specifically designed to output a DC voltage. Again, these are low output level devices which are intended for use primarily as control signals. If you are looking for something like a vehicle generator or alternator, or something to power a household appliance, you need to contact a company which specializes in these types of higher output devices.
Q: How do I know whether to use a planetary or spur gearhead? A: Planetary gearheads are typically used when high torques are needed in a limited space. Spur gear systems are used where low current consumption, low noise, and high efficiencies are needed. The negative tradeoffs for using planetary gearboxes are higher current consumption, lower effiency, and higher audible noise.
Q: How can I calculate the final operating conditions (current, speed, etc.) for a motor+gearhead combination given the torque load at the output shaft of the gearhead? A: To give you a generalized example, assume that the motor+gearhead combination 1724T012S+16/7, 43:1 is being used with 12 Volts applied to the motor terminals, and that a torque of 10 oz-in is desired at the output shaft of the gearhead.
Gearhead 16/7 with a 43:1 gear ratio has a data sheet efficiency value of 70%. This means that 30% of the power developed by the motor will be lost in the gearhead. The simplest method of accounting for gearhead losses is to increase the torque requirement by the appropriate amount and make the calculations as if the gearhead were 100% efficient. In this case, we increase the torque requirement at the gearhead output by 30% resulting in a torque (for calculation purposes) of 13 oz-in.
Total torque = 10 oz-in x 1.3 = 13 oz-in.
The torque reflected back to the motor is then simply the total torque divided by the gear ratio:
Motor torque = 13 oz-in / 43 = .302 oz-in.
The motor torque constant is the proportionality constant which defines the relationship between the torque at the motor shaft and the current in the motor windings. In this case, the torque constant for the motor 1724T012S is 2.01 oz-in / Amp. That is, for every 1 Amp in the motor windings, the motor will produce 2.01 oz-in of torque. The reciprocal of the motor constant in this case is .498 Amp / oz-in. Since we have already calculated the torque at the motor shaft to be .302 oz-in, we can use the reciprocal of the torque constant to calculate the motor current due to the external load:
Current = (.498 Amp / oz-in) x .302 oz-in = .150 A = 150 mA
The motor has a small amount of internal friction which requires a proportionate amount of current to drive it. This current is defined as the motor no-load current. In this case, the value is 8 mA (taken from the data sheet). Since the motor requires 150 mA to drive the external load and 8 mA to drive its own internal friction, the total current required for this application would be 158 mA.
The speed of a DC motor is a linear function of the load which it is driving. The proportionality constant relating motor speed to the motor torque load is the slope of the torque versus speed curve. This slope is calculated by dividing the listed no-load speed (actually a negative value) of the motor (maximum speed and zero external load) by the stall torque (zero speed and maximum torque). In the case of motor 1724T12S, the slope of the torque versus speed curve is given by the following:
Note that the slope of the line is a negative value, indicating that the speed losses will be greater with increasing motor load. In this case, we calculated a motor load of .302 oz-in. Therefore, the motor speed loss due to this external torque load will be:
Speed loss = (-5369 rpm / oz-in) x .302 oz-in = -1621 rpm
With no load on the motor shaft, the motor speed will be 8,000 rpm. With a load of .302 oz-in, the motor will lose 1621 rpm from the no-load value. Therefore, in this application the motor speed is rendered by:
Motor speed = 8000 rpm - 1621 rpm = 6378 rpm
The speed of the motor at the output shaft of the gearhead under load is simply the motor speed divided by the gear ratio. In this case:
Output speed = 6378 / 43 = 148 rpm
We accounted for the power losses in the gearhead at the beginning of this exercise, so we need not be concerned about this factor again.
If you want help working out your particular application, please call one of our Applications Engineers, toll-free from the USA or Canada, at 1-800-807-9166.
Precision Gearhead Application Considerations
A full range of planetary and spur gearheads are available to go with our DC micro motors. With a few exception, the input gears of most gearheads are plastic to reduce noise at high speed. All metal input stages are available for high torque applications. In addition to our standard product range, many options are available to suit individual applications requirements.
In selecting a gearhead, one must be mindful that gearbox slection will impact more than just the output speed and torque level at the output shaft. Some of the considerations to keep in mind should include:
Backlash: Backlash is a characteristic of gearhead and gear train construction that allows bidirectional shaft play. It can be caused by generous tolerances in gear design, tooth wear over time, slight machining errors in the gear cutting process, etc. It is measured at the output shaft of the gearhead and can vary typically from 1-7 degrees. Backlash is load dependent and will increase as the load increases. Backlash can cause significant error in positioning system and should be compensated for. Typically, shaft encoders are mounted on the motor shaft and not the output gear shaft of small DC gear-motors. This means that the motor armature position can be different than the expected position of the output gear shaft by the level of backlash in the gear. 3 degrees at the output shaft of a gearhead could mean hundreds of encoder pulses at the motor depending on the resolution of the encoder and the ratio of the gearhead. For example, if you are using a 512 pulse shaft encoder and you have a gearhead ratio of 43:1, 3 degrees of backlash at the gearhead output shaft could mean up to 183 encoder pules of error systematically.
Backlash can be eliminated in 1 direction by placing load tension on the shaft before initiating a move. For more dynamic bidirectional applications backlash can be compensated for electronically by using an external absolute encoder for comparison to the shaft encoder. The motion control electronics can the be programmed to correct position error. The FAULHABER Group also offers zero backlash gearheads to eliminate backlash mechanically. They are dual pass spur gearheads in which the individual passes are preloaded against one another thereby eliminating shaft backlash. Consult with a MicroMo applications engineer about eliminating backlash in your application.
Bearing Choice: Ball bearings are typically specified in applications where high radial and axial shaft loads are present. Be advised, however, that using ball bearings can increase audible noise in some cases. Please refer to the gearhead datasheet for shaft loading specifications. Sintered bearings are available for lower torque applications characterized by lower radial shaft loading and constant load characteristics. Ceramic bearings are an alternative for cost-sensitive applications in which extended life and enhanced radial load-bearing capabilities are important. MicroMo and the FAULHABER Group have developed a proprietary series of ceramic sleeve bearings. These bearing systems allow the user to increase radial loads beyond the levels allowed in traditional sintered bronze bearing systems. Costs of the ceramic bearings are also considerably below that of ball bearings.
Care should be taken when press fitting components to a gearhead output shaft. We recommend not exceeding the press fit force ratings specified in the gearhead datasheet. This can damage the bearings and the internal gears themselves. In some cases, gearhead shaft bearings (ball bearings only) are preloaded with a small wave washer under the retaining ring on the bearing. Exceeding the press fit force specification on the datasheet can damage this wave washer and negate the preload on the bearing. This will effect the performance of the bearing and should always be avoided.
Lubrication: The gear and bearing lubrication can be a defining factor in gearhead performance. All FAULHABER brand gearhead bearing systems and gear trains are lubricated for life. Re-lubrication is not needed, and is not recommended. The use of non-approved lubricants on or around the gearheads or motors can negatively impact their function and life expectancy. The standard lubricants for reduction gears are formulated to provide optimum life performance with minimum current consumption at no-load condition. For extended life and severe performance requirements, all metal gears and special heavy duty lubricants are available. Specially lubricated gearing systems are also available for extended temperature and vacuum environments. Contact a MicroMo applications engineer to discuss modifying the lubrication for special environmental requirements.
Input Speed and Direction of Rotation: The input speed specification on the FAULHABER precision gearhead datasheet refers to the input speed recommended in order to maximize gearhead life. This specification is not intended to limit the gearheads to input speeds below the specification. It can be considered to be a safe mean value for operation. Your application may not require the maximum lifetime performance of the gearhead and this input speed specification may be safely exceeded depending on the performance requirements. Contact your MicroMo application engineer for assistance if you have any questions on the gearhead input speed. All gearheads ofered by MicroMo are reversible. In the datasheets you may see an equal or not equal symbol. Don't let this confuse you. This simply means that when positive voltage is applied to the positive terminal of the motor and negative to the negative terminal that the output shaft of the gearhead, depending on the ratio, is equal to the direction of rotation of the motor or is not equal to the direction of rotation of the motor. If you have any question on any of the specifications in our datasheets, don't hesitate to contact one of our application engineers for assistance.
Blocking, Stalling, and Backdriving: In general we do not recommend that our gearheads are blocked while the motor is under power. Due to the wide range of ratios available for gearheads and it is highly probably that the motor has enough power, even at low current, to "overpower" the gearhead if it is blocked or stalled. This means that the torque generated at the motor is enough to strip the gears in the later stages of the gearhead or even to shear off the output shaft. Careful consideration should be paid to setting up the appropriate current limits in an application if the gearhead must be blocked to stop it.
Backdriving our gearheads is not recommended. Backdriving means that a torque is applied to the gearhead output shaft which in turn will reverse drive the input stages of the gearhead. This can damage the gearhead in a myriad of ways including causing it to jam or simply breaking off the output shaft.
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