Yes, our planetary gearheads and spur gearheads allow for bidirectional operation.
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."
To give you a generalized example, assume that the motor + gearhead combination 1724T012SR + 16/7, 43:1 is being used with 12 Volts applied to the motor terminals, and that a torque of 71 mNm 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 torque 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 92 mNm.
Total torque = 71mNm x 1.3 = 92 mNm.
The torque reflected back to the motor is then simply the total torque divided by the gear ratio:
Motor torque = 92 mNm / 43 = 2.1 mNm
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 1724T012SR is 14.3 mNm/A. That is, for every 1 A in the motor windings, the motor will produce 14.3 mNm of torque. The reciprocal of the motor constant in this case is .070 A/mNm. Since we have already calculated the torque at the motor shaft to be 2.1 mNm, we can use the reciprocal of the torque constant to calculate the motor current due to the external load:
Current = .070 A/mNm x 2.1 mNm = 147 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 147 mA to drive the external load and 8 mA to drive its own internal friction, the total current required for this application would be 155 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 of the motor (speed at nominal voltage and zero external load) by the stall torque (zero speed and maximum torque). In the case of motor 1724T012SR, the slope of the torque versus speed curve is given by the following:
Slope = DY/DX = -7900 rpm / 10.5 mNm = -752 rpm / mNm
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 2.1 mNm. Therefore, the motor speed loss due to this external torque load will be:
Speed loss = -752 rpm / mNm x 2.1 mNm = -1579 rpm
With no load on the motor shaft, the motor speed will be 7900 rpm. With a load of 2.1 mNm, the motor will lose 1579 rpm from the no-load value. Therefore, in this application the motor speed is rendered by:
Motor speed = 7900 rpm - 1579 rpm = 6321 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 = 6321 rpm / 43 = 147 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 contact us.
A full range of planetary and spur gearheads are available to go with our micro motors. With a few exceptions, 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 selection 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. FAULHABER 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 FAULHABER 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. FAULHABER has 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 FAULHABER 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 FAULHABER application engineer for assistance if you have any questions on the gearhead input speed. All gearheads offered by FAULHABER 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.