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Step 9The Center of the World
Your entire motor revolves around its center shaft.
No, really, it does.
Inside-out motors like hub motors have the advantage that their "shaft" is actually stationary. It is also the only mechanical connection to the outside world, because... well, everything else is moving around it. So, the shaft must be stout and resistant to deformation or bending. An off-axis, bent, or otherwise incorrectly constructed & used shaft will cause wobble, stress the bearings, and with your weight on it, could exceed the strength of your fasteners.
Single Supported vs. Double Supported
There are two top-level arrangements, and they have some implications with respect to vehicle compatibility and shaft design.
ý overhung, single supported, or "car" style. The most common style for large hub motors, like those used on... cars. Only mounted on one side. The shaft is thus used in bending. Shafts and bearings for motors of this style need to be much thicker and stronger to avoid damage than...
ý double supported, or "bike" style. The most common for small hub motors. The vehicle weight bears down on both sides of the stationary shaft, and the bearing loads appear between these two points. For short distances between supports, the shaft is used in shear. This is a better arrangement for stiffness, but its not as serviceable because the motor is surrounded by vehicle on both sides.
I will focus on double supported shafts for now, since the single supported designs are quite literally just half of the former.
Single Bearing vs. Double Bearing
Uh oh. There's even a distinction here? Yes! The rotor assembly can be supported only on one side, that is, one endcap, or have two endcaps and be fully enclosed.
ý Single bearing systems represent the vast majority of your average R/C outrunners. While most of those use a live shaft, the principles are the same: the rotor is supported only on one end, and the other is open to air.
Besides exposing the internals of your motor to weather and debris, knowledge of some intermediate mechanical engineering principles is needed to correctly design a single-bearing system. I will not consider single bearing motors, because they are mechanically less durable than an equivalent sized double bearing motor.
You CAN have a single bearing motor with double frame attachment, but then it's just pointless, no?
ý Double bearing, or two-endcap rotors are what essentially all production hub motors are. Even if they are single-supported (car style), there is still a front endcap and a rear endcap, both of which hold bearings. These provide the idea symmetric loading that prevents rotor deformation and magnet-stator collisions.
General overview of shaft design
Refer to Figure 1 for a basic cross sectional diagram of a generic hub motor center shaft.
From left to right:
ý the External Mounting Surface is the main means of attachment to the vehicle. This may be an externally threaded bolt-like protrusion, or a square clamping surface, whatever. This may not be present in compact motors, but are almost always found on bike-style motors, because they are designed to drop right in place of the nonmotorized rear wheel.
ý the External Mounting Clearance is a shoulder to provide spacing between the vehicle frame and the rotor surfaces. May or may not be the same physical diameter as...
ý the Bearing Seats are precision-machined surfaces onto which the motor bearings are fitted. Tight tolerances (1 to 2 thousandths or less!) are required for proper bearing use.
ý the Internal Bearing Clearance serves as a backstop for the bearings so they cannot shift axially.
ý the Stator Mounting Surface may directly couple to the stator, or can support a hub or other mechanism to retain the stator. Generally the largest diameter the shaft occurs here.
ý the Internal Mounting Surface performs the same function as the EMS, but is on the interior of the shaft. This typically takes the form of a threaded hole into which you can tighten a screw against the vehicle frame. Any practical combination of EMS or IMS features can be used - this is a matter of design.
However, there is one very important aspect of internal features that you have to be aware of.
Getting the wires out
Without an electrical connection to the outside world, your motor cannot operate! At the minimum, you need provisions for running three heavy gauge wires out from the internals of the motor. If you plan on using Hall Effect sensors, this could increase to eight total wires: 3 large and 5 small signal wires.
Most generally speaking, two methods exist for running conductors to your windings:
ý Through the shaft center. The shaft is hollow, and the motor mounts using external features. This requires drilling out the center of a shaft while remaining concentric and on-axis. A cross hole or slot is drilled internal to the motor, usually near the stator mounting surface, to bridge the interior of the motor with the outside. Then, wires are run through this center hole.
ý Besides the shaft. In RazEr's case, I elected to use this method of cutting a small keyway (actually a flat) and just running the wires out through it. While easier, this method causes wires to run very close to rotating surfaces, and also means that a section of the motor bearing has no shaft contact. This is mechanically suboptimal.
Examples of each method are in figures 3 through 5 below.
Shaft Size
Yeah, I know, we have to talk about this eventually. The fact that you have to provide enough space to run cables means the motor shaft cannot be too small in diameter. Small diameter shafts are also nonconducive to stiffness.
For hub motors, the old adage rings true: Bigger IS better. Use the largest diameter you have available to you, or the design allows!
Both iterations of RazEr's motor used 15mm diameter shafting. I found this adequate for the roughly 2 inch span they had to bridge.
Shaft size directly correlates with what bearings you can use. Speaking of bearings...
No, really, it does.
Inside-out motors like hub motors have the advantage that their "shaft" is actually stationary. It is also the only mechanical connection to the outside world, because... well, everything else is moving around it. So, the shaft must be stout and resistant to deformation or bending. An off-axis, bent, or otherwise incorrectly constructed & used shaft will cause wobble, stress the bearings, and with your weight on it, could exceed the strength of your fasteners.
Single Supported vs. Double Supported
There are two top-level arrangements, and they have some implications with respect to vehicle compatibility and shaft design.
ý overhung, single supported, or "car" style. The most common style for large hub motors, like those used on... cars. Only mounted on one side. The shaft is thus used in bending. Shafts and bearings for motors of this style need to be much thicker and stronger to avoid damage than...
ý double supported, or "bike" style. The most common for small hub motors. The vehicle weight bears down on both sides of the stationary shaft, and the bearing loads appear between these two points. For short distances between supports, the shaft is used in shear. This is a better arrangement for stiffness, but its not as serviceable because the motor is surrounded by vehicle on both sides.
I will focus on double supported shafts for now, since the single supported designs are quite literally just half of the former.
Single Bearing vs. Double Bearing
Uh oh. There's even a distinction here? Yes! The rotor assembly can be supported only on one side, that is, one endcap, or have two endcaps and be fully enclosed.
ý Single bearing systems represent the vast majority of your average R/C outrunners. While most of those use a live shaft, the principles are the same: the rotor is supported only on one end, and the other is open to air.
Besides exposing the internals of your motor to weather and debris, knowledge of some intermediate mechanical engineering principles is needed to correctly design a single-bearing system. I will not consider single bearing motors, because they are mechanically less durable than an equivalent sized double bearing motor.
You CAN have a single bearing motor with double frame attachment, but then it's just pointless, no?
ý Double bearing, or two-endcap rotors are what essentially all production hub motors are. Even if they are single-supported (car style), there is still a front endcap and a rear endcap, both of which hold bearings. These provide the idea symmetric loading that prevents rotor deformation and magnet-stator collisions.
General overview of shaft design
Refer to Figure 1 for a basic cross sectional diagram of a generic hub motor center shaft.
From left to right:
ý the External Mounting Surface is the main means of attachment to the vehicle. This may be an externally threaded bolt-like protrusion, or a square clamping surface, whatever. This may not be present in compact motors, but are almost always found on bike-style motors, because they are designed to drop right in place of the nonmotorized rear wheel.
ý the External Mounting Clearance is a shoulder to provide spacing between the vehicle frame and the rotor surfaces. May or may not be the same physical diameter as...
ý the Bearing Seats are precision-machined surfaces onto which the motor bearings are fitted. Tight tolerances (1 to 2 thousandths or less!) are required for proper bearing use.
ý the Internal Bearing Clearance serves as a backstop for the bearings so they cannot shift axially.
ý the Stator Mounting Surface may directly couple to the stator, or can support a hub or other mechanism to retain the stator. Generally the largest diameter the shaft occurs here.
ý the Internal Mounting Surface performs the same function as the EMS, but is on the interior of the shaft. This typically takes the form of a threaded hole into which you can tighten a screw against the vehicle frame. Any practical combination of EMS or IMS features can be used - this is a matter of design.
However, there is one very important aspect of internal features that you have to be aware of.
Getting the wires out
Without an electrical connection to the outside world, your motor cannot operate! At the minimum, you need provisions for running three heavy gauge wires out from the internals of the motor. If you plan on using Hall Effect sensors, this could increase to eight total wires: 3 large and 5 small signal wires.
Most generally speaking, two methods exist for running conductors to your windings:
ý Through the shaft center. The shaft is hollow, and the motor mounts using external features. This requires drilling out the center of a shaft while remaining concentric and on-axis. A cross hole or slot is drilled internal to the motor, usually near the stator mounting surface, to bridge the interior of the motor with the outside. Then, wires are run through this center hole.
ý Besides the shaft. In RazEr's case, I elected to use this method of cutting a small keyway (actually a flat) and just running the wires out through it. While easier, this method causes wires to run very close to rotating surfaces, and also means that a section of the motor bearing has no shaft contact. This is mechanically suboptimal.
Examples of each method are in figures 3 through 5 below.
Shaft Size
Yeah, I know, we have to talk about this eventually. The fact that you have to provide enough space to run cables means the motor shaft cannot be too small in diameter. Small diameter shafts are also nonconducive to stiffness.
For hub motors, the old adage rings true: Bigger IS better. Use the largest diameter you have available to you, or the design allows!
Both iterations of RazEr's motor used 15mm diameter shafting. I found this adequate for the roughly 2 inch span they had to bridge.
Shaft size directly correlates with what bearings you can use. Speaking of bearings...
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