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Step 10Get your Bearings!
Smooth bearings make a world of difference for an electric motor. In a hub motor, they are even more important, because they have to support the full weight of a vehicle whereas a standard indirect drive motor might only have to put up with chain tension.
General Bearing Knowledge
In all likelihood, you'll end up using miniature metric single row deep groove ball bearings in your design, because they are the most common types around. Such bearings are rated using the 6000 system.
Bearings are rated by their Dynamic Radial Load Capacity. Dynamic means moving, and radial load is any direction orthogonal to the shaft axis - which is to say, any way you can think of loading it. Ball bearings are generally not rated for Thrust loads, which are coaxial to the shaft.
An average 6001 type bearing has a 12mm bore, a 28mm outer diameter, is 8mm wide, and has a DRL rating of about 1000 pounds. That might sound like alot, and it is.... if your application is applying constant loads with little to no shock, like in an industrial motor running a pulley or something. This is never true for hub motors.
What kills ball bearings is shock load. You hitting a pothole, the sidewalk, a small animal, etc. Even just sidewalk seams can exert impulse forces of thousands of pounds for a fraction of a second. Force is proportional to acceleration, and hitting something solid imparts very high accelerations into the colliding masses. Bearing failure is called by brinelling, or the balls putting divots into the bearing races from shock loads. This results in the "crunchy bearing" sound.
In the worst case, you can deform or shatter a ball, and your bearing usually seizes up. Hence, it never hurts to use the biggest bearings you can design into the motor. The above 6001 bearing is a good choice if you don't mind the limited shaft diameter.
Thin Profile Bearings
The 6800 and 6900 series describe "thin section" bearings which have a minimal difference between the bore and the OD. Bigger ones are some times called ring bearings.
They are convenient because they offer large shaft diameters, good for wire clearance, but without being excessively large in outer diameter or width. After, you don't want your bearings eating up all the precious space between your mounting surfaces.
However, the 6800 and 6900 series are "thin section" for a reason. They are designed for very light loads. The minimal difference in the outer and inner dimensions means that steel thickness is sacrificed for space saving. These bearings usually have DRLs no more than a few hundred pounds.
Yeah, that still sounds like alot, right? But the steel outer and inner races may be just two or three millimeters thick. Thin section bearings brinell easier than their beefier brethren because the thin steel races have less resistance to forceful incursions, like an overloaded ball.
I would caution against using the 6800 series at all. The 6900 series is slightly heavier in construction and represent a good intermediate between ring bearings and "normal" bearings.
For instance, a 6802 ball bearing has a 15mm bore and is only 24mm across. A 6902 bearing has the same bore but is 28mm in diameter, and has over twice the rated load in general purpose ABEC-1 style. Peace of mind for 4 more millimeters?
Sealed or Shielded?
When spec'ing out bearings, you will often find them in myriad flavors, regalia, and trim levels. The question usually boils down to "open, sealed, or shielded"?
Open bearings are open to the air. There's nothing covering the bearing races from dust, grit, and contamination. They also cannot retain lubricant. Open bearings will be destroyed very quickly in hub motor duty. You find these more inside motors or engines where they're bathed in oil and enclosed from the outside.
Shielded bearings are the next level of grime protection. A thin metal shield over the ball races keeps out most everything. However, metal shields do not contact the inner race, so over time, things still do get in. These are by far the most common ball bearings, though, because they represent a good compromise.
Sealed bearings use a rubber seal to accomplish the same goals with more security. The downside of a sealed bearing is more free-running drag, because the rubber seal rubs on the inner race as it moves.
If I had a choice, I would just go with sealed bearings. The price difference between them and shielded is usually minimal, they retain lubricants better, and generally speaking, metal shields can be deformed or damaged easier than a flexible rubber seall
Bearing fit
Ball bearings are precision devices, and thus need precision to be correctly mounted and used. Never use a hammer or mallet to install ball bearings. If they do not slip in, use a proper arbor press! Even a vise is better than nothing (and no, I don't mean vise grips).
Bearing installation must be straight (not crooked) and the difference between the bearing's OD and your mounting surface's bore should be less than 1 thousandth of an inch. That's 0.001 inches, or .02 millimeters. That's really precise.
Too tight fits will cause "crunchiness" and a hard to turn bearing. Using the bearing like this can destroy it quickly.
Loose fits, if under 5 thousandths, are generally rescuable using a retaining compound such as Loctite 609. Very loose fits are not recommended at all.
General Bearing Knowledge
In all likelihood, you'll end up using miniature metric single row deep groove ball bearings in your design, because they are the most common types around. Such bearings are rated using the 6000 system.
Bearings are rated by their Dynamic Radial Load Capacity. Dynamic means moving, and radial load is any direction orthogonal to the shaft axis - which is to say, any way you can think of loading it. Ball bearings are generally not rated for Thrust loads, which are coaxial to the shaft.
An average 6001 type bearing has a 12mm bore, a 28mm outer diameter, is 8mm wide, and has a DRL rating of about 1000 pounds. That might sound like alot, and it is.... if your application is applying constant loads with little to no shock, like in an industrial motor running a pulley or something. This is never true for hub motors.
What kills ball bearings is shock load. You hitting a pothole, the sidewalk, a small animal, etc. Even just sidewalk seams can exert impulse forces of thousands of pounds for a fraction of a second. Force is proportional to acceleration, and hitting something solid imparts very high accelerations into the colliding masses. Bearing failure is called by brinelling, or the balls putting divots into the bearing races from shock loads. This results in the "crunchy bearing" sound.
In the worst case, you can deform or shatter a ball, and your bearing usually seizes up. Hence, it never hurts to use the biggest bearings you can design into the motor. The above 6001 bearing is a good choice if you don't mind the limited shaft diameter.
Thin Profile Bearings
The 6800 and 6900 series describe "thin section" bearings which have a minimal difference between the bore and the OD. Bigger ones are some times called ring bearings.
They are convenient because they offer large shaft diameters, good for wire clearance, but without being excessively large in outer diameter or width. After, you don't want your bearings eating up all the precious space between your mounting surfaces.
However, the 6800 and 6900 series are "thin section" for a reason. They are designed for very light loads. The minimal difference in the outer and inner dimensions means that steel thickness is sacrificed for space saving. These bearings usually have DRLs no more than a few hundred pounds.
Yeah, that still sounds like alot, right? But the steel outer and inner races may be just two or three millimeters thick. Thin section bearings brinell easier than their beefier brethren because the thin steel races have less resistance to forceful incursions, like an overloaded ball.
I would caution against using the 6800 series at all. The 6900 series is slightly heavier in construction and represent a good intermediate between ring bearings and "normal" bearings.
For instance, a 6802 ball bearing has a 15mm bore and is only 24mm across. A 6902 bearing has the same bore but is 28mm in diameter, and has over twice the rated load in general purpose ABEC-1 style. Peace of mind for 4 more millimeters?
Sealed or Shielded?
When spec'ing out bearings, you will often find them in myriad flavors, regalia, and trim levels. The question usually boils down to "open, sealed, or shielded"?
Open bearings are open to the air. There's nothing covering the bearing races from dust, grit, and contamination. They also cannot retain lubricant. Open bearings will be destroyed very quickly in hub motor duty. You find these more inside motors or engines where they're bathed in oil and enclosed from the outside.
Shielded bearings are the next level of grime protection. A thin metal shield over the ball races keeps out most everything. However, metal shields do not contact the inner race, so over time, things still do get in. These are by far the most common ball bearings, though, because they represent a good compromise.
Sealed bearings use a rubber seal to accomplish the same goals with more security. The downside of a sealed bearing is more free-running drag, because the rubber seal rubs on the inner race as it moves.
If I had a choice, I would just go with sealed bearings. The price difference between them and shielded is usually minimal, they retain lubricants better, and generally speaking, metal shields can be deformed or damaged easier than a flexible rubber seall
Bearing fit
Ball bearings are precision devices, and thus need precision to be correctly mounted and used. Never use a hammer or mallet to install ball bearings. If they do not slip in, use a proper arbor press! Even a vise is better than nothing (and no, I don't mean vise grips).
Bearing installation must be straight (not crooked) and the difference between the bearing's OD and your mounting surface's bore should be less than 1 thousandth of an inch. That's 0.001 inches, or .02 millimeters. That's really precise.
Too tight fits will cause "crunchiness" and a hard to turn bearing. Using the bearing like this can destroy it quickly.
Loose fits, if under 5 thousandths, are generally rescuable using a retaining compound such as Loctite 609. Very loose fits are not recommended at all.
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