Bevel/Miter Gear Mounting Distances

[asid61]

Originally Posted by GeeTwo

This depends on what you are designing. If you are designing a swerve drive, yes, you need a thrust bearing so that the gearbox can support the robot weight. On the other hand, if all you are doing is turning the motor sideways then driving another gear or sprocket, or are transferring torque from one direction to another (e.g. a lobster drive), there will not be a significant axial load.

I thought miter/bevel gears by nature exert large axial forces? In a swerve drive you can have a thrust bearing on the module and a separate one on the miter/bevel gears to take two different thrust loads.

[AdamHeard]

Originally Posted by asid61

I thought miter/bevel gears by nature exert large axial forces? In a swerve drive you can have a thrust bearing on the module and a separate one on the miter/bevel gears to take two different thrust loads.

There is a little bit of hand waving here, but for the typical FRC swerve that uses 12DP 15T bevels with some reduction after to the wheel (the rough size modules 118, 1717, 973, 1625, etc... have run ) it's okay to use the radial bearing (typical an FR6/FR8 here) to handle these axial loads. If doing this make sure the gear is necked down such as the Vex bevel so it's not rubbing the outer race.

Starting fresh, and not having a massive pile of anecdotal data for these exact bevels to reference, I'd design in better thrust management (or do the math for what thrust you bearing can take and compare that to the loads generated).

[Aren Siekmeier]

Originally Posted by AdamHeard

There is a little bit of hand waving here, but for the typical FRC swerve that uses 12DP 15T bevels with some reduction after to the wheel (the rough size modules 118, 1717, 973, 1625, etc... have run ) it's okay to use the radial bearing (typical an FR6/FR8 here) to handle these axial loads. If doing this make sure the gear is necked down such as the Vex bevel so it's not rubbing the outer race.

Starting fresh, and not having a massive pile of anecdotal data for these exact bevels to reference, I'd design in better thrust management (or do the match for what thrust you bearing can take and compare that to the loads generated).

A 1" diameter bevel gear driven at the same speed as a 2" wheel delivering 100lb of force to that wheel still only sees a 50 lb thrust load. Increasing wheel size or reduction after the bevel stage, or reducing final force output, reduces this thrust load. Since the radial bearings typically take several hundred pounds radially, sounds reasonable that the groove is deep enough (min. ~30 degree span) to support that small of a thrust load as well. Sounds like some of those swerves did rely on the radial bearing in this way.

Edit: did wheel to bevel diameter ratio backwards. Fixed by multiplying thrust loads by 4.

[AdamHeard]

Originally Posted by Aren Siekmeier

Sounds like some of those swerves did rely on the radial bearing in this way.

AFAIK they all relied on the traditional radial bearing to handle these thrust loads.

[s_forbes]

Originally Posted by AdamHeard

AFAIK they all relied on the traditional radial bearing to handle these thrust loads.

One counter example is 1640, who has been re-iterating their swerve modules for the last few years and have one of the most fully developed systems (in my opinion). They still incorporate thrust bearings into their modules. It's only about $3.50 per bearing, so not really a large resource cost.

Anyone designing swerve modules should check out their wiki, it's a fountain of information. http://wiki.team1640.com/index.php?t...XI_Drive_Train

[InFlight]

Just dug out Shigley & Mithell Machine Design Text

For the VEX Miter gears the average gear tooth diameter is 0.926 inches, or a radius of 0.463 inch. For Miter gears both Pitch angles are 45°

Axial Bearing Load = (Torque / radius avg) tan (45°) sin(45°)
= (Torque/0.463) (1) (.707)

CIM Motor Torque at maximum power is 172 oz* in

Axial Gear Force = 172 (Ounce*In) * (.707) / 0.463 in = 262 oz or 16.4 lbs.

Directly driven by a CIM Motor. If there are typical swerve speed reduction stages of 4:1 or so ahead of the miter gears, you can multiply this axial force by that ratio as well.

[Aren Siekmeier]

Originally Posted by InFlight

Axial Bearing Load = (Torque / radius avg) tan (45°) sin(45°)
= (Torque/0.463) (1) (.707)

The argument to tangent is the pressure angle of the teeth, not the pitch cone angle. tan(20°) is around 0.36 resulting in a reduction in your figures by a factor of 3.

[InFlight]

Axial Bearing Load = (Torque / radius avg) tan (20°) sin(45°)
= (Torque/0.463) (.364) (.707)


Axial Gear Force = 172 (Ounce*In) * (.364)(.707) / 0.463 in
= 95.6 oz or 5.9 lbs

With a Swerve drive gear reduction, this is still a substantial force on a small FR6ZZ

[AdamHeard]

Originally Posted by InFlight

Axial Bearing Load = (Torque / radius avg) tan (20°) sin(45°)
= (Torque/0.463) (.364) (.707)


Axial Gear Force = 172 (Ounce*In) * (.364)(.707) / 0.463 in
= 95.6 oz or 5.9 lbs

With a Swerve drive gear reduction, this is still a substantial force on a small FR6ZZ

Much smaller than the thrust loads applied to the bearings in all traction wheeled skid steers.

[ajlapp]

If you're particularly worried about the thrust loads and you're using the Vex miter gears...you'll notice they already include a small shoulder on the back side for a standard sized 0.0625" thick thrust bushing. Adding this bushing will not modify the mount distance.

They don't advertise this feature but it's there.

[Gdeaver]

Are thrust bearings required behind the bevel gears? No. Some teams have not used them and it will work. Our justification for them is long term protection from wear. Our robots rack up hours and hours of run time from many events, driver practice and demos. They are cheap failure protection. Also note that there are some very large dynamic loads. This years game was very tame for drive trains. And then there was 2014. We have never had any failures in this part of the module. We will always use thrust bearings. We do not use radial ball bearings on the vertical. We use needle bearings and the thrust bearings. The drive shaft is 4140 steel. We have contemplated going with a 7075 drive shaft with radial ball bearings. Have always questioned the durability of a 7075 shaft in this application. We try to maintain tight tolerances on the bevel gears. That said we have 1 module that is about .023" off on the vertical bevel. It's been bouncing around since 2013 and still works. Just beginning to have very slight wear. No matter how a team chooses to do the bevel gear and bearings the 1 most important thing for teams to do is design the lower module so that it does not "rack". Racking will lead to wear and failure.