How to calculate the rotational friction of square axle

[BenGuise]

How would I calculate the frictional torque of rotating an axle with a square cross section? The axles that I’m using to drive a gear box are rectangular instead of cylindrical like normal axles, so I’m trying to figure out how much more a motor has to work to turn a square axle than it does to turn a cyclindrical axle

 

[berkeman]

How would I calculate the frictional torque of rotating an axle with a square cross section? The axles that I’m using to drive a gear box are rectangular instead of cylindrical like normal axles, so I’m trying to figure out how much more a motor has to work to turn a square axle than it does to turn a cyclindrical axle

Welcome to the PF.

Can you post a sketch or more details? What do the bearings look like?

 

[Dr.D]

Is this a case of a square shaft in a round bearing, or something else?

 

[BenGuise]

Welcome to the PF.

Can you post a sketch or more details? What do the bearings look like?

They’re just bearing flats. Here’s a picture.

Welcome to the PF.

Can you post a sketch or more details? What do the bearings look like?

heres a picture.

 

[BenGuise]

Welcome to the PF.

Can you post a sketch or more details? What do the bearings look like?

Just to clarify, I’m trying to find the static friction. Thanks

 

[Dr.D]

That certainly does not tell us very much. If you want help, please tell us a lot more about your system.

 

[BenGuise]

Here’s a picture of the full system. I want to know how to find the minimum force that needs to be exerted by the motor in order to rotate the axle. The cones from the motor and passes through a whole of one of the plastic bearing flats I sent a picture of on both sides of the gear tower. I could probably figure it out if I were using cylindrical axles, but I’m not sure that rectangular axles behave the same way. Thanks.

 

[BenGuise]

Is this a case of a square shaft in a round bearing, or something else?

Yes, square shaft in a round bearing.

 

[Dr.D]

Assuming that there is some amount of clearance (as there must be if it is not to bind up), you will be limited to contact on two edges most of the time (this looks like a recipe for lots of wear!). The contact location will move as the shaft rotates, so the point of application of the friction force will move as well.

Why on Earth would anyone do this? This looks like a bad design from the get-go.

 

[BenGuise]

Assuming that there is some amount of clearance (as there must be if it is not to bind up), you will be limited to contact on two edges most of the time (this looks like a recipe for lots of wear!). The contact location will move as the shaft rotates, so the point of application of the friction force will move as well.

Why on Earth would anyone do this? This looks like a bad design from the get-go.

I compete in a robotics league called vex robotics and they have specific vex robotics branded parts that you’re limited to building with. And those are the shafts and bearing that they offer.

 

[JBA]

All, below is the link to the above illustration and discussion on "VEX bearing flats".

https://renegaderobotics.org/what-are-bearing-flats-for/

It would appear the root question may be "How much more friction will a square shaft rotating in a round hole have than a round shaft rotating in a round hole?"

If you want to understand more, google "VEX Robotics" which makes robotics erector kits and components; and, the student robotics competitions using robots built with the VEX system.

 

[BenGuise]

Assuming that there is some amount of clearance (as there must be if it is not to bind up), you will be limited to contact on two edges most of the time (this looks like a recipe for lots of wear!). The contact location will move as the shaft rotates, so the point of application of the friction force will move as well.

Why on Earth would anyone do this? This looks like a bad design from the get-go.

They also offer what they call high strength axles, which are just a thicker version of the ones in the picture, so I was thinking about taking those and shaving them down into smaller cyclindrical axles with a lathe because they would technically be legal to compete with since they’re made from vex issues parts

 

[Tom.G]

Try slipping a bearing flat on a square axle and spinning the bearing. I suspect it will spin rather freely. If it does, that means the friction is very close to zero. In your build, make sure all the axles are well aligned with the bearing flats. Any side loading on the bearings will force either the axle to bend or the bearing material to deform four times each revolution, eating up a noticeable amount of power.

Sounds like an interesting project. Please keep us updated on your progress and results... and Good Luck in the contest!

Tom

 

[Dr.D]

The bearing spinning freely on the axle may spin freely, but that is an artificial situation; there is no load on the bearing. It is the loaded case that really tells the story.

 

[CWatters]

Probably best to measure the friction. Calculating it is likely to be inaccurate.

 

[Dr.D]

Probably best to measure the friction. Calculating it is likely to be inaccurate.

I certainly agree, but be sure to measure it when the system is under load, not in a free state.

 

[Nidum]

Are the axles actually sharp cornered square section or are they really cylindrical but with four flats cut on them leaving a significant part of the original cylinder intact ?

 

[CWatters]

A bit of googling suggests that VEX make collars to fit around shafts. Do these collars fit the ID of their bearings?

https://www.vexrobotics.com/shafts-and-hardware.html
https://www.vexrobotics.com/bearings.html

Want a smoother ride? Use Bearings and Shaft Collars to keep your mechanisms and wheels moving smoothly, and locked into place.

 

[Dr.D]

Looks like that photograph partially answers the previous question from Nidum about the shaft corners. It looks like they could well be part of a circular shaft with four large flats.

 

[BenGuise]

Looks like that photograph partially answers the previous question from Nidum about the shaft corners. It looks like they could well be part of a circular shaft with four large flats.

Those black axle pieces are intended to be used as axle couplers to connect two axles and they don’t fit into the holes of the bearing flat. I drilled a bigger hole into one of the plastic bearing flats though to see if it would work as a circular shaft and it actually works great.

 

[CWatters]

I didn't mean fit into the bearing flats I meant do they fit into the ball bearings they sell?

 

[BenGuise]

I didn't mean fit into the bearing flats I meant do they fit into the ball bearings they sell?

They don’t sell ball bearings..

 

[CWatters]

I posted a link to them earlier. Is that not them?