Sliding axle tension system

[IKE]

Originally Posted by Al Skierkiewicz

Remember that the forces on the cross screws are greatest when the tensioner is receiving forces from the chain in motion. You may not bend the screws while tensioning but I bet you will in a pushing match with anotheer robot.

Let's throw some numbers at this assume that while being pushedmost of the wieght is transferred to one wheel (100 pounds) and it is slipping give a coefficient of friction at 1.2(ish) and you have 120 pounds. for a 6 inch wheel this is 360 inch-pounds of torque in the system (120 pounds x 3 inch radius). If it is a 2 inch sprocket at the wheel, the chain force is then 1080 pounds. This gets added onto the tension that you put into the system. You can then run the calculation for the bending stress in a round beam constrained on both ends and then loaded in the middle (make sure you use the root diameter of the bolt if it is all thread).

We use two tensioners on each dead axle so that they are essentially in shear. Not only do they share the lods, but then you can directly connect them to the axle.

[Travis Covington]

Originally Posted by IKE

Let's throw some numbers at this assume that while being pushedmost of the wieght is transferred to one wheel (100 pounds) and it is slipping give a coefficient of friction at 1.2(ish) and you have 120 pounds. for a 6 inch wheel this is 360 inch-pounds of torque in the system (120 pounds x 3 inch radius). If it is a 2 inch sprocket at the wheel, the chain force is then 1080 pounds. This gets added onto the tension that you put into the system. You can then run the calculation for the bending stress in a round beam constrained on both ends and then loaded in the middle (make sure you use the root diameter of the bolt if it is all thread).

We use two tensioners on each dead axle so that they are essentially in shear. Not only do they share the lods, but then you can directly connect them to the axle.

It should be noted that the 2 bearing block screws provide a significant clamping force on the side rail. The friction between the bearing blocks and the side rail adds a ton of resistance and limits the amount of bending force the tensioning screw sees while driving. The clamping force alone will hold the bearing block in place better than you might imagine. The more serious concern is a high speed impact on a wheel, which will provide significantly more stress on that screw than anything our little motors can provide. Even still, you'll likely be able to replace a broken/bent screw if it does fail.

For reference, we used this style of tensioning in 2005 and 2006 and had no issues while using a #10 screw.

However, more importantly (in my eyes, and as Cory mentioned), the lack of axial alignment between the inner and outer bearings/blocks IS an issue and should get more attention than the load this screw sees from drivetrain forces.

[R.C.]

Originally Posted by Don Rotolo

Very nice design! I think I'll be stealing it during build season!

Don

ME THREE!

Originally Posted by rc_cola1323

ME THREE!

RC: If you'd like a different version of this design that takes a bit less machine time, hit me up and I'll have something for you...

To the OP: I'm guessing that the hex you're using is from the McMaster standoffs? To make it easier to machine, you might want to drill the perpendicular hole onto the flat of a hex. Getting a drill to not walk, and to bite right while on a point is a little bit difficult.

Keep working with this design, I bet you'll land at the same place that Madison and I ended up after a few iterations!

[minisimon]

IKE: I think that your math might be a bit off. I don't see how a 2" sprocket would turn 360 in*lbs of torque into 1080 lbs of force. In fact, I think that a 2" diameter sprocket would result in 360 lbs of force in the chain. This is still a large number, but not quite as scary as 1080 lbs.

However, Travis has made a number of good points that mean any calculations we do only serve to give an idea of the forces the tensioning system (both the tightening screw and the clamp screws) will see.

[Al Skierkiewicz]

Ike,
I was trying to visualize the forces transferred to that part during a collision. (I am trained for failure analyzing) If both robots are traveling towards each other then for an instant we can believe the motors can be considered to be in locked rotor mode. That transfers all of the potential energy of both of the two robots to the drive train of each. It is in that moment of collision (with another robot or the wall) that would then transfer the maximum energy to the part in question. Still your 1080 number is pretty impressive when considering the forces involved.