More Mule then Rover?

[tician]

Ever try adding a small grating in front of the reel mowers to catch debris and push it to the side (think cattle catcher on a steam locomotive)? Or maybe an auger, like on the front of a combine, to direct debris to the sides? Or invert the auger to direct debris to a small conveyor belt in the center to collect debris into a container (pine cones friggin' hurt when they make it out from under a rotary lawnmower)? Or a 'jamming' gripper on a small-ish arm to grab debris sighted by a camera observing its path (collect or throw away)?

Can you tell I've wanted to build a robotic lawnmower? I had once considered using a reciprocating-type clipper akin to a hedge trimmer, but that seems to have the worst parts of both a reel and rotary mower (still needs some sort of protection like a reel mower and a power source like a rotary mower).

[Tommy_T]

Ever try adding a small grating in front of the reel mowers to catch debris and push it to the side (think cattle catcher on a steam locomotive)?

tician, good idea!
I have not tried so far, one reason a rotary style mower cut so good is it sucks the grass up as it's cutting,
a rake in front of a reel mower may help stand the grass up for a better cut, and help keep the barbie dolls
out of the blades.

Tommy

[Tommy_T]

Would seem I have a design problem.

in last two years I'v broken each of the drive axles once, the axle is 1.000" dia. steel turned to .800" dia. for the
bearing, each failure occurred at this turned surface. the first was in the middle between the bearings, yesterday
it was the other side at the output edge of the bearings.

I'll change the bearings to 1.000" ID dia. and use retaining rings, in place of the turned .800" shoulder.

Tommy

[Th232]

You've already got a fix lined up, but what was the corner radius at the shoulder? Sharp internal corners are great stress concentrators.

The first failure sounds interesting though, did you figure out a cause?

[tician]

Any sort of surface defect on a shaft will become a stress concentration point, and not just the shoulder - although the shoulder radius is an extremely important parameter during design. Get any sort of wear in one location of the shaft, and odds are that will be where it fails.

What really stands out to me as a likely cause of failure is directly supporting the wheel with the output shaft. Shafts are intended to withstand the large shear stress of torsion, and having the wheel cantilevered so far out on the shaft will cause significant bending (tension/compression) stress that they are not so capable of withstanding.

Another question to ask is: what type of steel were the shafts? Because not all steel is created equal.

[Tommy_T]

The first failure sounds interesting though, did you figure out a cause?

Th232, it seems when I made the bearing blocks I didn't make sure the bearings were fully seated
(press fit), one bearing was cocked a little, over time it caused the axle fail.

but what was the corner radius at the shoulder?

.020" is the corner radius the failure happened @.090" from the corner radius.

what type of steel were the shafts? Because not all steel is created equal.

tician, the shafts used were 1045 steel with a hardness of brinell 150(Rockwell 80).

Get any sort of wear in one location of the shaft, and odds are that will be where it fails.

when they were assembled there was a slip fit between the bearings and the shaft, now there seems to be .012" over on the outside bearing.

Tommy

[Tommy_T]

What really stands out to me as a likely cause of failure is directly supporting the wheel with the output shaft.

The reason I don't suspect the shaft steel or wheel placement.
The wheel hub shoulder is .670" dia. , the bearing shoulder is .800" dia.
none of the failures occurred at the smallest dia.(wheel hub).

I believe all the failures are a result of the bearings, or bearing fit/installation.

on a side note:
if you gotta cut grass with batteries the reel style seems to work best.


Tommy

[Tommy_T]

What really stands out to me as a likely cause of failure is directly supporting the wheel with the output shaft.

tician, I'm being to see it(I get slower as I get older).

each of the failures showed excess wear on the shaft at outside bearing, with no measurable wear on the inside
bearing. the bearings are rated at 2900lb, even with 140lb of sand bags(when wet heavy snow) the load on the
wheel is under 350lb, but because I'm powering the wheel(motor sprocket) on the same side as the load carrying
wheel it is causing excess wear at that bearing(outside bearing).

I increased the dia. at that bearing from .800" to 1.00" and I'm beginning to see all I did was postpone the next failure.

A better fix would be to make room to move the motor sprocket to the other side of the spindle(inside bearing side)
in hopes of evening out the wear between inside and outside bearing surfaces.

Tommy

[tician]

Honestly, this thread has been stuck in my head since I first read about the shaft failure. Shortening the shaft so that the wheels are closer to the frame should help increase the life a bit as it should decrease the bending stress a bit (the bending forces are what causes the bearing to eat into the shaft). Ideally there would be a space frame of some sort to connect the wheel to the bot's frame (to support the weight and forward forces) instead of using the shaft to do it all. This would permit a smaller diameter shaft since it would no longer be supporting the weight of the bot, just the torque between the drive sprocket and the wheels (with a not so small radial force caused by the drive sprocket). If I have time this weekend, I may break out the mechanical engineering textbooks and notebooks and do the math (mostly been EE the last while, so the ME has suffered).

If I am thinking correctly, the axle can be approximated as a cantilever beam. The wheel end of the shaft has an upward reaction force equal to the weight of the bot applied at the bearings, the forward reaction force caused by the wheel (due to the torque applying a force at the ground via the wheel), and the reaction to the applied torque. The wheel end is unconstrained (free to rotate and translate in all dimensions). There will be a very small bending moment at the end of the shaft when the weight of the bot causes the wheel to canter inward (caused by the reaction force of the weight not being applied directly to the end of the shaft, but slightly further out), but given the width and conformal nature of the pneumatic tires you are using, I think it should be safe to assume this bending moment is negligible.

The bearing end of the shaft is fixed in translation three dimensions and fixed in rotation in two dimensions. It will have the downward force of the bot at the bearing and a backward reaction force to counter the forward force from the wheel end. Because the bearing end is fixed in 5 degrees of freedom, there will be reaction moments to counter the bending moments caused by the two forces at the wheel end (the upward reaction force to the weight and the forward force to drive the bot). The longer the shaft and/or the larger the reaction forces at the wheel, then the larger these reaction moments at the wheel (higher stress) and the larger the deflection of the wheel end (the edge of the bearing will bite into the shaft as it deflects). This is of course ignoring the torque and reaction force from the drive sprocket, but its location will affect where these forces are found.

[Tommy_T]

Shortening the shaft so that the wheels are closer to the frame should help increase the life a bit

tician, because I'm using a 42" wide snow plow, with two wheeled prepositional steering, shortening the wheelbase would have
an adverse effect on controlling the plow's path.

the surface for the bearings is 2.75" wide, the ball bearing surface used is 0.5"(1.0" total for both bearings), if I used roller bearings
in place of the ball bearings, it would increase the surface used and in turn reduce the wear at them locations.

lucky for me when the shaft failed it was near my place(50ft), if it was further away recovering the device would have been very
difficult because of the weight of the device. I need to come up with a recovery option(other then calling to tow truck).
maybe something like picture below.

Tommy