progress
Tim & Gina Woodburn
for an exhibition at the NEC in July. The firm we work for sponsored us
100%! We'll be competing against another firm on the final day of the
exhibition in a fight to the death, but for all the other days we'll be
competing in speed and agility trials.
Checkout http://www.focalpoint.freeserve.co.uk/robotpics.htm for a rough and
ready look on how wer're getting on with our exhibition robot. Fraid its a
boring wedgie this year, we didnt have time to build the elastic axe we were
planning. The good news on that though is that we've purchased the
components for the weapon so we'll be able to assemble the beast after July.
And yes, it it heavier than 80kg...
Tim
various
Tim & Gina Woodburn
http://www.focalpoint.freeserve.co.uk/robotpics.htm
Sorry the page is slow to load.
Anvil weighs in at 81kg, bloomin miracle since we weren't even being careful
with the weight.
We'll be exhibiting the robot next week from Wednesday at the NEC. We'll be
fighting another robot to destruction on Sunday - thats if it survives a
week of punishment from the general public - yes the public get to drive
him...
So if youre in the area, pop in and take a look.
Tim
Codehead
be
> fighting another robot to destruction on Sunday - thats if it survives a
>*yes the public get to drive him...*
And we DO have a cut-out to stop wayward loons from taking out half the
arena. :)
Codehead
Codehead
For those that are interested, Anvil 'won' it's first scrap at the NEC
Skills 2000 show.
We spent the week letting kids drive it around. The arena was pitifully
small and took a serious beating from the 'little darlings' attempts at
driving. Our kill switch, and reducing the gain on the speed controllers to
20% couldn't compensate for their homicidal tendencies. We had a slight
problem before the fight as one of our motor spindles sheared off after the
week's abuse, but we managed to bodge it back together enough to drive
again. The opposing bot, 'Gow Chundoy' was a 85kg monster built of 1/4"
plate, it's main tactic was to spin on the spot and wait for us to bunder
into as the thing had directional control problems and an AM controller.
There was no clear winner, and the spindle sheared again, leaving us with
only one motor driving, but Anvil was much more aggressive and was doing all
the work in the fight, and so was awarded the win on points. Video footage
was taken, and I'm sure Tim will be putting it up when he can.
The spindle problem may be of interest to anyone who has or is considering
machining down Bosch 750 spindles. Ours were turned down to 9mm (75% of the
original size) and also had a 3mm keyway. The break occured right at the
base of the turned portion. We are unsure if the break occured because of
axial mis-alignment (unlikely) or simply because the reduced diameter of the
spindle couldn't take the torque of the motor. We are currently redesigning
the drive modules with flexible couplings to remove the need to machine the
spindle and prevent mis-alignment problems in future.
Cheers,
Codehead
Greg Middleton
<code...@SPAMSPAMSPAMyahoo.com> wrote:
>The spindle problem may be of interest to anyone who has or is considering
>machining down Bosch 750 spindles. Ours were turned down to 9mm (75% of the
>original size) and also had a 3mm keyway. The break occured right at the
>base of the turned portion. We are unsure if the break occured because of
>axial mis-alignment (unlikely) or simply because the reduced diameter of the
>spindle couldn't take the torque of the motor. We are currently redesigning
>the drive modules with flexible couplings to remove the need to machine the
>spindle and prevent mis-alignment problems in future.
The GPA motors are not over designed, far from it, they have several
marginal features. Bosch wouldn't have used a 12mm shaft unless it was
absolutely necessary.
Incidentally you almost halved the cross sectional area by reducing it
to 9mm 8-(
Greg
Codehead
> marginal features. Bosch wouldn't have used a 12mm shaft unless it was
> absolutely necessary.
>
> Incidentally you almost halved the cross sectional area by reducing it
> to 9mm 8-(
>
> Greg
With the benefit of hindsight, we shouldn't have done it that way. However,
we were on a tight timescale and it was the simplest and most compact
approach. We'll know next time.
Codehead
andy the pugh
> The spindle problem may be of interest to anyone who has or is considering
> machining down Bosch 750 spindles. Ours were turned down to 9mm (75% of the
> original size) and also had a 3mm keyway. The break occured right at the
> base of the turned portion. We are unsure if the break occured because of
> axial mis-alignment (unlikely) or simply because the reduced diameter of the
> spindle couldn't take the torque of the motor. We are currently redesigning
> the drive modules with flexible couplings to remove the need to machine the
> spindle and prevent mis-alignment problems in future.
It is probably not a good idea to modify the motor shafts at all, though
in many cases you will get away with it.
What radius did you have at the root of your change in radius? You need
to consider the stress intensity factor, sharp changes in shape of a
shaft make the effective stress there much greater. If your drive
arrangement adds any superimposed bending stress on the shaft then it
becomes so much worse.
Your reduction of the radius of the shaft is also a lot more serious
than it appears. I don't have my books of sums here at home, but I seem
to recall that stress at the outside of a shaft at a given torque is a
function of 1/radius^3 or ^4. I'll look up the equation tomorrow to get
the real answer. I suspect that by reducing the diameter from 12 to 9
you reduced the strength by a factor of 4, and superimposed a stress
intensity of 2x unless your radius was large. The keyway adds another
stress intensity factor as well, by the way.
If you add flexible couplings make sure you support them correctly.
For connecting things to motor shafts with no keyways (like the Bosch
750s) the best bet are probably Transtorque bushes. The clamp on the an
unmachined shaft, are rated to 50% more than the motor peak torque, are
easily removed and cost 15 quid each.
On a positive note, your robot has fought and survived, more than can be
said of our, which has fought once and failed dismally.
--
ap
andy the pugh
pugh) wrote:
>Your reduction of the radius of the shaft is also a lot more serious
>than it appears. I don't have my books of sums here at home, but I seem
>to recall that stress at the outside of a shaft at a given torque is a
>function of 1/radius^3 or ^4. I'll look up the equation tomorrow to get
>the real answer.
Right, stress at the surface of a solid round bar in torsion is T.d/2J
where T is torque, d is diameter and J is the polar moment of inertia
of the shaft section.
For a rod J = pi.d^4/32
so substituting back in to the first equation, stress at the surface
of a bar is 16.T/pi.d^3
So, by reducing the diameter from 12mm to 9mm you increased the stress
in the shaft by a factor of 2.4.
Codehead
>
> For connecting things to motor shafts with no keyways (like the Bosch
> 750s) the best bet are probably Transtorque bushes. The clamp on the an
> unmachined shaft, are rated to 50% more than the motor peak torque, are
> easily removed and cost 15 quid each.
>
These Transtorque bushes sound interesting, can you reccomend a supplier?
>
> On a positive note, your robot has fought and survived, more than can be
> said of our, which has fought once and failed dismally.
>
Thanks, however, our opponent wasn't exactly going to chase us down, so it
was an easy fight.
Cheers,
Codehead
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andy the pugh
<code...@SPAMSPAMSPAMyahoo.com> wrote:
>These Transtorque bushes sound interesting, can you reccomend a supplier?
I got mine from Charles Walker Transmissions in Sheffield, but RS
carry them (http://rswww.com part number 814-893 )
I think I underestimated their torque capacity, though, The 12mm bore
version is rated at 40Nm, 4x the stall-torque of a Bosch 750.