Torque needed to generate lateral force with eccentric

On Fri, 19 Feb 2010 12:17:36 -0800, Wizzz wrote:


Yes, you can use moments -- assuming no friction, the weight on the point
of contact with the eccentric, times the horizontal displacement between
the point of contact and the eccentric shaft axis, will be the torque due
to lifting.

Or calculate the speed that the object will lift or fall with respect to
the speed of the shaft, and divide -- that's the effective moment arm.

But the real kicker here is "assuming no friction".  If that eccentric is
rubbing, then most of the torque will be due to friction.  You'll have to
take that into account.

--
www.wescottdesign.com

Great point about the friction...
...any hint on how to calculate/estimate that? I assume it depends on
the materials used, and there must be standard friction coefficients
or something, right?

Thanks for the help.
Regards

On Fri, 19 Feb 2010 12:54:54 -0800, Wizzz wrote:


Sure, there are lots of ways to calculate friction.  Each one gives a
different answer, and most of them are wrong!!

Seriously, yes it can be done to a rough approximation.  But friction
coefficients aren't simple -- a single material doesn't have one, only
pairs of materials, and the surface finish matters, and wear matters, and
lubrication, and, and, and.

The answer to friction questions is generally "get an expert" and/or
"testing, testing, testing".  I'm not an expert at this -- I'm usually in
the happy position of standing there with my hands in my pockets,
smirking, while the experts beat their brains out on friction issues,
although if they fail to overcome them then the smirk disappears when
someone says "Tim, can you get this system working right anyway?".

--
www.wescottdesign.com

OK... then I'll just consider 80% of my torque is lost to friction and
I should be safe, right? ;-)

Wizzz wrote:

Do I hear 95? Going once ...

Build the system with a pair of good bearings where the gearbox will be.
Mount the cam* on a shaft run through the bearings. Attach a lever arm
to the shaft and determine what weight is needed to move the object in a
series of positions along its path. Remember that the cam will exert a
lateral as well as a lifting thrust.

Jerry
________________________________________
* "Cam" because an eccentric won't provide constant distance for
constant angle. You may prefer a snail cam (Archimedean spiral).

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Engineering is the art of making what you want from things you can get.
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No!  There are books and tables that have the coefficient of friction
between different materials.
http://en.wikipedia.org/wiki/Friction
It is easy to find the static friction values but harder to find the
dynamic friction vales.   Then you must find a way to blend the
together if you want do know a dynamic response other wise you can
simply assume the friction is dynamic and the angular speed is
constant.
Do some searching for coefficient of friction.  There is lots of
information.
Don't worry too much about being exact. ( Never let perfect be the the
enemy of good ).
Just know that there may be variances.   The lubricated metal on metal
coefficient of friction is 0.16.  This is much lower than the 0.8 or
80% you mentioned.

There are many ways to generate friction coefficients. If you use
modeling software they may include more than one model for friction.
I don't care if my estimate for friction is 0.15 or 0.17 instead of
0.16 because I will try them all.  The kinetic friction will be less
and maybe as low as 0.1 but I can't find the right answer.   If you
use 0.16 you should be safe.  Now how much force will be required to
accelerate the mass?  Obviously we need to know the mass but also the
change in radius with respect to the rotation and then we need to know
the rotational speed.  The chain rule will provide the acceleration as
a function of angle and frequency. In your case the eccentric will
make contact at slightly different angles so it will vary even at a
constant speed.

This sounds like a test system and I/we get involved with a lot of
them.  Usually a linear motor or hydraulic cylinder applies a
sinusoidal force but at high frequencies moving something between two
cams seems like the easiest but not the most flexible way to go.

When modeling remember that the sum of forces is equal to 0 unless
something is accelerating and the energy in must be equal to the work
done+heat generated.  If everything balances then you probably have it
right.

Search the internet.  There is a lot of info

Peter Nachtwey

Wizzz wrote:

It should come clear if you draw a force diagram. Accounting for
friction will be a major difficulty whatever you calculate. Have you
considered actual tests on a full-scale model?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On Fri, 19 Feb 2010 16:36:06 -0500, Jerry Avins wrote:


Jerry's an old retired fart so he may not realize that there are computer
aided design tools that will do all of this in mathemagic land, without
you ever needing to build anything.

Which doesn't explain why, when I see mechanical types congregating in a
lab, they're generally doing measurements to determine friction, or to
verify friction calculations.

:-)

Note that in addition to any difficulties that friction presents to you
for determining the motor size, it also makes it hard to precisely
position that cam, which in turn makes it hard to precisely locate your
object.  Nothing in life is free...

--
www.wescottdesign.com

Tim Wescott wrote:


[%X]


With a distance of movement of 3mm or so, and if your required profile of
motion is suitable for it, you could make the cam by putting a roller or
ball bearing assembly onto an off-centre round shaft. The bearing outer can
interface with the object and roll around the off-centre shaft to minimise
the friction. I think others have given clues for the rest of the
calculations you need.

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On Sat, 20 Feb 2010 11:58:35 +0000, Paul E. Bennett wrote:


I know I thought that real hard, even if I didn't actually point it out.

Even roller bearings have friction effects, and depending on how tightly
you need to locate within that 3mm a roller bearing (or, for that matter,
any motor/cam arrangement) may not be suitable.

But a roller bearing is probably going to be better than a sliding
contact for all but the least (or most) possible required precision.

--
www.wescottdesign.com

Tim Wescott wrote:

Instead of a cam (with or without a rolling surface) and gearbox, I
would first consider a fine-pitch screw. If there's no room under the
object, it might be possible to put the screw(s) to one side. I once
built a carriage for wafer examination that implemented fine focus with
three short screws bearing sprockets and linked by roller chain. I still
have the dies we made for the 1/8x100 thread (and the commercial taps
that made them). The focus assembly was about 1/2" thick.

If friction is reduced to the point that the drive is more than 50%
efficient, power will be needed to keep it from lowering under its own
weight. A block and tackle is more efficient than 50%, a self-locking
chain hoist is less.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

¯AF¯AF¯AF¯AF¯AF¯AF¯AF¯AF¯AF¯AF¯AF

I agree with Jerry.  I love a good statics problem, but if I had a
similar problem I would no doubt use a stepper motor with a lead
screw.

On Fri, 19 Feb 2010 12:17:36 -0800, Wizzz wrote:


By the way: your total stroke is 3mm, but what precision are you looking
for?

+/- 0.1mm is going to be easy, but 0.01mm is starting to put some pretty
stringent demands on bearings, surface finishes, and whatnot.

--
www.wescottdesign.com