: When using Mechanism and setting up 2 gears, set the number of teeth etc.
: done this. When drag rotation is done everything moves fine - HOWEVER - the
: teeth don't mesh graphically - is there a trick to this to give a more
: realistic appearance?
Steve, I'm not sure if I understand the problem. It sounds like you're saying
they start out meshed okay, but when you define them as gears, with the ratios
all, that somehow they don't stay meshed when you move them. Is that about it?
you're not gonna T me for this, so it's just as well you handed
them out IA :-)
I've never been able to understand the fascination people have
for modelling gear teeth. Are you doing this just so your boss
can whack his weewee rapidly ? If so, that's okay. Continue on
as before. Otherwise, is this for Real Engineering (tm) ? If so,
forget it. Think 'GIGO.'
a) How does the software create the involute curve ? Does it
use a mathematical formula to derive an involute from the
base circle ? if not, think GIGO. If you don't KNOW, how
can you trust your end result ?
b) how does the software deal with the tooth thinning that's
necessary in Real Life (tm) for gears to mesh without
interference due to manufacturing tolerances ? GIGO.
c) Let's ASSume that you are trying to solve the most basic
of gear tooth failure problems : pitting at the pitch line
leading to spalling, loss of case, and eventual failure.
This is caused by loads exceeding the film strength of the
lubricant, leading to micro-welding which then pulls little
pieces out of the mating parts usually starting at the pitch
line then continuing to catastrophe. You would think that
if you had good tooth profile curves and smart software that
takes into account the contact ratio, thus the varying stress
on the teeth that occur as the number of teeth carrying the
load varies from 1.5 to 2 (approximately, in a decent design)
then you might get some fairly accurate results. IF the
software takes these factors into account and IF you have
the profiles down pat and IF the teeeth aren't modified with
tip and root relief to overcome the bending loaded-tooth
problem, then maybe you could do this in Pro/E. Even tho
it'd take about fifty times as long as using an HP-41, you
could probably do it. That's IF you knew that all these items
were calculated correctly in the software. Otherwise, GIGO.
d) How about a more real-world (hate to keep bringing these
ugly complications in here) situation ? There are two
common modes for tooth failure. The first is the one we
just discussed - overload leading to oil film failure.
The second is failure due to fatigue starting in the root.
If it is uncommon to have a correctly-defined involute
in most software, it is almost unique to have the trochoids
that comprise the root fillets done correctly in software.
The only programs which I know of that do that are two-D
specialized gear programs which create the fillet by an
iterative generation creating a shape conjugate to the
correctly- defined cutter geometry. If your fillet isn't
the correct shape, how the hell can you trust the analysis ?
Especially when it's the fillet shape which is the critical
portion of the analysis ? I't common to gain up to 15% in
fatigue strength just by modifying the root fillet shape.
Does Pro/E do this ? CAN Pro/e do this ? Oh, by the way -
hobbed roots are not the same shape at all as shaper-cut
roots. So do you know which process will be used to make
the parts ? Again, will the parts be finish-shaved with
tip and root relief ? Or cut by a hob with tip and root
relief ? Or crowned ? unless you know ALL these factors,
AND how the software creates its little Virtual gear,
your analysis will be useless. No, WORSE than useless
because someone will probably believe it. After all, it
looks so good on-screen and it came out of a computer,
right ? Sigh.
There are many more factors besdies these involved; I've just
touched on the most basic ones. Suffice to say, using solid
modelled gear teeth for anything beyond amazing the masses is
not yet ready for prime time.
I'm sure there are smart people playing with solid-modelled gear
tooth analysis, but they already know the problems. I can think
of one PhD. who is using APL/2 for a research project, but it's
most certainly not commercial software. The people who ask "how
can I model a gear tooth so I can test it in Mechanica ?" are the
ones who should beware. GIGO. At the present time it is *much*
faster and more accurate to analyze gear teeth using traditional
methods - the good ol' calculator approach. Leave the tooth-meshing
display for the times when you want to amaze your ignorant boss -
and better hope he *is* ignorant enough to not start asking the
obvious questions. Otherwise, don't waste your time playing with
your pecker in Pro/E by modelling gear teeth. It's useless. Worse
than useless - it gives false results.
wow hamei! Did this guy touch a nerve!? I was going to say that I've
seen some pretty decent involutes get drawn, but you're right... it was
a specialized 2D Gear application that knew about most of your points.
The graphics wouldn't really help too much if you didn't heed the other
warnings coming out of the software.
Actually, I think Steve was just trying to animate some gears that he
tossed approximate teeth on (nothing really wrong with that) and he
wanted to see them stay lined up. They didn't... because he didn't have
any real way to line them up. Probably an initial condition on the joint
would have gotten them lined up. I'll bet they were always the same
amount of mis-allignment, right? Not incrementally getting worse.
Sorry all - been away...
To answer a few questions..
The teeth were generated within Pro/E for the sole purpose of motion
representation only. I don't plan to do gear design or analysis in Pro/E. I
don't have the formulas I used in front of me presently - but if you need to
know then post and I'll post back. Your comment about modeling gear teeth -
depending on the business etc. modeling of teeth is required for drawing
purposes and thus will be in the assembly as well
Yes you are correct ... If I assemble in assembly mode - you can use a
number of different items and configurations to simulate the gear teeth to
mesh in a reasonably accurate fashion. However when you convert to Mechanism
and create pin connections the orientation of the teeth from the 2 gears
doesn't match up as if they were properly meshed, movement is OK - however
if you are using this assembly as a presentation - it is noticeably not
What are you using for the driving dimensions ? You should be
using the pitch diameters. At the pitch diameter two gears MUST
move at the same pitch-line speed (otherwise the teeth break and
fall off :-)
i don't know Mechanism at all but what you basically have with
a pair of gears in mesh is a pair of rolls friction-driving each
other with no slip. Think of it that way rather than teeth-as-pegs
pushing on each other, maybe you can find a way to simulate the
motion more effectively. You may be getting screwed up because
the sliding velocity everywhere *except* at the pitch point of
the mesh is constantly changing. If Mechanism has a way to
represent friction rolls you'll be home free.
I still say it's a waste of time tho :-)
i don't see what the problem with generating gear teeth is, we managed
to do this years ago using proe using proper involute curves,
compensating for correction factors, class, tooth root radius etc &
all done in accordance with iso6336 parts 1 to 5. Unfortunately we are
not going to be developing this into a commercial product, but it's
fast and easy to customise any aspect of the gears. Just a little bit
of hard work is all that is called for.
Right. Only problem is GIGO.
There is no such thing as a "tooth root radius." If you just want
to make a cute animation, fine. If you want to do analysis, forget
it. You are not including some of the most critical factors in an
analysis. Gear roots are *not* all created equal. The manufacturing
methods make a huge difference in the shape of the root. The roots
are *not* a simple "radius," either.
btw, there's no way in hell you could be "in accordance with iso
6336 parts 1 thru 5" when ISO Method A (which you erroneously call
part 1) "...requires precise measurements of actual test gearing ..."
iso6336 part 1 is "calculation of capacity of spur and helical gears,
basic principles, introduction and general influence factors"
iso6336 part 2 is "calculation of surface durablility (pitting)"
iso6336 part 3 is "calculation of tooth bending strength"
there is no part 4
iso6336 part 5 is "strength and quality of materilas"
Method A that you quote is used throughout this spec and relies on
physically measuring already produced gears. It is therefore pointless
to use Method A.
The "tooth root radius" is generated by the hob used to cut the gear.
All of our hobs are to BS978 part 1 fig.1. The tooth root radius is
accounted for in iso6336 part 3 section 5.
Our proe software does not use mechanica to do the simulation. All the
relevant data we require is generated using relations.
That's right. It is pointless unless you wish your results to
match the real world (tm).
Or the shaper cutter. And it isn't a radius, it's a trochoid
because, as you said, it's *generated.* The 'radius' is not
a copy of the tool radius because during the cutting action
the tool and part move relative to each other. That's what
generated *means*. This is not like milling with a form tool.
I'm sure you accounted for that in your software, right ?
the fact that the "radius" in the root is not in fact a copy
of the radius at the tip of the cutting teeth ?
And all your hobs use 'standard' dimensions ? Which 'standard' ?
DP ? Module ? Shaper depth ? Preshave depth ? Pregrind ? Split
pitch ? Old AGMA ? new AGMA ? Composite tooth ? What do you do
about tooth thinning for backlash, ignore it ? What about long
and short addendum gearsets, you don't do that ? Never compensate
for equal wear ? never run extra-depth teeth for smoother running
and less noise ? Never use full radius tip hobs for extra fatigue
strength ? You stick with a 20* pressure angle ALL the time ?
you're aware (no, probably not) that there is no such thing as
a "standard" tooth ? Nobody who is a professional gear person
sticks with "standard" tooth relations or "standard" proportions
which means that in gears there IS no "standard" except for the
convenience of buying off-the-shelf cutting tools for commercial
grade work. "Standard" cutting tools which one then uses at non-
'standard' depths to create non-'standard' teeth for the purpose
required. In a real gear design 'standards' are *starting* points.
For that matter,if you are doing production you don't even use
"standard" pitches. Go look at an automotive gearbox some time.
Or a set of plastic gears. Or timing gears. Or powder metal gears.
Or anything intended to be quiet. Or carry large loads with tip
and root relief. In fact, you can even buy off the shelf cutters
set up with protuberance for tip and root relief ... I suppose
you account for that, too ?
There is no such thing as "standard" in the gear world, except
for beginners who don't know any better.
That's really really nice. And I'm sure you know that shaper-cut
teeth are deeper than hobbed teeth ? And a totally different shape
in the root ? I'm also sure you know that the roots of even hobbed
teeth are NOT simple radii ? Did you know that a large percentage
of gear tooth breakage occurs because of cracks starting in the
corners of the roots ? And that often deeper roots with larger
curves are effective in increasing fatigue strength ? Usually by
around an 'insignificant' 15% ? Cut with 'standard' full-radius
(on the tip) cutting tools ? You're aware that there are *several*
'standards' for different gear cutting tools and *several* 'standard'
tooth depths so that just about every set of gears is *different* ?
You account for this in your software, do you ? unh-hunh.
As I said, GIGO.
Yeah, all the relevant information probably *could* be generated
using relations. It *could* be, but dollars to donuts in your case
it *isn't* because you aren't even aware of what the relations are.
And that was my initial point : this all *can* be done in software
(in fact some people are doing it in software, but not off-the-shelf
pro/e, that's for sure) but it's a lot more work than it sounds like
and if you don't know what you are doing the results won't match the
real world which is sort of the point of analysis, so in effect it's
meaningless. In fact, worse than meaningless because it gives people
who think they know everything just because they looked in a book
and pulled some numbers off a chart the impression that they know
what they're doing. Which, at the end of the day, could be very
destructive. My point was not that gear tooth analysis *can't* be
done, but that people asking the question "How can I model gear
teeth in ProE so I can analyze them ?" really *shouldn't* be doing
this because if they don't already know what they're doing they'll
get bad results. Unless, of course, they just want to make a pretty
picture to impress the boss, in which case it's fine (as long as
no one mistakes the results for accurate.)
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