A few people expressed interest in seeing the spur gear differential I'm
building for my grandson's pedal car so I put some photos up. They can be
The text editor wasn't being very cooperative when I did this so there is no
write up there yet. You can tell the size by comparing to the six inch scale
included in the photos. The pinions are 1" OD and the side plates are 3.6"
OD. The bearings on the output shafts are 6204. It will be enclosed in a
16ga "can" and sealed with a small amount of oil and a sprocket or toothed
pulley will be bolted to the side of the pinion carrier as the input.
Lots of well done parts here, great work.
Can you explain how it works? When I think of a differential, I think of one
driven input shaft able to split power to either of two output shafts. Is
the planetary assembly the input shaft? How does it hook up to the output
shafts in the other pics? Or am I totally off base here?
The ones Northern sells look like Tecumseh axles. They are fine but are too
heavy for this application. This one is for a pedal car for my grandson. It
will be a scaled down version of the car I built for myself. You can see it
here if you want to.
Yes. A sprocket will be bolted to the side of the planetary carrier and
connected via roller chain to the pedals. The short shafts extending from each
side with the bearings on them are the output shafts. The planetary carrier
floats on those shafts. The bearings will be mounted in the frame and the
shafts will be extended out to the wheels. It's easier to see how it works if
you compare it to the more common bevel gear type. Just picture the side gears
being changed from bevel to spur and each spider gear being replaced by a pair
of pinions that are meshed with each other only in the space between the side
I first saw that type in a Cletrac style steering transmission for an old
crawler. It took me a while to figure out that "yeah, that really is a
For all who would like to get a spur gear differential, there is a similar unit
used on Snapper walk-behind lawnmowers. You might get one from a mower with a
blown engine -- cheap. The differential/axle assembly is almost ageless,
having been produced for over 30, and the design and some early production are
still going strong. To see an exploded view, go to:
Hmm... In a symmetrical open differential, that is, one that has side gears of
size and it has no limited slip elements added, there is no reduction in the
sense of the word. In an auto rear axle there is a reduction and direction
stage at the input provided by the ring and pinion gears but it has nothing to
the action of the differential. Now the interesting thing about a differential
the sum of the outputs will always be 2 times the input speed. For example if
input is turning at 1 RPM and the load on the output shafts is equal, the left
will be turning at 1 RPM and the right shaft will be turning at 1 RPM. If you
the input at 1 RPM and unbalance the load on the output shafts by completely
the left shaft so its output is 0 RPM and also remove the load from the right
will turn at 2 RPM. Another way to look at it is to lock the carrier and rotate
the shafts 1 RPM clockwise. The other shaft will rotate 1 RPM
this case the _difference_ is 2 ( 1- (-1)). If you use the shafts as inputs
carrier as the output it becomes the mechanical analog of an electronic
amplifier with a gain of 1.
In an earlier post I mentioned a Cletrac style steering transmission for an old
crawler. They have an interesting arrangement with two sets of planetary gears
side. When the steering brake is fully applied to one side that track doesn't
a complete stop. The brake actually operates on a planetary carrier and the
that side slows to about one-fourth to one-tenth (depending on ratios of the
of the input speed. The other side speeds up to 1 3/4 to 1 9/10 of the input
no brake is applied it's just an open differential. More modern tracked
much more sophisticated systems that allow them to spin around their own center
having multiple inputs and travel at higher speeds while maintaining stability
are still just a big box full of heavy duty planetary gears.
Marks and Machinery's have some information on epicyclic gears and there is
stuff on the net. I really like to work with them but sometimes the ratios
readily apparent. It's usually 1+ something or 1- something. I hope that
this rambling is the answer to your question.
On 25 Apr 2004 04:37:56 GMT, firstname.lastname@example.org (Richard Coke) shouted from
Olds used a planetary diff in the original Toronado FWD transaxle.
"An honest man doesn't need a long memory"- Jesse Ventura
Did it use helical cut gears? I'm guessing that it did since they are much
quieter than straight cut gears. Somewhere I saw a limited slip diff. that used
the axial thrust of helical gears as the sensing and/or locking mechanism...
only a vauge memory, can't remember where.
Just to be a little bit pedantic, as far as I know _all_ mechanical diffs use
planetary gears. Even a bevel gear diff. is a planetary system. The axis of
the spider gears is a right angles to the axis of the side gears but it is
still a planetary arrangement. It has the advantage of having fewer number of
parts than a spur gear system to accomplish the same thing.
The Torsen differential does something like that.
howstuffworks.com has some good info about it.
Jim Wygralak | PGP (or gpg) signed messages get a free ride past my spam
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