mechanical engineers - help me out with some geometry stuff!

[gmacrae]

Hey guys, im puzzling myself stupid (well, more stupid) over a problem I've come up with;

I'm changing a factory twin wiper system to run just a single big wiper from the centre of the windscreen, basically removing the drivers side wiper/rotor/linkage and having the centre one sweep as much of the windshield as possible by modding it to sweep more than it's current 180degrees (to say 240 degrees). It's a pretty simple task except working out how much Ill need to shorten the little arm on the wiper rotor by (cut + weld).

Allow me to dazzle you with my paint skillz:



So currently the wiper sweep 180 degrees, from 0 to 90 then back to 0 as the wiper motor does its 360degree cycle. Because the arm on the wiper motor is 40mm long, and the arm on the rotor is 55mm long (centres to centres).

If the rotor arm was the same length as the motor arm (40mm), the wiper would do 360 instead of 180.

So toyota making the rotor arm 15mm longer cut the wiper's sweep by 180 degrees...

I only want the sweep cut by 120 degrees

so (15mm divided by 180) X 120 = 10mm...

So to get 240 degree sweep (0 to 120 then back to 0) the rotor arm needs to be only 10mm longer than the motor arm (50mm instead of its current 55mm).

Only thing is, i'm not completely sure I've got this right, the brain doesn't handle this shit too well these days so I don't trust my reasoning, anyone care to chime in?

Cheers

Gene

[sergei]

Have you considered that the swipe angle also depends on the length of the light blue arm?
I think to calculate this properly you will need to involve pi sin and maybe cosin .
It is not linear proportion sort of thing....

[sergei]

http://en.wikipedia.org/wiki/Piston_motion_equations



Join another diagram (circle part) on the top of this diagram and you have your model to pull the formula out...

But you need to find the measurement of the long arm (blue on your diagram).

Once you do, I can calculate it this for you in the evening....
EDIT: also it is very important to find one of the angles when the arm is fully extended (eg: between blue arm and motor arm).

[gmacrae]

uh...

haha sweet man thanks, I just measured the arm/link - 500mm. Nice easy number to work with (for you )

If you could work that out later on that'd be awesome, thanks again!

[sergei]

Actually I need is another measurement -> distance between rotating centres (centre of the motor to fixed pivot on the wiper, instead of measuring angle I was talking about above, as it is easier to measure).

[gmacrae]

that measurement is also 500mm... its a bit tricky to measure but that's within 5mm or so (between rotating centres).

Cheers

[sergei]

that measurement is also 500mm... its a bit tricky to measure but that's within 5mm or so (between rotating centres).

Cheers

aha so the centres are offset.
Will make a construct when I get home and post the findings...

[gmacrae]

you da man Mr Mad Russian. 'preciate the help aye, shit's way over my head, was either drunk or asleep thru school c maths, dammit

[rx7guy]

Pictures are an easy way

Original:



47mm arm:

[gmacrae]

woah, RX7GUY, that's sweet man thanks... but I'm not completely following it... the little arm on the right, i assume is the wiper motor arm, it's not moving/rotating is it? Are you saying i need the wiper rotor arm (on the left) to be 47mm?

[gmacrae]

oh wait, yea i see it is moving 180, still confused lol

it definitely only moves the wiper 0 - 90 - 0 (within a few degrees) as it is, so i'm struggling to follow the diagram

[gmacrae]

ahhh! 138-45=93*

149-33=116*

Sorry, the degrees from horizontal was throwing me off...

Thats great though thanks! 47mm arm eh, might go for 46 i think, few degrees past 120 wont hurt.

What software did you use to do that?

[fevil]

Using this calculator http://ostermiller.org/calc/triangle.html to find the base of a triangle that will give you the travel distance to make the wiper arm go through 120deg of arc. Equals 97mm, then the throw at the wiper motor has to be half that, 47.5mm.

[sergei]

Here are my calculations on your current setup:

Two triangles involved (close position and far position):
http://www.wolframalpha.com/input/?i=triangle+55%2C+540%2C+500

4.163 134.5 41.3


http://www.wolframalpha.com/input/?i=triangle+55%2C+460%2C+500

4.511 41.13 134.4

The angles we are concerned with are following (in bold above):

134.5 and 41.13, current sweep angle is 134.5-41.13= 93.2 degrees.

[sergei]

Here is the construct I used to calculate those triangles:

[sergei]

By my calculations the 46mm arm will give you 120.9' (which is 241.8' by your terms).
the 47mm arm will give you 116.6' (which is 233.2' by your terms).

Which is in accordance with previous posts...

[sergei]

Using this calculator http://ostermiller.org/calc/triangle.html to find the base of a triangle that will give you the travel distance to make the wiper arm go through 120deg of arc. Equals 97mm, then the throw at the wiper motor has to be half that, 47.5mm.

Although this calculation (using base of the triangle of the wiper arm) is approximately close, but is incorrect as there are two triangles involved (see my construct above), and in this case works ok because of the proportion of long to short arms is very large (thus making angles very narrow).

Plugging my numbers in calculator reveals that this method is off by ~1 degree, which is is close enough (but still not exactly correct ).

Saying that, the things at work here (I am fairly certain the mechanism has at least 1 mm of play, if not more, the measurements where at least +/- 1mm accurate) thus if OP makes the thing at ~ 45mm to 48mm, he will be in the right ballpark.

[gmacrae]

yea that's mint guys, my 50mm arm wouldn't have been close at all

Thanks again aye, i'll try to post up pics of the process in this thread if any others want to copy. Its in an AW11, which works well because the centre wiper is mounted literally dead centre, but should be pretty easy to apply it to other cars if you wanted to i guess

[wde_bdy]

You might have trouble getting the swept area far enough over to the drivers side in order to keep it legal/safe, factory setups tend to have a blade that isn't fixed in relation to the arm to get the appropriate coverage. This is especially a problem with short screens.

Callum

[gmacrae]

Yea I was thinking that but after doing a few checking on the winscreen with the 120degree sweep and the longest wiper blade that will fit, it actually looks pretty good. I'll double check this though