# equation driven sketch?

• posted

I am trying to develop a sketch of a timing belt with a fixed length, running over 2 pulleys and an idler. I want to see how much the idler moves when the 2 pulleys center distance is changed. I believe that there was a tutorial on this a few releases ago but I can't find it using sw2003. To date I haven't used equations so any help would be greatly appreciated.

• posted

Equations are relatively simple. They follow all the rules of mathematics Parentheses first then mult and div then add and subtract. You can use PI and sin cos and tan but this is in radians so you may have to convert to degrees first. One thing that I use often to save time is you can get an arc length dimension by selecting an arc and then selecting its endpoints. Then you can just select the dimension and it is inserted into your equasion instead of calculating it out. Here is an example of an equasion that I used to draw a deformed end of a tube that becomes the shape of a slot.

"Length of Flat@Sketch2" = ( "Diameter of Tube@Sketch1"*pi-("Arc Length@Sketch2"*2))/2

This will always keep the "circumference" if you will, of the flattened sketch the same as the circumference of the Origional tube.

Corey Scheich

appreciated.

• posted

I appreciate the response! Unfortunately I'm still at a loss! What I have is a sketch of the pitch line of a timing belt, the driving dimensions are the center to center of 2 pulleys and their pitch diameters, the pitch dia. of the tension pulley and its center point coincident to a construction line representing its movement to take up the slack.

The current way I get the profile of the pitch line is by measuring the sum of all the segments, then moving the tension pulley to get close as possible to the length of the belt. It would be nice to be able to see the length change dynamically while moving the location of the tension pulley, or even better to input the length of the belt as a driving dimension.

• posted

or even

This can be done with a little bit of setup. The sweep sketch of the belt you want to have a finite length. So what you will need to do to accomplish this is.

Put arc length dims on the 3 radii of the belt the 2 driving pulleys and one that would represent the arc of the tensioner pulley.

Also add a dimension to the line between the 2 driving pulleys

Once you have done this there are only 2 lengths left to figure out. The length of the two tangent lines that connect the arc of the tensioner pulley and Each pulley. If you subtract the lengths of the defined arcs and lines from the belt length you are left with the combined length of the final line. The only problem is you will have to watch out because as your angle increases so does the length of each arc because of the tangents.

I can't test this all out to refine but it should get you started.

• posted

I had trouble with this a while back too. I was not able to figure out how to set up an equation that used driven dimensions, namely, the arc lengths around the pulleys as the center to center distance changed. I had only a few fixed belt lengths to try, and I was wanting to know what my c-c distance would be without the use of an idler.

Don't tell anybody, but I ended up using GrafiCalc. Worked pretty slick...

• posted

You can do exactly what you want by using the solver in Excel. It will require the use of the little-known but extremely useful 'Excel to SW link template' available on the SW website at:

This tool allows you to link dimensions in SW to cells in Excel and drive them in either direction.

A simple belt/idler sketch will probably have 3 arcs and 3 lines. Give each of these belt segments a driven length dimension and link each of these 6 dimensions to a corresponding cell in Excel. Choose to have SW drive Excel. In Excel, add these cells together in another cell which will be the total belt length.

Next, link the driving dimension that positions the idler to a cell, choosing to have Excel drive SW this time. Now you can change the idler position from Excel and the segment length and total length cells will adjust accordingly.

This is where the Excel solver comes in. The solver is an optional add-in that you select when installing Excel. (You can also add it later.) The solver allows you to say 'make cell A1 equal to 300mm by changing the value of cell A6' (or whatever). Excel will then automatically drive SW through multiple iterations until it finds the solution. (It only takes a few seconds.) In your case, of course, you would ask it to solve for total belt length by changing the idler position.

Basically, you get Excel to do automatically what you're doing manually, that is, to move the idler a little bit at a time until the belt length is correct.

• posted

Would someone mind downloading the ExcelLink.zip and emailing it to me sometime. I let my maintenance (since 95) expire when 2003 came out and I haven't been inspired to renew it yet (although 2004 looks promising).

TIA, Gary Knuts> You can do exactly what you want by using the solver in Excel. It will

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The people on this group are the greatest!!!!!!!!!!!!!!!

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Exellent! Thanks so much.

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If dimension click on each endpoint of an arc then the arc is self you get an arc length dimension you can use in your equation.

steve

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