Designing Gear In Catia V5r14 Howto

DESIGNING GEAR IN CATIA V5R14 HOWTO

(HOW-TO)

This is step by step guide how to create gear using CATIA. I'm not CATIA expert. Someone might find that CATIA doesn't have much with TCP/IP and FreeBSD. However I'm B.Sc.M.E. and I find playing with CATIA very interesting. I coulnd't find much about gear design using GOOGLE or YAHOO. Since I enjoy learing and practicing what I learn, I also enjoy sharing that with others (often seen on overnet/edonkey: please share :) don't be selfish. Here is step by step guide how to make spur gear using CATIA. Note that this was inspired by document I have found at website www.piovision.com (credits go to the unknown designer).Also note that spur gear design we use today was first sugested by Swiss mathematician Leonhard Euler (15.4.1707-18.9.1783)

(Euler was also credited for this on old 10 Franc Swiss banknote, checkout Euler on wikipedia).

Robert Willis of Cambridge Universty was the first who demonstrated advantage of involute shaped teeth over popular epicycliod shaped teeth. Epicycloidal shaped teeth design belongs to Danish scientis Ole Romar, but it is also worth noticing that Nicholas of Cusa (1401-1464) and Albreth Durer (1471-1525) studied epicycloid, so it is not clear who was the first who invented gear. But let's start. This document assumes that you have basic CATIA experience and that you are not a complite moron. Also, this document assumes that you know basic spur gear geometry. Here is excellent document from www.bostongear.com in case you are not familiar. Some basic notations: rb - base cylinder radius r - pitch circle radius rk - outside circle radius rf - root radius a - presure angle (20deg) m - moduo (in our example 20) m=p/3.14159 where p is circular pitch 2r=m*z z - number of teeth (in our example 20)

When you start CATIA, go to TOOLS->OPTIONS>infrastructure-> part infrastructure and in Display select Paramteres and Relations.

Then in Options->General in Parameters and Measures select with value and With formula in Parameters Tree View.

Now it is time to go to Generative shape design:

You will see something like:

Bring Knowledge toolbar out:

Then by clicking on arrow pointed down near table icon:

Fog and f(x) are two most important things you will use for gear design:

Now it is time to enter some basic parameters that define gear This is done by clicking at f(x) icon:

And then when you see dialog box: Formulas: Part1 fist select Parameter type (real, lenght or angle) click new parameter of type and then edit value. You can do this until all parameters are enered.

When you enter parameters it is time to enter some formulas. For, r, rb, rk and rf we enter formulas by naming them and by clicking Add Formula. Formula editor will appear:

After typing all formulas and expanding specification tree you will see something like this:

It is time to add laws that will define our involute. Click on fog icon, name law x add parameters, t and x select their types and add law:

Same should be done for y. This law will help us to create points that define spline for our involute. Involute is line that is trajectory of point belonging to line that is always

tangent to base gear cylinder. It is used for tooth profile. If gears had profiles formed by straight lines they wouldn't work.

After expanding specification tree you should be able to see something like this:

Now it is time to start creating our points for involute spline.

Click on point icon, select xy plane for support and when asked to enter H and V cordinates right button click should bring menu where you should chose edit formula:

You will be prompet by edit formula dialog. Type in: Relations\x .Evaluate(0)

You should do same for V except you should use Relations\y .Evaluate(0).

You will get starting point for involute:

After repeating this step for .Evaluate(0.1)-. Evaluate(0.5) you will get this:

Click on spline icon and chose all 6 points:

Your involute is created! This is how it looks in sketcher:

From this point everything is more or less simple. Create base cirle by clicking on cirle icon. Right click on radius and chose rb from formula editor:

Now it is time to extrapolate our involute. For lenght also right click and chose formula (rb-rf)*1.5

You can use View-> zoom and pan to see what actually you are doing. Create plane. Use formula: inv(360/z)/4. You will get -4.5deg angle offset from ZY plane.

After this create rf circle and from inser menu use corner to create corner. Corner dialog will apear:

This is what you should get:

Now use trim and result should be:

It is time for simetry. Tooth starts to get shape:

Create rk circle and use trims to get tooth shape:

Circular pattern:

First select tooth (trim.3) and then for reference element select Z axis.

Gear is almost done now it is time for joining all teeth: Turn off check manifold and check consistancy:

Apply one more trim and here is our gear in normal plane:

So in case that you want all process as .CATPart here it is: SpurGEAR If you find this useful buy me a beer ;)

Designing parametric spur gears with Catia V5 Published at http://gtrebaol.free.fr/doc/catia/spur_gear.html Created by Gildas Trébaol on June 10, 2005. Part design rebuilt on October 31, 2005. Tutorial rewritten on April 9, 2007. Zipped part: spur_gear.zip (100 KB). Zipped demo: spur_gears.zip (800 KB). VRML gear: spur_gear.wrl (330 KB). The powerful CAD system Catia version 5 has no built-in tool for designing gears. When you are making a realistic design, you may need a template spur gear.

Since the geometry of a spur gear is controlled by a few parameters, we can design a generic gear controlled by the following parameters: • • •

The pressure angle a. The modulus m. The number of teeth Z.

This tutorial shows how to make a basic gear that you can freely re-use in your assemblies.

1. Gears theory and standards

1.1 Sources, credits and links • • • • • •

Most of my tutorial is based on a nice tutorial on helical gears in English at http://ggajic.sbb.co.yu/pub/catia/. I improved it a little for making an exactly symmetric tooth. The mathematic description of the involute curve is visually explained in French at http://serge.mehl.free.fr/courbes/developC.html. The formulas of the involute curve can also be found in French at http://www.mathcurve.com/courbes2d/developpantedecercle/developpantedecercle.shtml. The gear technology is explained in French at http://casm.insa-lyon.fr/engrenag/. The conventional formulas and their names in French come from the pocket catalog Engrenages H.P.C, June 1999 edition.

1.2 Table of useful parameters and formulas Here is a table containing the parameters and formulas used later in this tutorial: • •

The table is given first so that you can use it for further copy/paste operations. All the units are defined in the metric system.

•

This figure shows the a, ra, rb, rf, rp parameters defined in the table:

# Parameter Type or unit

Formula

1 a

angular degree

20deg

2 m 3 Z

millimeter integer

— —

4 p

millimeter

m*π

5 e

millimeter

p/2

6 ha

millimeter

m

Description Name in French Pressure angle: technologic constant Angle de pression. (10deg ≤ a ≤ 20deg) Modulus. Module. Number of teeth (5 ≤ Z ≤ 200). Nombre de dents. Pas de la denture sur une Pitch of the teeth crémaillère génératrice on a straight generative rack. rectiligne. Circular tooth thickness, Epaisseur d'une dent measured on the pitch circle. mesurée sur le cercle primitif. Addendum = height of a tooth Saillie d'une dent. above the pitch circle.

7 hf

millimeter

8 9 10 11

rp ra rf rb

millimeter millimeter millimeter millimeter

12 rc

millimeter

13 t

floating point number

14 yd

millimeter

15 zd

millimeter

16 ro

millimeter

17 c

angular degree

18 phi

angular degree

Dedendum = depth of a tooth below if m > 1.25 Creux d'une dent. Plus grand the pitch circle. Proportionnally hf = m * 1.25 en proportion pour un greater else hf = m * 1.4 petit module (≤ 1.25 mm). for a small modulus (≤ 1.25 mm). m*Z/2 Radius of the pitch circle. Rayon du cercle primitif. rp + ha Radius of the outer circle. Rayon du cercle de tête. rp - hf Radius of the root circle. Rayon du cercle de fond. rp * cos( a ) Radius of the base circle. Rayon du cercle de base. Radius of the root concave Congé de raccordement à la corner. racine m * 0.38 (m * 0.38) is a normative d'une dent. (m * 0.38) vient de formula. la norme. 0≤t≤1

Sweep parameter of the involute curve.

Y coordinate rb * ( sin(t * π) of the involute tooth profile, cos(t * π) * t * π ) generated by the t parameter. rb * ( cos(t * π) + Z coordinate sin(t * π) * t * π ) of the involute tooth profile. Radius of the osculating circle of rb * a * π / 180deg the involute curve, on the pitch circle.

Coordonnée Y du profil de dent en développante de cercle, généré par le paramètre t. Coordonnée Z du profil de dent en développante de cercle. Rayon du cercle osculateur à la courbe en développante, sur le cercle primitif. Angle du point de la sqrt( 1 / cos( a )2 - Angle of the point of the développante à 1)/ involute l'intersection avec le cercle PI * 180deg that intersects the pitch circle. primitif Angle de rotation pour obtenir atan( yd(c) / zd(c) Rotation angle used for making un )+ a roue symétrique par rapport au 90deg / Z gear symetric to the ZX plane plan ZX

1.3 Notes about the formulas (in French) Formule N°11: explication de l'équation rb = rp * cos( a ): • • • •

La crémaillère de taillage est tangente au cercle primitif. Au point de contact, a définit l'angle de pression de la ligne d'action. La ligne d'action est tangente au cerce de base. On a donc un triangle rectangle à résoudre.

Formule N°12:

Paramètre de balayage de la courbe en développante.

•

Entre le cercle de pied et les flancs des dents, prévoir un petit congé de raccordement pour atténuer l'usure en fatigue.

Formules N°14 et N°15: explication de zd = rb * cos( t ) + rb * t * sin( t ): • • • •

La développante est tracée sur le plan YZ, qui correspond à la vue de face dans Catia. Le premier terme rb * cos( t ) correspond à une rotation suivant le cercle de base. Le second terme rb * t * sin( t ) correspond au déroulement de la développante. Cette expression rappelle que le rayon de coubure de la développante vaut rb * t.

Formule N°16: •

Pour simplifier le dessin d'un engrenage, on peut éventuellement remplacer la développante de cercle par un arc de cercle.

A good approximation of a curve at a given point is the osculating circle.

Une bonne approximation d'une courbe en un point donné est le cercle osculateur.

The osculating circle of a curve at a point Le cercle osculateur à une courbe en un point shares with the curve at that point: partage avec la courbe en ce point: •

A common tangent line (continuity of the 1stderivative).

•

Une même tangente (continuité au 1erdegré).

A common radius of curvature • Un même rayon de courbure nd (continuity of the 2 derivative). (continuité au 2nd degré). • Cercle osculateur à la courbe développante au niveau du diamètre primitif: o L'angle de la dévelopante est égal à l'angle de pression a. o Le rayon du cercle osculateur est donc: ro = rb * a * π / 180.

•

Formule N°17: • • •

En réalité, la développante est déphasée par rapport à la figure ci dessus. Pour exprimer ce déphasage, on calcule le paramètre angulaire c au point où la développante coupe le cercle primitif. On a alors: o o o o

zd(c)2 + yd(c)2 = rp2 rb2 * ( 1 + c2 ) = rp2 cos(a)2 * ( 1 + c2 ) = 1 c2 = 1/cos(a)2 - 1

2. Start and configure the generative shape design workshop •

The part design workshop is not sufficient for designing parametric curves.

•

So, we switch to the generative shape design workshop:

Next, we configure the environment for showing parameters and formulas:

•

We set the 2 highlighted check boxes:

•

Now the tree of your part should look like this:

3 Enter the parameters and formulas 3.1 Define the primary generation parameters •

Switch to the Generative Shape Design workshop and click on the f(x) button:

•

Then you can create the gear generation parameters: 1. Select the unit (integer, real, length, angle, …). 2. Press the create parameter button. 3. Enter the parameter's name. 4. Set the initial value, used only if the parameter has a fixed value.

•

•

Now your tree should look like this:

3.2 Define dependent parameters • • •

Most of the geometric parameters are related to a, m, and Z. You don't need to assign them a value, because Catia can compute them for you. So, instead of filling the initial value, you can press the add formula button.

•

Then you can edit the formula:

3.3 Check the primary and computed parameters •

Set the following option in order to display the values and formulas of each parameter:

•

Now your tree should display the following parameters and their formulas:

3.4 Parametric laws of the involute curve Up to now, we have defined formulas for computing parameters. Now we need to define the formulas defining the {Y,Z} cartesian position of the points on the involute curve of a tooth. We could as well define a set of parameters Y0, Z0, Y1, Z1, … for the coordinates of the involute's points. However, Catia provides a more convenient tool for doing that: the parametric laws. In order to create a parametric law:

•

click on the fog button:

•

Enter the formulas #14 and #15 of the 2 laws used for the Y and Z coordinates of the involute curve: o o

yd = rb * ( sin( t * PI * 1rad ) - cos( t * PI * 1rad ) * t * PI ) zd = rb * ( cos( t * PI * 1rad ) + sin( t * PI * 1rad ) * t * PI )

•

Notes about the formula editor of Catia: • •

The trigonometric functions expect angles, not numbers, so we must use angular constants like 1rad or 1deg. PI stands for the π number.

4. Create a geometric body and start inserting geometric elements In Catia, the PartBody is intended for mechanical surfaces. For geometric constructions, you need to work in a geometric body: •

Create it with the Insert / Open Body top menu:

•

Then, you can use the buttons on the right toolbar for inserting different geometric elements.

•

Catia assigns a default name to each geometric element, but you can rename it with a contextual dialog opened with the right button / properties menu of the mouse:

5. Make the geometric profile of the first tooth The following steps explain how to design a single tooth. The whole gear is a circular repetition of that first tooth. 5.1 Define the parameters, constants and formulas Already done in the section related to parameters and formulas.

5.2 Insert a set of 5 constructive points and connect them with a spline The position of each point is defined by the yd(t) and zd(t) parametric laws:

•

Define 5 points on the YZ plane.

•

•

In order to apply the involute formulas, edit the Y and Z coordinate of each point and enter the values of the parameter from t = 0 to t = 0.4 (most gears do not use the involute spiral beyond 0.4) For example, for the Y coordinate of the involute's point corresponding to t = 0.2:

•

Make a spline curve connecting the 5 constructive points:

5.4 Extrapolate the spline toward the center of the gear Why do we need an extrapolation ? • • •

The involute curve ends on the base circle of radius rb = rp * cos(20) ≈ rp * 0.94. When Z < 42, the root circle is smaller than the base circle. For example, when Z = 25: rf = rp - hf = rp - 1.25 * m = rp * (1 - 2.5 / Z) = rp * 0.9. So the involute curve must be extrapolated for joining the root circle.

Extrapolate the spline: •

Start from the 1st involute point.

•

The length to extrapolate is empirically defined by the formula f(x) = 2 * m:

5.5 Rotate the involute curve for the symmetry relative to the ZX plane Why do we need a rotation ? On the extrapolated involute curve designed in the Y, Z coordinate system … RED the contact point on the pitch circle has an unconvenient position. It is more convenient to draw a tooth that is symmetric on the ZX plane, because it makes it easier to control the angular position of a gear in a mechanism : LIME On the rotated involute curve … the two contact points of the tooth … that are located on the pitch circle at ± 90deg / Z … CYAN MAGENTA are symmetric relative to the ZX plane.

•

The colors above correspond to the following geometric elements:

•

For computing the rotation angle, we need first to compute the involute parameter or the pitch circle (formula #17):

Is it true ? In order to check it, you can build two temporary elements:

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Insert another point and apply the involute formula with the c parameter:

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Then, insert a half-circle having the radius of the pich circle rp.

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Check that the involute point with the c parameter is actually located on the intersection of the pitch circle and the extrapolated spline curve:

•

Once the c parameter is checked, the temporary point and the temporary circle can be deleted.

Now, we can rotate the extrapolated curve, so that the first gear tooth is symetric relative to the ZX plane: •

We use the formula #18 for computing the phi rotation angle in 2 steps: 1. The curve is rotated by atan( yd(c) / zd(c) ) so that the intersection between the involute and the pitch circle (the red point on the figure) is moved to the ZX plane. 2. Then, curve is rotated by ¼ of the gear period: 90deg / Z (the left lime point on the figure), so that the ZX plane corresponds to the median plane of the first tooth.

•

A rotation operation is applied to the extrapolated spline, using the phi rotation angle:

5.6 Draw the outer circle and the root circle •

We insert two half circles having a radius equal to ra and rf, respectively.

•

The figure below shows how to configure the outer circle:

5.7 Insert a rounded corner near the root circle •

The corner between the extrapolated involute curve and the root circle has a radius defined by the rc parameter.

•

Catia asks you to select an arc (in red) out of 4 possible geometric solutions (in blue):

5.8 Create the rounded corner of the next tooth Why are we going up to the next tooth ? •

Initially, I designed a symmetric profile for the first tooth and I duplicated it Z times:

•

But then, the generated profile was interrupted between each tooth by a fake edge:

•

For preventing that, I build now the whole profile between consecutive teeth on the root circle:

Now we can build the symmetric corner: •

On the figure above, you can see: o A vertical line tracing the ZX plane. o An oblique line tracing the median plane between consecutive teeth. o This plane corresponds to the ZX plane rotated by 180deg / Z around theX axis.

•

The following figure shows how to define that median plane:

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Now, this plane is used for defining a symmetric rounded corner on the root circle:

5.9 Assemble the different elements of the first tooth Now, we have to cut, fill and join the different elements of the 1st tooth:

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Cut the segment of the extrapolated spline between the outer circle and the rounded corner.

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Define a symmetric profile relative to the ZX plane, for the other side of the 1st tooth:

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We could cut the root circle and the outer circle, but instead we define two arcs having a radius equal to rf and ra, respectively:

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The last operation consists in joining all the elements of the 1st tooth:

6. Build the whole gear profile and extrude it The gear profile is just a circular repetition of the tooth: •

We define a repetition around the X axis.

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The number of instances is controlled by the Z parameter (number of teeth):

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The first tooth and the duplicated teeth are joined for making the whole gear profile:

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Now, we can switch back to the part design workshop (see the green arrow) and extrude the gear profile:

7. Cut the gear wheel The gear wheel is cut after the extrusion, because each application requires a specific wheel thickness: • •

In a real factory, the teeth of the gear would be machined after the gear wheel is cut on a lathe. In a CAD design, it is simpler to make the gear wheel with a groove, after the extrusion of the teeth.

•

That wheel design is semi-parametric: the external diameter and the 20deg chamfer are dependent of ra, but the bore diameter and the thickness are adjusted manually on the sketch:

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Now, you can add pocket(s) for transmitting the torque between the gear wheel and a key or a splined shaft.

8. Check the parametric generation Now you can play with the Z and m parameters and generate any spur gear:

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If Z is equal to 13:

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If Z is equal to 15:

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