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symmetric constraints

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symmetric constraints

Does anyone know if constraining two surfaces of a rotationally symmetric
part to a fixed theta (with the cylindrical coordinate system Z-axis aligned
with the axis of symmetry) is equivalent to a Cyclic Symmetry constraint? I
have seen 90 degrees of a part constrained to fixed X on one side and fixed
Y on the other side (Z is the rotation axis) as a recommended technique and
was wondering if smaller included angles would work using a Cylindrical
CSys.
I do understand that the loads would have to be rotationally symmetric also
in this technique, but I believe they would not if a Cyclic Symmetry
constraint was applied.

Can anyone confirm or refure these suppositions?

Thanks very much for the help,

Alan Carlson
Discern Engineering
1681 E. Hennepin Ave. Suite 180
Minneapolis, MN 55414-2562
(612)617-9258
(612)378-5503 FAX
(612)270-3926 cell
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2 REPLIES 2

RE: symmetric constraints

Alan,

Constraining the two cut sides of a "rotationally symmetric" body in theta
is NOT the same as using a cyclic symmetry constraint .... unless the loads
applied to the body are uniformly distributed about 360º in a radial and/or
axial manner. Any loads which are applied to the body in the "theta"
direction (torque about "Z") require a cyclic symmetry constraint set, even
if they are uniformly distributed, or "rotationally symmetric". This is
because torques applied about the axial direction would produce
circumferential deformations (theta direction). These deformations would
not be permitted if a "theta" constraint is used in place of cyclic
symmetry.

Regards,

Chris

Chris Kaswer
Sr. Analytical Engineer
The Timken Corporation
Automotive - Needle Bearing Solutions
59 Field Street
Torrington, CT 06790-1008
(860) 626-2426 - voice
(860) 496-3638 - fax
-

RE: symmetric constraints

Alan

The way I think of it is like this.

With cyclic symmetry you're looking at a pie slice. The two surfaces that
you attach the cyclic symmetry constraints to must be the same shape and
size (just rotated around the axis a bit) They do not have to be flat -
they can be a wavy surface if you want.

Theses two surfaces are then coupled - so that any deflection that occurs at
one is transferred to the other. So their stiffnesses effect each other.

By applying separate and independent constraints to two surfaces you won't
get this coupled action, which is an important part of cyclic symmetry.

But it all really depends on what you're trying to achieve. It could be
that independent cinstrainits are what you're after - they'll give more or a
"mirror" effect rather than a "rotated" effect

---
David Reid
davidr@optima-design.co.uk
Optima Design Services Ltd
Direct +44 1491 411369
Mobile +44 7760 253644
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