I've put two long rectangle at the both ends of a pulley joint, but I want the long rectangle to move only vertically without rotation. I've use the following method to make it's rotation fix as this.body.IsFixedRotation(true); and this.body.fixedRotation = true; does not work.

this.body.SetAngle(0);

and use this to make the platform not to be affected by the x velocity

this.body.SetLinearVelocity(new Box2D.Common.Math.b2Vec2(0,this.body.GetLinearVelocity().y));

However, the problem comes, I've placed the same codes at the the object that I place on top of the long rectangle, they seems to follow the rotation before the fixed rotation(long rectangle looks visually correct, rotation is set to 0 and moves vertically). making the top object slide and jittering. and got through the long rectangle. Is there a way for me to make the pulley to move stable-ly up and down without rotation? Thanks.